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Singlet fission in covalently-linked tetracenes

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Singlet fission in covalently-linked tetracenes

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Singlet fission in covalently-linked
tetracenes
by
Nadezhda V. Korovina
A Dissertation Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(CHEMISTRY)
December 2016

i

EPIGRAPH

"Ultimately, when you die, you're oxidized. You're only alive for kinetic reasons, not
thermodynamic reasons."

-MET

ii

DEDICATION

To my mother Olga Maier and everyone who helped me get to this
point in my life.

iii

ACKNOWLEDGEMENTS
I would like to thank my research advisor, Prof. Mark Thompson for accepting me into his
group two and a half years into my graduate work. During my four year stay in Prof.
Thompson’s lab, his invaluable mentorship has helped me advance not only as a scientist, but
also as a person. He took time to engage in scientific discussions and encouraged me to contend,
rather than blindly accept, the scientific opinions of others (including his own). He allowed me
freedom in my research endeavors, while providing the breadth and depth of his expertise as
guidance. His professionalism and kindness has created a comfortable social environment in his
group, which in turn imbues confidence in his students. It was an honor to be part of his research
group and have such a distinguished scientist as my advisor.
Additionally, I would like to thank Prof. Stephen Bradforth for his mentorship, and for the
fruitful scientific collaboration. He truly cares about the education that the department gives to
its students, as well as the wellbeing of the graduate students in the department. Despite being
incredibly busy during his time as the department chair, he always took the time to listen to my
concerns and provided his very honest opinion regarding all matters. His rigorous evaluation of
my work has made me a stronger scientist.
I also would like to thank my dissertation committee member Prof. Vitaly Kresin and my
qualifying exam committee members Prof. Smaranda Marinescu and Prof. Ralf Haiges for taking
the time to evaluate me as a scientist and provide helpful feedback. Prof. Kresin, Prof. Marinescu
and Prof. Haiges are also effective educators and it was a valuable experience to take their
courses.
Another person who has been instrumental in my Ph.D. work has been Prof. Peter
Djurovich. Peter is very knowledgeable in the area of photophysics and photochemistry, and was
always available to impart his expertise.
I also would like to thank the Burg Foundation for funding my teaching experience via the
Burg Teaching Fellowship, and Prof. Jessica Parr for being an excellent mentor for teaching
chemistry.
Science is becoming increasingly multidisciplinary, and my thesis work would not be
complete without the invaluable transient absorption contributions and theoretical insights
provided by the groups of Prof. Stephen Bradforth and Prof. Anna Krylov. The transient
absorption contributions from Dr. Saptaparna Das, Jimmy Joy, Laura Estergreen, and
computational insights from Xintian Feng were instrumental for helping us paint a complete
picture of singlet fission in the covalently-linked tetracenes.
I would like to express my gratitude to the entire remarkable chemistry faculty at USC with
whom I have had the honor of interacting. I have obtained a valuable education at USC, which I
can be proud of.
During my time in Prof. Mark Thompson’s lab, I had the honor of mentoring two
wonderful undergraduate students – Zachary Nett and Cassidy Feltenberger. They have
contributed in many ways to this thesis, but more importantly they have allowed me to share the
iv

joy of chemistry with them. Watching them learn and advance as scientists has been one of the
most rewarding aspects of my graduate work at USC.
I would like to thank all of my colleagues - post docs, graduate and undergraduate students,
who have made my stay at USC enjoyable. (Anastasia Gunina, Rasha Hamze, Betsy Melenbrink,
Dr. Ana Gamboa, Adam Ung, Antonina Nazarova, Dr. Anastasia Kadina, Eric McAnally,
Natalie Orms and many more).
The people who make this department function smoothly deserve a special thanks: Michele
Dea, Judy Fong, Magnolia Benitez, Jaime Avila, Allan Kershaw, and Frank Devlin.
I am thankful for my undergraduate advisors, Prof. John Spence and Prof. Benjamin
Gherman, who kept in touch during my Ph.D. and provided encouragement and support in my
scientific endeavors.
I would like to thank the people who were the closest to me and supported me in every way
during this journey: Eric Mansur and William Wellman. Eric provided support during the
difficult time of changing research groups, and encouraged me throughout. William has gone
above and beyond to help me get through this busy time of writing my thesis. He has done
everything from making me lunches to editing the grammar and formatting in my thesis. He was
there with me during the long nights of writing and assuaged my worries. His support
strengthened my confidence and helped me cross the finish line. I greatly appreciate his caring
for me, and look forward to our companionship in the time to come.
Finally, I would like to thank my loving mother. Her strength has been my inspiration. Her
life has shaped my character. She taught me to be thorough in tasks which require rigor and
discipline, but allowed me complete freedom in creative endeavors, which has translated into my
success as chemist. It is great honor to share genes with such a kind, caring, thoughtful, smart,
creative and beautiful person.

v

TABLE OF CONTENTS
Epigraph ......................................................................................................................................... i
Dedication ...................................................................................................................................... ii
Acknowledgements ...................................................................................................................... iii
Table of Contents ...........................................................................................................................v
List of Tables .............................................................................................................................. viii
List of Figures ............................................................................................................................... ix
Abstract ....................................................................................................................................... xiv
Chapter 1: Introduction to singlet fission ......................................................................................1
1.1 Organic photovoltaics and singlet fission ...........................................................................1
1.2 Definition and history of singlet fission...............................................................................2
1.3 Mechanism of single fission ................................................................................................6
1.3.1 Step 1: formation of the correlated triple pair ........................................................10
1.3.2 Step 2: formation of the separated triplets .............................................................15
1.3.3 Motivation for studying SF in covalent chromophore assemblies.........................18
1.3.4 Overview of SF in covalent dimers .......................................................................20
1.3.5 References ..............................................................................................................28
Chapter 2: Photophysical properties of ethynyltetracene. ..........................................................32
2.1 Introduction ........................................................................................................................32
2.2 Results and discussion .......................................................................................................33
2.2.1 Synthesis and
1
H NMR of ET-TMS and PET ......................................................33
2.2.2 Computational studies of ET-TMS and PET .........................................................36
2.2.3 Steady state photophysical data (solution) ............................................................37
2.2.4 Steady state photophysical data (thin film) ...........................................................42
2.2.5 Transient absorption and singlet fission rate of ET-TMS ......................................45
2.3 Conclusions ........................................................................................................................48
2.4 References ..........................................................................................................................50
Chapter 3: Singlet fission in cofacial dimers - o-BETB and BETX: the role of excimers .........51
3.1 Introduction ........................................................................................................................51
3.1.1 Excimers ................................................................................................................52
3.1.2 Excimers and SF ....................................................................................................55
3.2 Results and Discussion ......................................................................................................57
3.2.1 Synthesis and
1
H NMR of o-BETB and BETX .....................................................57
3.2.2 Molecular structures of o-BETB and BETX .........................................................61
3.2.3 Steady state photophysical properties of o-BETB and BETX ...............................66
3.2.4 Photophysical properties of neat thin films of o-BETB and BETX ......................69
vi

3.2.5 Transient absorption and the SF mechanism in o-BETB in various media ...........70
3.2.6 Transient absorption of BETX ...............................................................................81
3.3 Conclusions ........................................................................................................................83
3.4 References .........................................................................................................................85
Chapter 4. Singlet fission in ortho- , meta-, and para-BETB – the role of chromophore
coupling. ........................................................................................................................................88
4.1 Introduction ........................................................................................................................88
4.2 Results and discussion .......................................................................................................93
4.2.1 Synthesis and
1
H NMR of m-BETB, p-BETB-ethex and p-BETB-ohex ..93
4.2.2 Computational studies of m-BETB, p-BETB-ethex and p-BETB-ohex ....99
4.2.3 Steady state photophysical properties of m-BETB, p-BETB-ethex and
p-BETB-ohex ...........................................................................................102
4.2.4 Time resolved emission and kinetic model for SF in m-BETB ...............109
4.2.5 Transient absorption of the dimers in solution ........................................117
4.2.6 Photophysical properties of neat thin films of the dimers .......................125
4.3 Conclusions ......................................................................................................................127
4.4 References ........................................................................................................................129
Chapter 5. Singlet fission in tetra-(phenylethynyltetracenyl)benzene – the role of exciton
delocalization. .............................................................................................................................131
5.1 Introduction ................................................................................................................131
5.2 Results and discussion ...............................................................................................136
5.2.1 Synthesis and
1
H NMR of TETB, Ph-o-BETB and Ph-TETB ................136
5.2.2 Molecular and electronic structure of dimer vs. tetramer ........................145
5.2.3 Steady state photophysical properties of Ph-o-BETB and Ph-TETB ......147
5.2.4 Transient absorption of Ph-o-BETB and Ph-TETB .................................155
5.2.5 Kinetic model and rates of singlet fission in Ph-o-BETB and Ph-TETB 158
5.3 Conclusions ................................................................................................................160
5.4 References ..................................................................................................................161

Bibliography ...............................................................................................................................164
Appendix 1. Intramolecular charge transfer in an asymmetric tetracene dimer. ......................171
A1.1 Introduction .............................................................................................................171
A1.2 Results and discussion ............................................................................................174
A1.2.1 Synthesis and
1
H NMR of BTEY ............................................................174
A1.2.2 Photophysical properties of BTEY as a function of solvent polarity ......177
A1.2.3 Transient absorption of BTEY .................................................................181
A1.3 Conclusions .............................................................................................................183
vii

A1.4 References ...............................................................................................................184

Appendix 2. Experimental details. ............................................................................................186
A2.1 Instrumentation and general information ................................................................186
A2.1.1 NMR and mass spec. ...............................................................................186
A2.1.2 UV-Vis, fluorometer and TCSPC ............................................................186
A2.1.3 Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) ...186
A2.1.4 Femtosecond Transient Absorption .........................................................187
A2.2 Sample preparation details for optical experiments ................................................187
A2.3 Synthetic details for all compounds presented in this thesis ..................................189
A2.4 Crystallographic information ..................................................................................201
A2.5 References ...............................................................................................................203

viii

LIST OF TABLES

Table 2.1: S
1
and T
1
energies of tetracene, ET-TMS, and PET calculated using
B3LYP/6-31+G**. ....................................................................................................37
Table 2.2: The emission properties of tetracene, ET-TMS, and PET in a THF solution measured
at room temperature. .................................................................................................39
Table 2.3: The quantum yields and lifetimes of emission of ET-TMS and PET as a function of
concentration in THF. ..............................................................................................41
Table 3.1: Time constants ( 
i
=1/k
i
) for the kinetic model used to fit the TA data for o-BETB in
different media. .........................................................................................................79
Table 4.1: The quantum yields and lifetimes of the fluorescence of the monomer and the dimers
in a THF solution and the quantum yields of emission of the films of the same. ..106
Table 4.2: Electrochemical properties of the four dimers compared to the monomer, and the
optical gaps of these molecules. .............................................................................108
Table 4.3: The rate constants used to fit the fluorescence decay of m-BETB in three solvents, in
units of ns
-1
. .............................................................................................................115
Table 4.4: The lifetimes of the decays of the DADS used to fit the TA data of p-BETB-ohex and
p-BETB-ethex in THF, and p-BETB-ethex in PMMA. ..........................................122
Table 5.1: Emission properties of PET, Ph-o-BETB and Ph-TETB in solution. ........................150
Table 5.2: The lifetimes of the states used to fit the TA data of Ph-o-BETB and Ph-TETB. ...160
Table A1.1: The quantum yields of emission of BTEY in toluene, THF, CH
2
Cl
2
, acetone, and
DMF; the lifetimes of the 560 nm emission of BTEY in the above solvents, and the
values of the dielectric constants of these solvents. ................................................180
Table A2.1. Crystallographic parameters for BET-B and BETX crystals ..................................203

ix

LIST OF FIGURES

Figure 1.1 A schematic representation of the four stages of operation of OPV cells .....................2
Figure 1.2 A simplified schematic of singlet fission ......................................................................3
Figure 1.3 Delayed fluorescence in crystalline anthracene measured by Singh et al. in 1965 .......4
Figure 1.4 A plot of number of publications on the topic of singlet fission throughout the years,
using data obtained from SciFinder ..............................................................................4
Figure 1.5 The four stages of operation of OPV cells which incorporate SF materials as the
donor layers ..................................................................................................................5
Figure 1.6 A diagram of the spin portions of the electronic wave functions describing the S
1
and
the T
1
states ..................................................................................................................6
Figure 1.7 A Jablonski diagram for a pair of tetracenes with the relevant rates and states
involved in singlet fission ...........................................................................................10
Figure 1.8 Magnitudes of the overlap densities for various configurations of two chromophores.
(Figure reproduced from Ref. 1) ...............................................................................14
Figure 1.9 The a) crystalline morphology of tetracene b) depiction of amorphous morphology of
tetracene, which would have certain chromophore pairs oriented in geometries
conducive to exciton trapping, c) amorphous morphology of covalent dimers of
tetracene, in which the chromophores are preferentially oriented for singlet fission 18
Figure 1.10 The bis-tetracene molecules published by the Bardeen group in 2006 .....................21
Figure 1.11 Kinetic model used for fitting the fluorescence decay data by Muller et al ..............21
Figure 1.12 The indirectly coupled covalently linked dimers of 1,3-diphenylisobenzofuran
published by Johnson et al. in 2013 ...........................................................................22
Figure 1.13 The ortho-, meta- and para- bis(TIBS-pentacenyl)benzene dimers synthesized by
Dan Lehnherr and published by Zirzlmeier et al. in 2015 .........................................23
Figure 1.14 The linearly-linked bipentacenes synthesized and published by Sanders et al. in
2015 ............................................................................................................................23
Figure 1.15 The linearly-linked heterodimers synthesized and published by Sanders et al. in
2016 ............................................................................................................................24
Figure 1.16 The “bent -shaped” pentacene dimers published by Sakuma et al. in 2016 ...............25
Figure 1.17 The “cross -conjugated” TIPS -pentacene dimers synthesized by Dan Lehnherr and
published by Zirzlmeier et al. in 2016 .......................................................................26
Figure 1.18 The norbornyl-bridged tetracene dimer published by Cook et al. in 2016 ................27
Figure 1.19 The terrylenediimide dimers synthesized and published by Margulies et al. in 2016
....................................................................................................................................28
Figure 2.1 Chemical structures of a) ET-TMS and b) PET ..........................................................33
Figure 2.2 The aromatic regions of the
1
H NMR spectra of a) ET-TMS and b) PET in CDCl
3
at
25
o
C ...........................................................................................................................35
Figure 2.3 Molecular orbitals of a) tetracene, b) ET-TMS, and c) PET .......................................37
Figure 2.4 Steady state absorption (solid black line) and emission (dashed red line) spectra of
ET-TMS, and PET compared to tetracene .................................................................38
Figure 2.5 Molar absorptivities of tetracene (black), ET-TMS (red) and PET (blue) in THF .....40
Figure 2.6 Emission spectra as a function of concentration of a) ET-TMS and b) PET in THF ..41
Figure 2.7 The calculated potential energy surfaces of the S
0
, S
1
and Q
1
states of an
ethynyltetracene dimer ...............................................................................................42
x

Figure 2.8 A comparison of the solution (dashed red) and thin film (solid black) absorption (left)
and emission (right) spectra of ET-TMS (top) and PET (bottom) ............................43
Figure 2.9 Emission decays of ET-TMS and PET in THF solutions and as thin films ................44
Figure 2.10 The likely photo-degradation product of PET, based on the anthracene studies in ref.
16 ................................................................................................................................45
Figure 2.11 Transient absorption spectra of the monomer ET-TMS in a) PMMA and b) neat film
following excitation at 500 nm. The cyan dotted line shows the triplet spectrum
obtained from ET-TMS sensitization measurements with Pd(TPBP). c) The
extinction spectra for S
1
S
n
and T
1
T
n
transitions used to calculate singlet and
triplet populations (d) for ET-TMS ............................................................................46
Figure 2.12 The energy level scheme of the triplet sensitizer Pd(TPBP) and ET-TMS ...............47
Figure 2.13 The transient absorption of a 5 mol% Pd(TPBP) doped ET-TMS film ....................47
Figure 3.1 Visual representation of the molecular arrangement which maximizes the
) (
ˆ
1 1
1
0 1
T T H S S
el
coupling matrix element ..........................................................51
Figure 3.2 Chemical structures of the dimers presented in this chapter. a) ortho-bis-
ethynyltetracenyl-benzene – o-BETB and b) bis-ethynyltetracenyl-xanthene – BETX
....................................................................................................................................52
Figure 3.3 Potential energy surfaces of the ground and the S1 states, and the corresponding
orbital configurations. The minimum on the S
1
surface corresponds to the “excimer”
state .............................................................................................................................53
Figure 3.4 A simiplified illustration of the electronic configurations describing the excimer and
the
1
(T
1
T
1
) states using a four orbital, four electron approximation ..........................56
Figure 3.5 a)
1
H NMR of o-BETB and b) BETX in CDCl
3
at 25
o
C ............................................60
Figure 3.6 a) The syn and the anti conformers of BETX. b) The three conformers of o-BETB
and their corresponding computed energies, computed at B3LYP/6-31++G** level
of theory .....................................................................................................................62
Figure 3.7
1
H NMR spectra of a) o-BETB and b) BETX at 25
o
C (red), 10
o
C (yellow), -5
o
C
(green), -20
o
C (cyan), -40
o
C (navy) and -60
o
C (violet) .........................................63
Figure 3.8 a) and b) Crystal structure of o-BETB, and c) its crystal packing...............................64
Figure 3.9 a) crystal structure of BETX
1
(inter-tetracene spacing = 3.4 Å) and b) crystal
structure BETX
2
(inter-tetracene spacing = 3.5 Å). The view chosen here has the
xanthene moiety of the BETX structures perpendicular to the page, with the tert-
butyl groups removed for clarity. c) Simplified Crystal packing diagram of BETX,
with t-butyl groups removed .......................................................................................65
Figure 3.10 Molar absorptivities of ET-TMS, o-BETB, and BETX ............................................66
Figure 3.11 Absorption and emission of ET-TMS (top), o-BETB (middle), and BETX (bottom)
in a THF solution (black), doped into PMMA (red), neat films (blue), and in 2-
methyl-THF at 77 K (dashed grey). The intrinsic BETX solution emission spectrum
was obtained by subtracting the photooxidized impurity’s emission from the
measured BETX solution emission ............................................................................67
Figure 3.12 Emission decays of ET-TMS, o-BETB, and BETX in THF solutions ......................68
Figure 3.13 Comparison of absorption (black) and emission (red) spectra of the a) tetracene
dimers versus the b) anthracene dimers in a PMMA matrix. The areas of the
emission peaks for BETX, o-BETB and BEAX are normalized to an o-BEAB area of
xi

1.0. The photoluminescence efficiencies for BEAX and o-BEAB are 57 and 96%,
respectively .................................................................................................................69
Figure 3.14 Transient absorption of thin films of o-BETB doped with Pd(TPBP) at 5 mol%,
following photoexcitation at 626 nm ..........................................................................70
Figure 3.15 a) Transient absorption spectra of a neat film of o-BETB following excitation at 500
nm. The cyan line shows the triplet spectrum obtained from sensitization
measurements with Pd(TPBP). b) The singlet and triplet populations for o-BETB are
calculated using the extinction spectra for S
1
S
n
and T
1
T
n
transitions of BETB
....................................................................................................................................72
Figure 3.16 a) Transient absorption spectra of BET-B in THF. The inset shows the solvent
polarity independent dynamics at 560 nm for ¹(T
1
T
1
) state. b) The comparison of
spectral shape for S
1
, ¹(T
1
T
1
) and T
1
state of BET-B in THF ....................................73
Figure 3.17 Transient absorption spectra of o-BETB doped into DPA and DPT on exciting with
550 nm. The cyan line in the right plot shows the triplet spectrum obtained from
o-BETB sensitization experiments .............................................................................75
Figure 3.18 Comparison of simulated fits (red dotted line) with the TA data for o-BETB in DPT
film at 900 ps (black line) using basis sets for a) T
1
(o-BETB) + T
1
(DPT), b) T
1

(o-BETB) + T
1
(o-BETB) c) T
1
(DPT) + T
1
(DPT), and d) T
1
(o-BETB) + T
1
(DPT)
without the
1
(T
1
T
1
) (o-BETB) state ............................................................................76
Figure 3.19 Kinetic model for o-BETB in a) solution, PMMA, DPA and b) DPT, neat film. The
notation A denotes the tetracenes in the same dimer and B denotes the DPT for the
BETB in DPT film or third tetracene in another o-BETB dimer for the BETB neat
film. The calculated populations for S
1
(o-BETB) (black), ¹(T
1
T
1
) (o-BETB) (blue),
T
1
(o-BETB) (red) and T
1
(DPT) (green dashed) extracted from the kinetic model are
shown in the bottom panel for a) o-BETB in THF and b) o-BETB in DPT & neat
o-BETB film ...............................................................................................................77
Figure 3.20 a) Transient absorption spectra of o-BETB and BETX in THF solutions. b) Overlay
of the 200 fs TA trace of BETX with 50 ps TA trace of o-BETB, to demonstrate the
resemblance in the spectral shapes .............................................................................82
Figure 3.20 Computed energies of the S
1
and the
1
(T
1
T
1
) states of BETX at the ground state and
relaxed S
1
geometries. Details of the calculations provided in ref. 22 .......................83
Figure 4.1 Depiction of possible routes of chromophore coupling in o-BETB: a) through space,
b) through bond ..........................................................................................................88
Figure 4.2 The possibility of conjugation through ortho-, meta- and para- ethynylbenzenes .....89
Figure 4.3 The meta-BETB and para-BETB dimers which were synthesized and studied here .91
Figure 4.4 The four para-BETB dimers which were synthesized, with only the p-BETB-ohex
and p-BETB-ethex soluble enough for photophysical characterization .....................95
Figure 4.5
1
H NMR spectrum of m-BETB in CDCl
3
at 25
o
C ......................................................97
Figure 4.6
1
H NMR spectrum of the aromatic region of p-BETB-ethex in CDCl
3
at 25
o
C ........97
Figure 4.7
1
H NMR spectrum of the aromatic region of p-BETB-ohex in CDCl
3
at 25
o
C .........98
Figure 4.8
1
H NMR spectra of the aromatic regions of the dimers compared to the monomer
(PET) .........................................................................................................................98
Figure 4.9 The HOMO and LUMO of a) o-BETB, b) m-BETB, c) p-BETB-ethex, and d)
p-BETB-ohex, calculated at the B3LYP/6-31+G** level of theory ........................101
xii

Figure 4.10 The extinction spectra of m-BETB, p-BETB-ethex, and p-BETB-ohex compared to
o-BETB and PET in a THF solution ........................................................................103
Figure 4.11 The steady state absorption and emission spectra of m-BETB, p-BETB-ethex, and
p-BETB-ohex compared to o-BETB and PET in a THF solution (black) and as neat
thin films (red) .........................................................................................................105
Figure 4.12 Cyclic voltametry plots of a) o-BETB, b) m-BETB, c) p-BETB-ethex, and d)
p-BETB-ohex in CH
2
Cl
2
, using ferrocene as an internal reference .........................108
Figure 4.13 The comparison of emission decays of a) m-BETB in air free, and ambient
conditions, and b) between m-BETB and PET .........................................................110
Figure 4.14 The emission decays of m-BETB in THF at four concentrations ............................111
Figure 4.15 The emission decays of m-BETB in THF at room temperature and in a solid solvent
glass at 77 K .............................................................................................................112
Figure 4.16 The emission decays of m-BETB in various media ................................................112
Figure 4.17 The excited state kinetic model used to fit the emission decay of m-BETB ...........114
Figure 4.18 The emission decays of m-BETB overlaid with the fits in the three solvents .........115
Figure 4.19 The a) fs and b) ns transient absorption of m-BETB ...............................................118
Figure 4.20 The fs transient absorption of p-BETB-ohex and p-BETB-ethex and in THF ........119
Figure 4.21 The fs transient absorption of p-BETB-ethex in PMMA ........................................119
Figure 4.22 The DADS and concentrations from the target analysis of the TA data of a)
p-BETB-ohex and b) p-BETB-ethex and in THF, and c) p-BETB-ethex in PMMA
using a 3 state sequential decay model .....................................................................121
Figure 4.23 The overlay of State 3 from target analysis of p-BETB-ethex in THF (blue) with the
shifted and normalized sensitized T
1
T
n
absorption of ET-TMS (black) .............122
Figure 4.24 The proposed mode of decoupling of the correlated T
1
in p-BETB-ethex ..............123
Figure 4.25 A cartoon diagram showing the closer lying orbitals of the linker and the tetracenes
in a) p-BETB-ohex compared to b) p-BETB-ethex .................................................125
Figure 4.26 The transient absorption spectra of thin films of a) m-BETB and b) p-BETB-ethex
..................................................................................................................................127
Figure 4.27 The decays of the different spectral regions of the TA spectra of thin films of a)
m-BETB and b) p-BETB-ethex ................................................................................127
Figure 5.1 Depiction of how initial S
1
exciton delocalization could provide the entropic driving
force for singlet fission in tetracene. The S
1
exciton corresponds to N microstates (N
= number of molecules over which S
1
is delocalized), whereas the localization of the
triplet pair has several possibilities, resulting in >N microstates ..............................132

Figure 5.2 An artist’s rendition of the SF process in o-BETB: delocalized S
1
exciton relaxes into
the correlated triplet pair (localized on the same chromophores as S
1
) ..................135
Figure 5.3 An artist’s rendition of the expected SF process in TETB: delocalized S
1
exciton has
four different options for relaxing into the correlated triplet pair ............................135
Figure 5.4 The chemical structures of the compounds presented in this chapter: a) TETB, b)
Ph-TETB, and c) Ph-o-BETB ..................................................................................136
Figure 5.5 The Diels-Alder reaction of molecular oxygen and tetracene; the predominant
degradation mechanism of acenes in ambient conditions ........................................137
Figure 5.6 Crystals of 5-bromo-11-phenyltetracene, obtained by slow crystallization from
chloroform layered with hexane ...............................................................................141
Figure 5.7
1
H NMR spectrum of TETB in CDCl
3
......................................................................142
xiii

Figure 5.8 Comparison of the
1
H NMR spectra of TETB (top) with o-BETB (bottom) in CDCl
3
..................................................................................................................................143
Figure 5.9
1
H NMR spectrum of Ph-o-BETB in CDCl
3
.............................................................144
Figure 5.10 Comparison of the
1
H NMR spectra of Ph-o-BETB (top) with o-BETB (bottom) in
CDCl
3
.......................................................................................................................144
Figure 5.11
1
H NMR spectrum of Ph-TETB in CDCl
3
................................................................145
Figure 5.12 B3LYP/6-31+G** optimized geometries of a) o-BETB and b) TETB ...................146
Figure 5.13 Molecular orbitals of a) o-BETB and b) TETB .......................................................147
Figure 5.14 Molar absorptivities of PET (black), Ph-o-BETB (red) and Ph-TETB (blue) .......149
Figure 5.15 Steady state absorption (solid) emission (dashed) spectra of PET (top), Ph-o-BETB
(middle) and Ph-TETB (bottom) in solution at room temperature ..........................150
Figure 5.16 The decays of Ph-TETB emission at 630 nm in a THF solution at room temperature
(black line) and in a 2-Methyl-THF solvent glass at 77 K (red line). The instrument
response function is shown as the grey line .............................................................151
Figure 5.17 The normalized excitation (solid) and emission (dashed) spectra of Ph-TETB at
77 K (black) and at room temperature (red) ............................................................152
Figure 5.18 a) The emission spectra of the same sample of Ph-TETB in 2-MeTHF at 77 K and at
room temperature, without normalization. b) Photographs showing the emission of
Ph-TETB at 77 K (top) and at room temperature (bottom) .....................................153
Figure 5.19 The emission spectra of a) p-BETB-ohex and b) Ph-TETB upon exposure to 480 nm
light under ambient conditions. The absorption spectra of c) o-BETB and d)
Ph-TETB after several days of exposure to room lights, under ambient conditions
..................................................................................................................................155
Figure 5.20 Transient absorption spectra of Ph-o-BETB and Ph-TETB excited at 500 nm.......157
Figure 5.21 The decay of the TA signal of a) Ph-o-BETB and b) Ph-TETB at various
wavelengths ..............................................................................................................158
Figure 5.22 The decay associated spectra obtained by target analysis of the TA data of a)
PhoBETB and b) PhTETB. c) The populations of the S
1
and the
1
(T
1
T
1
) states of
PhoBETB and PhTETB obtained from target analysis ............................................159

xiv

Abstract
Organic photovoltaics (OPVs) are a cheaper, sustainable alternative to their inorganic
counterparts, but their low power conversion efficiencies have hampered their utilization.
Recently Nozik and Michl proposed incorporating singlet fission (SF) materials into OPVs to
harvest the high energy photons and convert them into pairs of low energy triplet excitons to
reduce thermal relaxation losses. Singlet fission is an excited state process which occurs in
organic materials whose S
1
state energy is twice that of the T
1
state. During this process, the S
1

exciton which was generated by a single photon absorption rapidly converts into a pair of triplet
excitons located on adjacent chromophores, and coupled into an overall singlet spin state. SF was
discovered in crystalline anthracene and tetracene in the 1960s, however, the potential uses of SF
materials in enhancing the performance of organic electronics has spurred renewed interested
among the scientific community. The field of singlet fission has experienced rapid growth in the
past decade as researchers have explored new materials for SF and attempted to answer
important questions such as which factors govern the rate of singlet fission in crystalline and
amorphous materials, which other electronic states facilitate SF, how should the chromophores
be oriented next to each other for optimal SF, and which factors influence the separation of the
triplet excitons from the correlated triplet pair.
This thesis presents the design strategies, synthesis, and photophysical properties of a series
of covalently-linked tetracene arrays, which allowed us to learn about the effects of relative
chromophore orientation and electronic coupling on SF dynamics, the separation of the triplet
excitons from the strongly coupled correlated triplet pair state, and the role of exciton
delocalization in driving SF in tetracene systems.
We observed that in ethynyltetracene SF proceeded much faster than in unsubstituted
tetracene, and we initially constructed covalently linked dimers of ethynyltetracenes. First, the
effects of π orbital overlap in two cofacial dimers with different degrees of π overlap –
bis(ethynyltetracenyl)-xanthene, and ortho-bis(ethynyltetracenyl)-benzene, BETX and o-BETB,
respectively. It was found that too much π ove rlap in BETX results in excited state decay via
excimer, while some π overlap was ideal for rapid (< 10 ps) relaxation into the
1
(T
1
T
1
) state in
o-BETB. The triplet excitons can be separated from the correlated triplet pair via energy transfer
to neighboring chromophores in bulk or in a diphenyltetracene host. We then explored the effects
of through-bond chromophore coupling on SF efficiencies in meta-, and
para-bis(ethynyltetracenyl)benzenes. It was found that the conjugating para-diethynylbenzene
linkers facilitate rapid singlet fission, while the meta-diethynylbenzen bridged tetracenes
primarily relax via fluorescence from the S
1
state. Furthermore, in solution, the para- dimers
exhibited spectral signatures of the monomeric T
1
state absorption, which suggests that the
rotational flexibility about the linker axis in the para- dimers resulted in decoupling of the
tetracenes in the excited state. Lastly, the effect of exciton delocalization on SF rate was
explored in a chemically stabilized, phenylated o-BETB dimer (Ph-o-BETB) and a structurally
analogous tetramer (Ph-TETB). It was found that exciton delocalization in the tetramer resulted
in a SF rate that was ten times faster than that in the dimer.
The discoveries of SF dynamics in these systems are inspiring the development of next
generation of optimized SF materials for use in OPVs and other organic electronic applications.
1

Chapter 1: Introduction to singlet fission
1.1 Organic photovoltaics and singlet fission
Organic photovoltaics (OPVs) are an emerging sustainable energy harvesting technology
which holds the promise to improve on the deficiencies of the inorganic counterparts. The most
common photovoltaic devices on the market today are those made of crystalline and
polycrystalline silicon.
2
Even though these solar cell modules provide fairly good power
conversion efficiencies of 15-20 %,
2
their production requires stringent processing conditions.
Furthermore, due to the indirect bandgap, silicon is a poor absorber, therefore thick layers of
silicon are required to collect sufficient amount of sunlight. This results in bulky and inflexible
form factors of silicon solar cells.
An alternative to silicon photovoltaics, are solar cells made of organic chromophores.
Beginning in the early 2000s this field has experienced a lot of interest. OPVs present several
advantages compared to their silicon counterparts. First, due to high extinction coefficients,
organic dyes absorb the light well; therefore very thin layers are capable of absorbing large
amounts of sunlight. Since only thin layers of the material are required, organic solar cells can in
theory be made on flexible substrates and their light weight can open up possibilities of their use
in various applications for which light weight and flexibility are required.
Unfortunately, OPV devices suffer from lower power conversion efficiencies than their
inorganic counterparts, and much effort has been devoted to understanding the underlying
physics of OPV devices and devising ways to improve their efficiencies. The process of photon
conversion to current by an OPV device can be exemplified by four steps, as shown in Figure
1.1: 1) light absorption, 2) exciton diffusion to the donor-acceptor interface, 3) charge separation
at the donor-acceptor interface, and finally 4) charge extraction by the electrodes.
2

In order to improve the efficiencies of OPV devices, researchers have been targeting all
four of those steps. For instance, certain materials such as squaraines whose molar absorptivities
can be as high as 10
5
M
-1
cm
-1
are being developed to improve the light absorption step in OPVs.
3

Other groups have focused on finding the optimal layer morphologies to improve the exciton
diffusion step in OPV, and materials undergoing symmetry breaking charge transfer are being
explored for enhancement of charge separation in OPV.
4, 5
Furthermore, a process called singlet
fission has the potential to improve OPV efficiencies by converting high energy photons into
pairs of lower energy excitons, thereby improving the photocurrent in OPVs.
1, 6, 7

1.2 Definition and history of singlet fission
Singlet fission is a process which occurs in materials consisting of organic chromophores
whose first excited singlet (S
1
) state energy is approximately two times greater than its lowest

Figure 1.1 A schematic representation of the four stages of operation of OPV cells.
3

triplet (T
1
) state energy.
1, 8
Absorption of a high energy photon by these materials leads to the
population of a single S
1
state, which then rapidly converts into a triplet pair state with an overall
singlet character –
1
(T
1
T
1
). When this process takes place in a crystal or an aggregate, the triplet
excitons can hop via Dexter energy transfer and spatially separate away from each other.
9-18
A
simplified depiction of the singlet fission process is shown in Figure 1.2.

In 1965, researchers at the National Research Council of Canada invoked singlet fission to
explain the microsecond delayed fluorescence of crystalline anthracene (see Figure 1.3).
19

Following that report, singlet fission was used to explain the low fluorescence quantum yield of
crystalline tetracene.
20
Singlet fission in tetracene was later confirmed with magnetic field
dependent delayed fluorescence measurements.
21
The process was then discovered in several
other molecules, including perylene and carotenoids.
22, 23
By the 1980s, the scientific community
had largely lost interest in the phenomenon. However, in 2004, Arthur Nozik and Josef Michl
proposed the utilization of singlet fission materials for improving photovoltaic efficiencies.
24, 25

As a result of this proposition, scientific interest in singlet fission has burgeoned, and the number
of articles exploring the subject is increasing annually, as shown in the plot in Figure 1.4.

Figure 1.2. A simplified schematic of singlet fission.
4

Incorporation of singlet fission materials as donor or acceptor layers in organic photovoltaic
devices holds promise for increasing the photocurrent and improving the overall performance of
the devices. Specifically, incorporation of SF materials as active layers in PV devices could
allow the efficiencies of such devices to bypass the Shockley-Queisser efficiency limit of 33 %

Figure 1.4. A plot of number of publications on the topic of singlet fission throughout the
years, using data obtained from SciFinder.

Figure 1.3 Delayed fluorescence in crystalline anthracene measured by Singh et al. in 1965.
19

5

for single junction devices to 46 %.
24
The high energy photons absorbed by these materials are
converted into two lower energy excitons, rather than being wasted as heat, as depicted in Figure
1.5. The two triplet excitons can then migrate to a donor-acceptor (D-A) interface and produce
two charge carriers. Singlet fission also presents the advantage of rapid generation of triplet
excitons in organic materials, which are desirable due to their long lifetimes; a feature that
increases their probability of reaching the D-A interface.

In the past half of a decade SF materials have been incorporated in various organic
electronics including organic
26, 27
and hybrid solar cells,
28, 29
photodetectors,
30
and dye sensitized
solar cells (DSSCs).
31
OPV devices containing tetracene, diphenyltetracene have been reported
to exhibit efficiencies between 0.29 and 1.27 %.
26, 27, 32
Photovoltaic devices using pentacene as
the donor and PbS nanoparticles have been reported to reach efficiencies close to 1%.
29, 33-35

Congreve et al. recently demonstrated that pentacene based devices can exhibit external quantum
efficiencies greater than 100 %.
36
Lastly, DSSCs using diphenylisobenzofuran as a sensitizer
have been reported to have an internal quantum efficiency of 70 % and an efficiency of 1.1 %.
31

Although progress in this field has been steady, the efficiencies of the SF based devices are still

Figure 1.5 The four stages of operation of OPV cells which incorporate SF materials as the
donor layers.
6

low, and it is necessary to gain a deeper understanding of the SF mechanism in order to optimize
SF materials for the purpose of solar energy harvesting.
1.3 Mechanism of singlet fission
Materials must satisfy three basic requirements in order for singlet fission to become viable:
1) the energy of the S
1
state must be approximately twice that of the T
1
state for a given material,
2) the [S
1
S
0
] and
1
(T
1
T
1
) states must be adequately electronically coupled, 3) the rate of SF must
be faster than the rates of other excited state deactivating processes.
To understand the energy difference between the S
1
and the T
1
states, we must first look at
the wave functions describing them. We can use the simplest two-electron, two-orbital model to
build the excited state wave functions. An electronic wave function is a product of the spatial
component, which is a function of the electron position, and the spin component. The spin
components are shown in Figure 1.6. In order for the wave function to be appropriate, the overall
wave function has to be anti-symmetric. Therefore, the spatial components of the S
1
and the T
1

wave functions have given by equations 1.1 and 1.2, respectively.

  ) ( ) ( ) ( ) (
2
1
1 2 2 1
r r r r
b a b a
      

(1.1)
  ) ( ) ( ) ( ) (
2
1
1 2 2 1
r r r r
b a b a
      

(1.2)

Figure 1.6 A diagram of the spin portions of the electronic wave functions describing the S
1

and the T
1
states.
7

In equations 1.1 and 1.2, label a corresponds to orbital a, and label b, corresponds to a higher
lying orbital b, r
1
and r
2
are the coordinates of electrons 1 and 2. The product of the

 spatial
wave function with the spin wave function of the S
1
state then produces an anti-symmetric wave
function which describes the S
1
state, and analogously for

 and the T
1
wave function.
To obtain the energy of the S
1
and the T
1
states, we apply the two electron Hamiltonian
operator, given by equation 1.3.
2 1 0
2
0
4
ˆ ˆ
r r
e
H H
el

 

(1.3)
where
0
ˆ
H is the 1 electron Hamiltonian operator, e is the elementary charge of the electron, and
ε
0
is the permittivity of free space. The resulting integrals for evaluating the energies of the S
1

and the T
1
wave functions are shown in equations 1.4 and 1.5, respectively.
  ) 1 ( ) 2 (
4
) 2 ( ) 1 ( ) 2 ( ) 1 (
4
) 2 ( ) 1 ( 2
) 1 ( ) 2 ( ) 2 ( ) 1 (
4
ˆ
) 1 ( ) 2 ( ) 2 ( ) 1 (
2 1 0
2
2 1 0
2
2 1 0
2
0
1
b a
r r
e
b a b a
r r
e
b a E E
b a b a
r r
e
H b a b a E
b a
S



  


  
 

(1.4)
  ) 1 ( ) 2 (
4
) 2 ( ) 1 ( ) 2 ( ) 1 (
4
) 2 ( ) 1 ( 2
) 1 ( ) 2 ( ) 2 ( ) 1 (
4
ˆ
) 1 ( ) 2 ( ) 2 ( ) 1 (
2 1 0
2
2 1 0
2
2 1 0
2
0
1
b a
r r
e
b a b a
r r
e
b a E E
b a b a
r r
e
H b a b a E
b a
T



  


  
 

(1.5)
where the wave functions and the electron coordinates are represented as their labels, E
a
and E
b

are the energies of orbitals a and b, respectively. The remaining two electron integrals in the
above equations are called the Coulomb (J) and the exchange (K) integral, given by equations
1.6 and 1.7, respectively. The Coulomb integral represents the Coulomb repulsion energy
between the electrons, due their charge. The exchange integral, on the other hand, does not have
8

an interpretation in terms of classical mechanics, and is a purely quantum mechanical
construction.
) 2 ( ) 1 (
1
) 2 ( ) 1 (
4
12 0
2
b a
r
b a
e
J

 (1.6)
) 1 ( ) 2 (
1
) 2 ( ) 1 (
4
12 0
2
b a
r
b a
e
K

 (1.7)
Equations 1.4 and 1.5 can then be written more concisely in the following manner:
  K J E E E
b a S
    2
1
(1.8)
  K J E E E
b a T
    2
1
(1.9)
The energy gap between the S
1
and the T
1
levels for a given chromophore is then equal to
twice the exchange integral (K). Conceptually, this energy difference can be understood as
follows: the spatial wave function of the T
1
state approaches zero as the distance between the two
electrons gets closer, meaning that the probability of finding two electrons very close to each
other is small in the T
1
state. The S
1
wave function, on the other hand, is maximized at close
inter-electron distances, meaning there is high probability of finding two electrons near each
other in the S
1
state. Therefore, in the T
1
state, since the electrons are more likely to be farther
away from each other (compared to the S
1
state) the electron repulsion is minimized, and vise
versa for the S
1
state.
It is evident that the magnitude of the exchange integral depends upon the amount of
overlap between the orbitals participating in the electronic transition. For Class I molecules such
as acenes (see Smith and Michl for classification of organic chromophores),
1
the absorption of a
photon with appropriate energy brings the chromophores into an optically bright S
1
state which is
dominated by a transition from the highest occupied molecular orbital (HOMO) to the lowest
9

unoccupied molecular orbital (LUMO). Therefore, the exchange integral in these systems is
dependent on the overlap of the HOMO and the LUMO orbitals. The orbital overlap becomes
sufficiently large to achieve the desired S
1
and T
1
energies in linear acenes (tetracene, and
longer) and biradicaloids (such as diphenylisobenzofuran).
The second requirement is a sufficient amount of electronic coupling between the [S
1
S
0
]
and the correlated triplet pair state
1
(T
1
T
1
). This coupling matrix element is a function of the
orbital overlap between two adjacent chromophores, and can be achieved via through-bond or
through-space interactions. A more detailed discussion of ) (
ˆ
1 1
1
0 1
T T H S S
el
is provided
below.
Lastly, the rate of singlet fission in a given system must be faster than the rates of
competing S
1
deactivating processes, in order for it to be efficient. Excited state relaxation
processes such as charge transfer and excimer formation can take place on a picosecond time
scale; consequently, when designing structures for singlet fission, the probability of these
processes should be minimized.
The exact details of the singlet fission mechanism are the subject of this thesis. The
simplest description of SF is represented by Equation 1.10,
(1.10)
where S
0
is the electronic ground state, S
1
is the first excited singlet state, and T
1
is the lowest
triplet state of a given chromophore. A pictorial view of Equation 1 is provided by the
Jablonski-like description of SF shown in Figure 1.7. In Equation 1.10, the S
1
state, localized on
one chromophore, relaxes into a correlated triplet pair state
1
(T
1
T
1
), followed by the separation
and/or de-correlation of the triplet excitons. Singlet fission is defined here to be the first step –
10

the formation of the correlated triplet pair state
1
(T
1
T
1
) – however to take advantage of singlet
fission in optoelectronic devices, the second step – formation of the separated triplets is also
necessary. The following paragraphs expound on the two steps of singlet fission (with a
particular focus on SF in acenes).

1.3.1 Step 1: formation of the correlated triplet pair
The overall process of singlet fission and free triplet formation is governed by the
Hamiltonian H = H
el
+ H
spin
, where

is the electronic Hamiltonian, operating on the spatial
part of the electron wavefunction, and H
spin
is the spin Hamilonian (akin to the one used to
describe the Zeeman effect).
1
The first step of singlet fission – the formation of the correlated
triplet pair
1
(T
1
T
1
) – is a spin allowed process, and therefore can be described exclusively by

.
In essence, the relaxation of S
1
S
0
into the
1
(T
1
T
1
) can be thought of as internal conversion. Given

Figure 1.7. A Jablonski diagram for a pair of tetracenes with the relevant rates and states
involved in singlet fission.
1,13,57
11

that the energy difference between the S
1
and the
1
(T
1
T
1
) levels is small, singlet fission can
proceed very rapidly, on a picosecond (in tetracene)
12, 37
and sub-picosecond (in pentacene) time
scale.
16, 18
The upper limit for the energy of the
1
(T
1
T
1
) state can be well approximated by
2×E(T
1
), or by the E(
5
(T
1
T
1
)),
38
and admixture of the S
1
configurations into the
1
(T
1
T
1
) wave
function can lower the
1
(T
1
T
1
) energy.
38, 39

An expression of the rate of formation of the correlated triplet pair from the initially excited
singlet state, which takes into account the energetics of the process as well as the electronic
coupling between states was derived analogously to the Marcus rate equation by Yost et al.
40

This rate expression is given in Equation 1.11, where V is the electron exchange interaction
matrix element which couples the S
1
S
0
and the
1
(T
1
T
1
) states residing on two adjacent
chromophores, λ is the average molecular reorganization energy between the S
1
S
0
and the
1
(T
1
T
1
) states, and G  and is the energy difference between the
1
(T
1
T
1
) and the S
1
S
0
states.





  
 
kT
G
kT
V k
SF




4
) (
exp
4
1 2
2
2

(1.11)
This rate equation for SF is only valid in the case of weak coupling, and a more general rate
equation was presented by Yost et al. based on the scheme by Bixon and Jortner.
41
In the more
general equation, the rate is determined by Equation 1.11 in the weak coupling regime, and is
limited by the adiabatic timescale for the large coupling values.
The quantity G  , the exo- or endothermicity of the singlet fission process, is largely
determined by the difference between the energy of the S
1
state and the
1
(T
1
T
1
) state energy.
From Equation 1.11, it is evident that exothermic singlet fission should be faster than
endothermic singlet fission. A comparison of the experimentally reported rates and energies of
pentacene and tetracene supports this observation. Crystalline pentacene, whose S
1
energy is
12

1.83 eV and whose T
1
energy is 0.83 eV, produces triplets on a timescale of 80 fs.
17, 18
The
energetics of singlet fission in tetracene are slightly endothermic, with an S
1
energy of 2.32 eV,
and a T
1
energy of 1.25 eV, hence the timescales of singlet fission in crystalline and
polycrystalline tetracene has been measured to be in the range of 9-300 ps.
13, 14, 42-46
However, if
the energy gap between the S
1
and the
1
(T
1
T
1
) is too large, such as in hexacene, the process can
be presumed to be in the Marcus inverted region, which would result in an overall reduction of
the rate. Transient absorption studies of singlet fission in substituted hexacene reported a rate of
0.196 ps
-1
.
40, 47, 48
Although fast (τ ≲ picosecond) singlet fission is desirable, a large exothermic
driving force (large S
1
-
1
(T
1
T
1
) gap) would result in some energy loss upon singlet fission. For
use in solar energy harvesting applications, it is desirable to minimize the energy loss, therefore
singlet fission materials in which SF is isoergic or slightly endoergic are beneficial, provided that
singlet fission still occurs rapidly (on a ps timescale). The E(S
1
) ≲ 2E(T
1
) energy alignment
presents the advantage of reduced energetic losses upon SF, which would improve open circuit
voltages in OPV devices. Furthermore, E(S
1
) ≲ 2E(T
1
) materials exhibit enhanced T
1
diffusion
lengths as a result of long distance Forster exciton hopping, which is possible due the S
1
-
1
(T
1
T
1
)
equilibrium; whereas in materials with E(S
1
) > 2E(T
1
), annihilation to the S
1
state is not
favorable, therefore the excitons only travel by short range Dexter hopping mechanism.
49

Tetracene, perylene, and isobenzofuran are some of the materials that satisfy this condition.
Another important quantity in the rate equation is the electronic coupling between the initial
(S
1
S
0
) and the final
1
(T
1
T
1
) states. The electronic coupling term in the above equation,
) (
ˆ
1 1
1
0 1
T T H S S
el
assumes the direct (as opposed to mediated) mechanism for singlet fission. In
the direct mechanism, the S
1
directly converts into
1
(T
1
T
1
) without populating any transition
states during the process. In the mediated mechanism, however, the charge transfer state
13

(described by a hole on one chromophore, and an electron on the neighboring chromophore) is
transiently populated. The early literature regarding this matter suggests that both of these
mechanisms are plausible for SF in acenes.
1
The theoretical work by Kolomeisky et al.
postulates that the correct way to articulate the role of CT states in SF is as configurations
admixed into the S
1
or
1
(T
1
T
1
) states, or the virtual states, rather than the adiabatic states capable
of being populated.
50
In that regard, the direct pathway is enhanced by admixture of the CT
configurations in the electronic wave-functions of the S
1
and the
1
(T
1
T
1
) states.
The electronic coupling matrix element, ) (
ˆ
1 1
1
0 1
T T H S S
el
, can be understood qualitatively
in terms of the orbital overlap between the neighboring chromophores. Smith and Michl have
used a simple model to qualitatively estimate the magnitudes of the coupling for different
configurations of two chromophores. In this model, only the HOMO and the LUMO of
individual chromophores are considered, and the coupling term can then be simplified to the
expression provided in Equation 1.12. The overlap integrals within Equation 1.12 can be thought
of as representing two simultaneous charge transfers, akin to the Dexter energy transfer
mechanism.







 
A B B A A B B A el
h l
r
e
h h l h
r
e
l l T T H S S
12
2
12
2
1 1
1
0 1
2
3
) (
ˆ
(1.12)
Figure 1.8 provides a visual representation of the coupling matrix element
) (
ˆ
1 1
1
0 1
T T H S S
el
computed based on the HOMO/LUMO model by Smith and Michl. It is
evident that the magnitude of ) (
ˆ
1 1
1
0 1
T T H S S
el
is maximized in geometries where there is a
substantial overlap between two neighboring chromophores. In symmetric chromophores,
however, perfect π overlap results in the cancelation of the overlap integrals; but slip stacking the
14

chromophores in the direction of the transition dipole moment (μ) can produce large amplitudes
of the ) (
ˆ
1 1
1
0 1
T T H S S
el
. Additionally, the coupling can be maximized in a perfectly overlapping
orientation if the molecular orbitals of the chromophores are polarized by substituents and the
symmetries are broken. A large cofacial overlap between the chromophores, however, also
facilitates the formation of excimers, which could compete with singlet fission. This point is
addressed in detail in Chapter 3.

Linearly-linked chromophores can also exhibit nonzero values of ) (
ˆ
1 1
1
0 1
T T H S S
el
, albeit
much smaller than in the π stacked case. In the linearly -linked systems, the magnitude of
) (
ˆ
1 1
1
0 1
T T H S S
el
is large if the amplitudes of the chromophores’ orbitals are large at the linking
atoms.
The kinetics of the first step of singlet fission, the formation of the correlated triplet pair,
1
(T
1
T
1
), has not been studied extensively, as it is not trivial to spectroscopically distinguish
between the first and the second steps of SF. A majority of the solid state studies of SF reported
S
1
converting directly to two T
1
states, without population of any observable intermediate states.
Furthermore, the
1
(T
1
T
1
) state cannot be sensitized using an external sensitizer, so the
determination of its spectroscopic signature is not trivial in solid state studies. Recent studies of

Figure 1.8 Magnitudes of the overlap densities for various configurations of two
chromophores. (Figure reproduced from Ref. 1)
15

covalently-linked dimers of singlet fission chromophores have provided insight into the
spectroscopic character of the
1
(T
1
T
1
) state, and will be discussed in greater detail below.
1.3.2 Step 2: formation of the separated triplets
Traditionally, SF has been described as taking place in two steps, given by Equation 1.10.
The 2
nd
step, the separation of the correlated triplet pair
1
(T
1
T
1
) into uncorrelated or spatially
separated triplets has not been studied in detail, primarily due to the inability to spectrally
distinguish the
1
(T
1
T
1
) state from the T
1
---T
1
state in most systems. As a result, little is known
about the 2
nd
step of SF, specifically: at what point does the correlation end and the triplets
separate into independent entities. Smith and Michl propose that the time scale for de-
correlation should be slow, as the 2
nd
step is dictated by the spin Hamiltonian (H
spin
), and should
be inversely proportional to the magnitude of the electronic coupling between the chromophores:
if the chromophores are strongly coupled the
1
(T
1
T
1
) will be much lower than the
5
(T
1
T
1
), and if
they are weakly coupled, the
1
(T
1
T
1
) and the
5
(T
1
T
1
) states will exhibit a small energy splitting.
8

Equation 1 implies that only two chromophores are necessary to produce two independent triplet
excitons via singlet fission, however, this may not be valid in the case of strongly coupled
chromophores.
In his work on crystalline tetracene, Bardeen has explained the 2
nd
step of SF in terms of the
spin states using the four orbital four electron ansatz.
12, 13
In the absence of a magnetic field, the
 x | ,  y | , and  z | are eigenstates of the zero field Hamiltonian (but not of the total spin Ŝ
2

operator). In a two molecule four electron system, the nine possible triplet states are product
combinations  xx | ,  xy | , etc. The diagonalization of the Ŝ
2
operator in the triplet pair basis
produces one singlet, three triplets, and five quintet eigenstates. The singlet state is a
superposition of the  xx | ,  yy | , and  zz | states, given by Equation 1.13.
16

  zz yy xx   
3
1
singlet (1.13)
This spin wave function provides the basis for how a singlet state can be projected onto a triplet
pair spin manifold. This superposition state is not a stationary state, but will evolve in time,
through spin-lattice interactions. The final product is a wave function which has collapsed into
one of the  xx | ,  yy | , and  zz | states, and it is presumed to be complete when the two T
1
have
diffused apart from each other.
Additional experimental work carried out by the Bardeen and the Friend groups has
distinguished between the two steps of singlet fission through temperature dependent studies of
polycrystalline tetracene.
51, 52
Burdett and coworkers have used time resolved emission as well
as transient absorption spectroscopies to study the excited state dynamics of tetracene at
temperatures ranging from 4 K to 298 K.
51
They observed that the fast component of the
emission decay (100 ps) did not vary with temperature. The delayed fluorescence due to T
1
---T
1

annihilation was only present at room temperature and absent at lower temperatures. They
concluded from these findings that the first step of singlet fission, the formation of the
1
(T
1
T
1
)
state, is temperature independent in tetracene; however, the separation of the correlated pair into
independent triplets requires activation energy. The Friend group has reached the same
conclusions about the formation of the
1
(T
1
T
1
) state, but observed a sharp decline in the T
1

population in the 100 ps – 1 ns time range at very low temperatures (< 40 K).
52

One possible explanation for the temperature dependent triplet formation via singlet fission
in solid tetracene could be the effect of entropy. SF in tetracene is energetically uphill by
0.17 eV; to explain the rapid rate of SF in tetracene, Chan and coworkers have proposed that an
increase in entropy upon SF provides the thermodynamic driving force necessary for this
process.
53
They performed 2PPE experiments on crystalline tetracene from which they concluded
17

that the initially excited S
1
state is in coherent superposition with the
1
(T
1
T
1
) state. They
observed that the dynamics of this S
1
state were temperature independent, consistent with what
Burdett et al. have observed in polycrystalline tetracene. The conversion of the coherent
superposition state into a pair of triplet excitons was observed to be complete by 200 ps by Chan
et al., which suggests that the 0.17 eV barrier was overcome through interaction with the
environment. They propose that the energetic barrier is overcome through entropic contribution
to the overall thermodynamics of the process. According to their model, the S
1
exciton is
initially delocalized over N molecules of tetracene, and therefore corresponds to

microstates. The correlated triplet pair state,
1
(T
1
T
1
), which serves as the intermediate in the SF
process, corresponds to

microstates, which can form within the radius of
the initially delocalized S
1
state, with the factor of three coming from the superposition of the xx,
yy, and zz eigenstates, described by Bardeen earlier. The entropic gain upon SF can then be
calculated using Equation 1.14.
)) 1 ( 3 ln(
ln ln
) (
1 1
1
1
  
    


N k S
k k S
B
T T
B
S
B
(1.14)
The rate of SF can then be expressed by Equation 1.15, using transition state theory:





 






 
 





 
 
  
T k
H
k
S
T k
G
k
B B B
SF
exp exp exp
0 0
  (1.15)
where

G ,

H , and

S are the Gibbs free energy change, enthalpy change and entropy
change, respectively, from the singlet exciton to the transition state.

H corresponds to the
thermal barrier to singlet fission, (0.17 eV in tetracene). The entropy gained upon the formation
of the
1
(T
1
T
1
) state then supplies sufficient thermodynamic force for the second step of SF, the
separation of this state to free triplets, according to the model proposed by Chan et al.
18

Kolomeisky and coworkers took the concept of the entropic contributions to SF further, and
derived

S for each step of the SF process.
1.3.3 Motivation for studying SF in covalent chromophore assemblies
In order to harvest the benefits of SF in OPV applications, the optimization of both steps of
SF is necessary. Since the first step of SF is heavily dependent upon the relative orientation of
the chromophores, it will depend greatly upon the morphology of SF materials in the active
layers of the OPV devices. The morphology of the active layers in the devices is often difficult to
control, and the materials in these layers often end up amorphous due to the processing
conditions (e.g. vacuum/vapor deposition or solution processing/spin-casting). While the
molecules in a crystalline environment (such as tetracene) exhibit similar molecular environment
(see Figure 1.9a), in an amorphous film many configurations are possible (Figure 1.9b).

Until recently, singlet fission (SF) has been primarily studied in crystalline systems.
11, 13
In
crystals like tetracene, the isotropism of a crystalline lattice ensures that every molecule
experiences the same molecular environment. As a result, molecules oriented in an optimal
geometry for SF in crystals are equally likely to undergo SF upon absorption of a photon.
a) b) c)
Figure 1.9 The a) crystalline morphology of tetracene b) depiction of amorphous
morphology of tetracene, which would have certain chromophore pairs oriented in
geometries conducive to exciton trapping, c) amorphous morphology of covalent dimers of
tetracene, in which the chromophores are preferentially oriented for singlet fission.
19

Additionally, theoretical studies suggest that crystal lattice vibrations facilitate SF.
54
For
instance, the triplet yield in crystalline tetracene and pentacene is 200 %.
11, 17, 55
However, in
solids where the relative geometries deviate from those of crystal structures, the SF yields are
substantially lower.
43
Due to the commonly amorphous packing of active layers in OPV devices,
it is pertinent to understand the SF dynamics in amorphous materials in order to make use of
these materials in OPVs.
The recent work by Roberts et al. presents the dynamics of SF in an amorphous film of
5,12-diphenyltetracene (DPT).
42
The kinetics of singlet fission in this material show a fast
exponential (~ 1 ps) triplet rise followed by a slower power law rise of the triplet population
spread over hundreds of picoseconds. The two phases are associated with direct and diffusive
singlet fission processes, respectively. The direct process originates from excitation on or
adjacent to the sites with DPT molecules arranged in a preferred orientation for singlet fission,
whereas the diffusive phase arises when migration of a singlet exciton is required from the
excitation site to the preferred fission site. It has been predicted that the fast singlet fission sites
have two DPT molecules in a preferred dimer arrangement; however, there are < 5% of such
dimer sites present in the amorphous films.
42
Nevertheless, the singlet fission rate at the preferred
DPT sites is markedly faster than those reported for tetracene polycrystalline thin films (9-90 ps)
and single crystals (40-300 ps).
13, 14, 42-46
Thus, to eliminate the slower, diffusive process in favor
of faster singlet fission, it is advantageous to arrange every pair of molecules in a thin film in the
preferred orientation, especially since the diffusion process is a potential source of exciton loss.
Ideally, singlet fission should outcompete other (loss) processes.
These results suggest that in amorphous films, some of the photons which are absorbed by
non-ideally oriented chromophores produce excitons which can end up in a trap site prior to
20

reaching the optimal singlet fission site. Therefore, to design efficient SF materials for use in
OPV, the favorable relative orientation for SF must be ensured in all chromophores despite the
amorphous morphology of the layers. This can be achieved through the incorporation of
covalently linked dimers (Figure 1.9c) of SF chromophores in the active layers of OPV.
As the rates and efficiencies of SF are contingent upon the relative orientation of the
chromophores within dimers, determination of this optimal orientation is paramount in
developing effective SF materials for OPV devices. This thesis presents the quest for the optimal
relative geometry in covalently-linked ethynyltetracenes. First, the viability of ethynyltetracene
(ET) as a SF chromophore is presented in Chapter 2; then, the synthesis and photophysical
properties of two cofacially oriented ET dimers are described in Chapter 3, with a particular
focus on the effect of excimers on the kinetics of SF in these systems. The 2
nd
step of SF, the
separation of the triplets from the correlated pair, is also addressed in Chapter 3. Chapter 4
expounds on the role of through-bond versus through-space coupling in facilitating SF in four ET
dimers, and touches on the effect of charge transfer interactions in these systems. In the final
chapter, the synthesis and photophysical properties of a covalently linked tetramer of ETs are
presented, with a focus on the effect of entropy and S
1
exciton delocalization on the first step of
singlet fission. Lastly, the properties of an asymmetric tetracene dimer are given an Appendix I.
1.4 Overview of SF in covalent dimers
Prior to presenting the data of our covalently-linked ETs, it is useful to first give an
overview of SF in other covalently-linked chromophores published in the literature. The next
section will summarize SF in covalently-linked systems in a chronological order.
Linearly-linked tetracene dimers, 2006
The first reported study of SF in covalent systems was done by Muller and coworkers in the
Bardeen lab using the molecules produced by the Klaus Mullen group, shown in Figure 1.10.
56, 57

21

In these dimers, the through-space overlap is not possible and only the through-bond coupling is
anticipated. The magnitude of this coupling is expected to be small because the tetracenes are out
of plane of the linking ring. The authors used time resolved emission to quantify SF. Delayed
fluorescence was observed for these dimers in solution, akin to that in the crystalline anthracene
and tetracene from older works.
58, 59
The authors fit the S
1
decay using the kinetic model shown
in Figure 1.11, and obtained yields of 2.8 % and 2.9 % for dimers 1 and 2, respectively.

Linearly-linked diphenylisobenzofuran dimers, 2013
The groups of Josef Michl and Justin Johnson have focused on SF materials based on the
diradicaloid electronic structure. In 2013 they published their findings on indirectly coupled
covalent dimers of 1,3-diphenylisobenzofuran, whose structures are shown in Figure 1.12.
60

Figure 1.11. Kinetic model used for fitting the fluorescence decay data by Muller et al.
56,57

Figure 1.10 The bis-tetracene molecules reported by the Bardeen group in 2006.
56,57

22

They used transient absorption to study the excited state dynamics of these dimers in various
solvents. It was found that the dynamics, as well as the triplet yield, were dependent on the
polarity of the solvent, such that the triplet yields were greater in more polar solvents. A T
1
yield
of 9 % was measured for dimer 3 (see Figure 1.11) in DMF at 230 K, while dimer 2 gave a
maximum of 4 % T
1
yield.

Ortho-, meta-, para-bis(TIBS-pentacenyl)benzene dimers, 2015
Since the publication of the diphenylisobenzofuran dimers described above, our group was
in the process of synthesizing the ethynyltetracene dimers, and by the beginning of 2015 we had
developed an understanding of the SF kinetics in dimers presented in Chapter 3. In the
meantime, the Guldi group had studied the kinetics of SF using transient absorption in a series of
pentacene dimers prepared by Dan Lehnherr (see Figure 1.13), and published these results in
2015.
61
They reported a very rapid formation of the T
1
in the ortho and the para dimer, such that
the clear spectroscopic signature of the T
1
state was never isolated in these systems. The T
1

formation and decay in the meta dimer were much slower. The T
1
formation appeared to be
biexponential and variable with the polarity of the solvent, which lead them to assign a rate of
6.10×10
10
s
-1
to CT, and 1.59×10
10
s
-1
to SF in benzonitrile, and 3.97×10
10
s
-1
to CT, and
1.11×10
10
s
-1
to SF in toluene. The calculated T
1
yield in toluene was 125 %, and 156 % in

Figure 1.12 The indirectly coupled covalently linked dimers of 1,3-diphenylisobenzofuran
reported by Johnson et al. in 2013.
60
23

benzonitrile. They used the solvent dependence of the T
1
generation to suggest that the
mechanism of SF could proceed in a stepwise manner, via population of the CT state.

Linearly-linked bipentacenes, 2015
Around the same time, Sanders et al. published TA studies of a series of pentacene dimers
linked at the 2 position (see Figure 1.14).
62
They observed a decay of S
1
and a rise of T
1
on a
timescale of τ = 0.7 ps, and a subsequent decay of the T
1
signal on a timescale of τ = 450 ps.
Considering that the sensitized T
1
(one T
1
per dimer) has a lifetime of >18 μs, they concluded
that the observed T
1
in the dimer must be a result of SF, and therefore the two T
1
on the adjacent
chromophores within the dimer annihilate in 450 ps.

Figure 1.14 The linearly-linked bipentacenes synthesized and reported by Sanders et al. in
2015.
62

Figure 1.13 The ortho-, meta- and para- bis(TIBS-pentacenyl)benzene dimers synthesized by
Dan Lehnherr and reported by Zirzlmeier et al. in 2015.
61

24

Linearly-linked heterodimers of pentacenes, 2016
Sanders et al. then went on to study analogous heterodimers of pentacenes, substituted at
the 2 position with TIPS and TIBS anthracene, tetracene, and hexacene (see Figure 1.15).
63
They
observed that the pentacene-anthracene (PA) dimer does not undergo SF, and determined that the
pentacene-tetracene (PT) dimer forms T
1
states on a timescale of τ = 0.83 ps, with a lifetime of
2.4 ns. Despite the increased thermodynamic driving force for SF in the pentacene-hexacene
(PH) dimer, the T
1
states form more slowly, on a timescale of τ = 1.2 ps, and decay more quickly
(0.21 ns). They concluded that the case of isoergic SF in the PT dimer is the best for practical
uses.

“Bent-shaped” pentacene dimers, 2016
During the same period, Sakuma et al. published their studies of several “bent -shaped”
TIPS-pentacene dimers,
64
analogous to those of Sanders et al. (see Figure 1.16).
62
They observed
the fastest rate of SF (8.5×10
10
s
-1
) in PcD-4Ph, with a T
1
yield of 200 %. The PcD-3Ph
compound produced T
1
at a much slower rate of 2.9×10
9
s
-1
with a yield of 188 %. The

Figure 1.15 The linearly-linked heterodimers synthesized and reported by Sanders et al. in
2016.
63
25

biphenyl-bridged dimer, PcD-Biph turned out to be the slowest at producing triplets (k
sf
=
1.8×10
9
s
-1
), and a yield of 176 %.

Cross-conjugated pentacene dimers, 2016
In 2016 Zirzlmeier et al. published their studies of another set of pentacene dimers prepared
by Dan Lehnherr. This time, the acenes were not in direct conjugation, but rather “cr oss-
conjugated” (see Figure 1.17 for chemical structures), such that through-space interactions were
possible, but through-bond coupling was minimized.
65
Based on the linear absorption spectra,
they determined that the pentacenes in XC1 were more strongly coupled than in XC2. According
to the TA data, the rate of T
1
formation via SF was slightly faster in XC2 (k
sf
= 0.70×10
12
s
-1
)
than in XC1 (k
sf
= 0.65×10
12
s
-1
). The T
1
signal also decayed faster in XC2 (k
T1decay
= 2.54×10
9
s
-
1
) than in XC1 (k
T1decay
= 2.08×10
9
s
-1
). Furthermore, the yields of T
1
were determined to be
dependent on the solvent polarity, 162 % for XC2 in benzonitrile, and 128 % in toluene, while
XC1 yielded 127 % in benzonitrile and 119 % in toluene. This led the authors to conclude that
SF proceeds via a mediated mechanism with intermediate CT population in the weakly,
through-space coupled pentacene dimers.

Figure 1.16 The “bent -shaped” pentacene dimers rep orted by Sakuma et al. in 2016.
64
26

Norbornyl-Bridged Tetracene Dimer, 2016
In 2016, almost ten years after the original tetracene dimer work had been published,
another tetracene dimer emerged in the literature, this time from the Damrauer group. They had
been working on the synthesis of the norbornyl-bridged tetracene dimer (BT1) (Figure 1.18),
inspired by the naphthalene dimers of Scholes and Paddon-Row.
66
The initially published
theoretical work on this system, predicted that although the electronic coupling of BT1 is larger
than in the dimers studied by Muller et al.,
67
the key nonhorizontal electron transfer matrix
element used to determine the SF rate is zero, due to the symmetry of this dimer.
68
Then in 2016
Cook et al. published the excited state kinetics of BT1.
69
Using TCSPC Cook et al. measured a
biexponential fluorescence with τ
1
= 4.3 ns (91.5%) and τ
2
= 11.5 ns (8.5%). By utilizing a
kinetic model analogous to that published by Muller et al. for linearly-linked tetracene dimers,
they calculated a SF yield of 6.3 %. Furthermore, like Muller et al., they found the rate of T
1

fusion (k
fus
) in their dimer to be greater than the rate of SF (k
SF
) by a factor of 7.9 for their
system. They attributed this to free energy difference of the
1
(T
1
T
1
) and S
1
, with S
1
being lower
by 52 meV than the
1
(T
1
T
1
) state.

Figure 1.17 The “cross -conjugated” TIPS -pentacene dimers synthesized by Dan Lehnherr
and reported by Zirzlmeier et al. in 2016.
65
27

π-stacked covalent terrylenediimide dimers, 2016
Lastly, the latest dimer study published this year by Margulies et al. from the Wasielewski
lab has examined a series of covalently-linked terrylenediimides (TDIs) with progressively larger
degrees of slip-stacking (Figure 1.19).
70
Using transient absorption, they observed that the
dimers in which the TDIs were closely packed to each other (small degree of slip, 0.8 Å in 0, and
5.3 Å in 1) form the excimer state on the timescale of the instrument response (<200 fs), which
then relaxes into a CT state. The formation of the CT induced absorption features is faster in the
polar solvent (CH
2
Cl
2
) than in the nonpolar solvent (toluene), and is accompanied by an increase
in the ground state bleach. The same CT features were observed for 2 in CH
2
Cl
2
, but the
dynamics differed in toluene. In the nonpolar solvent, 2 exhibits T
1
T
n
induced absorption
features on a timescale of 2 ps, with additional persistent induced absorption features in the near
IR region (which are not present in the T
1
T
n
spectrum of the monomer). The rapidly formed
T
1
T
n
features in the dimer decay on a timescale of 1 ns.

Figure 1.18 The norbornyl-bridged tetracene dimer reported by Cook et al. in 2016.
69
28

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Figure 1.19 The terrylenediimide dimers synthesized and published by Margulies et al. in
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70
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32

Chapter 2: Photophysical properties of ethynyltetracene
2.1 Introduction
The synthesis of tetracene dimers is not trivial, as there are few locations on the tetracene
core at which the substitutions can be made easily. As was discussed in the previous chapter, the
ideal tetracene dimers would be those in which cofacial π overlap between tetracenes is possible.
The prohibitively large steric interactions of two tetracenes in close proximity have hindered the
synthetic development of many theoretical dimer structures. For this reason, we chose to
substitute the tetracene core with an alkyne group. This substitution reduces steric bulk at the
position of attachment and provides the synthetic advantage of utilizing Sonogashira coupling
reactions to attach the tetracenes in a close-packed covalent manner.
1, 2

Prior to our studies of ethynyltetrcenes (see Figure 2.1 for the chemical structure), singlet
fission (SF) had not been explored in alkyne-substituted tetracenes. From the previous studies of
substituted tetracenes it is evident that the substituents on the tetracene core can not only
influence the morphology of the material, but also have a significant impact on the relative S
1

and T
1
energies of these chromophores.
3
Since SF is slightly endothermic in unsubstituted
tetracene,
4
substituents which increase the energy gap between the correlated triplet pair
[
1
(T
1
T
1
)] and the S
1
states could result in very slow or non-existent singlet fission in these
materials. The energy gap between the
1
(T
1
T
1
) and the S
1
states has the largest influence on the
SF rates, as demonstrated by Yost et al.,
5
therefore when designing chromophores for SF, it is
important to ensure the isoenergicity or slight exothermicity of SF in the material.
Given the conjugating nature of the alkyne, it was not immediately obvious how the relative
S
1
and T
1
energies (and therefore the rates of SF) would be impacted when it is added as a
substituent to tetracene. Absorption spectra of previously reported bis-ethynyl-acenes indicate
33

that the S
1
energy is significantly lowered by the alkynes;
6-8
however, the effect on T
1
energies
has not been explored.
Prior to examining the dimers of ethynyltetracene, we explored the photophysical properties
of monomeric 5-ethynyltetracene derivatives: 5-(TMS-ethynyl)-tetracene (ET-TMS) and
5-(phenylethynyl)-tetracene (PET), whose chemical structures are shown in Figure 2.1 a, and b,
respectively. It was observed that the single alkyne substitution leads to a red-shift of more than
31 nm in absorption and emission spectra compared to tetracene, representative of a decrease in
the S
1
energy of ~0.2 eV. Despite the decrease in the S
1
energy, rapid SF was observed by
transient absorption in an amorphous thin film of ET-TMS, suggesting that both the S
1
and T
1

states were lowered by the alkyne substitution, such that SF is thermodynamically viable in this
material. Furthermore, SF takes place on a subpicosecond timescale in ET-TMS, making it the
fastest tetracene based SF material. The maximum T
1
yield, however, is only 92 % in ET-TMS,
and with additional room for improvement (for instance, by controlling the morphology). Given
the promising properties of this chromophore, carefully designed dimers of it could prove to be
efficient at producing triplets via SF.

2.2 Results and Discussion
2.2.1 Synthesis and
1
H NMR of ET-TMS and PET
The ethynyltetracene (ET) was primarily chosen as a SF chromophore simply because the
synthetic hurdles toward tetracene dimers were avoided in molecules based on this chromophore.
The electronic state energies in ET, and therefore its viability as a SF material were unknown. As

Figure 2.1 Chemical structures of a) ET-TMS and b) PET.
34

a result, the ET’s monomeric form prompted the examination first. The monomer ETs chosen for
this study are those in which the alkyne is attached to a TMS group (ET-TMS), as this was used
as a starting material for the dimers, and one in which a phenyl group (PET) is attached to the
alkyne, as this monomer is more representative of the dimers (in which the ETs are attached to
benzene rings). The synthetic scheme for ET-TMS and PET is provided in Scheme I. All
reactions were carried out under an inert atmosphere of N
2
gas. First, in order to reduce the costs
of the materials, tetracene was synthesized using the reported analogous procedure for pentacene
synthesis.
9
The tetracene was then brominated using a modified, reported protocol.
10, 11
The
yields of this reaction can reach as high as 90 %. The bromotetracene can then be reacted with
TMS-acetylene or phenylacetylene using the high-yielding Sonogashira reaction to give
ET-TMS and PET, respectively.
12

The comparison of the
1
H NMR spectra of ET-TMS and PET (Figure 2.2) indicates that the
substituent on the other end of the alkyne has electronic effects on the acene, as the peaks
corresponding to analogous protons in these molecules are slightly shifted. This difference
suggests a conjugating nature of the alkyne. For instance the peak corresponding to proton ‘a’ is
more downfield in PET compared to the analogous proton in ET-TMS.
Scheme I: Synthetic scheme for the synthesis of ET-TMS and PET.

35

Figure 2.2 The aromatic regions of the
1
H NMR spectra of a) ET-TMS and b) PET in
CDCl
3
at 25
o
C.
36

2.2.2 Computational studies of ET-TMS and PET
The electronic structure of ETs can be further understood by comparison of their molecular
orbitals. An unsubstituted tetracene is highly symmetric, belonging to the D
2h
point group;
however, a TMS-acetylene substitution at the 5 position reduces the symmetry to C
1
. The
molecular orbitals reflect the altered symmetry of the acene. The HOMO and the LUMO in
ET-TMS and in PET shown in Figure 2.3 are delocalized onto the alkyne group. In PET, the
HOMO and the LUMO also extend to the phenyl substituent on the alkyne, which suggests that
the alkyne allows for partial conjugation to the phenyl. The extension of the HOMO and LUMO
onto the alkyne results in reduction of the S
1
energy in these molecules, relative to tetracene. The
calculated S
1
and T
1
values for tetracene, ET-TMS and PET are shown in Table 2.1.
Interestingly, the T
1
energies are lowered more by the alkyne substitution than the S
1
energies.
As a result, the S
1
/T
1
ratio is not significantly affected, compared to tetracene. This is a
surprising effect, considering that the S
1
state is usually more localized, and is typically more
affected by substitutions, whereas the T
1
is commonly more localized and unaffected by
substitutions. In the case of the alkyne substitution, perhaps the considerable MO amplitudes on
this substituent effectively extend the π network and therefore the size of the chromophore.
37

2.2.3 Steady state photophysical data (solution)
The alkyne substitution on the acene significantly alters its electronic properties, as
demonstrated by the comparison of the steady state absorption and emission spectra of ET-TMS,
PET and tetracene overlaid in Figure 2.4. Although the line shape of the vibronic progression of
diminishing intensity is retained in the substituted tetracenes, the energy of the S
1
is lowered
substantially. In comparison with tetracene, the absorption of ET-TMS is red-shifted by 31 nm
(0.161 eV), while that of PET is red-shifted by 39 nm (0.200 eV); the emission of ET-TMS is
Table 2.1: S
1
and T
1
energies of tetracene, ET-TMS, and PET calculated using
B3LYP/6-31+G**.
S
1
T
1
S
1
/T
1
Tetracene 2.70 1.44 1.87
ET-TMS 2.53 1.31 1.93
PET 2.45 1.27 1.93

Figure 2.3 Molecular orbitals of a) tetracene, b) ET-TMS, and c) PET.
38

red-shifted by 42 nm (0.212 eV), and that of PET is red-shifted by 50 nm (0.249 eV). The
vibronic spacings in the absorption spectra of tetracene, ET-TMS and PET differ slightly,
indicating variation in the potential energy surfaces of their excited states. Tetracene exhibits a
spacing of 1400 cm
-1
in the absorption spectrum, while that in the ET-TMS and PET absorption
spectra is approximately 1300 cm
-1
. It is also noteworthy that the absorption and emission of
PET differs from ET-TMS, further indicating that the alkyne is slightly conjugating, allowing for
electronic coupling to any substituent attached to the other end of the alkyne. This point will be
important when analyzing the coupling between ethynyltetracenes in covalent dimers.

The quantum yield of emission offers another significant difference between the
alkyne-substituted tetracenes and the unsubstituted tetracene. The quantum yields of tetracene,

Figure 2.4 Steady state absorption (solid black line) and emission (dashed red line)
spectra of ET-TMS, and PET compared to tetracene.
39

ET-TMS, and PET are listed in Table 2.3. The quantum yield of emission of tetracene in THF
was measured to be 14 %, while the quantum yield of the emission of PET was 67 % and that of
ET-TMS was 69 % under the same conditions. The relatively low quantum yield of tetracene
emission is ascribed to the fairly rapid rate of intersystem crossing, due to the close energetic
proximity of the T
2
state to its S
1
state.
13
The considerable increase in the quantum yield in
alkyne-substituted tetracenes indicates that the relative energies of the T
2
and S
1
states have been
altered, such that rapid population transfer to the T
2
from the S
1
is not possible in these systems.
One consideration for designing a chromophore for SF is the minimization of non-radiative
pathways in the isolated chromophore in order to ensure that the rates of non-radiative decay do
not outcompete that of SF. In this respect, the alkyne-substituted tetracenes, ET-TMS and PET
appear to be more promising chromophores for SF than the unsubstituted tetracene.

The radiative rates of the chromophores can be derived from the quantum yield and the
lifetime using Equation 1.

r
nr r
r
k =
k k
k
=


fl
(1)
where Φ
fl
is the quantum yield of emission, k
r
is the radiative rate, k
nr
is the non-radiative rate,
and τ is the lifetime of the emission. The values of the radiative rates of tetracene, ET -TMS, and
PET in a THF solution are given in Table 2.2. The radiative rate of the alkyne-substituted
tetracenes is faster than that of tetracene by nearly a factor of two, which suggests that the
magnitude of the S
1
transition dipole moment in these systems is greater than that in tetracene, in
Table 2.2: The emission properties of tetracene, ET-TMS, and PET in a THF solution
measured at room temperature.
Φ
fl
τ (ns) k
r
(μs
-1
)
Tetracene 0.14 5.43 26.2
ET-TMS 0.69 16.1 42.9
PET 0.67 11.9 56.1

40

accordance with the Strickler-Berg relation.
14
This is further supported by the relative molar
absorptivities (ε) of these molecules shown in Figure 2.5. The molar absorptivities of ET -TMS
(1.3×10
4

cm
-1
M
-1
at 504 nm) and PET (1.6×10
4

cm
-1
M
-1
at 512 nm ) are more intense in the
visible region compared to that of tetracene (1.0×10
4

cm
-1
M
-1
at 473 nm). These results are
unsurprising, as the transition dipole moment of the lowest energy transition in acenes is along
the short axis of the molecule. Therefore, addition of conjugating substituents along that axis
should increase the magnitude of the transition dipole moment.

The quantum yield of emission of ET-TMS and PET is dependent on the concentration of
these chromophores in solution. Upon increasing concentration, the quantum yield decreases,
and the emission lifetime becomes multi-exponential, as shown in Table 2.3. Although the
emission lifetime becomes non-linear, the line shape of the emission spectrum remains
consistent. The most notable difference in the emission spectra of different concentrations of
acenes is the effect of re-absorption at high concentrations in the blue wavelengths, which results
in filtered spectra, shown in Figure 2.6.

Figure 2.5 Molar absorptivities of tetracene (black), ET-TMS (red) and PET (blue) in THF.
41

Two possible emission quenching mechanisms at higher concentrations of acenes are
excimer formation or SF. A recent study by Stern et al. has looked at TIPS-tetracene excited
state dynamics in concentrated solutions, and observed these molecules undergoing SF via the
excimer state in highly concentrated solutions.
15
Since the core of these molecules is a tetracene,
in which SF is already endothermic, it is surprising that upon forming an excimer, these systems
are energetic enough to split into two triplets. TD-DFT calculations suggest that formation of the

Figure 2.6 Emission spectra as a function of concentration of a) ET-TMS and b) PET in THF.
Table 2.3: The quantum yields and lifetimes of emission of ET-TMS and PET as a function of
concentration in THF.
Concentration (μM) Φ
fl
Lifetime
ET-TMS
71 57.2 (0.08) 6.9 ns, (0.92) 16.9 ns
41 59.0 (0.05) 7.3 ns, (0.95) 16.7 ns
15 61.0 16.2 ns
5.8 62.8 16.1 ns
2.2 69.0 16.1 ns
PET
40 59.3 (0.35) 4.6 ns, (0.65) 12.1 ns
12 63.0 (0.15) 5.5 ns, (0.85) 12.5 ns
4.7 66.7 11.9 ns

42

excimer would make SF substantially energetically uphill in ethynyltetracenes. The calculated
potential energy surfaces (PESs) of the S
0
, S
1
and the quintet states

of the dimer of
ethynyltetracene, as a function of intermolecular distance, is shown in Figure 2.7. The S
1
state
has a minimum at 3.5 Å, while the ground state of PET is repulsive, and reaches a minimum at
4.7 Å. The quintet PES, which is representative of the
1
(T
1
T
1
) state, is also repulsive. This
suggests that relaxation into an excimer can be detrimental for SF in these systems, as SF would
be uphill by ~0.34 eV upon excimer formation.

2.2.4 Steady state photophysical data (thin film)
The photophysical properties of the amorphous thin films of ET-TMS and PET are quite
different from those in solution. The absorption and emission spectra of solutions and thin films
of ET-TMS and PET are shown in Figure 2.8, and the emission decays are presented in Figure
2.9. The vibronic progression in the absorption spectra of thin films of ET-TMS and PET is

Figure 2.7 The calculated potential energy surfaces of the S
0
, S
1
and Q
1
states of an
ethynyltetracene dimer.
43

retained; however, the overall line-shape is broadened and red-shifted by 19 nm compared to
their corresponding solution spectra. Since the absorption spectra of these molecules were not
dependent on solvent polarity or polarizability, this red-shift in the absorption of the neat films is
indicative of Davydov splitting, which in turn reflects the electronic coupling between
neighboring chromophores in the solid state. The magnitude of electronic coupling can then be
estimated to be ~0.09 eV in the thin films. Although large values of electronic coupling are
beneficial for increasing the rate of SF, large values of Davydov splitting could substantially
lower the S
1
energy and upset the S
1
/T
1
energy ratio, making SF in a given system
thermodynamically unfavorable. In the case of ET-TMS thin film, the lowest S
1
transition is at
approximately 2.37 eV, which would require the T
1
state energy to be < 1.19 eV in order for SF
to be thermodynamically viable in this system.

0.0
0.4
0.8
400 500 600 700
0.0
0.4
0.8
Solution
Thin film
Intensity (norm).
Wavelength (nm)
Solution
Thin film
Solution
Thin film
0.0
0.4
0.8
Solution
Thin film
500 600 700
0.0
0.4
0.8
Intensity (norm).
Wavelength (nm)
Figure 2.8 A comparison of the solution (dashed red) and thin film (solid black) absorption
(left) and emission (right) spectra of ET-TMS (top) and PET (bottom).
44

The biggest difference between the photophysical properties of the thin films and those of
the solution is in the emission spectra. The neat film emission line shapes of both ET-TMS and
PET lack the defined vibronic progression, and are rather broad and featureless, with an ample
Stokes shift of 65 nm. The intensity of the neat film emission is very low, with a quantum yield
of ~0.6 %. The decay of this emission is multi-exponential (see Figure 2.9) with a majority of
the emission decaying within the timescale of the instrument response (0.4 ns). The source of
this emission is likely trap cites in the amorphous thin films which correspond to two
chromophores oriented in a configuration which makes them amenable to excimer relaxation.
The low intensity of the emission, coupled with rapid excited state decay indicates that
aggregation of these chromophores in thin films introduces a non-radiative pathway which was
not available to isolated chromophores in solution.
Although the thin film absorption, emission line shape, intensity and lifetime are similar for
ET-TMS and PET, the two follow very different non-radiative relaxation pathways in the solid
state. Specifically, PET thin films photodegrade upon excitation with 500 nm light under inert
atmosphere while the ET-TMS films do not. Based on previous studies of phenyl-
Solution
Film
0 20 40
10
-4
10
-3
10
-2
10
-1
10
0
Decay
Time (ns)
0 20 40
10
-4
10
-3
10
-2
10
-1
10
0
Decay
Time (ns)
Solution
Film

Figure 2.9 Emission decays of a) ET-TMS and b) PET in THF solutions (black) and as thin
films (red).
45

ethynylanthracene,
16
it is likely that PET undergoes a 2+4 cycloaddition between the tetracene
core of one chromophore and the alkyne of a neighboring chromophore in the excited state.

2.2.5 Transient absorption and singlet fission rate of ET-TMS
It is not possible to quantify SF in a given material by relying entirely on steady state
spectroscopic techniques. Therefore, transient absorption (TA) spectroscopy must be invoked in
order to determine whether ET-TMS undergoes SF in the solid state. Due to the
photo-degradation of PET in thin film, no transient absorption spectra were collected for this
material, and the remainder of this section will be devoted to ET-TMS.

Figure 2.10 The likely photo-degradation product of PET, based on the phenyl-
ethynylanthracene studies in ref. 16
46

As anticipated from the steady state emission data, the excited state dynamics of ET-TMS
neat film are drastically different from those in solution. Transient absorption spectra of ET-TMS
in PMMA (Figure 2.11a) resemble those of the monomeric tetracenes. Upon excitation with 500
nm light, transient absorption spectra of ET-TMS in PMMA show the S
1
S
n
induced absorption
features (~407 nm) similar to those observed for tetracene derivatives,
17
with no spectral
evolution over 1 ns. The TA data of neat ET-TMS in Figure 2.11b shows an immediate
appearance of S
1
S
n
induced absorption peak at ~ 415 nm following excitation at 500 nm.
a)
400 450 500 550 600 650
-2
0
2
4
6
8
10

A (mOD)
Wavelength (nm)
0.2 ps
1 ps
10 ps
300 ps
1300 ps
b)
400 450 500 550 600 650
0
2
4
6
8
Wavelength (nm)
A (mOD)

0.2 ps
1 ps
10 ps
50 ps
200 ps
ET-TMS T
1

c)
400 450 500 550 600 650
0.0
2.0x10
4
4.0x10
4
6.0x10
4
8.0x10
4
Wavelength (nm)
 (M
-1
cm
-1
)

S
1
T
1
x 4
d)
1 10 100 1000
0.0
5.0x10
5
1.0x10
6
1.5x10
6
2.0x10
6
2.5x10
6
Time (ps)
Excitations ( m
-3
)

T
1
S
1

Figure 2.11. Transient absorption spectra of the monomer ET-TMS in a) PMMA and b)
neat film following excitation at 500 nm. The cyan dotted line shows the triplet spectrum
obtained from ET-TMS sensitization measurements with Pd(TPBP). c) The extinction
spectra for S
1
S
n
and T
1
T
n
transitions used to calculate singlet and triplet populations (d)
for ET-TMS.
47

However, within 1 ps this peak decreases and a new induced absorption feature grows at ~510
nm. The spectral shape of the new induced absorption feature is similar to the T
1
T
n
transition
in DPT.
Sensitization experiments were performed to confirm the assignment of the T
1
T
n

transition. Specifically, the ET-TMS film was doped with palladium tetraphenylbenzoporphyrin
[Pd(TPBP)], which undergoes efficient intersystem crossing with a triplet lifetime of
143-195 μs.
18
The S
1
and T
1
energy alignment of ET-TMS and Pd(TPBP) is such that excitation
of the Pd(TPBP) populates its S
1
state, which is lower in energy than that of ET-TMS. The S
1

state of Pd(TPBP) then rapidly relaxes into its T
1
state, which is higher than that of ET-TMS, and
is therefore capable of undergoing triplet energy transfer to ET-TMS. The energy level scheme
is shown in Figure 2.12. The TA spectra of the sensitization experiment are shown in Figure
2.13. The two intense negative peaks at early times (<10 ps) correspond to the Pd(TPBP) ground
state bleach (GSB), which decay within 50 ps, and give rise to T
1
T
n
induced absorption
features of ET-TMS. The induced absorption features at 500 nm, which emerge consecutive to
the Pd(TPBP) GSB decay, correspond to the T
1
T
n
absorption features of ET-TMS.

Figure 2.13 The transient absorption of a 5
mol% Pd(TPBP) doped ET-TMS film.

Figure 2.12 The energy level scheme of
the triplet sensitizer Pd(TPBP) and
ET-TMS.
48

The absorption profile (peaks at 475 and 510 nm) from the transient spectrum of neat
ET-TMS film at long delays matches with the T
1
spectrum of ET-TMS from sensitization
measurements. The decays of the S
1
S
n
and rise of the T
1
T
n
transitions corresponding to the
S
1
and T
1
populations, respectively, have two timescales. This is analogous to the S
1
and T
1

dynamics in amorphous DPT films. The fast timescale (<1 ps) of T
1
formation corresponds to
SF taking place at the sites in the amorphous ET-TMS film where the molecules are oriented in a
geometry which facilitates rapid SF, whereas the slow timescale (10 ps) corresponds to the
diffusive pathway for SF. However, unlike in the DPT results, the formation and the subsequent
decay of the T
1
population in the ET-TMS thin film occurs much faster. The sub-ps formation of
the triplets in this material makes it the fastest tetracene-based SF material. Such fast rates of SF
could indicate better coupling between the ET-TMS molecules within the thin film. Large
coupling could increase the forward and the reverse rates of SF. The decay of the T
1
population
on <1 ns timescale is also very fast for tetracene-based materials. The fast T
1
decay could be a
result of efficient triplet annihilation due to large coupling between the acenes, or fast diffusion
to exciton trap cites.
Using the extinction spectra for S
1
S
n
and T
1
T
n
transitions, the TA data for ET-TMS
film was fit to extract the singlet and triplet populations and the maximum SF yield was
determined to be ~ 92% (detailed calculations can be found in the SI of ref. 19). The relatively
low SF yield could be due to fast T
1
recombination and a large amount of exciton trap sites
throughout the film.
2.3 Conclusions
The substitution of the tetracene core with an alkyne group in the 5 position altered the
electronic structure of the tetracene. The extension of the conjugation, and therefore the
49

delocalization of the HOMO and LUMO orbitals of the tetracene onto the alkyne resulted in a
red-shift of the absorption, an increase in oscillator strength, and a decrease in non-radiative
rates. Despite the decrease in the S
1
energy by ~0.2 eV (compared to tetracene), SF was still
thermodynamically viable in an amorphous thin film of ET-TMS. In fact, the triplet formation
via SF takes place within 1 ps in a thin film of ET-TMS, possibly suggesting that the T
1
energy
was also lowered by the alkyne substitution, such that E(S
1
)≈2×E(T
1
).
The T
1
yield in the thin film of ET-TMS is fairly low, at 92 %, likely due to exciton trap
sites present in the amorphous thin film and due to rapid T
1
recombination. To eliminate the
unfavorable exciton trap sites in amorphous acene films, it is beneficial to construct covalent
dimers in which the chromophores are pre-oriented in a geometry which is conducive to fast SF.
These experimental data presented in this chapter show that the energy level criterion
[E(S
1
)≈2×E(T
1
)] is fulfilled in ethnyltetracenes and thus the E(T
1
)/E(S
1
) ratio in the covalent
dimers of ethynyltetracenes should be sufficient for them to undergo SF. The subsequent
chapters present data for various covalent arrangements of ethynyltetracene and address several
important questions regarding SF in tetracenes, such as the role of excimer formation, the role of
chromophore coupling, and the role of exciton delocalization in dictating SF kinetics.

50

2.4 References
1. Sonogashira, K., Journal of Organometallic Chemistry 2002, 653 (1–2), 46-49.
2. Korovina, N. V.; Chang, M. L.; Nguyen, T. T.; Fernandez, R.; Walker, H. J.; Olmstead,
M. M.; Gherman, B. F.; Spence, J. D., Organic letters 2011, 13 (14), 3660-3663.
3. Murov, S. L.; Carmichael, I.; Hug, G. L., Handbook of photochemistry. CRC Press: 1993.
4. Smith, M. B.; Michl, J., Chem. Rev. 2010, 110, 6891.
5. Yost, S. R.; Lee, J.; Wilson, M. W.; Wu, T.; McMahon, D. P.; Parkhurst, R. R.;
Thompson, N. J.; Congreve, D. N.; Rao, A.; Johnson, K., Nature chemistry 2014, 6 (6),
492-497.
6. Levitus, M.; Garcia-Garibay, M. A., The Journal of Physical Chemistry A 2000, 104 (38),
8632-8637.
7. Maulding, D. R.; Roberts, B. G., The Journal of Organic Chemistry 1969, 34 (6), 1734-
1736.
8. Nakatsuji, S. i.; Matsuda, K.; Uesugi, Y.; Nakashima, K.; Akiyama, S.; Fabian, W.,
Journal of the Chemical Society, Perkin Transactions 1 1992, (7), 755-758.
9. Pramanik, C.; Miller, G. P., Molecules 2012, 17 (4), 4625-4633.
10. Müller, A. M.; Avlasevich, Y. S.; Schoeller, W. W.; Müllen, K.; Bardeen, C. J., J. Am.
Chem. Soc. 2007, 129, 14240.
11. Sun, T.; Shen, L.; Liu, H.; Sun, X.; Li, X., Journal of Molecular Structure 2016, 1116,
200-206.
12. Waldman, D. A.; Kolb, E. S.; Hutchinson, K. D.; Minns, R. A., Sensitizer dyes for
photoacid generating systems. Google Patents: 2011.
13. Turro, N. J., Modern molecular photochemistry. University science books: 1991.
14. Strickler, S. J.; Berg, R. A., The Journal of Chemical Physics 1962, 37 (4), 814-822.
15. Stern, H. L.; Musser, A. J.; Gelinas, S.; Parkinson, P.; Herz, L. M.; Bruzek, M. J.;
Anthony, J.; Friend, R. H.; Walker, B. J., Proceedings of the National Academy of
Sciences of the United State of America 2015.
16. Becker, H.-D.; Andersson, K., Journal of photochemistry 1984, 26 (1), 75-77.
17. Roberts, S. T.; McAnally, R. E.; Mastron, J. N.; Webber, D. H.; Whited, M. T.; Brutchey,
R. L.; Thompson, M. E.; Bradforth, S. E., Journal of the American Chemical Society
2012, 134 (14), 6388-6400.
18. Rogers, J. E.; Nguyen, K. A.; Hufnagle, D. C.; McLean, D. G.; Su, W.; Gossett, K. M.;
Burke, A. R.; Vinogradov, S. A.; Pachter, R.; Fleitz, P. A., The Journal of Physical
Chemistry A 2003, 107 (51), 11331-11339.
19. Korovina, N. V.; Das, S.; Nett, Z.; Feng, X.; Joy, J.; Haiges, R.; Krylov, A. I.; Bradforth,
S. E.; Thompson, M. E., Journal of the American Chemical Society 2016, 138 (2), 617-
627.

51

Figure 3.1 Visual representation of the
molecular arrangement which maximizes
the ) (
ˆ
1 1
1
0 1
T T H S S
el
coupling matrix
element.
Chapter 3: Singlet fission in cofacial dimers - o-BETB and BETX: the role of excimers
3.1 Introduction
When designing a tetracene dimer for fast
and efficient singlet fission, it is advantageous to
maximize the coupling matrix element
) (
ˆ
1 1
1
0 1
T T H S S
el
representative of the direct
coupling between the

and the
1
(T
1
T
1
) states.
1

As explained in Chapter 1, this matrix element is
maximized when there is sizeable cofacial π overlap between the chromophores, see Figure 3.1.
However, if the chromophores are perfectly symmetric, perfectly cofacial overlap between them
results in a cancelation of the overlap integrals. In such a case, the chromophores must be slip-
stacked in the direction of the transition dipole moment. In cases where the orbitals of the
chromophores are polarized due to molecular substitutions, it may not be necessary to slip-stack
them in order to achieve large values of the coupling matrix element. In the previous chapter it
was demonstrated that ethynyltetracenes are energetically competent to undergo singlet fission.
To test whether cofacial π overlap indeed facilit ates singlet fission, we designed and synthesized
two ethynyltetracene dimers with different degrees of π overlap. The structures of these dimers
are shown in Figure 3.2. In one dimer (bis-ethynyltetracenyl-xanthene – BETX), the
ethynyltetracenes are bridged by a xanthene unit, and are confined to a parallel cofacial
orientation, while in the other dimer (ortho-bis-ethynyltetracenyl-benzene – o-BETB), the
ethynyltetracenes are situated ortho- on a benzene ring, and exhibit approximately one ring of
overlap. In both of these dimers, the symmetry of the tetracene orbitals is perturbed by the
alkyne substitution; therefore, the perfect cofacial overlap in BETX is not detrimental.
52

3.1.1 Excimers
Another excited state process which becomes efficient when two chromophores exhibit a
large degree of cofacial π overlap is relaxation into an excited dimer, otherwise known as an
‘excimer.’
2
An excimer represents a minimum in the excited state potential energy surface,
which corresponds to a geometry in which the chromophores are closer together than they are in
a minimum energy ground state structure. Figure 3.3 supplies the rationale for excimer
formation. In this figure, r is the distance between the chromophores, labeled ‘a’ and ‘b,’
respectively. In the ground state, two chromophores exhibit a repulsive potential, meaning that at
large distances r, the energy of the system is equal to twice the energy of an individual
chromophore; however, as r becomes shorter and orbital overlap increases, the electron repulsion
between the chromophores raises the total energy of the system. At shorter distances r, the
interaction of the orbitals of individual chromophores results in orbital splitting to produce super-
molecular orbitals. The ground state electron configuration for two chromophores in close
proximity (bottom left diagram in Figure 3.3) shows that the HOMO and HOMO’ are equally
populated, resulting in no net energy gain. The scenario changes when one of the chromophores
is in the excited state. At large values of r, the chromophores do not interact, and the total energy
of the system is equal to

. At shorter distances, the population of the super-
molecular orbitals results in an electronic configuration in which the highest lying electron is in
a) b)
Figure 3.2 Chemical structures of the dimers presented in this chapter. a) ortho-bis-
ethynyltetracenyl-benzene – o-BETB and b) bis-ethynyltetracenyl-xanthene – BETX
53

an orbital which is lower than the orbital of the isolated chromophore, resulting in a lower total
energy of the system than in the case of infinitely spaced chromophores. Therefore, the lowest
excited state of two chromophores in the vicinity of each other is bound, whereas the ground
state of the same system is unbound. This means that the emission which takes place from an
excimer state is likely to be significantly red-shifted (from the monomeric chromophores’
emission), and featureless (lacking vibronic structure in its spectral shape).

The net energetic gain in the excited state increases with increasing cofacial overlap of the
π orbitals. Excimer formation has been shown in an array of polycyclic aromatic hydrocarbons
such as perylene, pyrene, anthracene, etc.
3-6
In solutions of these molecules, the rate of excimer
formation is limited by the rate of diffusion, and is therefore a function of the solvent viscosity.
For instance the rate of formation of pyrene excimers in a tetradecane solution at room
temperature is 100 ns.
7
However, if the two chromophores are already confined to the vicinity of
each other, the rate of relaxation into an excimer can be on the timescale of 10 ns.
8
In cases of
chromophores pre-oriented for excimer formation, excimer emission can appear 100 ps after

Figure 3.3 Potential energy surfaces of the ground (lower) and the S
1
(upper) states, and the
corresponding orbital configurations. The minimum on the S
1
surface corresponds to the
“excimer” state.
54

excitation.
6
Furthermore, if the two chromophores are confined in close proximity to each other
in the ground state, the ground state becomes bound, and the excimer emission can be
structured.
9

Another characteristic feature of excimer emission is a slow radiative rate. The radiative
rate of excimer emission is much slower than that of typical π  π* fluorescence. The slower
rate stems from the lower oscillator strength of the transition, which is comprised of the linear
combination of LUMO
a
 HOMO
b
and LUMO
b
 HOMO
a
transitions. Given that the orbital
overlap of these transitions is lower than that for a π  π* localized on the same chromophore,
the oscillator strength drops significantly, and along with it, the radiative rate, in accordance with
the Strickler-Berg relation.
10
The lifetimes of the excimer decays can be as high as 430 ns.
11

Excimer emission has been observed in solution and in the solid state for various polycyclic
aromatic hydrocarbons (PAHs). In the solid state, excimer emission has been associated with
compounds which crystallize in a cofacial manner, such as perylene, peropyrene and
dichlorotetracene.
12-14
In crystalline peropyrene for instance, the excimer emission is red-shifted
to 610 nm from 575 nm of the monomeric emission.
14

Given the relative singlet and triplet energies of tetracene, and the endothermicity of singlet
fission in this system, relaxation into an excimer could prove to be detrimental for singlet fission
in a cofacial tetracene dimer. Tetracene excimer emission has been reported for a cofacial pair
of tetracenes suspended in a PMMA matrix at 15 K by Iannone and coworkers.
9
The authors
observed a structured, red-shifted emission with a peak at 560 nm. Therefore, in a close, co-
facial configuration of tetracenes, the

relaxes by 0.4 eV. Considering that the energy of

in
tetracene is 1.25 eV, formation of two triplets via singlet fission would be ~0.3 eV uphill in this
configuration.
55

3.1.2 Excimers and SF
Although the singlet excimer state electronic configuration is different from that of the
1
(T
1
T
1
) state (as shown in Figure 3.4), the S
1
configurations can admix into the
1
(T
1
T
1
) state
wave function, thereby lowering its energy. The energy of two independent triplets (most closely
approximated by the
5
(T
1
T
1
) state) is likely to be equal to 2×E(T
1
), the energy of the
1
(T
1
T
1
)
state, however, can be lower, due to the admixture of low-lying S
1
configurations into the
1
(T
1
T
1
)
wave function. This is best exemplified by the excited states of trans-butadiene, in which the
5
(T
1
T
1
) and
3
(T
1
T
1
) states are greater in energy than the
1
(T
1
T
1
) state by 1-2 eV.
15
Furthermore,
it is not immediately obvious whether the
1
(T
1
T
1
) state potential energy surface (PES) will most
closely follow the
5
(T
1
T
1
) or the S
1
PES. For tetracene-based systems, if
1
(T
1
T
1
) traces the
5
(T
1
T
1
) PES, then relaxation into an excimer along the S
1
surface will move the excited state
population energetically downhill from the
1
(T
1
T
1
) state, and additional energy would be
required in order for SF to take place. In this scenario, once the
1
(T
1
T
1
) state is formed, the
correlated triplets are energetic enough to behave as independent triplets. If the
1
(T
1
T
1
) state
follows the S
1
surface, however, singlet fission does not require extra energy, and is likely to
proceed very rapidly. Once the
1
(T
1
T
1
) state is generated in this scenario, the population may be
stuck in this state, and the triplets will not behave as independent entities. This latter scenario
could render SF materials (dimers, in particular) to be useless for applications, as the
1
(T
1
T
1
)
could have rapid non-radiative decay channels to the ground state, as it is still in the singlet spin
manifold. Smith and Michl have postulated that the splitting between the
1
(T
1
T
1
) and the
5
(T
1
T
1
)
is likely to be a function of the coupling between the chromophores; this subject will be further
addressed in the following chapter.
56

The role of excimers with relation to SF is still debated, as several groups have reported
conflicting results. Nichols and coworkers have reported that the observed excimer emission in
crystalline peropyrene is the predominant excited state decay pathway, and singlet fission was
not detected in this system.
14
Excimer formation has also been determined to be the predominant
relaxation pathway for several cofacially oriented perylenediimide and terrylenediimide dimers
by the Wasielewski group, preventing singlet fission from taking place in these systems.
16-18
The
same group, however, reported that singlet fission takes place via the excimer state in
diketopyrrolopyrrols.
19

In principle, relaxation into an excimer should not inhibit singlet fission in systems in
which singlet fission is strongly exothermic (such as pentacenes), provided that the excimer is
not deeper in energy than 2×E(T
1
). In fact, triplet formation via singlet fission has been observed
to proceed from an excimer state in concentrated solution of TIPS-pentacene.
20
Interestingly,
triplet formation via singlet fission has also been reported to take place from an excimer state in
a concentrated TIPS-tetracene solution, despite the anticipated energy barrier. It is possible that
the

energy of the TIPS-tetracene could have been substantially lowered by the TIPS groups
compared to tetracene, thereby making singlet fission isoergic or less endothermic. Furthermore,
as these experiments were carried out in solution, it is not known which relative geometry of the
chromophores lead to singlet fission.

Figure 3.4 A simiplified illustration of the electronic configurations describing the excimer
and the
1
(T
1
T
1
) states using a four orbital, four electron approximation.
57

In amorphous films and crystals with multiple relative chromophore orientations, it is not
immediately obvious which relative orientation of the chromophores leads to fast SF. Therefore,
in order to understand the effects of a specific geometry on singlet fission dynamics in acenes, it
is prudent to confine the two chromophores to a certain relative geometry in a covalent manner.
These dimers can then be studied in solution, thus eliminating any solid state effects. This
chapter presents the synthesis as well as structural and photophysical characterization of two
cofacial covalently-linked ethynyltetracene dimers, (bis-ethynyltetracenyl-xanthene – BETX)
and (ortho-bis-ethynyltetracenyl-benzene – o-BETB). The tetracenes in BETX exhibit maximum
overlap, such that intramolecular excimer formation could be possible; however, in o-BETB the
tetracenes are overlapping at an angle relative to each other, and excimer interactions are not
likely to be strong.
3.2 Results and Discussion
3.2.1 Synthesis and
1
H NMR of o-BETB and BETX
The literature scope of synthetic modification of tetracene is very limited. The most
common route to functionalization of tetracene is via bromination, which selectively occurs first
at the 5 position, and second second bromination follows at the 11 or 12 positions of tetracene.
Tetracene is easily oxidized, and therefore reactions carried out with tetracene and its derivatives
must be carried out under inert atmosphere and preferably in the dark. Once substituted at the
active positions, the tetracene becomes less prone to photooxidation, but not immune, and should
be kept out of ambient conditions in solution. As a solid, however, tetracene and its derivatives
are more stable.
Given the demanding conditions for carrying out tetracene chemistry, very few reports of
covalently-linked tetracene dimers existed in the literature prior to the work reported in this
58

thesis. The only precedence for tetracene dimers at that time was for the benzene-bridged
tetracene dimers synthesized by the Mullen group. Due to the large steric repulsion between the
tetracenes, only the para- and the meta- dimers were successfully synthesized.
To alleviate the steric bulk issues encountered by Mullen and to facilitate the synthesis, we
chose to modify the tetracene core with a single alkyne group at the 5 position. The substitution
with an alkyne provides several benefits: 1) it protects the tetracene from rapid photooxidation,
by lowering the energy of the LUMO and reducing the electron density at the 5,12 positions
(thereby reducing the rate of the Diels-Alder reaction of acene’s central ring with O
2
),
21
2) it
lends the tetracene amenable to high yielding Sonogashira reactions, which opens an array of
synthetic possibilities for creating dimers of tetracene, 3) because the alkyne’s π orbitals are
slightly conjugating to the tetracene π orbitals, the orbitals of tetracene b ecome polarized thus
relaxing the symmetry requirements mentioned above, and finally 4) it should reduce the triplet
energy of the chromophore more than it reduces the singlet energy based on the calculated S
1
and
T
1
energies,
22
which would make singlet fission in these systems more energetically favorable
and therefore more rapid.
The synthetic route toward the dimers, presented in Scheme I, is based on several other
analogous reported synthetic schemes.
23-25
Initially, the tetracene core was functionalized by
bromination using N-bromosuccinimide (NBS) to give 5-bromotetracene. The reaction yields for
this step can be as high as 85 %. A TMS-acetylene group was then coupled into the place of the
bromine using the high yielding Sonogashira reaction with standard reaction conditions
[Pd(PPh
3
)
4
as the catalyst, CuI as the co-catalyst, THF as a solvent and Et
3
N as a base and a co-
solvent]. The resulting product, 5-(TMS-ethynyl)-tetracene (ET-TMS) is fairly chemically stable,
and is used as a monomer reference throughout the studies reported here. In order to attach the
59

ethynyl-tetracene to various bridging dihalides, the TMS group was first removed by
anaerobically stirring ET-TMS with K
2
CO
3
in THF and MeOH for two hours. The resulting
product, 5-ethynyltetracene (ET) is very unstable, and to minimize its decomposition, must be
used in the next step immediately. The coupling of ET with various aryl dihalides was carried
out using the standard Sonogashira reaction conditions. The yields for the final dimer products
ranged from 53 to 82 %. Slower addition of the ethynyltetracene resulted in higher yields, due to
the minimization of the alkyne self-coupling.

The tetracene moieties in the two dimers differ electronically, as evidenced by the
differences in the
1
H NMR spectra of these compounds, shown in Figure 3.5. Proton ‘b’, for
instance, is more upfield in BETX (Figure 3.5b), suggesting its increased exposure to electron
density. Additionally, the spectra of the dimers are different from the monomers (Chapter 2)
primarily in the drastic upfield shifts of protons j, k, l, m of o-BETB (Figure 3.5a) and protons h,
i, j of BETX (Figure 3.5b), which is likely due to the shielding of the tetracene protons exposed
to the aromatic ring current of the neighboring tetracene.
Scheme I:

60

Figure 3.5 a)
1
H NMR of o-BETB and b) BETX in CDCl
3
at 25
o
C.
61

3.2.2 Molecular structures of o-BETB and BETX
Due to the flexibility of the bridging alkyne groups, the tetracene moieties in the resulting
dimers can sample a range of rotational conformers. BETX has two minima in the ground state
potential energy surface (PES) which correspond to the syn (long sides of the tetracenes pointing
the same way, as illustrated in the crystallographic forms) and the anti (long sides of the
tetracenes pointing in opposite directions), whose DFT optimized structures are shown in
Figure 3.6, with the anti conformer being lower in energy by 0.64 kcal/mol (from optimized
B3LYP/6-31+G** geometries). The barrier to interconversion between the conformers is
estimated to be 10.5 kcal/mol from a molecular dynamics simulation. This suggests that at room
temperatures all rotational conformers of BETX are sampled at room temperature in solution on
the NMR timescale. The o-BETB dimer, on the other hand, has three minima in its ground state
potential energy surface, corresponding to minimum, medium, and maximum overlaps of the
tetracene moieties (shown in Figure 3.6b). The maximum overlap conformer is the lowest in
energy, with the medium and the minimum being 0.1 kcal/mol and 1.6 kcal/mol higher in
energy, respectively. The barriers to rotation between the isomers are ~1 kcal/mol between min.
and med., ~3 kcal/mol between med. and max. These values suggest that all three conformers are
freely interconverting at room temperature in solution on the NMR timescale (μs).
62

The experimental evidence for rapid rotation of the tetracenes in the dimers gleaned from
variable temperature (VT)
1
H NMR spectroscopy is consistent with the rotation barriers
estimated by the MD calculations. The VT
1
H NMR spectra for o-BETB and BETX are shown in
Figures 3.7a and b, respectively. It is evident that at temperatures as low as -60
o
C, the tetracene
moieties in o-BETB are still able to freely rotate on the NMR timescale, as the peaks
corresponding to tetracene protons do not de-coalesce. This implies that the experimental
rotation barrier in these compounds is lower than ~6.5 kcal/mol. In BETX, the proton peaks also
do not exhibit de-coalescence upon cooling. However, the proton peaks in the -60
o
C spectrum
appear broad and reduced in intensity, which likely suggests self-aggregation.

a) b)
Figure 3.6 a) The syn and the anti conformers of BETX. b) The three conformers of o-
BETB and their corresponding computed energies, computed at B3LYP/6-31++G** level of
theory.
63

Figure 3.7
1
H NMR spectra of a) o-BETB and b) BETX at 25
o
C (red), 10
o
C (yellow), -5
o
C
(green), -20
o
C (cyan), -40
o
C (navy) and -60
o
C (violet).
a)
b)
64

Even though all rotational conformations are sampled at room temperature in solution, only
the maximum overlap conformers are observed in the crystal structures of both o-BETB and
BETX. Crystals of o-BETB were obtained by slow evaporation of THF under an inert
atmosphere and found to be burgundy prisms. The tetracene moieties in o-BETB are oriented at
~20  angle with respect to each other, as illustrated in Figure 3.8a. The long sides of the
tetracene moieties overlap in the center of the molecule. The maximum overlap conformer of
o-BETB belongs to the C
2
point group and exhibits approximately one ring of overlap between
the tetracenes (Figure 3.8a). The distance between the tetracenes within o-BETB is 3.1 Å at the
closest point of interaction, and 3.8 Å at the farthest point, as indicated in Figure 3.8b. The
intermolecular packing of o-BETB in the crystal places tetracenes from neighboring molecules in
a slip-stacked arrangement (along the long and the short axes) with a π -facing orientation (see
Figure 3.8c). The intermolecular tetracene distance in o-BETB is 3.4 Å.

Crystals of BETX were grown by slow evaporation of toluene under an inert atmosphere,
giving bright red rods. The asymmetric unit of BETX has two structurally distinct forms, shown
in Figure 3.9a and 3.9b. The acenes of one BETX dimer, BETX
1
(Figure 3.9b), are slanted at a

Figure 3.8 a) and b) Crystal structure of o-BETB, and c) its crystal packing.
65

50 angle relative to the xanthene core resulting in a slip-stacked geometry with three acene rings
overlapping. In the other BETX dimer, BETX
2
(Figure 3.9a), the acenes are nearly perpendicular
to the xanthene core, with all four rings overlapping. In both of the BETX dimers the distance
between the tetracenes is 3.4-3.5 Å. In crystals of BETX, all intermolecular pairs of tetracenes
are perpendicularly oriented, and no intermolecular tetracene-tetracene    packing is observed
(see Figure 3.9c). In an amorphous film, however, intermolecular overlap of the tetracenes is
possible.
The intermolecular distances in substituted tetracene crystals, such as diphenyltetracene,
dithiophenyltetracene, and rubrene range from 3.2 to 3.8 Å.
26-30
The intramolecular tetracene
distances in our dimers, and the intermolecular tetracene distance in o-BETB are similar to those
between the tetracenes in other substituted tetracene crystals.

Figure 3.9 a) crystal structure of BETX
1
(inter-tetracene spacing = 3.4 Å) and b) crystal
structure BETX
2
(inter-tetracene spacing = 3.5 Å). The view chosen here has the xanthene
moiety of the BETX structures perpendicular to the page, with the tert-butyl groups removed
for clarity. c) Simplified Crystal packing diagram of BETX, with t-butyl groups removed.
66

3.2.3 Steady state photophysical properties of o-BETB and BETX
The extent of the chromophore coupling in the dimers and their excited state decay
pathways can be gleaned from the comparison with the monomer chromophore (ET-TMS). The
steady state photophysical properties of o-BETB and BETX are significantly different from those
of ET-TMS. The absorption spectra of o-BETB and BETX in solution and PMMA are
red-shifted by 18 nm (687 cm
-1
) relative to ET-TMS, and their absorption line-shape (Figure 3.10
and 3.11) has an enhanced 0-1 peak, and a diminished 0-0 peak. This has previously been
observed in covalent dimers with cofacially oriented chromophores, and attributed to Davydov
interactions.
16, 17, 31, 32
Additionally, the molar absorptivities of the dimers (see Figure 3.10) are
greater than that of ET-TMS in the visible region. The integrated areas of the absorption peaks of
the dimers are approximately twice that of the monomer (ET-TMS).

300 400 500 600
0
50k
100k
150k
200k

 cm
-1
mol
-1
L)
Wavelength (nm)
ET-TMS
BET-X
BET-B
400 500 600
0
5k
10k
15k
20k
25k

 cm
-1
mol
-1
L)
Wavelength (nm)
ET-TMS
BET-X
BET-B

Figure 3.10 Molar absorptivities of ET-TMS, o-BETB, and BETX.
67

The most striking difference between the dimers and the monomer in solution is in their
emission properties. The emission intensities of o-BETB in a THF solution (0.6 %) and BETX in
the same medium (<0.5 %) are weak, suggesting that an excited state relaxation pathway is
available in isolated dimers, which is not present in ET-TMS. The emission decays presented in
Figure 3.12 are reflective of the differences in the emission quantum yields of these compounds.
ET-TMS exhibits a mono-exponential emission decay, with a lifetime of 17 ns, while the
emission of both o-BETB and BETX decays very rapidly (τ < 0.5 ns), but also carries a
long-lived component of τ ≈ 12 ns, likely resulting from small amounts of monomeric impurities
in the samples. Although the o-BETB Stokes shift of 23 nm has increased compared to ET-TMS,
the emission line-shape retains vibronic structure suggesting that emission is not due to a deep-

Figure 3.11. Absorption and emission of ET-TMS (top), o-BETB (middle), and BETX
(bottom) in a THF solution (black), doped into PMMA (red), neat films (blue), and in 2-methyl-
THF at 77 K (dashed grey). The intrinsic BETX solution emission spectrum was obtained by
subtracting the photooxidized impurity’s emission from the measured BETX solution emission.
68

trap excimer state. At 77 K, the methyl-THF glass solution of o-BETB produces a similar
emission shape to that at room temperature, but the vibronic structure is more pronounced.

Although the emission intensity of BETX is also weak (Φ
fl
= 2.6 % in PMMA, <0.5 % in
THF) the line shape is substantially different from that of o-BETB. In all media, the BETX
emission is broad and featureless, with a large gap between 
max
values for absorption and
emission (87 nm), suggesting that BETX undergoes emission from an excimer-like structure. In
a solvent glass at 77 K, the emission still has a large energy shift, but vibrational features are
apparent, indicating that the excimer-like structure is only partially formed.
9
This emission
spectrum is very similar to that observed for a pair of cofacial tetracenes at 15 K by Iannone et
al.
9
The lifetime of the BETX emission at 77 K is 53 ns (compared to 19 ns for ET-TMS, and 16
ns for o-BETB at 77 K), supporting the excimeric nature of the emission.
33
Similar emission
characteristics have been observed for an isolated, cofacially stacked pair of tetracenes at 15 K,
9

and a broad featureless emission with a long lifetime was observed for cofacial anthracenes in
anthracenophanes.
33, 34

10 20 30 40 50
0.01
0.1
1

Intensity (norm.)
Time (ns)
ET-TMS
BET-B
BET-X

Figure 3.12 Emission decays of ET-TMS, o-BETB, and BETX in THF solutions.
69

The energy requirement for singlet fission [E(S
1
)  2E(T
1
)] is met for the tetracene-based
dimers, but not for their anthracene-based analogs. The comparison of the emission properties of
o-BETB and BETX with their anthracene analogs, o-BEAB and BEAX (see Figure 3.13)
provides information about the excited state relaxation channels in the tetracene dimers. The EA
dimers exhibit high emission intensities when suspended in a rigid polymethylmethacrylate
(PMMA) matrix (57 % for BEAX, and 96 % for o-BEAB). The emission intensity of the
tetracene analogs, on the other hand, is strongly quenched in the same media (2.6 % for BETX,
and 5.0 % for o-BETB). This suggests that the lower E(T
1
)/E(S
1
) ratio in o-BETB and BETX
opens an additional channel of excited state deactivation, presumably singlet fission, which is
energetically inaccessible in o-BEAB and BEAX.

3.2.4 Photophysical properties of neat thin films of o-BETB and BETX
The thin films of both the monomer, ET-TMS and the dimers all exhibit a significant
red-shift and broadening in the absorption spectrum. The red-shift could be indicative of
inter-chromophore coupling in the solid state and possible exciton delocalization. Furthermore,
Figure 3.13. Comparison of absorption (black) and emission (red) spectra of the a) tetracene
dimers versus the b) anthracene dimers in a PMMA matrix. The areas of the emission peaks
for BETX, o-BETB and BEAX are normalized to an o-BEAB area of 1.0. The
photoluminescence efficiencies for BEAX and o-BEAB are 57 and 96%, respectively.
70

in the solid state, these compounds lose the structured emission, and what remains is a very low
intensity, broad, featureless emission which is red-shifted from the absorption by ~120 nm. The
low intensity, broad, and featureless emission from the neat films indicates that the intrinsic
fluorescence of all of these compounds is totally quenched in the solid state, and only the
emission from intermolecular exciton trap sites emanates from the neat films.
3.2.5 Transient absorption and the SF mechanism in o-BETB in various media
Prior to delving into the transient absorption spectra of the dimers in various media, it is
necessary to determine the spectral shape of the T
1
→T
n
absorption. The T
1
state of o-BETB was
sensitized using Pd(TPBP), in an analogous manner to the method reported in Chapter 2 for
ET-TMS T
1
sensitization. The resulting spectra in Figure 3.14 show a structured T
1
→T
n

absorption of o-BETB at 400 ps, with peaks at 450, 475 and 525 nm.

The transient absorption spectra of a thin film of o-BETB in Figure 3.15 following a
500 nm excitation show a typical S
1
→S
n
induced absorption spectral peak at 420 nm and a
ground state bleach between 475-575 nm at early times (< 1 ps). The S
1
S
n
peak decreases
within 1 ps and a new structured, induced absorption feature grows in at 450 nm. This new
induced absorption feature is assigned to the T
1
T
n
absorption, based on the sensitization
measurements of a Pd(TPBP) doped BETB film (cyan line in Figure 3.15a, see Ref.
22
for

Figure 3.14 Transient absorption of thin films of o-BETB doped with Pd(TPBP) at 5 mol%,
following photoexcitation at 626 nm.
71

sensitization details). The triplet yield calculations were performed as described above for ET-
TMS film and a maximum triplet yield of 154±10% at 5 ps was obtained. As illustrated in
Figure 3.15b, the S
1
population has completely decayed in 10 ps when singlet fission is
complete. This is a major difference from what was observed for ET-TMS and our previous
work with DPT.
35
Furthermore, the decay of S
1
population as well as the rise of the T
1

population monoexponential, indicating that only one rate constant for singlet fission is possible
in the amorphous film of the dimer. This hints at the intramolecular nature of singlet fission in
this system. A second major difference is that most of the triplets generated decay rather rapidly,
~ 400 ps. This decrease in the T
1
T
n
absorption

band is accompanied by a recovery of the
ground state. This triplet lifetime is significantly shorter than that observed for the sensitized
triplets (3 µs) and is independent of the excitation fluence between 8 - 64 µJ/cm
2
(see SI of ref.
22
). These facts suggest geminate triplet-triplet annihilation to reform S
0
as the dominant triplet
decay pathway in neat o-BETB film.
72

The excited state dynamics can now be isolated to the intramolecular processes occurring in
an isolated dimer by studying o-BETB in dilute solution. Excitation of a THF solution of
o-BETB at 500 nm leads to immediate appearance of a ground state bleach between 450-550 nm
and a familiar S
1
S
n
induced absorption band at 407 nm (Figure 3.16). However, with
increasing time, the S
1
S
n
absorption decreases and a new transient state appears within 1 ps,

Figure 3.15 a) Transient absorption spectra of a neat film of o-BETB following excitation
at 500 nm. The cyan line shows the triplet spectrum obtained from sensitization
measurements with Pd(TPBP). b) The singlet and triplet populations for o-BETB are
calculated using the extinction spectra for S
1
S
n
and T
1
T
n
transitions of o-BETB.
73

with new induced absorption most noticeable at 560 nm, and a considerably broader absorption
feature centered ~400 nm. The transient absorption of this state does not change shape between
50-200 ps, and does not resemble either the SADS line shape associated with o-BETB T
1
T
n
, or
with S
1
S
n
(Figure 3.16b). The spectrum associated with this latter transient decays with a
lifetime of 500 ps. The rate and magnitude of its formation is independent of solvent polarity and
polarizability (inset in Figure 3.16a), ruling out a charge transfer state. Since no excimer-like
steady state emission is observed in THF solution (Figure 3.11 (middle)), and no additional
induced absorption peaks are observed in the near IR (800-1400 nm) TA, a region generally
associated with excimers,
36, 37
an excimer state is ruled out as an intermediate. Instead, this
transient state is assigned as the ¹(T
1
T
1
) state (Figure 3.16b). Additional evidence for this
assignment is provided below.

Figure 3.16. a) Transient absorption spectra of o-BETB in THF. The inset shows the solvent
polarity independent dynamics at 560 nm for ¹(T
1
T
1
) state. b) The comparison of spectral
shape for S
1
, ¹(T
1
T
1
) and T
1
state of o-BETB in THF.
.
74

The fact that fast and efficient triplet generation is observed in neat films of o-BETB, but
only the ¹(T
1
T
1
) state is formed in THF solution suggests that the ¹(T
1
T
1
) state, once formed on
an isolated o-BETB, cannot further evolve into separated triplets, unless one of the triplets from
the pair can energy transfer onto an adjacent molecule. To test this hypothesis, we doped
o-BETB into a film of 5,12-diphenyltetracene, DPT, a material whose T
1
energy is low enough
that it can potentially accept a triplet from the ¹(T
1
T
1
) state of o-BETB. The S
1
energy of DPT is
higher than that of o-BETB, therefore o-BETB can be selectively excited, without the possibility
of singlet energy transfer. Calculations (Ref.
22
) suggest that the T
1
state of DPT (1.6 eV) is also
higher than that of o-BETB (1.4 eV), but triplet energy transfer is expected to be thermally
accessible from the o-BETB ¹(T
1
T
1
) state (2.8 eV).
A sample of o-BETB doped into 9,10-diphenylanthracene, DPA, was prepared as a control.
Unlike DPT, the T
1
state energy of DPA is expected to be markedly higher than that of o-BETB,
preventing triplet transfer from o-BETB to DPA. The DPA and DPT films were doped with
o-BETB at 26 and 17 vol%, respectively. The pump wavelength (550 nm) was chosen to avoid
direct excitation of the host molecules. o-BETB doped into DPA (Figure 3.17a) exhibits similar
transient behavior as o-BETB in solution. Upon initial excitation to the S
1
state, the same
transient ¹(T
1
T
1
) state (with induced absorption at 580 nm) is populated within 1 ps, followed by
its relaxation to the ground state with a lifetime of 500 ps. The decay of the ¹(T
1
T
1
) state of
o-BETB doped into DPA at 26 vol% is faster than in solution, likely because nearly all of
o-BETB are in contact with at least one more o-BETB at such a large doping ratio.
Interestingly, o-BETB doped into DPT (Figure 3.17b) initially gives similar spectral and
kinetic behavior to o-BETB in solution (and in DPA) up to 2 ps. At longer time delays (>10 ps),
however, the distinct spectral feature for the T
1
absorption of o-BETB (similar to that observed
75

for neat o-BETB film) becomes readily discernible. Simultaneously, a ground state bleach
(sharp peaks at 440, 470 and 500 nm) corresponding to the DPT host emerges, suggesting triplet
energy transfer from the ¹(T
1
T
1
) state of o-BETB to DPT. Singlet energy transfer is ruled out, as
the characteristic S
1
S
n
induced absorption for DPT is not observed
35, 38
and the linear
absorption spectra also suggest that the S
1
of o-BETB is lower in energy than the S
1
of DPT.
Thus, at longer delays the ¹(T
1
T
1
) state of o-BETB relaxes to generate one T
1
(o-BETB) and one
T
1
(DPT) as illustrated by the simulation of TA data of o-BETB in DPT film (Figure 3.18). The
combined triplet yield was found to be 120±10% at 1 ns based on the extinction spectra for the
T
1
T
n
transitions of o-BETB and DPT. This yield calculation is a lower estimate, as the triplet
transfer from ¹(T
1
T
1
) state to one T
1
(o-BETB) and one T
1
(DPT) is not fully complete within 1
ns. This slow triplet rise time is verified in the kinetic model described below.

Figure 3.17 Transient absorption spectra of o-BETB doped into DPA and DPT on exciting
with 550 nm. The cyan line in the right plot shows the triplet spectrum obtained from
o-BETB sensitization experiments.
76

Based on the TA studies of o-BETB in several media, a simple scheme for the excited-state
kinetics (Figure 3.19) is proposed where the time-dependent concentration of each transient state
can be calculated by solving the coupled rate equations:

(3.1)

(3.2)

Figure 3.18. Comparison of simulated fits (red dotted line) with the TA data for
o-BETB in DPT film at 900 ps (black line) using basis sets for a) T
1
(o-BETB) + T
1

(DPT), b) T
1
(o-BETB) + T
1
(o-BETB) c) T
1
(DPT) + T
1
(DPT), and d) T
1

(o-BETB) + T
1
(DPT) without the
1
(T
1
T
1
) (o-BETB) state.
77

(3.3)
where k
1
is the rate of formation of the ¹(T
1
T
1
) state from the S
0
S
1
state, k
2
is the rate of
formation of the separated triplets from the correlated triplet pair ¹(T
1
T
1
) state and k
3
is the
relaxation of ¹(T
1
T
1
) state to S
0
state. While time resolved photoluminescence shows a small
fraction (2-3%) of delayed fluorescence (equilibrium constant for S
1
⥂ ¹(T
1
T
1
) is ~49, Ref.
22
),
TA tracks the majority excited-state population, which relaxes back to the ground state. The
time-dependent concentrations obtained from solving these equations are multiplied with the
corresponding SADS extinction spectra of the S
0
S
1
, ¹(T
1
T
1
) and T
1
states to simulate the time
evolution of the TA spectra in different media. The SADS extinction spectra for these three
states in different media along with the procedure to obtain them are shown in supporting
information of ref.
22
.

Figure 3.19. Kinetic model for o-BETB in a) solution, PMMA, DPA and b) DPT, neat film.
The notation A denotes the tetracenes in the same dimer and B denotes the DPT for the
o-BETB in DPT film or third tetracene in another o-BETB dimer for the BETB neat film. The
calculated populations for S
1
(o-BETB) (black), ¹(T
1
T
1
) (o-BETB) (blue), T
1
(o-BETB) (red)
and T
1
(DPT) (green dashed) extracted from the kinetic model are shown in the bottom panel
for a) o-BETB in THF and b) o-BETB in DPT & neat o-BETB film.
78

The TA data of o-BETB in solution, PMMA and DPA, in which only the formation of the
¹(T
1
T
1
) state is possible, were fit using the scheme a) (Figure 3.19) to determine k
1
and k
3
, with
k
2
fixed at zero. These values are reported in Table II. The k
1
value is similar in all three media;
however, the internal conversion rate (k
3
) is slower in PMMA due to the different density of
vibronic states in solid PMMA. No separated triplets are formed in the isolated o-BETB, as they
cannot migrate away from one another.
The o-BETB doped into DPT experiment illustrates the need for a third acene to give the
separated triplets, i.e. T
1
(o-BETA) + T
1
(DPT). The selective excitation of o-BETB leads to
population of the S
1
state of o-BETB, followed by relaxation into the ¹(T
1
T
1
) state. From 20 ps
onward, distinct transient T
1
T
n
absorption spectral features of o-BETB and DPT were
observed. The kinetic model for singlet fission in this sample (Figure 3.19b) assigns two rate
constants for the triplet generation process. The first rate constant (k
1
) represents the conversion
from S
1
to

¹(T
1
T
1
)-(o-BETB), while the second (k
2
) represents the ¹(T
1
T
1
)-(o-BETB) to T
1

(o-BETB)+T
1
(DPT) rate. These values are reported in Table II. The k
1
and k
3
values were found
to be similar to those obtained for o-BETB in solution and DPA. The rate of formation (k
2
) of
individual triplets on o-BETB and DPT from the ¹(T
1
T
1
) state was estimated to be 6.7x10
9
s
-1
( 


= 150 ps). The second rate constant (k
2
) is dependent on both enthalpy and entropy. The triplet
energy of DPT is slightly higher than that of o-BETB (1.6 eV vs. 1.4 eV), such that
¹(T
1
T
1
)-(o-BETB) to T
1
(o-BETB)+T
1
(DPT) transformation is energetically uphill and the
¹(T
1
T
1
)-(o-BETB) state lives long enough to be experimentally detected. The driving force for
this transformation is thus entropic in nature.
39, 40
Assuming that each o-BETB molecule is
surrounded by multiple DPT molecules to which the energy transfer can occur, the entropy of the
T
1
(o-BETB)+T
1
(DPT) state is greater than that of ¹(T
1
T
1
)-(o-BETB), as there are several ways
79

in which the T
1
(o-BETB)+T
1
(DPT) state can be realized. Once the triplets, T
1
(o-BETB) and T
1

(DPT) were generated, no relaxation was observed in the 1 ns time window.

A similar kinetic model (Figure 3.19b) was used to fit the TA data for the neat o-BETB
film. The values for k
1
and k
3
from this fit are comparable to those for o-BETB in other media,
however the rate of separation to two triplets from the ¹(T
1
T
1
) state is ultrafast (k
2
~5x10
12
s
-1
, 


~ 0.2 ps). The faster k
2
results in rapid depopulation of the ¹(T
1
T
1
) state, such that at most only
~10% of the ¹(T
1
T
1
) population exists at 600 fs, in the presence of 36% of the T
1
(o-BETB)
population. As TA spectra are dominated by the T
1
 T
n
absorption of o-BETB, no spectral
evidence for the ¹(T
1
T
1
) state was detected in the TA data for neat o-BETB film, but the
population of this state was included in the model. This is likely because the ¹(T
1
T
1
)-(o-BETB)
to T
1
(o-BETB)+T
1
(o-BETB) transformation is isoenergetic or exergonic, unlike the endergonic
transfer from ¹(T
1
T
1
)-(o-BETB) to T
1
(o-BETB)+T
1
(DPT). The lifeime of the decay of the
¹(T
1
T
1
) state was kept at 500 ps, as it did not affect the fit much as long as it was longer than the
decay into separated T
1
. Additionally, to fit the TA data for a neat o-BETB film, the annihilation
of the generated triplets is modeled by an initial exponential decay (k
4
= 2.5x10
9
s
-1
, 

= 400 ps)
followed by an offset to account for the long time dynamics. This fast exponential relaxation of
Table 3.1: Time constants ( 
i
=1/k
i
) for the kinetic model used to fit the TA data for o-BETB
in different media.

1
(ps) 
2
(ps) 
3
(±50 ps)
THF 2 ± 0.5 -- 500
PMMA 1.7 ± 0.3 -- 1000
DPA 2 ± 0.2 -- 500
DPT 2 ± 0.3 150 ± 1 500
neat 0.8 ± 0.3 0.23 ± 0.05 500

80

the triplets is due to adjacent triplet-triplet annihilation between the triplets present on adjacent
o-BETB molecules and later when the triplets diffuse away, the recombination dynamics become
slower. Irrespective of the medium, o-BETB has a very fast formation rate (k
1
) of the ¹(T
1
T
1
)
state, indicating that the relative tetracene orientation in o-BETB allows for sufficient coupling
for the first step to proceed efficiently. The ab initio calculations presented in ref.
22
corroborate
the experimental results, as the formation rate for the ¹(T
1
T
1
) state in o-BETB is predicted to be
two orders of magnitude faster than in crystalline tetracene. These results suggest that the
relative geometry of the acenes in o-BETB is even more favorable for singlet fission than that in
crystalline tetracene.
Finally, it is important to comment on the nature of the bi-exciton state, ¹(T
1
T
1
). Such a
state has been identified computationally in various singlet-fission systems.
26, 39, 41-45
Its adiabatic
wave function is dominated by the ¹(T
1
T
1
) configuration (thus, bi-exciton).
43, 45
However, only
asymptotically this state can be described as a pure ¹(T
1
T
1
), in contrast to the
5
(T
1
T
1
) state which
almost always retains pure multi-exciton character. At typical chromophore orientations, the
wave function of ¹(T
1
T
1
) contains small contributions (4-6%) from other singlet configurations,
such as charge-resonance (A
+
B
-
+ A
-
B
+
) and excitonic (A*B+AB*) configurations.
43, 45
The
mixing of these configurations influences the couplings of ¹(T
1
T
1
) with the S
1
state and manifests
itself in the energy splitting between the
5
(T
1
T
1
) and ¹(T
1
T
1
) (E
b
), and importantly, in the optical
properties of the multi-exciton state. A pure multi-exciton state (such as
5
(T
1
T
1
)) is expected to
have transient absorption spectra similar to that of a triplet, whereas the singlet multi-exciton
state with a considerable mixing of other configurations should have a distinct spectroscopic
signature, different from that of a triplet. Configuration interaction with other singlet states can
also lead to intensity borrowing, which can give oscillator strength to the dark S
0
→¹(T
1
T
1
)
81

transition. Although we cannot compute transient absorption spectra for the ¹(T
1
T
1
) state using
the tools at our disposal, we can infer whether the multi-exciton state will have different transient
absorption based on the degree of mixing of other configurations. The computed E
b
values (0.02-
0.5 eV, see Table 1) suggest that this state features significant contributions of the charge-
resonance and excitonic configurations, which is confirmed by wave function analysis that
shows the weight of ¹(T
1
T
1
) configuration in the bi-exciton state is roughly 80%, and by
computed oscillator strengths for the S
0
-¹(T
1
T
1
) transition. For example, at the X-ray structure,
oscillator strengths to S
0
are 0.0004 and 0.0002 for o-BETB and BETX, respectively. At the
S
1
-excimer geometry, the oscillator strengths for both molecules increase to ~0.001. Thus, we
attribute the distinct transient absorption of the intermediate state to the ¹(T
1
T
1
) state, whose
wave function contains significant contributions from other singlet states.
3.2.6 Transient absorption of BETX
The transient absorption spectra of BETX in toluene with 500 nm excitation show
immediate (  200 fs) generation of a new transient state with photo-induced absorption between
380-700 nm (peak at 400 nm) and ground state bleach between 450-550 nm [Figure 3.20]. This
transient feature is spectrally different from the S
1
S
n
induced absorption feature observed for
ET-TMS and o-BETB at early time (~200 fs) delays (Figure 3.20), but it is spectrally similar to
the ¹(T
1
T
1
) induced absorption feature of o-BETB [see Figure 3.20b]. With increasing time
delay, this induced absorption feature diminishes with a concurrent ground state bleach recovery
within 500 ps. During this time period, no evolution of the spectral line shape was observed in
the probe window. The TA spectra of BETX in the near-infrared (850-1400 nm) region only
contain a broad tail of the induced absorption feature between 380-700 nm and no spectrally
distinct peaks are observed to evolve within 1 ns. Additionally, the spectral shape and dynamics
82

were invariable with solvent polarity and polarizability, indicating no charge transfer state
involvement in the excited state dynamics of BETX.

Interestingly, the transient spectral feature observed for BETX in solution at early time
delays appears to be similar to the spectral signature of ¹(T
1
T
1
) state observed for o-BETB in
solution (Figure 3.20). Based on this spectral similarity and the computed excited state energies
(Figure 3.20) for BETX, we posit that upon excitation in solution, BETX undergoes structural

Figure 3.20 a) Transient absorption spectra of o-BETB and BETX in THF solutions. b)
Overlay of the 200 fs TA trace of BETX with 50 ps TA trace of o-BETB, to demonstrate the
resemblance in the spectral shapes.
83

relaxation to form an excimer-like structure that may decay into the correlated triplet pair state
¹(T
1
T
1
) within 200 fs (time resolution for TA). The steady state emission of BETX observed in
rigid media could be originating from the excimer S
1
state in equilibrium with the ¹(T
1
T
1
) state,
or from the ¹(T
1
T
1
) state whose wave function contains a significant admixture of the bright S
1
state (the computed oscillator strength of the
1
(T
1
T
1
)→S
0
transition at the excimer geometry of
BETX is 0.001). Since the ¹(T
1
T
1
) state in BETX is lower in energy than that in o-BETB (see ref.
22
for calculated energies) the triplet separation is expected to be markedly slower for BETX than
o-BETB. Moreover, at the excimer geometry, the coupling between ¹(T
1
T
1
) and S
0
is larger for
BETX than for o-BETB, leading to faster decay to the ground state. Attempts to measure the TA
spectra of BETX in a neat thin film and BETX doped into DPT were unsuccessful due to
photodegradation of BETX.

3.3 Conclusions
The excited state dynamics of the covalent tetracene dimers are highly sensitive to the
relative chromophore orientation. The relative orientation of the chromophores determines the

Figure 3.21 Computed energies of the S
1
and the
1
(T
1
T
1
) states of BETX at the ground state
and relaxed S
1
geometries. Details of the calculations provided in ref. 22.
84

energies of the singlet, triplet, and ¹(T
1
T
1
) states, and the couplings between those states, and as a
result, the excited state relaxation pathway. In a covalent tetracene dimer in which the
chromophores have a large amount of overlap (BETX), the singlet excited state decays rapidly
either via the excimer pathway or a ¹(T
1
T
1
) state trapped at the excimer-like structure from which
the free triplets are no longer energetically accessible. In a covalent tetracene dimer in which the
chromophores exhibit approximately one ring of π overlap, i.e. o-BETB, the system does not
relax into a deep excimer configuration. for a spectroscopic signature of a state that precedes
triplet generation via singlet fission was observed, and this state was assigned as the ¹(T
1
T
1
) or
the multi-excition state. In o-BETB, without forming a strongly bound excimer, the tetracenes
possess sufficient orbital overlap to couple the S
0
S
1
and ¹(T
1
T
1
) states, and to promote rapid
conversion of S
0
S
1
to ¹(T
1
T
1
).
Once the ¹(T
1
T
1
) state is formed in an isolated dimer (o-BETB), the system cannot separate
the correlated triplets, and the state decays to the ground state by radiationless relaxation.
However, when o-BETB is placed into a tetracene-rich matrix comprised of DPT or other
o-BETB molecules, triplets efficiently transfer from ¹(T
1
T
1
) to the host. This could be due to the
strong coupling between the chromophores, and therefore large energy splitting between the
¹(T
1
T
1
) and the
5
(T
1
T
1
) states. This result could also indicate the importance of entropy in singlet
fission in strongly coupled systems. In the tetracene systems, singlet fission is presumably
slightly endothermic, therefore the entropy gain on producing the separated triplets by singlet
fission could play a key role. The data presented in this chapter suggests that for systems in
which the chromophores are strongly coupled or singlet fission is endothermic, triplet energy
transfer is an essential second step required for the production of free triplet excitons.
85

Covalent dimers hold promise for exploiting singlet fission in photovoltaic materials. We
have observed ultrafast, efficient, intramolecular singlet fission in a neat amorphous film of
o-BETB. The triplet yield in the neat film of the dimer, o-BETB, is 154±10 %, while the neat
film of the monomer analog, ET-TMS produces triplets with only 90±8 % efficiency. The
elimination of a diffusive phase in the singlet fission kinetics suggests that by pre-orienting all
chromophores into a preferred singlet fission geometry, the singlet fission efficiency can be
increased in disordered materials. This will relax the constraints placed on the film
manufacturing process enabling low-cost, high efficiency singlet fission based OPVs.
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88

Chapter 4: Singlet fission in ortho-, meta-, and para-BETB – the role of chromophore
coupling
4.1 Introduction
In the previous chapter it was shown that rapid (τ ≈ 10 ps) excited state relaxation to the
1
(T
1
T
1
) state is possible for dimers of ethynyltetracene in which the chromophores are cofacially
oriented.
1
In the tetracene dimers previously reported by other groups, the tetracenes were
linearly linked, such that cofacial π overlap was not possible. Accordingly, those dimers
exhibited very slow rates of SF (k
SF
≈ 10
6
s
-1
).
2-4
For the first time, we showed that given
sufficient coupling, SF can be fast in an isolated tetracene dimer.
However, we presumed that the mechanism of coupling in o-BETB was through-space, due
to significant π overlap of the tetracenes (Figure 4.1a). Another route to chromophore coupling in
o-BETB could be through-bond (Figure 4.1b), given the conjugating nature of the
ortho-diethynylbenzene unit linking the tetracenes. It is worth noting that the previously reported
tetracene dimers, in which SF was reported to be slow, the tetracenes were linked with
non-conjugating linkers.

Diethynylbenzenes have interesting electronic structure, and their excited state properties
have been extensively studied.
5
The three possible substitutions of two alkynes on the benzene

Figure 4.1 Depiction of possible routes of chromophore coupling in o-BETB: a) through
space, b) through bond.
a) b)
89

rings are at the ortho-, meta- and para- positions. The position of the alkyne substitution on the
benzene ring heavily influences the electronic structure of molecules containing these
diethynylbenzene units.
5
The ortho and the para arrangements can be thought of as conjugating,
while the meta cannot. The conjugation provided by these linkers can be understood in terms of
the chemical structures provided in Figure 4.2. The cumulene-like resonance form of the ortho-
and para-diethynylbenzenes strongly resembles the excited state electronic structure in these
molecules.
6, 7
If the substituents on the alkynes in the ortho-diethynylbenzenes are bulky, the
alkynes will bend out of the plane of the benzene ring (as is the case in o-BETB); this would
likely disturb the conjugation through the benzene linker, or at least reduce its effect when
compared with the totally planar para-diethynylbenzene.

The ortho-diethynylbenzenes have garnered a lot of interest due to their ability to undergo
Bergman cyclization.
8-10
Depending on the substituents on the alkynes, compounds containing
the ortho-diethynylbenzenes bridge have been reported to undergo either thermal or light
activated cyclization.
11

Compounds containing the para-diethynylbenzenes show properties conducive for charge
transport as well as high luminescence yields. The para-diethynylbenzenes unit facilitates

Figure 4.2 The possibility of conjugation through ortho-, meta- and para-
ethynylbenzenes.
90

efficient electronic communication between the chromophores it bridges;
12-14
as a result,
compounds based on this chemical motif have been incorporated into unltrasensitive sensors,
15

energy harvesting materials,
16
and molecular electronic devices.
17, 18

Unlike the previous two arrangements, the meta-diethynylbenzenes is non-conjugating, and
has been demonstrated to provide weak coupling in the ground state between the chromophores
tethered to it.
19, 20
For instance, Thompson et al. observed that following intramolecular CT,
charge recombination in meta-diethynylbenzene bridged donor-acceptor complexes was 10 times
slower than that in analogous para-diethynylbenzene bridged systems.
21
Additionally, the work
by Lee et al. showed that the charge transfer ability in donor-acceptor diads bridged by multiple
alkynylbenzene linkers decreased with increasing number of meta linkages versus para
linkages.
22

To study the effects of through-bond coupling in ethynyltetracene dimers, we synthesized
meta-bis(ethynyltetracenyl)benzene, m-BETB, and two versions of the
para-bis(ethynyltetracenyl)benzene, p-BETB-ethex and p-BETB-ohex, with different
substituents on the bridging benzene ring, ethylhexyl and hexyloxy, respectively (see Figure 4.3).
In p-BETB-ethex, the alkyl substituents on the benzene ring are not expected to have a
significant effect, and the through-bond coupling is expected to be similar to that in o-BETB,
based on similar conjugation properties of ortho- and para-diethynylbenzenes. In the other para
dimer, p-BETB-ohex, the electron donating alkoxy groups are expected to raise the occupied
orbital energy of the bridging benzene ring and introduce a quinoidal character. The higher
occupied orbital energy of the bridging benzene ring in p-BETB-ohex could result in stronger
coupling between the tetracenes, as the orbitals of the acenes and the linker will be closer.
91

In m-BETB, however, the through-space and through-bond coupling is expected to be
weak. The tetracenes in m-BETB can be accommodated with little steric hindrance, thereby
minimizing through-space interactions. As described above, the meta-diethynylbenzenes bridge
is non-conjugating, and minimal through-bond interaction is expected.

The advantage of tetracene dimers, versus their pentacene analogs, is that the kinetics of
singlet fission are slower, and the spectral features of the excited states which are populated
during the S
1
decay can be resolved using standard transient absorption techniques with 200 fs
pulses.
2, 23, 24
Additionally, the thermodynamics of SF in tetracene systems, in theory, should
allow for T
1
-T
1
annihilation and delayed fluorescence;
2, 3, 23, 25-27
in which case time correlated
photon counting (TCSPC) could be another useful technique for quantifying SF in these systems.
However, as seen in the previous chapter, the fluorescence properties of the strongly coupled
dimers do not lend themselves to trivial interpretation, due to weak intensity and partial,
unavoidable contamination from photooxidized impurities.
Nonetheless, the study of the ortho-, meta- and para-BETB dimers can provide mechanistic
details of singlet fission which were not discernible in the pentacene dimers, primarily due to the
energetics and slower rate of SF in the tetracene dimers.
2, 3, 28-32
Specifically, these dimers can

Figure 4.3. The meta-BETB and para-BETB dimers which were synthesized and studied
here.
92

help shed light on the effect of chromophore coupling on the states which are accessible during
the excited state decay.
Finally, dimers provide a clear route for studying the role of CT states in the SF
mechanism. Since dimers can be studied in solution, varying the dielectric of the solvent, and
therefore tuning the energy of the CT states in the dimer, is facile. Several researchers have
reported on the effects of CT states on the excited state dynamics in dimers. In the earliest works
on covalently linked SF chromophores, Johnson et al. reported higher triplet yields for
diphenylisobenzofuran dimers in more polar solvents.
33
In the weakly coupled pentacene dimers,
Zirzlmeier et al. reported an enhancement in the triplet yield in polar solvents, and allowed for
population of the CT state in the kinetic model used to fit their data.
28
The recent work of
Magulies et al. demonstrated that CT states can compete with SF in slip-stacked terylene dimers
in polar solvents.
34
Although we did not observe any effect of the solvent polarity on the
formation of the
1
(T
1
T
1
) state in ortho-BETB,
1
the interplay of chromophore coupling and CT
state involvement in SF in other tetracene dimers is worth exploring.
In this chapter the synthesis and spectroscopic properties of the three aforementioned
dimers are presented and compared with those of the monomer and of o-BETB. It was observed
that meta-diethynylbenzene is indeed a poorly conjugating linker, and provides non-significant
coupling between the tetracenes. As a result, meta-BETB in the excited state primarily decays
via fluorescence. However, a portion of delayed fluorescence, akin to that observed by Muller et
al. in the linearly linked tetracene dimers, was measurable. This delayed fluorescence was found
to be dependent on the solvent polarity and was absent at a low temperature of 77 K. The
para-diethynylbenzene linker, on the other hand, was observed to provide substantial coupling
93

between the tetracenes, as rapid triplet formation was determined to be the primary excited state
deactivation pathway in the para- dimers.
4.2 Results and discussion
4.2.1 Synthesis and
1
H NMR of m-BETB, p-BETB-ethex and p-BETB-ohex
In the previous chapters, the motivation for using alkynyl-modified tetracenes was
presented. These functional groups provide sufficient flexibility to alleviate the steric bulk issues,
and allow for the ortho- benzene coupled tetracene dimer to be synthetically possible.
Furthermore, the superior synthetic efficiency of the Sonogashira coupling reaction is an
undeniable advantage. Having successfully made two ethynyltetracene dimers, it was presumed
that all of the synthetic woes had past; however, a new impediment was encountered in the work
leading up to this chapter – solubility!
In the two dimers presented in the previous chapter, the tetracenes were oriented out of the
plane of the linker, which prevented them from stacking too densely in the solid state. The meta
and the para dimers, however, can achieve completely planar geometries along the plane of the
linker. The meta and the para dimers were synthesized according to Scheme I. Given the
previous experience with deprotected ethynyltetracene, and its rapid degradation, its necessity
was eliminated in the new synthetic scheme by placing the terminal alkynes on the linking
benzene ring, making the resulting terminal alkyne less reactive. The meta-diethynylbenzene is
commercially available, therefore the synthesis of the meta dimer required fewer steps than the
ortho dimer. The final product, meta-BETB exhibited slightly lower solubility than ortho, but
was still possible to purify by column chromatography on silica gel.
The synthesis of a soluble para dimer proved to be challenging. To combat the solubility
issues without impacting the tetracene moieties, we resorted to making substitutions on the
94

linking benzene ring. In total, four variations of the para dimer were synthesized, with different
substituents on the linking benzene, shown in Figure 4.4. The unsubstituted p-BETB was
absolutely unworkable; it was not soluble in any solvent at room temperature, and remained as
dark purple solids even after sonication. The TMS variant was inspired by the work of Lehrnherr
et al.,
35
but unfortunately proved to be an extremely insoluble orange solid. The alkoxy and alkyl
groups in the next two dimers, p-BETB-ohex and p-BETB-ethex, respectively, rendered these
molecules sufficiently soluble in order to characterize them by p-BETB-ethex variant was more
soluble than the p-BETB-ohex variant, likely due to additional disorder created by the ethyl-
hexyl groups.
The synthetic routes to these soluble para dimers were long and arduous. Although the final
Sonogashira coupling step of the bromotetracene and the di-alkyne linker (Scheme I) was facile,
the synthetic schemes of the 1,4-(diethynyl)-2,5-(dihexyloxy)-benzene and 1,4-(diethynyl)-2,5-
(bis-3-ethynylhexyl)-benzene linkers (Schemes II and II, respectively) were quite involved.

Scheme I: The synthetic scheme for meta- and para-BETB.

95

Figure 4.4. The four para-BETB dimers which were synthesized. Only the p-BETB-ohex
and p-BETB-ethex were soluble enough for photophysical characterization.
Scheme II: The synthetic schemes for substituted para-(bis-hexyloxy)-diethynylbenzene

Scheme III: The synthetic schemes for substituted para-(bis-ethylhexyl)-diethynylbenzene

96

The aromatic regions of the
1
H NMR spectra of m-BETB, p-BETB-ETHEX, and
p-BETB-ohex are shown in Figures 4.5, 4.6 and 4.7, respectively. Additionally, a comparison of
the aforementioned spectra with that of PET is shown in Figure 4.8. A majority of the peaks in
the dimers lie closely to those of PET. Proton a, which lies the closest to the alkyne is the one
most affected by the different substituents on the alkyne. Even though the tetracenes are
presumably not interacting with each other through space, the shift in the chemical shift of
proton a in different dimers is indicative of the different electronic properties exuded on these
dimers by the linking benzene rings. Also noteworthy are the differences between the
1
H NMR
spectra of p-BETB-ethex and p-BETB-ohex. It appears that the alkoxy groups introduce a large
perturbation to the electronic structure of p-BETB-ohex, as the chemical shifts of the aromatic
protons of the tetracenes in this molecule are the most shifted compared to PET. Furthermore,
the chemical shifts of the tetracene protons in p-BETB-ethex lie more closely to those of
m-BETB than those of p-BETB-ohex, which suggests that the effect of the alkoxy group is not
negligible.
97

Figure 4.6.
1
H NMR spectrum of the aromatic region of p-BETB-ethex in CDCl
3
at 25
o
C.

Figure 4.5.
1
H NMR spectrum of m-BETB in CDCl
3
at 25
o
C.
98

Figure 4.8.
1
H NMR spectra of the aromatic regions of the dimers compared to the
monomer (PET).

Figure 4.7.
1
H NMR spectrum of the aromatic region of p-BETB-ohex in CDCl
3
at 25
o
C.
99

4.2.2 Computational studies of m-BETB, p-BETB-ethex and p-BETB-ohex
A qualitative understanding of the electronic structure of the tetracene dimers presented in
this chapter can be gained from a comparison of the DFT computed molecular orbitals involved
in the excited state transitions. We have used B3LYP functional with 6-31G** basis set to
optimize the geometries of these dimers and with the 6-31++G** basis set to compute the
molecular orbitals and excited state transitions in these molecules. Although the B3LYP
functional tends to over-delocalize the orbitals, it was useful in pointing out the differences in the
dimers presented here. The alkyl chains in p-BETB-ethex and p-BETB-ohex were shortened to
methyl and methoxy groups, respectively, to reduce the computational costs. The HOMO and
LUMO of m-BETB, p-BETB-ethex, and p-BETB-ohex are shown in Figure 4.9, and the HOMO
and LUMO of o-BETB are included for comparison. The most notable difference is in the
amplitudes of the MOs on the linking benzene ring. In o-BETB, where we know the coupling to
be strong, despite the alkynes being out of plane of the benzene ring, there is sizeable amplitude
on the two benzene atoms connected to the alkyne groups. The MO amplitude on those atoms in
o-BETB becomes particularly prominent in the LUMO. The difference in MO densities between
the HOMO and the LUMO is indicative of a cumulene-like excited state in this molecule (the
density at the alkyne decreases, and the density on the two single bonds connected to the alkyne
increases upon HOMO→LUMO). In m-BETB, however, the MOs are localized on the
ethynyltetracenes, and no MO density is allocated on the benzene atoms directly linked to the
alkynes. A similar MO amplitude shift is observed between the HOMO and the LUMO, but the
increase in density on the single bonds near the alkyne in the LUMO is much smaller in m-BETB
compared to other dimers.
100

Similarly to o-BETB, the MO amplitude distribution in p-BETBs also extends onto the
benzene ring connecting the ethynyltetracenes; however, in p-BETBs, the benzene ring is
involved to a greater extent. In fact, the HOMO and the LUMO of p-BETBs appear to be
completely delocalized over the entire molecular backbone in these molecules. The MO
amplitude shift around the alkynes is similar to that in o-BETB, with a significant increase on the
single bonds around the alkyne in the LUMO of p-BETBs. The alternation in the HOMO and
LUMO amplitudes on the benzene in the p-BETBs suggests a greater involvement of the linker
in the electronic structure of these molecules. Studies of para-dialkynylbenzenes have reported
similar cumulene-like excited state structure, and this type of bond alternation has been used to
explain the propensity of these molecules to be planar in the excited state.
7

Lastly, it is worthy to note the similarities and differences in the MO structure between
p-BETB-ethex and p-BETB-ohex (Figure 4.9c and d, respectively). The LUMOs of these two
molecules look almost identical, although p-BETB-ohex has slightly reduced density on the
tetracenes in the LUMO. The HOMOs of these two molecules, however, differ substantially. In
p-BETB-ethex the HOMO has a node along the central bonds of the benzene linker, and no
amplitude on the methyl substituents on the benzene. In p-BETB-ohex, the HOMO has a node
centered on the two unsubstituted carbon atoms on the benzene, ortho to the alkynes.
Furthermore, the HOMO of p-BETB-ohex is considerably delocalized onto the oxygen atoms of
the alkoxy substituents, suggesting a quinoidal-like electronic structure in this dimer.
101

HOMO LUMO
a)

b)

c)

d)

Figure 4.9. The HOMO and LUMO of a) o-BETB, b) m-BETB, c) p-BETB-ethex, and d)
p-BETB-ohex, calculated at the B3LYP/6-31++G** level of theory.
o-BETB
m-BETB
p-BETB-ethex
p-BETB-ohex
102

4.2.3 Steady state photophysical properties of m-BETB, p-BETB-ethex and p-BETB-ohex
The steady state absorption and emission spectra can be used to further probe the electronic
structure of these molecules. The extinction spectra are representative of the magnitude of the
transition dipole moments in the molecules, and in concert with the shape of the absorption
spectra, can describe the extent of the coupling between the chromophores within the dimers.
The transition dipole moment in ethynyltetracenes is along the short axis of the acene and the
alkyne bond. Given the different orientations of the transition dipole moments of the acenes
within these dimers, the extinction spectra shown in Figure 4.10 are reflective of the
dipole-dipole coupling in these molecules.
As discussed in the previous chapter, the ET moieties in o-BETB are aligned such that the
transition dipole moments are coupled in an H-aggregate-like manner, as evidenced by the
enhanced ν
0-1
and diminished ν
0-0
absorption compared to the monomer (PET). The intensity of
the absorption is redistributed to longer wavelengths, and the integrated intensity in the visible
region is approximately twice that of the monomer. The absorption spectrum of m-BETB is
nearly identical to that of PET, but with a small red-shift of 4 nm (0.02 eV). The relative
intensities of the ν
0-0
and the ν
0-1
vibronic features of the PET and the m-BETB are identical. The
most notable difference between PET and m-BETB absorption properties is in their molar
absorptivity spectra: the molar absorptivity of m-BETB is approximately twice that of PET in the
visible part of the spectrum. This suggests that ETs in m-BETB are effectively independent of
each other and the electronic coupling between them is weak.
The absorption properties of the para dimers are different from those of o-BETB, m-BETB
and the monomer. In contrast to o-BETB, the ν
0-0
vibronic transition is enhanced relative to the
ν
0-1
transition in both of the p-BETB compared to the monomer. This feature is indicative of
103

J-aggregate-like interactions between the transition dipole moments of the chromophores. This is
consistent with the structure of these molecules, as the chromophores are linearly linked along
the axis of the transition dipole moment. The intensity of the molar absorptivity of
p-BETB-ethex in the visible region is approximately twice that of PET, however, the area under
the curve appears to be greater than twice that of PET. The increase in the integrated intensity
could suggest that the benzene linker is participating in the transition, which would be consistent
with the cumulene-like excited state.
The magnitude of the molar absorptivity of p-BETB-ohex is significantly greater than that
of any of the other dimers, and is accompanied by a substantial red-shift in the visible absorption.
Interestingly, the trend in the intensities of the higher energy absorption bands is reversed in
p-BETB-ethex and p-BETB-ohex. This further demonstrates that the electron donating alkoxy
substituents on the bridging benzene ring significantly alter the electronic structure of the dimers.

The emission properties of the dimers can also provide information about the magnitude of
coupling between the chromophores and hint at possible excited state deactivation pathways in
300 400 500 600
0
1x10
5
2x10
5
3x10
5
 cm
-1
mol
-1
L)
Wavelength (nm)
PET
o-BETB
m-BETB
p-BETB-ohex
p-BETB-ethex

400 450 500 550 600
0
1x10
4
2x10
4
3x10
4
4x10
4
5x10
4
 cm
-1
mol
-1
L)
Wavelength (nm)
PET
o-BETB
m-BETB
p-BETB-ohex
p-BETB-ethex

Figure 4.10. The extinction spectra of m-BETB, p-BETB-ethex, and p-BETB-ohex compared
to o-BETB and PET in a THF solution.
104

these dimers. The normalized absorption and emission spectra of all dimers in THF, and as neat
thin films are shown in Figure 4.11, and those of PET are provided for comparison. The quantum
yields of fluorescence and the fluorescence decay lifetimes are provided in Table 4.1. The most
notable difference can be seen in the quantum efficiency of emission properties of the dimers in
solution. As reported in the previous chapter, the Φ
fl
of o-BETB in THF was quite low, at
<0.5 %, compared to the 67 % for PET in THF. The quantum yield of emission of m-BETB, on
the other hand, is 62 % in THF, which is close to the value of PET. The spectral shape of the
m-BETB emission is also very similar to that of PET, and has a small Stokes shift. The high
intensity of this emission implies that the predominant excited state decay pathway in m-BETB is
fluorescence, and all other deactivation processes are expected to be slower in this dimer. In
contrast to the PET, the emission decay of m-BETB is multiexponential, with a majority (82 %)
of the emission decaying in 14.9 ns, 17 % decaying in 21.3 ns, and a very small amount (1 %) of
delayed fluorescence (170 ns). This delayed fluorescence is a feature common to weakly coupled
tetracene dimers, in which SF is slow,
2, 3
and is discussed in more detail in the following section
of this chapter.
The Φ
fl
of p-BETB-ethex is low, at 0.7 %, with a majority (88 %) of the emission decaying
on the timescale of the instrument response (<0.4 ns), 8 % decaying in 7.4 ns and 4 % decaying
in 3.5 ns. This long (7.4 ns) component could pertain to the small amount of the highly emissive,
partially oxidized dimer impurity. No detectible emission of p-BETB-ohex was measured. In
solution, only the photo-oxidized impurity produced a very low intensity signal, and no emission
signal was detected for a neat thin film of p-BETB-ohex. This suggests that an excited state
deactivation pathway faster than the fluorescence (~10 ns) is available in the para dimers. The
analogous para-bisethynylanthracenyl-benzene dimers have been reported to emit with high
105

quantum efficiencies: 97 % for para-bisethynylanthracenyl-benzene,
7
and 60 % for the
analogous hexyloxy substituted para-bisethynylanthracenyl-benzene.
36
Similarly to the
comparison of o-BETB to its anthracene analog in the previous chapter, the high quantum
efficiency of emission of the anthracene dimer compared to the tetracene dimers suggests that a
rapid excited state decay channel is available in the tetracene dimers, which is not available in
the anthracene dimers.

Figure 4.11. The steady state absorption and emission spectra of m-BETB, p-BETB-ethex,
and p-BETB-ohex compared to o-BETB and PET in a THF solution (black) and as neat thin
films (red).
106

Another technique offering an estimate of the amount of coupling between the
chromophores is cyclic voltametry (CV). During the sweep in the direction of oxidation, it is
expected that the first oxidation will be localized on one of the tetracenes in the dimer, and that
the second oxidation will take place on the second tetracene in the dimer. If the two tetracenes
are electronically coupled, there will be a stabilization of charge through the interaction with the
neighboring chromophore, and the second oxidation will take place at a voltage different from
the first oxidation. The splitting between the two lowest oxidation peaks then represents the
strength of the coupling between the chromophores. Two oxidation peaks were observed in all of
the dimers (see Figure 4.12); however the peaks were poorly resolved in m-BETB, due to their
close proximity to each other. The values of the splitting between the oxidation peaks are given
in Table 4.2. Based on these values, the chromophores are most strongly coupled in o-BETB and
in p-BETB-ohex, with values of 0.22 V. The coupling is the smallest in m-BETB, at 0.07 V, and
the oxidation peaks in p-BETB-ethex are spaced by 0.13 V.
The electrochemical properties of the dimers provide information about the energetics of
intramolecular charge transfer states. Several theoretical works have commented on the
Table 4.1: The quantum yields and lifetimes of the fluorescence of the monomer and the
dimers in a THF solution and the quantum yields of emission of the films of the same.
Molecule Φ (THF) τ (THF) Φ (thin film)

PET 0.67 11.9 ns <0.005
o-BETB <0.005
(0.57) 0.5 ns
(0.12) 1.7 ns
(0.31) 9.4 ns
<0.005
m-BETB 0.62
(0.82) 14.9 ns
(0.17) 21.3 ns
(0.01) 190 ns
<0.005
p-BETB-ethex 0.007
(0.88) < 0.4 ns
(0.08) 7.4 ns
(0.04) 3.5 ns
<0.005

107

importance of CT states in the SF mechanism. Yost et al. have suggested that the rate of SF is
likely to be enhanced if the CT states lie close enough in energy to the S
1
state,
37
and
Kolomeisky et al. have stated that the coupling between the S
1
S
0
and the
1
(T
1
T
1
) is larger when
CT states are admixed into the wave functions of those states.
38
Using CV and differential pulse
voltametry (DPV), the values of oxidation and reduction of the dimers can be obtained. The
approximate energy of the intramolecular CT state can then be roughly estimated from the
difference between the oxidation (E
ox
) and the reduction (E
red
) potentials. The values of E
ox
, E
red
,
and the electrochemical energy gap between them for the dimers and the monomer are provided
in Table 4.2. The values of the first excited state, estimated from the absorption spectra of these
dimers, are also supplied in Table 4.2 for comparison. In all molecules the electrochemical gap is
energetically similar to the optical gap, suggesting that CT states lie close enough in energy to
the S
1
to potentially influence SF kinetics. In the previous chapter, however, it was shown that
the kinetics of SF were independent of the solvent polarity in o-BETB, but in m-BETB the
scenario is different. The following section presents the fluorescence decays of m-BETB, which
were measured to be dependent on the solvent polarity.
108

Table 4.2: Electrochemical properties of the four dimers compared to the monomer, and the
optical gaps of these molecules.
Molecule Oxidation
peak
splitting
1
st
Oxidation Reduction E
gap, redox
E
gap, optical
PET --- 0.46 V -1.92 V 2.38 eV 2.42 eV
o-BETB 0.22 V 0.37 V -1.95 V 2.32 eV 2.37 eV
m-BETB 0.07 V 0.41 V -1.89 V 2.30 eV 2.40 eV
p-BETB-ethex 0.13 V 0.46 V -1.92 V 2.38 eV 2.34 eV
p-BETB-ohex 0.22 V 0.33 V -1.88 V 2.21 eV 2.28 eV

Fc
-0.5 0.0 0.5 1.0
0
10µ
20µ
Current (A)
Potential (V)
o-BETB
Fc
-0.5 0.0 0.5 1.0
0
10µ
20µ
30µ
Current (A)
Potential (V)
m-BETB

Fc
-0.5 0.0 0.5 1.0
0
10µ
20µ
30µ
Current (A)
Potential (V)
p-BETB-ethex
Fc
-0.5 0.0 0.5 1.0
0
10µ
20µ
30µ
Current (A)
Potential (V)
p-BETB-ohex

Figure 4.12. Cyclic voltametry plots of a) o-BETB, b) m-BETB, c) p-BETB-ethex, and d)
p-BETB-ohex in CH
2
Cl
2
, using ferrocene as an internal reference.
109

4.2.4 Time resolved emission and kinetic model for SF in m-BETB
The first ethynyltetracene dimer with high Φ
fl
to be presented in this thesis is m-BETB.
Unlike all other dimers, whose Φ
fl
are less than one percent in solution, the Φ
fl
of m-BETB in a
THF solution is 62 %. Interestingly, a small portion of the decay of this fluorescence has a very
long lifetime of hundreds of nanoseconds. A similar phenomenon was observed in the
weakly-coupled, highly-emissive dimers of tetracene reported by Muller et al., and was ascribed
to the delayed fluorescence due to intra-molecular triplet fusion, following intra-molecular SF.
To determine whether the triplet state is responsible for the delayed component of the
fluorescence decay of m-BETB, the fluorescence decay was measured under both air free and
ambient conditions. The energy of the T
1
state of tetracene derivatives is around 1 eV, making
them energetically viable for sensitization of singlet oxygen.
39
Therefore, if oxygen is present in
the system, it rapidly quenches the triplets of the acenes. The fluorescence decays of m-BETB in
an N
2
sparged THF solution and in a THF solution prepared under ambient conditions are shown
in Figure 4.13a. The distinct ~200 ns delayed fluorescence component present in an air free THF
solution of m-BETB is significantly smaller in the solution which was exposed to air. This result
indicates that the delayed fluorescence is originating from the interactions of the T
1
states of this
dimer.
The aforementioned result does not guarantee that the triplets responsible for the delayed
fluorescence were generated intramolecularly, but could be a result of annihilation of two
intermolecular T
1
states produced by intersystem crossing. The comparison of the fluorescence
decay of m-BETB with the monomer (PET) at the same optical density (Figure 4.13b) indicates
that the delayed fluorescence is a feature inherent only to the dimer, m-BETB, and is absent in
the PET fluorescence decay.
110

Furthermore, the measurements of the fluorescence decay of m-BETB at different
concentrations were performed to determine whether concentration, and therefore molecular
diffusion, has an effect on the delayed fluorescence. Four solutions, ranging from 21 μm to
1.5 μm were prepared, and their absorption spectra and fluorescence decays are shown in
Figure 4.14 a and b, respectively. The rate of T
1
---T
1
annihilation should be proportional to the
square of the concentration of the T
1
,

assuming that the diffusion of T
1
excitons toward each
other is the rate limiting step for generation of the delayed fluorescence. In that case, if the
delayed fluorescence was primarily due to inter-molecular T
1
annihilation, its rate would be
dependent on how frequently the molecules encounter each other, which would in turn be
dependent on the concentration of molecules in the sample. Therefore, in the case of
inter-molecular fusion, a concentration difference in one order of magnitude would result in a
two order of magnitude difference in the rates of annihilation. The experimentally measured
fluorescence decays at various concentrations of m-BETB do not reflect such a difference in the
annihilation rates. Instead, the decays of the four solutions of different concentrations of
m-BETB overlay almost identically onto each other. The lifetime of the delayed fluorescence of
0 250 500 750 1000
10
-5
10
-4
10
-3
10
-2
10
-1
10
0
Decay
Wavelength (nm)
m-BETB in THF (Air)
m-BETB in THF (N
2
)
0 250 500 750 1000
10
-5
10
-4
10
-3
10
-2
10
-1
10
0
Decay
Wavelength (nm)
m-BETB
PET

Figure 4.13. The comparison of emission decays of a) m-BETB in air free, and ambient
conditions, and b) between m-BETB and PET.
111

the most concentrated solution is τ = 170±5 ns, while that of the most dilute solution is
τ = 176±5 ns, with both of these numbers falling within the uncertainties of the values. This
provides strong evidence for the intramolecular origin of the T
1
---T
1
annihilation, in which both
of the triplets were generated intramolecularly via SF.

When this dimer, m-BETB, is frozen in a solvent (2-methyl-THF) glass at 77 K, the delayed
component of the fluorescence disappears (see Figure 4.15), and the decay can be fit to a single
exponential. This effect is not a result of the rigidity of the environment, as m-BETB dissolved in
a solid polymethylmethacrylate (PMMA) polymer matrix at <1% concentration by weight, still
exhibits the delayed fluorescence. Therefore, the lack of delayed fluorescence at 77 K is
suggestive of an activation barrier which needs to be overcome in order to produce the delayed
fluorescence. Given the endothermicity of singlet fission in tetracene, it is not unexpected that a
thermal barrier to SF is present in this dimer as well.
400 450 500 550
0.0
0.2
0.4
0.6
0.8
Absorbance
Wavelength (nm)
21 m
12 m
4.4 m
1.5 m

Figure 4.14. The emission decays of m-BETB in THF at four concentrations.
112

Lastly, the fluorescence decay of m-BETB appears to be dependent on the solvent polarity,
vide infra. The fluorescence decay of m-BETB was measured in cyclohexane, toluene, THF, and
DMF (see Figure 4.16), whose dielectric constants are 2.0, 2.4, 7.6, and 36.7, respectively. The
fluorescence in cyclohexane, toluene and THF appears to decay on similar timescales, while that
in DMF decays much more rapidly, both the prompt and the delayed fluorescence. It is worthy to
note that the delayed fluorescence component does not disappear in the most polar solvent, but
rather becomes more rapid.

Figure 4.16. The emission decays of m-BETB in various media

Figure 4.15. The emission decays of m-BETB in THF at room temperature and in a solid
solvent glass at 77 K.
113

In order to attain a qualitative understanding of the SF yields in m-BETB in different
solvents, the fluorescence decay data was fit using the crude kinetic model shown in Figure 4.17,
and the following set of coupled differential equations.
 
1 1
1
2T fus S fiss rad ic isc
S
N k N k k k k
t
N
     


(4.1)
1 1
1
S isc T trip
T
N k N k
t
N
  


(4.2)
 
1 1 1
1
2
2
T diff S fus T fus trip
T
N k N k N k k
t
N
    


(4.3)
where
1
S
N ,
1
T
N , and
1
2T
N are the populations of S
1
, T
1
, and
1
(T
1
T
1
) states, respectively, and the
rate constants are defined by Figure 4.17.
This model is analogous to that used by Muller et al. to calculate the SF yields in the
linearly linked tetracene dimers, however in the model used here the triplets generated by
intersystem crossing are also allowed to annihilate to the
1
(T
1
T
1
) state, and therefore produce
delayed fluorescence. This component was necessary in order to accurately model the data at
long times (>500 ns). The rate constants which were not varied include k
rad
, k
trip
, k
diff
, and the
sum of the k
ic
and k
isc
rate constants. The radiative rate, k
rad
, was obtained from the measured
quantum yield and the fast component of the fluorescence decay lifetime. The sum of the
non-radiative decay rates k
ic
and k
isc
was also obtained from the quantum yield and the fast
component of the fluorescence decay lifetime; even though the individual constants were
adjusted, the sum was kept as the total rate of the non-radiative decay. The k
trip
and k
diff
rate
constants were varied and only necessary for fitting the fail of the delayed fluorescence
(>800 ns), whose intensity is several orders of magnitude lower than the prompt fluorescence.
The data prior to 800 ns could be fit without the inclusion of these channels in the kinetic model.
114

The kinetic parameters were fit qualitatively to the data, and a least squares analysis is required
for more rigorous evaluation of the parameters. The remaining rate constants k
fiss
and k
fus
had a
large impact on the shape of the fluorescence decay, and were varied to achieve a qualitative fit.
The fits to the three fluorescence decays in toluene, THF and DMF are shown in Figure 4.18,
overlaid onto the experimental data, and the parameters used to generate the decays are given in
Table 4.3.

Figure 4.17. The excited state kinetic model used to fit the emission decay of m-BETB.
115

Using these parameters, the yield of SF can be calculated by using the following equation:
100 
 
 
ISC IC rad
fiss
SF
k k k
k
(4)
where Φ
SF
is the yield of singlet fission, and the values of the rate constants are defined by
Figure 4.17. Using the values from the fits, the yields of SF in m-BETB were estimated to be
approximately 1 % in toluene and THF, and slightly higher, 1.5 % in DMF. These yields are
Table 4.3: The rate constants used to fit the fluorescence decay of m-BETB in three
solvents, in units of ns
-1
.
k
rad
k
ic
k
isc
k
trip
k
diff
k
fiss
k
fus
Φ
SF

Toluene 0.047 0.017 0.003 0.0005 0.00001 0.00065 0.0056 ~1%
THF 0.042 0.010 0.010 0.0005 0.00001 0.00066 0.0066 ~1%
DMF 0.031 0.069 0.017 0.0005 0.00001 0.0017 0.012 ~1.5%

10
-5
10
-4
10
-3
10
-2
10
-1
10
0
10
-5
10
-4
10
-3
10
-2
10
-1
10
0
0 250 500 750 1000
10
-5
10
-4
10
-3
10
-2
10
-1
10
0
Toluene
Fit
Decay
THF
Fit
Time (ns)
DMF
Fit

Figure 4.18. The emission decays of m-BETB overlaid with the fits in the three solvents.
116

lower than those obtained by Muller et al. (~3 %) and by Cook et al. (~6 %) for their tetracene
dimers.
The rates of fusion to the S
1
state from the
1
(T
1
T
1
) state are greater than those of SF by
approximately one order of magnitude in all solvents. A similar observation was made for the
weakly coupled tetracene dimers reported in the literature. Using these two rate constants, an
equilibrium constant for the SF process in this dimer can be calculated. In turn, the equilibrium
constant can be used to estimate the ΔG of SF in m-BETB, by using Equation 5.
fus
fiss
k
k
RT G
K RT G
ln
ln
  
  
(5)
where R is the universal gas constant, T is temperature, K is the equilibrium constant for SF, and
the rate constants are as defined above. Using this equation and the data from Table 4.3, SF is
endothermic in m-BETB by approximately 0.06 eV, which is smaller than the 0.17 eV barrier to
SF in crystalline tetracene. The reduced energetic barrier is likely due to the lowering of the T
1

energies by alkyne conjugation.
It is important to emphasize the crude nature of the model. Even though the decay of
fluorescence is dependent on the solvent polarity, the model used here does not include the
population of a charge transfer state, which could serve as an intermediate to the SF or a decay
pathway for the
1
(T
1
T
1
) state. Therefore, the most conservative conclusion which could be drawn
from these results is that SF is very slow in m-BETB, and that the CT states are involved in the
excited state decay in this dimer, provided the dielectric constant is sufficiently high to make
those CT states energetically accessible.

117

4.2.5 Transient absorption of the dimers in solution
Transient absorption (TA) spectroscopy is a powerful technique which could be used to
probe the excited state dynamics of emissive and non-emissive compounds. Furthermore, it can
be used to discern between the various states which are populated during the decay, based on
their unique absorption features. Therefore, it is the tool of choice for studying SF dynamics in
covalently linked dimers, particularly those in which emission is very weak or non-existent. In
this section, the TA spectra of solutions of m-BETB, p-BETB-ohex and p-BETB-ethex are
presented, and kinetic models are devised from the experimental data.
The fs TA spectra of m-BETB in THF upon excitation at 515 nm are shown in
Figure 4.19a. There characteristic acene S
1
→S
n
absorption peaks at 390 nm, along with negative
ground state bleach (GSB) features at 516 nm and 470 nm. In the time window between 10 ps
and 1 ns, no interesting dynamics take place in this system, and the S
1
→S
n
absorption decays
with a concomitant recovery in the GSB. Therefore, we resorted to ns TA spectroscopy in order
to observe the formation of what is presumed to be the T
1
→T
n
absorption. The ns TA spectra in
Figure 4.19 show the excited state decay of m-BETB in a THF solution excited at 532 nm. In this
case, the S
1
→S
n
absorption is observed to decay, and the spectral shape morphs into a shape
which has a distinct peak at ~510 nm. A peak in this spectral region has been previously
observed for T
1
→T
n
absorption in 5,12-diphenyltetracene (DPT) and for ET-TMS.
1, 40
Given this
information, and in concert with the delayed fluorescence data in the previous section, it is likely
that the spectral shape observed at 135 ns for m-BETB in solution corresponds to that of the
T
1
→T
n
absorption. The rise of this absorption is slow and consistent with the kinetic model used
to fit the delayed fluorescence of m-BETB in the previous section. The fluorescence and the TA
data both suggest very slow triplet generation kinetics in m-BETB. In the introduction, it was
118

explained that both the through-bond and the through-space coupling are expected to be minimal
in this dimer. The rate of SF in m-BETB stands in stark contrast to that in the TIPS-pentacene
analog of this dimer, in which SF proceeded on a timescale of 200 ps.
28
A comparison of the
rates of SF in m-BETB and the pentacene dimer highlights the role of the energetic driving force
in dictating the rates of SF.

The excited state kinetics of the para dimers are not only different from those of m-BETB,
but also from those of o-BETB. The TA spectra of p-BETB-ohex and p-BETB-ethex in THF,
excited at 545 nm, and 530 nm, respectively, are shown in Figure 4.20a and b. At early times
(<50 ps), both of these compounds display the characteristic S
1
→S
n
absorption at ~425 nm, and
a GSB in the range of 520-540 nm. In 10s of picoseconds, the S
1
→S
n
absorption decays, and the
spectrum morphs to a spectral shape with an absorption peak at 549 nm in p-BETB-ohex and
540 nm in p-BETB-ethex.
In a THF solution, the S
1
decays rapidly in both of these dimers, however, when
p-BETB-ethex is suspended in a PMMA matrix, the excited state kinetics slow down drastically
(see Figure 4.21). In order to get an idea of the excited state dynamics in these systems, target
analysis was used to deconvolute the data. A three state model was used to fit the TA data for
375 450 525 600
0
2
4
6
8
A (mOD)
Wavelength (nm)
10 ps
24 ps
50 ps
100 ps
975 ps
375 450 525 600
0
5
10
15

A (mOD)
Wavelength (nm)
5 ns
7 ns
13 ns
17 ns
23 ns
27 ns
35 ns
135 ns

Figure 4.19. The a) fs and b) ns transient absorption of m-BETB
119

both p-BETB-ohex and p-BETB-ethex in solution and in PMMA. A two state model was not
adequate, as a significant amplitude of a residual spectrum was not accounted for by this model.

The decay associate difference spectra (DADS) obtained from the fit of the TA data of
p-BETB-ohex and p-BETB-ethex in THF are shown in Figure 4.22a and c, and their respective
populations are given in Figure 4.22b and d, with the lifetimes of these states listed in Table 4.4.
The spectral shapes of the DADS of p-BETB-ohex and p-BETB-ethex in THF are
qualitatively similar (Figure 4.22a and c). The shape of State 1 in both of these systems
resembles the typical acene S
1
→S
n
absorption, with an intense peak in the 375-470 nm, and a
375 450 525 600
-2
0
2
4
6

A (mOD)
Wavelength (nm)
1 ps
40 ps
100 ps
400 ps
600 ps
975 ps

Figure 4.21. The fs transient absorption of p-BETB-ethex in PMMA.
375 450 525 600
-6
-3
0
3
A (mOD)
Wavelength (nm)
1 ps
10 ps
24 ps
50 ps
100 ps
375 450 525 600
-5
0
5
10
15

1 ps
10 ps
24 ps
50 ps
100 ps
A (mOD)
Wavelength (nm)
Figure 4.20. The fs transient absorption of p-BETB-ohex and p-BETB-ethex and in THF.
120

large amplitude of the GSB. In both of these systems, State 1 rapidly decays to a state (State 2)
with diminished intensity in the 375-470 nm, similar GSB features to State 1, and increased
intensity in the induced absorption around 570 nm. Observe that the spectral shape of State 2 of
p-BETB-ohex and p-BETB-ethex in THF somewhat resembles that of the
1
(T
1
T
1
) state
absorption in o-BETB (see Chapter 3), with State 2 of p-BETB-ohex bearing more resemblance
to
1
(T
1
T
1
) absorption in o-BETB than State 2 of p-BETB-ethex. If State 2 corresponds to the
1
(T
1
T
1
) in p-BETBs, it could suggest that the spectral shape of this state is dependent upon the
coupling of the chromophores. Recall that based on the splitting in the oxidation potential peaks,
the coupling of the tetracenes was the strongest in o-BETB and p-BETB-ohex (0.22 V), and
significantly weaker (0.13 V) in p-BETB-ethex. The rate of formation of State 2, however, is
much faster than the formation of
1
(T
1
T
1
) in o-BETB. The target analysis gives values of 0.14 ps
(essentially on the timescale of the instrument response – 0.2 ps) for p-BETB-ohex and 0.39 ps
for p-BETB-ethex.
Unlike in o-BETB, State 2/
1
(T
1
T
1
) in p-BETBs decays, and gives rise to a State 3 with
prominent induced absorption peaks at 555 nm and 540 nm in p-BETB-ohex and p-BETB-ethex,
respectively. The shape of State 3 in p-BETBs in solution bears an uncanny resemblance to that
of the sensitized T
1
→T
n
induced absorption of the monomer, ET-TMS (see chapter 3). An
overlay of State 3 of p-BETB-ethex with a spectrally shifted sensitized T
1
→T
n
absorption of
ET-TMS is shown in Figure 4.23. Despite the slight discrepancy in the intensities in the
400-500 nm region, the relative peak positions and relative intensities overlay identically,
strongly suggesting that State 3 corresponds to T
1
→T
n
induced absorption in the p-BETB
dimers.

121

375 450 525 600
-15
-10
-5
0
5
10
p-BETB-ohex
in THF
Intensity
Wavelength (nm)
State 1
State 2
State 3
State 1
State 2
State 3
0.01 0.1 1 10 100 1000
0.0
0.2
0.4
0.6
0.8
1.0
p-BETB-ohex
in THF
Concentration
Time (ps)

375 450 525 600
-10
0
10
20
p-BETB-ethex
in THF
State 1
State 2
State 3
Intensity
Wavelength (nm)
State 1
State 2
State 3
1E-3 0.01 0.1 1 10 100 1000
0.0
0.2
0.4
0.6
0.8
1.0
p-BETB-ethex
in THF
Concentration
Time (ps)

375 450 525 600
-2.5
0.0
2.5
5.0
p-BETB-ethex
in PMMA
Intensity
Wavelength (nm)
State 1
State 2
State 3
State 1
State 2
State 3
1E-30.01 0.1 1 10 1001000
0.0
0.2
0.4
0.6
0.8
1.0
Concentration
Time (ps)
p-BETB-ethex
in PMMA

Figure 4.22. The DADS and concentrations from the target analysis of the TA data of a)
p-BETB-ohex and b) p-BETB-ethex and in THF, and c) p-BETB-ethex in PMMA using a
3 state sequential decay model.
122

The absorption of State 3 could correspond to either one or two triplets per dimer. There are
two factors which suggest that State 3 corresponds to absorption of two triplets on the same
dimer: 1) the rate of formation of State 3 is fast – 19.7 ps in p-BETB-ohex and 47.6 ps in
p-BETB-ethex. These rates of T
1
formation are too fast for ISC in acenes, but are in the range of
the measured SF rates in tetracene.
23, 24, 41, 42
2) More importantly, the rate of the decay of State 3
is very fast – 20.2 ps in p-BETB-ohex and 76.0 ps in p-BETB-ethex. Typically, a single triplet
exciton localized on an acene dimer decays on the timescale of microsceconds, due to the
forbidden nature of the transition. However, the recent studies of pentacene dimers showed that
when two triplet excitons are located on the same dimer, the T
1
→T
n
absorption decays on the
10-100 ps timescale, due to the favorable T
1
---T
1
annihilation pathway to the ground state.
28, 43

Table 4.4 The lifetimes of the decays of the DADS used to fit the TA data of
p-BETB-ohex and p-BETB-ethex in THF, and p-BETB-ethex in PMMA.
p-BETB-ohex
in THF
p-BETB-ethex
in THF
p-BETB-ethex
in PMMA
τ
1-2
0.14 ps 0.39 ps 1.23 ps
τ
2-3
19.7 ps 47.6 ps 400 ps
τ
3
20.2 ps 76.0 ps 11 ns

400 450 500 550 600 650
-4
-2
0
2
4
6
Intensity
Wavelength (nm)
p-BETB state 3
ET-TMS T
1
- T
n

Figure 4.23. The overlay of State 3 from target analysis of p-BETB-ethex in THF (blue)
with the shifted and normalized sensitized T
1
T
n
absorption of ET-TMS (black).
123

Therefore, the <100 ps decay of the T
1
absorption in the p-BETB dimers in THF is highly
suggestive of two triplets in close vicinity of each other in these molecules. The most likely
origin of these two triplets is via SF.
The comparison of the excited state dynamics of the ortho and the para dimers then raises
the question – why does the
1
(T
1
T
1
) state in the para dimers decay to form independent triplets
while that in the ortho dimer only relaxes to the ground state without producing independent
triplet excitons, despite the similar electronic coupling in these dimers? The biggest difference
between these dimers is their molecular geometry, and perhaps it holds the answer to the
discrepancies in their excited state behavior. Even though the tetracenes in o-BETB can rotate,
they always retain some amount of π overlap. Therefore, once the
1
(T
1
T
1
) state is formed in
o-BETB, the π overlap allows for the triplets to always be coupled to each ot her. In p-BETB, on
the other hand, the only source of electronic coupling is through-bond and it is expected to be
strongest when the entire molecule is planar. Upon twisting of one of the tetracenes
perpendicular to the plane of the rest of the molecules, the electronic coupling between the
chromophores could be ruptured, and the triplets from the correlated triplet pair state could
become independent of each other. The depiction of the possible molecular orientations
corresponding to the correlated and independent triplet pair states in p-BETB are shown in
Figure 4.24.

Figure 4.24. The proposed mode of decoupling of the correlated T
1
in p-BETB-ethex
124

If rotational motion of the tetracene in p-BETB provides the mechanism for generating free
triplets, then restricting that motion should reduce the rate and the yield of the formation of free
triplets. The comparison of the TA data of p-BETB-ethex in THF and in PMMA indeed reflects
this supposition. As evident in TA spectra of p-BETB-ethex in PMMA (see Figure 4.21), the
S
1
→S
n
absorption decays much more slowly, such that even at 975 ns a substantial S
1
population
remains. The characteristic T
1
→T
n
absorption peak at 550 nm still forms in this media; however,
the rate of its formation is much slower than for p-BETB-ethex in THF. A sequential three state
kinetic model was used in target analysis of this dataset; unfortunately, it incorrectly
deconvolutes the data. The spectral shape of State 3 obtained from this model still shows
significant amplitude around 400 nm (Figure 4.22e), where the S
1
→S
n
absorption is located. This
likely suggests that the there is a branching in the S
1
population, such that some of the molecules
in the S
1
state undergo SF, while others are stuck in a geometry not amenable to SF, and
therefore do not relax fast. Note that p-BETB-ethex in PMMA produces a substantial amount of
emission, with a quantum efficiency of ~10 %, which further supports that some of the
molecules are confined to a geometry in which SF is not possible, and therefore fluorescence
remains as the fastest decay channel available. In order to quantitatively describe the excited
state dynamics in this system, more advanced kinetic modeling is required and is currently
underway.
Of additional interest are the differences in the excited state kinetics of p-BETB-ohex and
p-BETB-ethex. All three of the processes: relaxation of S
1
into
1
(T
1
T
1
), conversion of
1
(T
1
T
1
) to
independent triplets, and the decay of the resultant triples are faster in p-BETB-ohex compared
to p-BETB-ethex. As discussed earlier, the coupling of the tetracene in p-BETB-ohex is greater
compared to p-BETB-ethex, and the alkoxy substituents on the benzene ring induce quinoidal
125

character, as evidenced by the molecular orbitals shown above. Another way to think about the
coupling through the ethynylbenzene linker is in terms of the relative orbital energies of the
tetracenes and the two bis-ethynylbenzene linkers. A qualitative diagram showing the relative
orbital energies of these molecules is provided in Figure 4.25. In the alkyl-substituted benzene
linker, the orbitals are expected to lie energetically far from those of the tetracenes and the
coupling is not expected to be very large. However, introducing the electron donating hexyloxy
substituents onto the benzene linker raises the energy of the HOMO, and therefore brings the
orbital energies of the linker closer to those of the tetracenes, resulting in more efficient
coupling.

4.2.6 Photophysical properties of neat thin films of the dimers
The photophysical properties of the thin films of m-BETB and p-BETB-ethex greatly differ
from those in solution. In the solid state, inter-molecular through-space coupling is available, as
evidenced by the red-shift in the absorption spectra in Figure 4.11. If the inter-molecular

Figure 4.25. A cartoon diagram showing the closer lying orbitals of the linker and the
tetracenes in a) p-BETB-ohex compared to b) p-BETB-ethex.
126

coupling is stronger than the intra-molecular coupling, then inter-molecular SF is likely to
outcompete inra-molecular SF.
The fs TA spectra of a thin film of m-BETB excited at 545 nm are shown in Figure 4.26a.
At early times, the usual S
1
→S
n
absorption features are observed at 400 nm, along with the GSB
at 545 nm. With increasing time delays, these features decay and the remaining spectral shape
bears a slight resemblance to the T
1
→T
n
absorption of o-BETB. The different spectral regions
decay with different rates (see Figure 4.27a), and this dataset could not be fit to a simple
sequential kinetic model. Advanced kinetic modeling of the thin film m-BETB transient
absorption data is currently underway. Qualitatively, it appears as though this compound
undergoes inter-molecular SF in the solid state, akin to monomeric ET-TMS.
The excited state decay of a thin film of p-BETB-ethex is surprisingly simple. The TA
spectra of the thin film of p-BETB-ethex excited at 568 nm are shown in Figure 4.26b. In these
spectra, even at early time delays, the distinct S
1
→S
n
absorption features are missing, and a
rather low intensity induced absorption feature is present in the 400 nm region instead. The
induced absorption features in the 400 nm and the 650 nm regions appear to be decaying with the
same rate (see Figure 4.27b), while the GSB is recovering at a faster rate than the induced
absorption features. During the excited state decay, no new states are populated, and the excited
state population returns to the ground state within several hundred picoseconds. Since the S
1
→S
n

absorption is absent, it is possible that the state immediately (within the instrument response
window) populated in a thin film of p-BETB-ethex is the
1
(T
1
T
1
) state, which simply decays to
the ground state without producing triplets. The lack of triplets in this material could be due to
the large Davydov interactions, which bring the energy of the S
1
state of the thin film of
p-BETB-ethex to 568 nm, which could be too low for SF.
127

4.3 Conclusions
The dimers presented in this chapter provide valuable insight about the effects of
through-space and through-bond coupling on SF dynamics in tetracene derivatives. It was
observed that SF is possible in purely through-bond coupled systems, without the contributions
from π interactions. The extent of conjugation provided by the linking group dictates the
magnitude of through-bond coupling, which in turn determines the rate of intra-molecular SF.
0.01 0.1 1 10 100 1000
-1
0
1
2
3

A (mOD)
Time (ps)
425 nm
550 nm
650 nm
0 50 100 150 200 250
-3
-2
-1
0
1
2
A (mOD)
Time (ps)
425 nm
550 nm
660 nm

Figure 4.27. The decays of the different spectral regions of the TA spectra of thin films of
a) m-BETB and b) p-BETB-ethex.
450 525 600 675
-1
0
1
2

A (mOD)
Wavelength (nm)
1 ps
10 ps
50 ps
100 ps
200 ps
500 ps
375 450 525 600 675
-6
-4
-2
0
2

A (mOD)
Wavelength (nm)
1 ps
10 ps
24 ps
50 ps
100 ps
290 ps

Figure 4.26. The transient absorption spectra of thin films of a) m-BETB and b)
p-BETB-ethex.
128

The dimer in which the tetracenes were coupled with a non-conjugating
meta-diethynylbenzene, SF was slow and the yield was low, as obtained from the fluorescence
decays, which exhibited a ~170 ns delayed component. The conjugating para-diethynylbenzene
linker, on the other hand, facilitated fast SF in solutions of the p-BETB-ohex and p-BETB-ethex
dimers. Furthermore, the formation of independent triplets from the correlated triplet pair was
possible due to the rotational motion of the tetracene about the axis of the alkyne bond.
Restricting this motion by placing the dimer in a rigid PMMA matrix significantly slowed down
the excited state decay of p-BETB-ethex, and particularly reduced the rate and the yield of the
formation of the independent triplets.
Stronger electronic coupling between the tetracenes and faster SF were observed in the para
dimer whose bridging benzene ring was substituted with the strongly electron donating alkoxy
groups (p-BETB-ohex). This is likely a result of closer energy spacing between the molecular
orbitals of the linker with the alkoxy groups and the molecular orbitals of the tetracenes,
compared to the linker with only alkyl groups attached.
The excited state behavior of the thin films of m-BETB and p-BETB-ethex is different from
that in solutions of these dimers. Triplet sensitization experiments are currently underway to
determine the spectral shape of the T
1
→T
n
absorption in these materials, and in turn, determine
whether triplets are forming via SF in the thin films. Qualitative analysis of the thin film data
hints at rapid inter-molecular SF in the thin film of m-BETB. In contrast, the S
1
excitons in the
p-BETB-ethex film are likely not energetic enough to undergo SF, as no change in the spectral
shape is observed.
The work presented in this chapter paints a complete picture of the interplay of
chromophore coupling and singlet fission rates in tetracene based systems. The next chapter
129

explores the role of exciton delocalization and entropy in the SF process by comparing the
excited state decays of a dimer and a tetramer of ethynyltetracenes.
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10113-10123.
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Matthew Y.; Campos, Luis M., Chem 2016, 1 (3), 505-511.
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8965-8972.

131

Chapter 5. Singlet fission in tetra-(phenylethynyltetracenyl)benzene – the role of exciton
delocalization
5.1 Introduction

Since the discovery of SF in the late 1960s and until the last decade, SF predominantly been
studied in crystalline or polycrystalline systems.
1-5
Only recently has SF been reported to proceed
efficiently in covalent assemblies of chromophores isolated in solution rather than in bulk
solids.
6-10
In bulk samples, solid state effects such as the dielectric and polarizability of the
material, and exciton delocalization could all influence the SF dynamics. Covalent dimers
eliminate these effects and simplify SF to solely a function of chromophore coupling and the S
1

and T
1
energies. However, if insight about solid state effects is desired, then to systematically
study the contributions from solid state effects, it is necessary to resort to cleverly designed
systems which allow for mimicry of the desired solid state properties in a controlled way.
The phenomenon which has recently been invoked to explain fast SF in crystalline
tetracene, a material with E(S
1
) ≲ 2E(T
1
) energy alignment, is exciton delocalization. Chan et al.
postulated that the delocalization of the initial singlet exciton results in an entropic gain upon SF
and in turn provides the driving force for SF in such systems.
11
Specifically, the initial state,
corresponding to the delocalized S
1
exciton can localize on any of the chromophores within the
delocalization radius (therefore the S
1
state can be described by N microstates, where N is the
number of chromophores within the delocalization radius), whereas the final state, localized pair
of T
1
excitons, can occur in one of several equal energy permutations of the location of the
1
(T
1
T
1
) biexciton within the sphere of the initial S
1
delocalization, as shown in Figure 5.1.
11, 12
If
the number of microstates for the final state is greater than that of the initial state, then there
should be a net entropic gain upon singlet fission. The entropic gain presumably allows
132

overcoming the enthalpic barrier to make the net process of SF in materials in which
E(S
1
) ≲ 2E(T
1
) thermodynamically favorable, based on the equation for Gibbs free energy
( ).

The delocalization of the S
1
state in crystalline tetracene has been quantified by Lim and
coworkers using time resolved fluorescence.
13
They measured the initially formed S
1
state to be
delocalized over 10 tetracene molecules. Other examples of exciton delocalization in organic
materials include pseudoisocyanine-halide dyes,
14
xanthene dye dimers,
15

oligophenylenevinylenes,
16
distyrylbenzene dendrimers,
17
poly(3-hexylthiophene),
18
and
biological systems.
19

Singlet exciton delocalization occurs via the coupling of the transition dipoles (μ) of
neighboring chromophores in an aggregate. As a result of this coupling, new inter-molecular
excited states emerge, and the splitting between them is referred to as Davydov splitting.
20
The
relative orientation of the transition dipole moments of the neighboring chromophores

Figure 5.1. Depiction of how initial S
1
exciton delocalization could provide the entropic
driving force for singlet fission in tetracene. The S
1
exciton corresponds to N microstates (N =
number of molecules over which S
1
is delocalized), whereas the localization of the triplet pair
has several possibilities, resulting in >N microstates.

133

determines the shift in the peak of the absorption spectrum. If the μ of neighboring
chromophores are oriented parallel to each other, the optically allowed state is one in which the μ
are pointing in the same direction, and is the higher lying state of the new pair of intermolecular
states; this is called the H type aggregate.
21, 22
The J type aggregate results from head to tail
arrangements of μ, and is characterized by sharp, red -shifted absorption.
14, 22-27
In cases where
the μ are not perfectly oriented in a parallel or head to tail fashi on, both of the effects in the
absorption spectrum will be observed, such as in crystalline tetracene which packs in a
herringbone crystal lattice.
23, 28, 29
The magnitude of the energy splitting is dependent on the
extent of coupling between the chromophores and the number of chromophores involved in the
transition. The greater the number of chromophores participating in the delocalization, the
greater the energy splitting will be. The triplet states typically do not experience delocalization,
because their μ are much lower.
30
Therefore, in materials with E(S
1
) ≲ 2E(T
1
) alignment,
excessive Davydov splitting could render the S
1
energetically incapable of undergoing SF.
31

Additionally, delocalization of the S
1
accompanies the delocalization of charge transfer
(CT) states, with subsequent lowering of their energies.
30
This could increase the CT character in
the S
1
(and possibly the
1
(T
1
T
1
)) wave functions. It has been suggested that increased admixture
of the CT states into the S
1
and
1
(T
1
T
1
) wave functions is also beneficial for SF.
32
If exciton
delocalization indeed provides a driving force for SF, it would be expedient to know how the rate
of SF is affected by the size of the delocalized exciton.
Controlling the size of a delocalized exciton in the solid state is not trivial; therefore it is
necessary to resort to covalent dimers and oligomers (of defined length) which can be studied in
solution or rigid solvent matrices without the contributions from solid state effects. The
photophysical properties of oligomers comprised of organic chromophores are analogous to the
134

quantum confinement effects in semiconductor nanoparticles, in that control over the size of the
molecular aggregate allows for tuning of the properties which are not present in isolated
chromophores or bulk solids.
33

In their recent work, Sanders and coworkers have studied SF in a series of linearly linked
pentacenes (ranging from 2 to 5 units).
9
Although exciton delocalization is not expected to be
significant in these weakly coupled systems, they observed an increase in the SF rate in longer
chains of pentacenes (1.2 ps
-1
for bis-pentacene, 2.1 ps
-1
for tetra-pentacene, and 2.3 ps
-1
in penta-
pentacene).
9
It would be interesting to see how this picture changes when the chromophore units
are strongly coupled and the exciton can delocalize over the entire oligomer.
In our o-BETB dimer presented in Chapter 3, the chromophores are strongly coupled
intramolecularly, and the S
1
state is likely to be delocalized over both of the tetracenes.
34-36
The
linear absorption spectrum of o-BETB supports excitation delocalization over the entire dimer.
Using the analogy in Figure 5.1, the number of possible configurations of the
1
(T
1
T
1
) state in this
system is one, as shown in Figure 5.2. Nonetheless, the
1
(T
1
T
1
) state formed on a timescale of
2 ps in this dimer. However, the rate of T
1
formation in the solid of o-BETB was markedly
faster, at 0.23 ps. Comparison of the linear absorption spectra shows that the thin film absorption
is red-shifted compared to the solution absorption spectrum, indicating that the chromophores are
coupled further, intermolecularly. The initial delocalization of the S
1
exciton over several
dimers
34
in a thin film of o-BETB could be a contributing factor to the fast SF rate in this
material.
135

To test the effects of exciton delocalization on the rate of SF in ethynyltetracenes we
devised an ethynyltetracene tetramer in which the relative orientation of the chromophores is the
same as in o-BETB, but the number of the chromophores able to participate in the excitation is
doubled. The structure of this tetramer is shown in Figure 5.3. Given our knowledge of the
conjugating nature of the ethynylbenzene linker, we can assume the chromophores to be strongly
coupled in this molecule. In such a case, the S
1
exciton is expected to be delocalized across all
four tetracenes, as shown in Figure 5.3. The
1
(T
1
T
1
) state would then have four possible options
for localization (teal shading in Figure 5.3).

Figure 5.3 An artist’s rendition of the expected SF process in TETB: delocalized S
1
exciton
has four different options for relaxing into the correlated triplet pair.

Figure 5.2 An artist’s rendition of the SF process in o-BETB: delocalized S
1
exciton relaxes
into the correlated triplet pair (localized on the same chromophores as S
1
).
136

In this chapter, the synthesis and photophysical properties of tetraethynyltetracenylbenzenes
are presented. Initially, the direct tetra- analog of o-BETB was synthesized (TETB – Figure
5.4a); however, it proved to be poorly soluble and chemically unstable, and a sturdier system was
required. To alleviate both the solubility and the instability issues, the phenylated analog of
TETB was synthesized (Ph-TETB – Figure 5.4b), and its excited state behavior is reported in this
chapter. The phenyl substituents are likely to affect the excited state energies of this molecule
compared to the unsubstituted one, and therefore to make a direct comparison of dimer vs.
tetramer, the phenylated version of o-BETB was also synthesized (Ph-o-BETB – Figure 5.4c)
and used to compare with the excited state properties of Ph-TETB.

5.2 Results and Discussion
5.2.1 Synthesis and
1
H NMR of TETB, Ph-o-BETB and Ph-TETB
The initial tetramer which was devised for this study is TETB, in which two sets of ETs are
coupled in an ortho arrangement on the benzene ring, and are expected to be oriented and
electronically coupled analogously to those in o-BETB. TETB was possible to synthesize
according to the route in Scheme I, analogously to the synthesis of o-BETB; however, the
purification and the characterization proved to be a challenge due to its poor solubility.
Furthermore, when in solution, this compound was more prone to photo-degradation than

Figure 5.4 The chemical structures of the compounds presented in this chapter: a) TETB, b)
Ph-TETB, and c) Ph-o-BETB
137

o-BETB, from qualitative observations. Its
1
H NMR spectrum is provided in this chapter for
comparison, but no photophysical data is shown.

The most likely route to degradation under ambient conditions in tetracene based molecules
is the 2+4 Diels-Alder cycloaddition reaction of the molecular oxygen at the two central rings of
the tetracene, as shown in Figure 5.5.
37
To combat the issue of photo-oxidation, the tetracenes
must be substituted at the ‘reactive’ positions: 5, 6, 11, and 12. As mentioned in Chapters 2 and
3, the alkyne substitution at the 5 position already provides protective benefits for the tetracene
by electronic effects.
38
The substituents on the tetracene core can provide protection from
photo-oxidation via the electronic and the steric effects. Substituents which draw the electron
density away from the reactive central rings of the tetracene make the Diels-Alder reaction with
molecular oxygen thermodynamically unfavorable, while sterically bulky substituents slow down
the kinetics of that reaction
38
.

Figure 5.5 The Diels-Alder reaction of molecular oxygen and tetracene; the predominant
degradation mechanism of acenes in ambient conditions.
Scheme I:

138

If bromination of tetracene is still the starting reaction of choice, then there are two
possibilities for double substitution on the tetracene: at the 5,11 positions, and at the 5,12
positions. The 5,11-dibromotetracene was chosen as the starting material because its purification
is easier. It crystallizes more readily than the 5,12 variant, and gives flocculent red needle
crystals which are less soluble than the 5,12-dibromotetracene.
An added benefit of the 5,11 substituted tetracene is that it is more resistant to
photo-oxidation than the 5,12 derivatives. In the 5,12 substituted tetracenes, the other middle
ring is unsubstituted, and it has been shown that the Diels-Alder reaction with oxygen primarily
takes place at the unsubstituted tetracene ring.
38
In the 5,11 substituted tetracene, both of the
rings are protected by one substituent, thereby reducing its predisposition for the 2+4
cycloaddition reaction.
The ‘protective’ substituent of choice for the molecules presented here is the p henyl group.
The phenyl substitution at the reactive positions of the tetracene leads to slower degradation of
the acene in molecules such as diphenyltetracene and rubrene.
39
Its sterics thwart oxidation, and
it is not strongly electron donating. The phenyl substituent is expected to be perpendicular to the
plane of the acene and therefore should not severely affect the relative S
1
and T
1
energies.
However, the effect may not be negligible, and in order to make more careful comparison
between the dimer and the tetramer, the phenylated version of o-BETB (Ph-o-BETB) was
synthesized as well.
The synthetic routes to Ph-TETB and Ph-o-BETB are shown in Scheme II. Initially, the
tetracene was doubly brominated using a modified reported protocol,
40, 41
followed by a single
Suzuki coupling with phenylboronic acid. Surprisingly, the optimization of this step proved time
consuming. The issue was the poor solubility of 5,11-dibromotetracene and the choice of the
139

palladium catalyst. The set of reaction conditions which finally produced satisfactory yields were
as follows: all reagents except for the Pd(PPh
3
)
4
were combined and the reaction mixture was
stirred and heated to 80
o
C until all of the 5,11-dibromotetracene was dissolved, then Pd(PPh
3
)
4

was added into the reaction flask against the positive flow of N
2
gas, and the reaction mixture
was left to react overnight.
140

The use of Pd(PPh
3
)
4
was crucial for the success of this reaction, as the use of PEPPSI-Ipr
under the same conditions lead to dehalogenation to produce phenyltetracene as the major
product. Furthermore, it was also necessary to start this reaction with isomerically pure
dibromotetracene. If both of the isomers of the dibromotetracene were present in the reaction
Scheme II:

141

mixture, the resulting isomeric mixture of bromophenyltetracene could not be separated by
column chromatography, due to the similar R
f
values of the products. Slow recrystallization gave
acceptable purity of the 5,11-dibromotetracene, which resulted in large, brick-shaped crystals,
shown in Figure 5.6. The remaining steps carried out similarly to those shown in Chapters 3 and
4, giving moderate yields. Lastly, it is important to note that tetra-ethynylbenzene is not stable,
and should be used immediately upon preparation.

The phenyl substituent proved to be a “two birds with one stone” approach to improvement
of the tetramer in that it protected the tetracenes from photo-oxidation (the chemical stability of
Ph-TETB will be discussed below) and it rendered the compound soluble in most solvents, such
that its photophysical characterization in solution was possible.
To demonstrate the electronic effects of increasing the number of ethynyltetracenes on the
benzene ring, the
1
H NMR spectrum of TETB is shown in Figure 5.7, and a comparison to that
of o-BETB is shown in Figure 5.8. The most notable difference between the two spectra is a
~0.1 ppm shift downfield of most of the TETB proton peaks compared to those of o-BETB. This
suggests that the protons on the tetracenes in TETB are more deshielded, possibly because some
of the electron density is shifted onto the center of the molecule – the tetra-ethynylbenzene

Figure 5.6 Crystals of 5-bromo-11-phenyltetracene, obtained by slow crystallization from
chloroform layered with hexane.
142

moiety. The comparison of these
1
H NMR spectra gives an indication that the electronic
environment of the tetracenes in the dimer and the tetramer is not quite identical.

Figure 5.7
1
H NMR spectrum of TETB in CDCl
3
.
143

The
1
H NMR spectrum of Ph-o-BETB (Figure 5.9) is also slightly different from that of
o-BETB, as evident from their comparison in Figure 5.10. The shift of the tetracene protons is
not in a uniform direction. For instance, the proton peaks presumed to be closest to the phenyl
substituent in Ph-o-BETB at 6.42 and 6.60 ppm are more upfield than the analogous proton
peaks of o-BETB at 6.72 and 6.89 ppm, likely because of the ring current effect on the protons in
the phenyl substituted derivative. The comparison suggests that the effect of the phenyl
substitution on the electronic structure of these molecules is not negligible

Figure 5.8 Comparison of the
1
H NMR spectra of TETB (top) with o-BETB (bottom) in
CDCl
3
.
TETB
o-BETB
144

Figure 5.10 Comparison of the
1
H NMR spectra of Ph-o-BETB (top) with o-BETB (bottom)
in CDCl
3
.

Figure 5.9
1
H NMR spectrum of Ph-o-BETB in CDCl
3
.
Ph-o-BETB
o-BETB
145

The
1
H NMR spectrum of Ph-TETB is provided in Figure 5.11. All of the peaks are
somewhat shifted from those of o-BETB, TETB, and Ph-o-BETB, but not in a systematic
manner. Nothing particularly unusual is noticeable in this spectrum, and the electronic structure
of this molecule is expected to be impacted by both effects: the phenyl substituents compared to
the unsubstituted version, and a tetramer structure rather than a dimer.

5.2.2 Molecular and electronic structure of dimer vs. tetramer
Although the crystal structure evaluation of these compounds has not yet been completed,
insight about their geometries can be gained from the computed geometries. The
B3LYP/6-31+G** optimized geometries of o-BETB and TETB are shown in Figure 5.12. Based
on these geometries, it is evident that the tetracenes tend to adopt the same relative orientation on
both the dimer and the tetramer. The biggest difference between o-BETB and TETB is a slightly

Figure 5.11
1
H NMR spectrum of Ph-TETB in CDCl
3
.
146

larger distance between the tetracenes in TETB at ~4.01 Å, versus ~3.90 Å in o-BETB. The
tetracenes in TETB should be able to reach similar configurations than those in o-BETB in the
ground state, and the

Even though the ortho tetracenes are structurally similar in both the dimer and the tetramer,
the orbital structure is drastically different. The frontier molecular orbitals (MOs) of these two
structures computed at the B3LYP/6-31++G** level of theory are shown in Figure 5.13. The
biggest difference between the MOs of these two structures is the extent to which the orbitals are
delocalized on the bridging benzene ring. In o-BETB the HOMO and LUMO have fairly small
orbital amplitudes on the benzene, whereas the MO amplitude on the benzene ring in TETB is
substantial in both the HOMO and the LUMO. Interestingly, the MO amplitudes on the
tetracenes in TETB are significantly smaller than those in o-BETB. This suggests that the tetra-
ethynylbenzene significantly polarizes the orbitals of the tetracene, likely because the orbital
energy of this molecular component is close to that of tetracene, such that significant interaction
between them is possible.
The calculations were performed for un-phenylated molecules (to minimize the
computational costs); however, for reasons discussed in the synthesis section, the
characterization of TETB was not optimal due to rapid degradation. Therefore, in the remainder
of this chapter only the phenylated (Ph-o-BETB and Ph-TETB) compounds will be compared.

Figure 5.12 B3LYP/6-31+G** optimized geometries of a) o-BETB and b) TETB.
147

5.2.3 Steady state photophysical properties of Ph-o-BETB and Ph-TETB
The biggest difference between the dimer and the tetramer is in their photophysical
properties. First, the absorption spectra of Ph-o-BETB and Ph-TETB are compared to PET and to
each other. The molar absorptivity spectra of these compounds, shown in Figure 5.14, are
significantly different from each other, both in shape and intensity. The absorption of
Ph-o-BETB is broader and red-shifted compared to that of the monomer, PET. The absorption of
the monomer in the visible region peaks at 512 nm, with an onset at 540 nm, whereas the lowest
energy vibronic peak of Ph-o-BETB is at 530 nm, with a much farther onset at 590 nm. The
broad spectral line-shape is indicative of structural disorder. The overall line-shape of the
Ph-o-BETB absorption is similar to that of o-BETB (see Chapter 3), with an enhanced ν
0-1

HOMO LUMO
a)

b)

Figure 5.13 Molecular orbitals of a) o-BETB and b) TETB.
148

feature and a diminished ν
0-0
feature, suggesting an H-aggregate like orientation of the transition
dipole moments in this molecule.
The red-shift of 0.18 nm (0.08 eV) in the lowest energy vibronic peak in the dimer
(compared to the monomer) is representative of the chromophore coupling in the dimer geometry
as well as exciton delocalization. The red-shift in the steady state absorption spectra has been
used to evaluate the extent of delocalization in coupled chromophore systems by Bakalis and
coworkers.
42

If 0.08 eV represents the delocalization over two chromophores in this orientation, then in
the case of four chromophores coupled in the same manner, the lowest energy absorption peak
would be expected to be shifted by approximately 0.16 eV. The shift in the lowest energy
absorption peak of the tetramer Ph-TETB versus the monomer (PET) is 0.18 eV, close to the
expected value. This suggests that the exciton is likely to be delocalized over all four tetracenes
in Ph-TETB.
The molar absorptivity of Ph-o-BETB is similar to that of o-BETB (Chapter 3). The
absorptivity in the visible region is approximately 2.4×10
4
M
-1
cm
-1
, slightly less than twice that
of the monomer (1.6×10
4
M
-1
cm
-1
), while the peak in the UV range of Ph-o-BETB (2.0×10
5
M
-
1
cm
-1
) is slightly greater than twice that of the monomer (0.9×10
5
M
-1
cm
-1
). The intensities of the
molar absorptivity spectrum of Ph-TETB in the visible (6.0×10
4
M
-1
cm
-1
), and the UV
(3.4×10
5
M
-1
cm
-1
) regions are slightly less than four times that of the monomer, possibly because
all of the tetracenes in Ph-TETB are sharing one benzene ring, which is presumably involved in
the electronic transitions, due to partial conjugation through the alkynes.
149

The steady state absorption and emission spectra of Ph-o-BETB and Ph-TETB (see
Figure 5.15) provide insight into the differences in the excited state properties of these
molecules. Both Ph-o-BETB and Ph-TETB exhibit low intensity emission (0.5% and 1.4%,
respectively) compared to the monomer (67%); however, the line-shapes of their emission
spectra are significantly different. The low intensity emission of Ph-o-BETB (Figure 5.15,
middle) exhibits a small Stokes shift and discernible vibronic features. The emission of
Ph-TETB, on the other hand, exhibits a very large shift of 67 nm (0.42 eV) from the first
vibronic peak of the absorbance. Furthermore, the line-shape of this emission is broad and
featureless (akin to the emission of BETX in solution and PMMA from Chapter 3). The lifetime
of this broad emission is multiexponential, with the largest component of the emission decay
τ~0.6 ps, and the smaller components of the emission decay corresponding to ~1 ns and ~5 ns.
Such broad and red-shifted emissions are often ascribed to emission stemming from an
excimer or a CT state. The emission of Ph-TETB is unlikely to emerge from a CT state, as the
line-shape, and particularly the peak of the emission, is not dependent on solvent polarity.
Emission from an intra-molecular excimer in Ph-TETB is also unlikely. Recall that the relative
tetracene orientation in the tetramer is similar to that in the dimer, therefore if the tetramer was

Figure 5.14 Molar absorptivities of PET (black), Ph-o-BETB (red) and Ph-TETB (blue).
150

emitting from an excimer state, the same should be observed for the dimer, which is not the case.
Furthermore, as mentioned in Chapter 3, the geometry of ethynyltetracenes coupled ortho to
each other on a benzene ring is not amenable to relaxation into a deep excimer.

Table 5.1 Emission properties of PET, Ph-o-BETB and Ph-TETB in solution.
Molecule Φ τ
PET 67% 11.9 ns
Ph-o-BETB 0.5% 0.55 ns (0.19) 3.2 ns (0.12) 11.4 ns (0.69)
Ph-TETB 1.4% 0.62 ns (0.61) 1.0 ns (0.36) 6.5 ns (0.03)

Figure 5.15 Steady state absorption (solid) emission (dashed) spectra of PET (top),
Ph-o-BETB (middle) and Ph-TETB (bottom) in solution at room temperature.
151

The emission properties of Ph-TETB in a rigid 2-methyl-tetrahydrofuran glass at 77 K
provide additional insight into its excited state behavior. The emission and excitation spectra of
Ph-TETB at 77 K and at room temperature are shown in Figure 5.17. It is evident that the
emission of Ph-TETB has a strong temperature dependence. Compared to the broad, red-shifted
room temperature emission, the emission of Ph-TETB at 77 K exhibits vibronic structure, with
an intense ν
0-0
vibronic feature. The peak of the 77 K emission is blue-shifted by 42 nm
(0.18 eV) from the room temperature emission peak. The first vibronic peak of the excitation
spectrum of Ph-TETB at 77 K, on the other hand, is red-shifted by 25 nm (0.10 eV) from that at
room temperature. A similar red-shift in the excitation spectrum upon cooling has been observed
by Schmieder et al. for bis-ethynylanthracenylbezene, and has been ascribed to increased
propensity for planarity of the molecules at 77K.
43

Figure 5.16 The decays of Ph-TETB emission at 630 nm in a THF solution at room
temperature (black line) and in a 2-Methyl-THF solvent glass at 77 K (red line). The
instrument response function is shown as the grey line.
152

Interestingly, the intensity of the emission of Ph-TETB at 77 K is about ten times greater
than that at room temperature. Figure 5.18 provides a comparison of the emission intensities of
the same sample of Ph-TETB in a 2-MeTHF solution at 77 K and at room temperature. The peak
at ~630 nm of the 77 K spectrum is slightly broader than the peak at 587 nm, and qualitatively, it
appears as though the same broad spectrum which is dominant at room temperature is also
present in the 77 K emission, but at lower relative intensity; however, the vibronically resolved
spectrum is dominant at 77 K. It is likely that the emission at 77 K is representative of the
emission from the S
1
state of this molecule, as it looks spectrally similar to the emission of
Ph-o-BETB at room temperature. Assuming that to be true, the S
1
state of Ph-TETB is then
quenched more efficiently than that of Ph-o-BETB at room temperature. This raises two
questions: 1) why is the quenching of the S
1
more efficient in Ph-TETB than in Ph-o-BETB at
room temperature, and 2) from which state does the room temperature emission of Ph-TETB
emerge?

Figure 5.17 The normalized excitation (solid) and emission (dashed) spectra of Ph-TETB at
77 K (black) and at room temperature (red).
153

Based on the comparison of the emission spectra of Ph-TETB at 77 K and at room
temperature, and between Ph-TETB and Ph-o-BETB, it is possible to hypothesize two scenarios
for the origin of the broad, red-shifted room temperature emission in Ph-TETB: 1) the emission
at room temperature originates from the S
1
state potential energy surface, that has a minimum at
a molecular geometry which is significantly different from that of the ground state, presumably
more excimer-like. Such a relaxed geometry cannot be accommodated in a rigid solvent glass,
and the emission at 77 K could predominantly take place from a non-geometrically relaxed state.
2) The emission at room temperature could originate from an electronic state other than the S
1

state. The conversion to that state from the S
1
state would either require thermal activation
energy, or large amplitude molecular motions, and therefore molecules at 77 K cannot reach this
state and instead emit from the S
1
state.
If the former is correct, then similar emission would be expected for Ph-o-BETB, as the
chromophores should be able to structurally relax in a similar way in these two molecules.
However, no broad emission is observed for Ph-o-BETB in solution. This leaves option 2 as the
more plausible explanation. The remaining question is then: what is the nature of the electronic
a) b)

Figure 5.18 a) The emission spectra of the same sample of Ph-TETB in 2-MeTHF at 77 K
and at room temperature, without normalization. b) Photographs showing the emission of
Ph-TETB at 77 K (top) and at room temperature (bottom).
154

state which is emitting from Ph-TETB in solution? Although the answer to that question requires
additional experiments, the nature of the states which are populated in the excited states of these
molecules can be gleaned from transient absorption (TA) studies.
Prior to presenting the TA data, it is worth commenting on the chemical stability of the
phenylated tetramer. As mentioned in the synthesis section, photo-oxidation is the primary
degradation pathway for acene based materials. In the non-emissive/poorly-emissive tetracene
dimers, the evidence for photo-oxidation comes in the form of increased intensity of the
monomer-like emission. In strongly coupled tetracene dimers, SF is the dominant non-radiative
excited state decay channel. However, when one of the chromophores within the dimer has
undergone a Diels-Alder reaction with molecular oxygen, the remaining acene exhibits
monomer-like emission properties and becomes strongly emissive. As an example of such, the
emission of p-BETB-ohex (from Chapter 4) as a function of irradiation time with 480 nm light (3
nm slit widths) under ambient conditions is shown in Figure 5.19a. The rapid increase in the
structured emission indicates that the dimer is degrading. Contrary to p-BETB-ohex, Ph-TETB
does not degrade at such a rapid rate. The emission of Ph-TETB under similar conditions
(irradiation with 480 nm light with 3 nm slit width under ambient atmosphere) is shown in
Figure 5.19b. Even after 28 minutes, very few signs of degradation are evident in the emission
spectra. After ~35 minutes of irradiation, the intensity of the emission begins to slightly decrease
(by about 10 percent). These results indicate that the phenyl substitution at the 5 position of the
11-ethynylbenzene is effective at reducing the rate of photodegradation, thereby making these
molecules more chemically stable than their un-phenylated counterparts. Regarding the scope of
OPV applications of these molecules, it is expedient to reduce their chemical reactivity. The
155

phenyl substitution at the 5 position is a good route for doing so and should be incorporated in
future acene-based materials intended for OPV use.

5.2.4 Transient absorption of Ph-o-BETB and Ph-TETB
As mentioned in the previous chapters, transient absorption (TA) is an essential technique
for studying SF, particularly for covalent systems in solution. The femtosecond TA spectra of
Ph-o-BETB and Ph-TETB in THF after excitation with 500 nm light are shown in Figure 5.20a
and b, respectively. The TA spectra of Ph-o-BETB are very similar to those of o-BETB (see

Figure 5.19 The emission spectra of a) p-BETB-ohex and b) Ph-TETB upon exposure to 480
nm light under ambient conditions. The absorption spectra of c) o-BETB and d) Ph-TETB after
several days of exposure to room lights, under ambient conditions.
156

chapter 3). At short time delays between the pump and the probe pulses, a prominent S
1
→S
n

absorption feature is present at ~410 nm, along with a ground state bleach (GSB) in the region of
480-560 nm, and a low intensity absorption at ~625 nm. At increasing time delays, the S
1
→S
n

absorption peak at 410 nm decays, leaving behind a broad, low intensity induced absorption peak
in that region. The induced absorption at 570-630 nm increases with increasing time delays,
analogously to what was observed for o-BETB (chapter 3). This induced absorption feature to
the red of the GSB is representative of the
1
(T
1
T
1
) absorption, as explained in chapter 3.
The fs TA spectra of Ph-TETB (Figure 5.20b) are qualitatively similar to those of the
dimers. At short time delays a vague resemblance of an S
1
→S
n
absorption feature is present at
425 nm, and at increasing time delays broad, featureless induced absorption bands dominate the
spectrum. The S
1
→S
n
absorption feature at 420 nm in Ph-TETB at early times is less intense
than that in the TA of Ph-o-BETB, rather, the induced absorption features in the TA spectra of
Ph-TETB look fairly broad and exhibit low intensity. This possibly indicates that the S
1
state of
Ph-TETB is decaying faster than in the dimers. As early as 1 ps, the overall line-shape already
looks like that of the
1
(T
1
T
1
) absorption.
Another notable difference in the TA spectra of Ph-o-BETB and Ph-TETB is in the
dynamics of the GSB. In Ph-o-BETB, the GSB recovers relatively slowly (red trace in Figure
5.21a) compared to the decays of the regions of induced absorption (black and blue traces in
Figure 5.21a). In the Ph-TETB spectra, the GSB recovery is very fast at early times, but then
follows the decay rates of the induced absorption regions at 410 and 650 nm. If the initial S
1
is
indeed delocalized in the tetramer, then upon SF the two tetracenes would host the
1
(T
1
T
1
) state,
while the remaining two should return to the ground state. The rapid recovery of the GSB in
157

Ph-TETB at short time delays could be suggestive of the delocalization of the initial S
1
exciton
over four tetracenes in this system.
The comparison of the dynamics of the induced absorption bands in the TA spectra of
Ph-o-BETB and Ph-TETB reveals the differences in the excited state kinetics of these
compounds. In Ph-o-BETB, the S
1
→S
n
band (420 nm, black trace in Figure 5.21a) decays
rapidly, and the
1
(T
1
T
1
) absorption band at 625 nm (blue trace in Figure 5.21a) appears to rise on
the timescale of the S
1
→S
n
band decay at short time delays, followed by a slow (~600 ps) decay,
simultaneously with the 410 nm induced absorption band. In the Ph-TETB TA spectra, however,
the 410 and 650 nm bands start at similar intensities and decay with similar rates. Since we know
that the
1
(T
1
T
1
) state has similar absorption intensities in both the 410 and 650 nm regions, it is
likely that at short time delays the excited state population of Ph-TETB is already mostly in the
1
(T
1
T
1
) state.

Figure 5.20 Transient absorption spectra of Ph-o-BETB and Ph-TETB excited at 500 nm.
158

5.2.5 Kinetic model and rates of singlet fission in Ph-o-BETB and Ph-TETB
To generate a better idea of the excited state kinetics in these systems, the TA data was
deconvoluted by target analysis using a two-state sequential model. The basis spectra obtained
from fitting the TA data of Ph-o-BETB and Ph-TETB using this model are shown in
Figure 5.22a and b, respectively, and the populations of the corresponding spectra are shown in
Figure 5.22c. The spectra corresponding to the Ph-o-BETB and Ph-TETB S
1
and
1
(T
1
T
1
)
absorption look qualitatively similar. State 1 in Ph-TETB, representative of the S
1
state
absorption is lacking the sharp peak at 410 nm, likely because a significant population of S
1
has
decayed within the instrument response (200 fs) and target analysis does not have a clean S
1
→S
n

spectrum from which to start. Therefore, the SF rate for Ph-TETB obtained with this fit is a
lower bound.
a) b)
Figure 5.21 The decay of the TA signal of a) Ph-o-BETB and b) Ph-TETB at various
wavelengths.
159

Using the two-state model, we obtain a lifetime of 3.76 ps for the formation of the
1
(T
1
T
1
)
state in Ph-o-BETB, which is almost a factor of 2 slower than that in unphenylated o-BETB. This
could be indicative of slightly less favorable S
1
/T
1
energetics resulting from the phenyl
substitution.
The rate of formation of the
1
(T
1
T
1
) state in the tetramer, Ph-TETB, however, is
approximately ten times greater than that in Ph-o-BETB. Target analysis predicts the
1
(T
1
T
1
) to

Figure 5.22 The decay associated spectra obtained by target analysis of the TA data of a)
PhoBETB and b) PhTETB. c) The populations of the S
1
and the
1
(T
1
T
1
) states of PhoBETB
and PhTETB obtained from target analysis.
160

form within 0.3 ps in the tetramer. This value comes close to the triplet formation in the neat film
of o-BETB (0.2 ps).
The rates of the decay of the
1
(T
1
T
1
) state in both, the dimer Ph-o-BETB and the tetramer
Ph-TETB are identical, as shown in Figure 5.21c. The target analysis extracts a value of 668 ps
for the lifetime of the
1
(T
1
T
1
) state. As a result, the yield of
1
(T
1
T
1
) in Ph-TETB appears to be
greater- unfortunately, the extinction spectra for this state cannot be measured, which leaves the
yield claims qualitative, at best.

5.3 Conclusions
The tetra-(5-phenyl-11-ethynyltetracenyl)benzene (Ph-TETB) is the pinnacle of the
covalently-linked tetracene evolution. The phenyl substitution at the 5 position proved to be
effective at protecting the acenes in this molecule from photo-oxidation. The steady state
absorption spectra of Ph-TETB in comparison to the dimer, Ph-o-BETB, point to the
delocalization of the S
1
state over all four of the tetracenes within Ph-TETB.
The electronic structure of Ph-TETB influences its excited state decay. The TA spectra of
this compound in solution show very rapid recovery of the GSB, and an ultrafast formation of
the
1
(T
1
T
1
) state (τ = 0.3 ps). The lifetime of the formation of the
1
(T
1
T
1
) state in the tetramer is
comparable to that in the thin film of the dimer, o-BETB.
Lastly, Ph-TETB exhibits interesting emission properties in solution and in a rigid solvent
glass. The room temperature solution spectrum of Ph-TETB is broad, and significantly
Table 5.2 The lifetimes of the states used to fit the TA data of Ph-o-BETB and Ph-TETB.

τ [S
1

1
(TT)] τ [
1
(TT) ]
Ph-o-BETB 3.76 ps 668 ps
Ph-TETB 0.303 ps 668 ps

161

red-shifted from the absorption. Interestingly, the emission of this state mostly decays with a
lifetime of 0.6 ns, close to the value determined for the decay of the
1
(T
1
T
1
) state via transient
absorption (0.67 ns). Additional experiments to decipher the origin of this emission in Ph-TETB
are underway.
Given the chemical stability and rapid SF in Ph-TETB, it could prove to be a superior donor
material for OPV devices. The thin film properties of this material require characterization in
order to determine its viability as a solar cell material and its OPV properties will be explored in
the future. Furthermore, we hope to inspire the development of analogous materials with
conscientiously prescribed chemical substitutions for increased stability of the acenes.
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171

Appendix 1: Intramolecular charge transfer in an asymmetric tetracene dimer
A1.1 Introduction
In the first SF review,
1
Smith and Michl pointed out that there are two possible mechanisms
for singlet fission: 1) direct mechanism, and 2) charge transfer mediated mechanism. In the direct
SF mechanism, the S
1
state relaxes directly to the
1
(T
1
T
1
) state without the population of any
intermediate states (red arrow in Figure A1.1). In the mediated mechanism, the population of the
S
1
state first transfers to a charge transfer state in which the neighboring chromophore is
involved (teal arrows in Figure A1.1). The resultant CT state then relaxes to the
1
(T
1
T
1
) state, by
one electron transfer (blue arrows in Figure A1.1).

Although Figure A1.1 shows the mediated SF mechanism as proceeding via actual
population of the CT states, the CT states can be important for SF through configuration
interaction with the S
1
or the
1
(T
1
T
1
) states.
2
Several researchers have shown that the one
electron coupling matrix elements between the S
1
and the CT wave functions can be used as a
proxy to evaluate the coupling, and therefore the rate, for SF.
3-5
Whereas, Kolomeisky et al. have
emphasized the importance of configuration interaction of the
1
(T
1
T
1
) state with the CT
configurations for enhancing the rate of SF in acene systems.
6

Figure A1.1 Direct (red arrow) and charge transfer mediated (teal and blue arrows)
mechanisms of singlet fission; figure reproduced from ref. 1.
172

The attempts to determine the mechanistic pathway of singlet fission have produced
conflicting results. Using ab initio methods Zimmerman et al. have evaluated the
) (
ˆ
1 1
1
0 1
T T H S S
el
matrix element, and concluded that CT states are not very relevant for SF,
because they admixed very weakly into the locally excited states.
7
Havenith et al. on the other
hand, posited that the CT states were important for SF, based on their separate evaluation of
direct and indirect contributions.
8

The recent studies of weakly coupled covalently linked dimers have also demonstrated the
involvement of CT states in SF kinetics. Johnson et al. observed an increase in the T
1
yield in
linearly-linked diphenylisobenzofuran dimers in polar solvents.
9
Zirzlmeier et al. reported an
enhancement in SF rate and yield for weakly coupled TIPS-pentacene dimers in more polar
solvents.
10, 11
In slip-stacked weakly coupled terrylene dimer, however, Margulies et al. have
shown CT process to be out-competing and preventing SF in polar solvents, and SF proceeding
rapidly without population of the CT states in non-polar solvents.
In Chapter 4 it was shown that the SF kinetics in the strongly coupled o-BETB dimer were
not dependent on the solvent polarity, suggesting the irrelevance of CT states for SF in this
system. In the weakly coupled m-BETB dimer, the very poor SF yield/rate was slightly enhanced
in polar solvents, suggesting the importance of CT states in SF kinetics in weakly coupled bi-
chromophore systems.
If energetically low lying CT states enhance SF in weakly coupled systems, then it could be
beneficial to enhance the admixture of such states into the S
1
and
1
(T
1
T
1
) wave functions of such
systems by lowering the energies of the CT states. A facile way of tuning the energies of the CT
states in a covalently linked dimer is by tuning the dielectric of the solvent medium. In the case
of the weakly coupled m-BETB dimer, the increase in the SF yield in the most polar solvent
173

(DMF, ε = 37) was only marginal compared to the non-polar solvent (toluene, ε = 2). Another
way of lowering the energy of the CT states is by desymmetrizing the chromophores within the
dimer.
The energy of the CT state is given by Equation A1.1,

solv IP EA CT
E C E E E      (A1.1)
where E
IP
is the ionization potential of the electron donating chromophore, E
EA
is the
electron affinity of the electron accepting chromophore, ΔE
solv
is the change in energy upon
solvation and is greater for more polar solvents, the constant C is the Coulomb attraction of the
resulting anion and cation, and is inversely proportional to the distance between the
chromophores on which the charges are localized.
12

In terms of designing chromophore dimers with lower E
CT
the pair of chromophores can be
chosen such that the difference between the E
EA
and E
IP
is small. To achieve this with tetracenes,
a combination of variously substituted tetracenes can be used. As shown in Chapter 2, the alkyne
substitution on the tetracene lowers the LUMO (and therefore the E
EA
) of the acene.

Figure A1.2 Graphical representation of the relative HOMO and LUMO levels of
chromophores in dimers comprised of a) identical chromophores (symmetric dimer), and b)
electronically different chromophores (asymmetric dimer).
174

In this appendix the photophysical properties of an asymmetric tetracene dimer
bis-teracenyleneyne (BTEY) are presented and the implications for SF discussed. The two
chromophores in this dimer are tetracene and ethynyltetracene, as shown in Figure A1.3a. The
two chromophores are oriented perpendicular to each other, as shown in the 3D structure in
Figure A1.3b. Although the relative E
EA
and E
IP
in this dimer may be similar to that of m-BETB
(as the HOMO energies, and therefore the E
IP
, are not very different between tetracene and
ethynyltetracene), the two chromophores are closer to each other in BTEY than in m-BETB. The
closer arrangement of the chromophores should result in larger C, and therefore lower E
CT.

A1.2 Results and Discussion
A1.2.1 Synthesis and
1
H NMR of BTEY
BTEY was obtained as a byproduct in the synthesis of BETX in which an excess of the
ethynyltetracene was used (Scheme I), and was not synthesized intentionally. The
homo-coupling of terminal arylalkynes to produce ene-ynes has been reported to proceed with
varying efficiencies using different catalysts. A ruthenium-based catalyst has been reported to
give 5% yield of head-to-tail ene-yne (one in which the aryl and ethynylaryl groups are on the
same side of the double bond) by Ozturk et al.
13
Perrone and coworkers reported an 8% yield of
the head-to-tail ene-yne using Pd(OAc)
2
with PPh
3
as the catalyst.
14
Yields greater than 90%
a) b)
Figure A1.3 The a) chemical and b) 3-dimensional structures of BTEY.
175

were reported by Beatrice et al. using a thallium-based catalyst,
15
by Komeyama et al. using a
silanamide yittrium(III) salt,
16
and by Dash et al. using methylaluminoxane.
17
Furthermore,
Zatolochnaya et al. have published a comprehensive study of the palladium-catalyzed reactions
of terminal alkynes and proposed a mechanism for the head-to-tail ene-yne formation,
reproduced in Figure A1.4.

Scheme I: The synthetic scheme for the synthesis of BETX (from chapter 3), in which
BTEY is the byproduct.

176

The
1
H NMR spectrum of BTEY is shown in Figure A1.5. The two characteristic terminal
alkene protons are labeled k and l in the spectrum.

Figure A1.4 A mechanism of palladium-catalyzed eneyne formation proposed by
Zatolochnaya et al.
177

A1.2.2 Photophysical properties of BTEY as a function of solvent polarity
The steady state photophysical properties can provide information about whether CT states
are involved in the excited state decay of a given molecule. The steady state absorption and
emission spectra of BTEY in five solvents (toluene, THF, CH
2
Cl
2
, acetone, and DMF) are shown
in Figure A1.6. In the absorption spectra it is evident that the first vibronic feature is lower than
the second vibronic feature. The first vibronic feature is located at 513 nm (which is typical for
monomeric or weakly coupled ethynyltetracenes), while the second vibronic peak is at 486 nm,
very close to the λ
max
of tetracene. Therefore, it may be that the resulting vibronic structure of the
absorption spectrum could be due to a linear combination of the uncoupled absorption spectra of
the ethynyltetracene and tetracene within the BTEY dimer. The emission spectra in non-polar

Figure A1.5 The
1
H NMR spectrum of BTEY.
178

solvents appear to be stemming from the lower energy chromophore – ethynyltetracene, with a
maximum at 527 nm.

The absorption spectra do not appear to be dependent on the polarity of the solvent, as the
peak wavelengths and relative intensities do not alter in different solvents. The emission spectra
of BTEY, on the other hand, are dependent on the solvent polarity. In solvents of increasing
polarity, the intensity of emission beyond 530 nm increases with increasing solvent polarity,
however the λ
max
does not change up to acetone. In DMF, however, the emission spectrum is
much broader in the region beyond 530 nm, but is also accompanied by a red-shift of the λ
max

from 527 nm to 533 nm. The independence of the absorption spectra and the red-shift and
broadening of the emission spectra in more polar solvents indicate that the CT states are
important in the excited state dynamics of BTEY.
The lifetime of the emission of BTEY is also solvent polarity dependent. A plot showing
the decay of the 560 nm emission of BTEY in the five aforementioned solvents is provided in
Figure A1.7a. The emission decay could only be fit with four exponentials, and these values are

Figure A1.6 The steady state a) absorption and b) emission spectra of BTEY in five
different solvents: toluene (black), THF (red), CH
2
Cl
2
(blue), acetone (pink), and DMF
(green).
179

given in Table A1.1, along with the emission quantum yield values. It is evident that the
quantum yield of emission is generally lower in more polar solvents. The lifetimes of the
emission, however, do not exhibit such a clear cut trend.

From the steady state emission spectra, we know that BTEY produces two overlapping
emission spectra, whose relative contributions are dependent on solvent polarity. The relative
contribution of the 25-29 ns lifetime increases with increasing solvent polarity, which could

Figure A1.7 a)The decay of the 560 nm emission of BTEY in five different solvents: toluene
(black), THF (red), CH
2
Cl
2
(blue), acetone (pink), and DMF (green). b) The comparison of
the 560 nm emission of BTEY in toluene and acetone, and c) the comparison of the 560 nm
emission of BTEY in toluene and DMF.
180

suggest that this is the lifetime of the CT state emission. The other lifetimes do not follow a trend
with increasing solvent polarity. The most qualitatively different decays are those in acetone
(Figure A1.7b) and in DMF (Figure A1.7c). In acetone, the major component of the emission
decay is slower than in other solvents, however, in DMF, majority of the emission decays faster
than in other solvents, but the relative amplitude of the delayed emission is larger. The emission
decay in DMF can be ascribed to increased interaction with CT states, however, the explanation
of the emission decay is acetone is not obvious. It is possible that SF takes place in this dimer,
but additional experiments are necessary to confirm this. Specifically, magnetic field dependence
of the emission decay could be helpful in resolving the excited state dynamics in this dimer.

The steady state photophysical properties of BTEY at 77 K are not drastically different
from those at RT. The emission spectrum of BTEY at 77 K is red-shifted by 4 nm, and the
excitation spectrum at 77 K is red-shifted by 6 nm from the absorption spectrum at RT. It is not
immediately obvious why the spectra red-shift at 77 K, but one possibility could be increased
planarity at 77 K. The dihedral angle between the alkene group and the ethynyltetracene could be
smaller in the rigid solvent glass at 77 K.
Table A1.1 The quantum yields of emission of BTEY in toluene, THF, CH
2
Cl
2
, acetone,
and DMF; the lifetimes of the 560 nm emission of BTEY in the above solvents, and the
values of the dielectric constants of these solvents.
ε Φ
fl
τ
1
τ
2
τ
3
τ
4

Toluene 2.4 0.47
2.8 ns
(0.047)
10.3 ns
(0.698)
27.9 ns
(0.003)
5.8 ns
(0.252)
THF 7.6 0.48
2.3 ns
(0.034)
10.8 ns
(0.694)
27.3 ns
(0.004)
5.4 ns
(0.268)
CH
2
Cl
2
8.9 0.33
2.0 ns
(0.008)
9.7 ns
(0.798)
25.5 ns
(0.006)
4.7 ns
(0.188)
Acetone 20.7 0.25
1.7 ns
(0.009)
11.8 ns
(0.760)
25.1 ns
(0.006)
5.7 ns
(0.225)
DMF 36.7 0.08
2.5 ns
(0.034)
12.8 ns
(0.159)
29.2 ns
(0.009)
7.03 ns
(0.797)

181

A1.2.3 Transient absorption of BTEY
The transient absorption spectra of BTEY in CHCl
3
upon excitation with 490 nm pump
beam at room temperature are shown in Figure A1.9. With increasing time delays between the
pump and the probe beams, the S
1
→S
n
peak at 410 nm decreases, but the ground state bleach
(GSB) at 516 nm becomes slightly more negative. Additionally, the broad peak in the
575-675 nm region becomes slightly narrower. The varying intensity at 490 nm is due to scatter.
The decay of the S
1
→S
n
peak and simultaneous growth of the GSB could be suggestive of
charge transfer from tetracene to ethynyltetracene or energy transfer from tetracene to
ethynyltetracene. The charge transfer scenario is more likely, as the ethynyltetracene also has the
S
1
→S
n
peak at 410 nm, so energy transfer from tetracene to ethynyltetracene should not result in
a significant decrease of the S
1
→S
n
peak.

Figure A1.8 The comparison of the absorption of BTEY at room temperature (solid black),
excitation at 77K (solid red), emission at room temperature (dashed black) and emission at
77 K (dashed red).
182

In the more rigid PMMA media (Figure A1.10a), at room temperature the decay of the
S
1
→S
n
peak is faster and is accompanied by a recovery of the GSB. With increasing time delays,
the GSB also exhibits a red-shift. The broad peak in the 575-675 nm region becomes more
structured at 460 ps. The TA spectra of BTEY in PMMA at 77 K (Figure A1.10b) indicate that
the excited state decays even faster than in PMMA at room temperature. The S
1
→S
n
peak decays
more rapidly than at room temperature, and the GSB recovers equally more rapidly. Typically,
increase in the rigidity of the solvent and decrease in the temperature slow down the
non-radiative decay pathways and increase the overall lifetime of the excited state. The reverse
trend in BTEY is interesting and begs for further investigation of this system.

Figure A1.9 The femtosecond transient absorption spectra of BTEY in CHCl
3
after the
excitation with 490 nm pump pulse, with the steady state absorption spectrum overlaid for
comparison (dashed black line).

183

A1.3 Conclusions
This dimer appears to be undergoing intra-molecular CT in polar solvents, but it is unclear
if SF is occurs in this dimer. The emission decays are multi-exponential with the longest
component of 25 ns. This is much shorter than the delayed fluorescence observed in m-BETB. It
could be that since only one of the chromophores is an ethynyltetracene while the other is just a
tetracene, that the energy of the
1
(T
1
T
1
) state is higher than it would have been for two ET
chromophores.
The TA spectra indicate subtle changes in the excited state absorption during the excited
state decay of BTEY. In solution, the decay of the S
1
→S
n
peak and increase in the GSB at
513 nm could suggest that CT from tetracene to ethynyltetracene is taking place in the excited
state. In rigid media (PMMA), the decay of the S
1
→S
n
peak is faster and is accompanied by GSB
recovery, and the decay becomes even faster when the PMMA film of BTEY is cooled to 77 K.
It could be that in BTEY in polar solvents, the decay via CT competes with SF, and in non-
polar solvents the
1
(T
1
T
1
) state is energetically inaccessible and the mediate coupling via CT is
not possible. In that case, when designing asymmetric dimers for SF, the endothermicity for SF

Figure A1.10 The transient absorption spectra of BTEY in PMMA a) at room temperature,
and b) at 77 K, after excitation with at 490 nm.
184

and the driving force for CT should not be too large. It is possible that the CT states can only
help if the S
1
and
1
(T
1
T
1
) are close in energy. In light of that, several asymmetric structures are
proposed below (Figure A1.11); in these structures both of the chromophores are
ethynyltetracenes, therefore they should have comparable T
1
energies.

A1.4 References
1. Smith, M. B.; Michl, J., Chem. Rev. 2010, 110, 6891.
2. Smith, M. B.; Michl, J., Annu. Rev. Phys. Chem. 2013, 64, 361.
3. Akdag, A.; Havlas, Z. k.; Michl, J., Journal of the American Chemical Society 2012, 134
(35), 14624-14631.
4. Havlas, Z.; Michl, J., Israel Journal of Chemistry 2016, 56 (1), 96-106.
5. Vallett, P. J.; Snyder, J. L.; Damrauer, N. H., The Journal of Physical Chemistry A 2013, 117
(42), 10824-10838.
6. Kolomeisky, A. B.; Feng, X.; Krylov, A. I., The Journal of Physical Chemistry C 2014, 118
(10), 5188-5195.
7. Zimmerman, P. M.; Bell, F.; Casanova, D.; Head-Gordon, M., J. Am. Chem. Soc. 2011, 133,
19944.
8. Havenith, R. W. A.; de Grier, H. D.; Broer, R., Mol. Phys. 2012, 110, 2445.
9. Johnson, J. C.; Akdag, A.; Zamadar, M.; Chen, X.; Schwerin, A. F.; Paci, I.; Smith, M. B.;
Havlas, Z.; Miller, J. R.; Ratner, M. A., J. Phys. Chem. B 2013, 117, 4680.
10. Zirzlmeier, J.; Casillas, R.; Reddy, S. R.; Coto, P. B.; Lehnherr, D.; Chernick, E. T.;
Papadopoulos, I.; Thoss, M.; Tykwinski, R. R.; Guldi, D. M., Nanoscale 2016, 8 (19),
10113-10123.

Figure A 1.11 Proposed structures for asymmetrical weakly coupled ethynyltetracene dimers
in which the energy of the charge transfer states can be tuned by a substituent on one of the
acenes.
185

11. Zirzlmeier, J.; Lehnherr, D.; Coto, P. B.; Chernick, E. T.; Casillas, R.; Basel, B. S.; Thoss,
M.; Tykwinski, R. R.; Guldi, D. M., Proceedings of the National Academy of Sciences 2015,
112 (17), 5325-5330.
12. Grabowski, Z. R.; Rotkiewicz, K.; Rettig, W., Chemical Reviews 2003, 103 (10), 3899-4032.
13. Öztürk, B. Ö.; Karabulut, S.; İmamoğlu, ., Applied Catalysis A: General 2012, 433–434,
214-222.
14. Perrone, S.; Bona, F.; Troisi, L., Tetrahedron 2011, 67 (38), 7386-7391.
15. Batrice, R. J.; McKinven, J.; Arnold, P. L.; Eisen, M. S., Organometallics 2015, 34 (16),
4039-4050.
16. Komeyama, K.; Kawabata, T.; Takehira, K.; Takaki, K., The Journal of Organic Chemistry
2005, 70 (18), 7260-7266.
17. Dash, A. K.; Eisen, M. S., Organic Letters 2000, 2 (6), 737-740.

186

Appendix 2. Experimental details
A2.1 Instrumentation and general information
A2.1.1 NMR and mass spec.:
1
H and
13
C NMR spectra were collected in CDCl
3
using
CHCl
3
peak (7.26 ppm in
1
H and 77.16 ppm in
13
C) as an internal standard on Varian 400-MR 2-
Channel NMR Spectrometer. The molecular weights were obtained with Voyager DE-STR
Maldi MS.
A2.1.2 UV-Vis, fluorometer and TCSPC: The UV–visible spectra were recorded on a
Hewlett-Packard 4853 diode array spectrophotometer. The steady-state emission at room
temperature and 77 K was measured with Photon Technology International QuantaMaster QM-
400 spectrofluorometer. Fluorescence lifetimes at room temperature and 77 K were determined
by the time-correlated single-photon counting technique (TCSPC), using IBH Fluorocube with a
405 nm LED excitation source, and an IRF value of 0.4 ns. Quantum efficiency measurements
were carried out using a Hamamatsu C9920 system equipped with a xenon lamp, calibrated
integrating sphere, and model C10027 photonic multichannel analyzer. The error in the emission
lifetime measurements is ±5% and for the quantum yields is ±10%.
A2.1.3 Cyclic voltammetry (CV) and differential pulse voltammetry (DPV): were
performed using an EG&G Potentiostat/Galvanostat model 283. Freshly distilled CH
2
Cl
2
(VWR)
was used as the solvent under inert atmosphere with 0.1 M tetra(n-butyl)ammonium
hexafluorophosphate (Aldrich) as the supporting electrolyte. A glassy carbon rod, a platinum
wire, and a silver wire were used as the working electrode, counter electrode, and
pseudoreference electrode, respectively. Electrochemical reversibility was established using CV,
while all redox potentials were determined using DPV and are reported relative to a
ferrocenium/ferrocene (Fc
+
/Fc) redox couple used as an internal standard. A scan rate of 100
mV/s was used for all measurements.
187

A2.1.4 Femtosecond Transient Absorption: The apparatus has been described previously.
1

In brief, pump and probe pulses were derived from the output of a Ti:sapphire regenerative
amplifier (Coherent Legend, 1kHz, 4 mJ, 35 fs). Excitation pulses centered at specified
wavelengths (between 500 and 556 nm) were generated using a type-II OPA (Spectra Physics
OPA-800C). White light supercontinuum probe pulses, spanning the visible (320-950 nm) were
obtained by focusing a small amount of the amplifier output into a rotating CaF
2
disk. To avoid
any orientational contribution to the observed dynamics,
2
the polarization of the supercontinuum
probe was set at the magic angle (54.7º) with respect to the pump polarization. The probe was
collimated and focused with a pair of off-axis parabolic mirrors into the sample whereas the
pump pulse was focused using a CaF
2
lens. The cross correlation between the pump and probe in
a quartz substrate matched to that used to support films had a FWHM of 150 fs averaged across
the probe spectrum for 500 nm excitation. A slightly longer instrument response of 170 fs was
found for 550 nm excitation. The instrument response for the solution measurements is slightly
higher, at 180 fs. The supercontinuum probe was dispersed using a spectrograph (Oriel MS127I)
onto a 256-pixel silicon diode array (Hamamatsu). Spectra were measured for a range of
excitation fluences from 12 to 96 μJ/cm
2
for ET-TMS thin film and 8 to 64 μJ/cm
2
for BET-B
thin film. The TA data shown in Figure 5 are reported at 24 μJ/cm
2
and 16 μJ/cm
2
for ET-TMS
and BET-B, respectively. The solution measurements were performed with a pump fluence of
~25 μJ/cm
2
. Samples were slowly translated perpendicular to the path of the pump and probe
using a linear stage to prevent photodamage.
A2.2 Sample preparation for optical experiments
All steady state solution measurements were performed in 1 cm quartz cuvettes. The
solutions were prepared such that the optical density in the visible region was approximately 0.1.
188

All solutions were sparged with nitrogen gas for 5 minutes and sealed prior to collecting
emission, lifetime and quantum yield measurements. All transient absorption solution
measurements were performed in 1 mm quartz cuvettes. De-aerated solutions with an optical
density of 0.2-0.4 were loaded into the 1 mm cuvettes and sealed inside of the glove box with a
nitrogen atmosphere. All thin film samples were made on quartz substrates. The neat films were
prepared by dissolving 0.005 g of acene in 0.4 mL of freshly distilled THF, filtering the resulting
solutions through a 0.22 μm syringe filter, and spin casting the solutions onto quartz substrates at
3500 rpm. The thickness of the films were between 60 nm and 100 nm, with an optical density
of 0.3 to 0.2 at 500 nm.
The PMMA samples were prepared by dissolving 0.001 or fewer g of acene and ≥0.100 g
of PMMA in 0.8 mL of toluene inside of the N
2
filled glove box. The vials containing the toluene
suspensions were sealed tightly and wrapped with parafilm inside of the glove box. The
suspensions were then sonicated for more than an hour. The resultant viscous solutions were
filtered through a 0.22 μm syringe filter and spun cast onto quartz substrates at 700 rpm under
yellow lights or under an inert atmosphere.
The triplet sensitization film samples were prepared by spin casting a 0.4 mL THF solution
of 0.0053 g of acene and 0.0007 g of Palladium tetraphenyltetrabenzoporphyrin (Pd(TPBP)) at
3500 rpm. The doped films were prepared by the same spin casting conditions, with the DPA
film doped with o-BETB 25.7 vol%, and the DPT film doped with o-BETB at 16.8 vol%. All
thin films were protected from photo-oxidation by placing a quartz window on top and sealing
with epoxy along the edges, inside the glove box under a nitrogen atmosphere. Emission
quantum yield measurements made inside of an integrating sphere using un-sealed thin films,
under a stream of nitrogen gas.
189

A2.3 Synthetic details for all compounds presented in this thesis
Tetracene was prepared from 5,12-Naphthacenequinone using a method analogous to the
one reported in the literature.
3
The 5,12-Naphthacenequinone was purchased from TCI
Chemicals, N-Bromosuccinimide, 4,5-Dibromo-2,7-di-tert-butyl-9,9-dimethylxanthene, and 1,2-
Diiodobenzene were purchased from Sigma Aldrich. All starting materials were used as
received without additional purification.
5-bromotetracene: was prepared from tetracene using a modified procedure from those
described in the literature.
4, 5
Tetracene (1.00 g, 4.38 mmol) was suspended in 250 mL of
freshly distilled CH
2
Cl
2
under nitrogen. The resulting suspension was sonicated for 30 minutes.
An additional funnel containing a solution of 0.86 g (8.82 mmol) of N-Bromosuccinimide (NBS)
in 15 mL of dimethylformamide and 70 mL of freshly distilled CH
2
Cl
2
under nitrogen was
installed onto the flask containing the tetracene suspension. The flask was heated to 50
○
C, and
the solution of NBS was added slowly, over a period of 5 hours. The resulting reaction mixture
was allowed to stir overnight at 50
○
C, in the dark, under nitrogen. The reaction mixture was
then cooled to room temperature, and CH
2
Cl
2
was removed by rotary evaporation in vacuo.
30 mL of methanol was added to the remaining slurry, and cooled to 0
○
C. 5 mL of water was
added dropwise and the solids were collected by vacuum filtration. The resulting solids were
rinsed with cold water and methanol. Red solids. Yield: 88 %.
5,11-dibromotetracene: same reaction procedure as above, but twice the equivalents of
NBS. Purification entailed flash chromatography on silica gel using hexanes and CH
2
Cl
2
,
followed by recrystallization from toluene. Pinkish-red solid. Yield: ~50 %.
5-phenyl-11-bromotetracene: 5,11-dibromotetracene (0.135 g, 0.35 mmol), phenylboronic
acid (0.038 g, 0.31 mmol), Cs
2
CO
3
(0.507 g, 1.55 mmol) were loaded into a Schlenk flask
190

equipped with a stirring bar. Air free toluene (7.0 mL), THF (6.0 mL), ethanol (1.5 mL), and
water (0.5 mL) were added to the Schlenk flask. The mixture was sparged with N
2
for 20
minutes. The mixture was heated to 70
o
C under N
2
atmosphere until the dibromotetracene
dissolved. The catalyst, Pd(PPh
3
)
4
(0.036 g, 0.03 mmol) was added to the reaction flask against
the positive flow of N
2
and the reaction mixture was stirred at 70
o
C, under N
2
, in the dark
overnight. Once cooled to room temperature, poured into water and extracted with CH
2
Cl
2
. The
combined organic layer was dried with Na
2
SO
4
and the solvent was removed in vacuo. The
product was purified by flash chromatography on silica gel using hexanes as the eluent. The
product was then recrystallized from CH
2
Cl
2
layered with hexanes. Red prisms. Yield: 58 %.
1
H
NMR (400 MHz, CDCl
3
, 25
○
C): δ = 9.28 (s, 1H), 8.49 (dd, J
1
= 9.0 Hz, J
2
= 1 Hz, 1H),
8.32 (s, 1H), 8.14 (d, J = 9.0 Hz, 1H), 7.79 (d, J = 9.0 Hz, 1H), 7.65-7.60 (m, 4H),
7.54-7.45 (m, 4H), 7.36-7.30 (m, 2H) ppm. MALDI-TOF: m/z calculated: 382.04 [M]+; found:
382.21 [M]+
TMS-ethynyltetracene (ET-TMS). A Schlenk flask equipped with a stirring bar was
loaded with 5-Bromotetracene (1.05 g, 3.42 mmol), CuI (0.18 mmol) and Pd(PPh
3
)
4
(0.14 mmol)
under a nitrogen atmosphere, and dissolved with 30 mL of freshly distilled THF. The resulting
solution was sparged with nitrogen for 10 minutes. In a separate Schlenk flask, a solution of
TMS-acetylene (0.90 mL, 6.37 mmol) in 25 mL of triethylamine was sparged with nitrogen for
15 minutes. The flask containing 5-Bromotetracene was heated to 60
○
C, and the solution of
TMS-acetylene was added dropwise. The reaction mixture was stirred at 60
○
C, under an inert
atmosphere, in the dark, overnight. Upon cooling to room temperature, the reaction mixture was
washed with a saturated solution of NH
4
Cl, and extracted with CH
2
Cl
2
. The combined organic
layers were dried with MgSO
4
and the solvent was evaporated in vacuo. The product was
191

purified by flash chromatography on silica gel using hexanes. Orange needles. Yield: 90 %.
1
H
NMR (400 MHz, CDCl
3
, 25
○
C): δ = 9.17 (s, 1H), 8.65 (s, 2H), 8.55 (d, J=8.87 Hz, 1H), 8.10-
8.06 (m, 1H), 8.03-7.97 (m, 2H), 7.55-7.50 (m, 1H), 7.47-7.40 (m, 3H), 0.49 (s, 9H) ppm.
13
C
NMR (100 MHz, CDCl
3
, 25
○
C): 133.4, 132.3, 131.8, 131.0, 130.9, 129.7, 129.0, 128.9, 128.3,
128.2, 127.2, 127.0, 126.8, 125.8, 125.7, 125.6, 125.5, 117.0, 107.4, 102.3, 0.49 ppm. MALDI-
TOF: m/z calculated: 324.13 [M]+; found: 324.1 [M]+ Elemental analysis (%), calculated for
C
23
H
20
Si: C 85.13, H 6.21; found: C 85.35, H 6.26.
Phenylethynyltetracene (PET): A Schlenk flask equipped with a stirring bar was loaded
with 5-Bromotetracene (0.450 g, 1.5 mmol), CuI (0.018 g, 0.09 mmol) and Pd(PPh
3
)
4
(0.080 g,
0.07 mmol) under a nitrogen atmosphere, and dissolved with 13 mL of freshly distilled THF.
The resulting solution was sparged with nitrogen for 10 minutes. In a separate Schlenk flask, a
solution of ethynylbenzene (0.2 mL, 1.8 mmol) in 10 mL of triethylamine was sparged with
nitrogen for 15 minutes. The flask containing 5-Bromotetracene was heated to 60
○
C, and the
solution of TMS-acetylene was added dropwise. The reaction mixture was stirred at 60
○
C, under
an inert atmosphere, in the dark, overnight. Upon cooling to room temperature, the reaction
mixture was washed with a saturated solution of NH
4
Cl, and extracted with CH
2
Cl
2
. The
combined organic layers were dried with Na
2
SO
4
and the solvent was evaporated in vacuo. The
product was purified by flash chromatography on silica gel using hexanes. Red fibres. Yield:
56 %.
1
H NMR (400 MHz, CDCl
3
, 25
○
C): δ = 9.27 (s, 1H), 8.69 (s, 2H), 8.65 (d, J = 8.8 Hz,
1H), 8.12-8.10 (m, 1H), 8.02 (d, J = 8.8 Hz, 2H), 7.86-7.84 (m, 2H), 7.56-7.42 (m, 7H) ppm.
ppm. MALDI-TOF: m/z calculated: 328.13 [M]+; found: 328.06 [M]+.
5-Ethynyltetracene (ET). A Schlenk flask equipped with a stirring bar was loaded with
ET-TMS (0.128 g, 0.394 mmol), and K
2
CO
3
(0.200 g, 1.45 mmol) under an inert atmosphere.
192

THF (3.8 mL) and methanol (2.4 mL) were added to the flask, and the resulting suspension was
sparged with nitrogen for 15 minutes. The reaction mixture was allowed to stir under an inert
atmosphere, in the dark, at room temperature for two hours. The solvent was removed in vacuo,
and the product was passed through a silica plug using CH
2
Cl
2
as an eluent. The solvent was
removed in vacuo, and the product was transferred to Schlenk flask under an inert atmosphere
and was used completely and immediately in the next reaction. Orange solids. Yield: 99 %.
1
H
NMR (400 MHz, CDCl
3
, 25
○
C): δ = 9.21 (s, 1H), 8.69 (s, 1H), 8.68 (s, 1H), 8.57 (d, J= Hz, 1H),
8.09 (m, 1H), 8.01 (m, 2H), 7.53 (m, 1H), 7.55 (m, 3H), 4.14 (s, 1H) ppm. The compound was
unstable under ambient conditions, and further characterization of it was not possible.
General procedure for a Sonogashira coupling of ET with dihalides: Dihalide (0.001
mol) was placed into a Schlenk flask with CuI (0.2 mmol) and Pd(PPh
3
)
4
(0.15 mmol) under a
nitrogen atmosphere, and dissolved with 30 mL of freshly distilled THF. The resulting solution
was sparged with nitrogen for 15 minutes. In another flask a solution of the ethynyl-tetracene
(0.0024 mol) in 15 mL of THF and 25 mL of Et
3
N was sparged with nitrogen for 20 minutes.
The flask containing the dihalide was heated to 60
○
C, and the solution of ethynyl-tetracene was
added dropwise over a period of four hours. The reaction mixture was stirred in the dark for 24
hours under an atmosphere of nitrogen at 60
○
C. Upon cooling to room temperature, the reaction
mixture was washed with a saturated solution of NH
4
Cl, and extracted with CH
2
Cl
2
. The
combined organic layers were dried with MgSO
4
and the solvent was evaporated in vacuo. The
product was purified by flash chromatography on silica gel using hexanes:dichloromethane in
3:1 ratio as an eluent.
Ortho-bis(ethynyltetracenyl)benzene (o-BETB). Burgundy prisms. Yield: 82 %.
1
H
NMR (400 MHz, CDCl
3
, 25
○
C): δ = 9.06 (s, 2H), 8.72 (d, J=8.6 Hz, 2H), 8.46 (s, 2H), 8.28 (s,
193

2H), 8.05 (m, 2H), 7.88 (d, J=8.5 Hz, 2H), 7.72 (d, J=8.8 Hz, 2H), 7.58 (m, 2H), 7.24 (d,
J=8.7 Hz, 2H), 7.l7 (m, 2H), 7.03 (m, 2H), 6.90 (d, J=8.6 Hz, 2H), 6.72 (m, 2H).
13
C NMR (100
MHz, CDCl
3
, 25
○
C): 133.2, 132.4, 131.2, 130.8, 130.6, 129.4, 128.7, 128.5, 128.3, 127.9, 127.6,
127.1, 126.50, 126.46, 126.36, 125.5, 125.3, 125.2, 124.9, 116.8, 101.0, 91.8 ppm. MALDI-
TOF: m/z calculated: 578.20 [M]+; found: 577.7 [M]+ Elemental analysis (%), calculated for
C
46
H
26
○ ½ CH
2
Cl
2
: C 89.91, H 4.38, Cl 5.71; found: C 89.76 , H 4.45.
Bis(ethynyltetracenyl)xanthene (BETX). Red rods. Yield: 63 %.
1
H NMR (400 MHz,
CDCl
3
, 25
○
C): δ = 8.83 (s, 2H), 8.09 (d, J= 8.34 Hz, 2H), 7.83 (s, 2H), 7.79 (s, 2H), 7.72 (d, J=
8.68 Hz, 2H), 7.62 (d, J= 2.32 Hz, 2H), 7.57 (d, J= 2.32 Hz, 2H), 7.38 (m, 4H), 7.07 (m, 2H),
6.98 (m, 4H), 6.85 (m, 2H), 1.86 (s, 6H), 1.44 (s, 18H).
13
C NMR (100 MHz, CDCl
3
, 25
○
C):
149.3, 145.8, 131.6, 131.1, 130.6, 129.9, 129.3, 129.0, 128.4, 127.8, 127.5, 126.8, 126.7, 126.0,
125.5, 125.4, 125.3, 124.7, 124.5, 124.4, 123.6, 116.6, 112.3, 97.1, 91.3, 35.1, 34.8, 32.8, 31.7
ppm. MALDI-TOF: m/z calculated: 822.39 [M]+; found: 823.26 [M]+.
General procedure for Sonogashira coupling of di-ethynylbenzenes with bromo-acenes:
Bromo-tetracene, Pd(PPh
3
)
4
and CuI were placed into a Schlenk flask equipped with a stirring
bar. The flask was evacuated and refilled with N
2
three times. Freshly distilled THF was added to
the reaction flask and the solution was sparged with N
2
for 15 minutes. In a separate flask an air
free solution of the di-ethynylbenzene (or tetra-ethynylbenzene) in Et
3
N was prepared. The
reaction flask containing the bromotetracene and the catalysts was heated to 60
o
C and the
solution of the diethynylbenzene (or tetraethynylbenzene) was added dropwise. The reaction
mixture was stirred in the dark, under N
2
at 60
o
C overnight. The reaction mixture was cooled to
room temperature, poured into a saturated NH
4
Cl solution, and extracted with CH
2
Cl
2
. The
194

combined organic layer was dried with Na
2
SO
4
and the solvent was removed in vacuo. The
product was purified by flash chromatography on silica gel using hexanes and THF.
meta-bis(ethynyltetracenyl)benzene (m-BETB): A Schlenk flask equipped with a stirring
bar was loaded with 5-Bromotetracene (0.337 g, 1.1 mmol), CuI (0.010 g, 0.05 mmol) and
Pd(PPh
3
)
4
(0.060 g, 0.05 mmol) under a nitrogen atmosphere, and dissolved with 13 mL of
freshly distilled THF. The resulting solution was sparged with nitrogen for 10 minutes. In a
separate Schlenk flask, a solution of 1,3-diethynylbenzene (0.06 mL, 0.39 mmol) in 10 mL of
triethylamine was sparged with nitrogen for 15 minutes. The flask containing 5-Bromotetracene
was heated to 60
○
C, and the solution of diethynylbenzene was added dropwise. The reaction
mixture was stirred at 60
○
C, under an inert atmosphere, in the dark, overnight. Upon cooling to
room temperature, the reaction mixture was washed with a saturated solution of NH
4
Cl, and
extracted with CH
2
Cl
2
. The combined organic layers were dried with Na
2
SO
4
and the solvent
was evaporated in vacuo. The product was purified by flash chromatography on silica gel using
hexanes and THF as the eluent. Red solids. Yield: 51 %.
1
H NMR (400 MHz, CDCl
3
, 25
○
C): δ =
9.33 (s, 2H), 8.70-8.72 (m, 3H), 8.31 (t, 1H), 8.19-8.14 (m, 2H), 8.05-8.02 (m, 4H), 7.91 (dd, J
1

= 7.90 Hz, J
2
= 1.75 Hz, 2H), 7.65-7.56 (m, 3H), 7.51-7.43 (m, 6H) ppm.
13
C NMR (100 MHz,
CDCl
3
, 25
○
C): 137.7, 133.2, 132.4, 131.84, 131.79, 131.2, 130.8, 129.8, 129.1, 128.9, 128.5,
128.3, 127.4, 127.0, 126.9, 126.0, 125.8, 125.6, 125.5, 124.5, 116.8, 100.9, 87.9 ppm. MALDI-
TOF: m/z calculated: 578.20 [M]+; found: 577.74 [M]+
para-bis(ethynyltetracenyl)-bis(ethylhexyl)benzene (p-BETB-ethex): The product was
purified by flash chromatography on silica gel using hexanes and THF as the eluent. Purple
solids. Yield: 19 %.
1
H NMR (400 MHz, CDCl
3
, 25
○
C): δ = 9.35 (s, 2H), 8.76 -8.67 (m, 6H),
8.14-8.10 (m, 2H), 8.05 (d,J = 8.3 Hz, 4H), 7.78 (s, 2H), 7.58 (dd, J
1
= 7.6 Hz, J
2
= 1.2 Hz, 2H),
195

7.51-7.44 (m, 6H), 3.17 (d, J = 7.3 Hz, 4H), 2.10 (sept, 2H), 1.61-1.20 (m, 16H), 0.96 (t,
J = 7.5 Hz), 0.73 (td, J
1
= 7.2 Hz, J
2
= 1.3 Hz) ppm. MALDI-TOF: m/z calculated: 801.45 [M]+;
found:801.17 [M]+.
para-bis(ethynyltetracenyl)-bis(hexyloxy)benzene (p-BETB-ohex): The product was
purified by flash chromatography on silica gel using hexanes and THF as the eluent. Purple
solids. Yield: 15 %.
1
H NMR (400 MHz, CDCl
3
, 25
○
C): δ = 9.42 (s, 2H), 8.86 (d, J = 8.8 Hz,
4H), 8.71 (d, J = 4.1 Hz, 2H), 8.19-8.16 (m, 2H), 8.05 (d, J = 8.6 Hz, 4H), 7.59-7.55 (m, 2H),
7.50-7.45 (m, 6H), 7.42 (s, 2H), 4.34 (t, J = 6.8 Hz, 4H), 2.14 (q, 4H), 1.69-1.61 (m, 4H),
1.41-1.35 (m, 4H), 1.31-1.22 (m, 4H), 0.80 (t, J = 7.3 Hz, 6H) ppm. MALDI-TOF: m/z
calculated: 378.38 [M]+; found: 378.50 [M]+
ortho-bis(5-phenyl-11-ethynyltetracenyl)benzene (Ph-o-BETB): The product was
purified by flash chromatography on silica gel using hexanes and THF as the eluent. Burgundy
solids. Yield: 35 %.
1
H NMR (400 MHz, CDCl
3
, 25
○
C): δ = 9.16 (s, 2H), 8.80 -8.77 (m, 2H),
8.09-8.07 (m, 2H), 8.06 (s, 2H), 7.75-7.73 (m, 2H), 7.61-7.59 (m, 2H), 7.55-7.52 (m, 4H),
7.31-7.23 (m, 8H), 7.18-7.15 (m, 4H), 6.98-6.94 (m, 2H), 6.60 (d, J = 8.4 Hz, 2H),
6.44-6.40 (m, 2H) ppm.
Tetra(5-phenyl-11-ethynyltetracenyl)benzene (Ph-TETB): The product was purified by
flash chromatography on silica gel using hexanes and THF as the eluent. Purple solids. Yield:
16 %.
1
H NMR (400 MHz, CDCl
3
, 25
○
C): δ = 9.26 (s, 4H), 8.98(d, J = 8.6 Hz, 4H), 8.75 (s, 2H),
8.13 (s, 4H), 7.81 (d, J = 8.5 Hz, 4H), 7.59-7.55 (m, 12H), 7.45-7.27(m, 12H), 7.21-7.18 (m,
8H), 7.00 (t, J = 9.0 Hz, 4H), 6.66 (d, J = 8.6 Hz, 4H), 6.45 (t, J = 8.5 Hz, 4H) ppm.
13
C NMR
(100 MHz, CDCl
3
, 25
○
C): 94.5, 100.8, 107.4, 116.5, 124.3, 125.1, 125.2, 126.3, 127.6, 128.4,
196

128.38, 129.2, 129.5, 130.5, 131.2, 131.6, 133.2, 137.1, 138.8 ppm. MALDI-TOF: m/z
calculated: 1383.49 [M]+; found: 1379.94 [M]+
Tetra(ethynyltetracenyl)benzene (TETB): The product was purified by flash
chromatography on silica gel using hexanes and THF as the eluent. Purple solids. Yield: <5%
after purification.
1
H NMR (400 MHz, CDCl
3
, 25
○
C): δ = 9.14 (s, 4H), 8.85 (d, J = 8.8 Hz, 4H),
8.66 (s, 2H), 8.52 (s, 4H), 8.31 (s, 4H), 7.92 (d, J = 8.8 Hz, 4H), 7.74 (d, J = 8.5 Hz, 4H),
7.32-7.12 (m, 12H), 6.96 (d, J = 8.5 Hz, 4H), 6.73 (t, J = 7.5 Hz, 4H) ppm. MALDI-TOF: m/z
calculated: 1078.36 [M]+; found: 1078.93 [M]+
para-(hexyloxy)benzene: hydroquinone (2.000 g, 18.2 mmol) and K
2
CO
3
(15.002 g, 108.6
mmol) were loaded into a Schlenk flask equipped with a stir bar, and pumped/purged with N
2

three times. The flask was filled with N
2
and 30 mL of DMF were added to the flask under inert
atmosphere. The solution was sparged with N
2
for seven minutes. Bromohexane (6.4 mL, 7.6 g,
46 mmol) was added to the reaction flask slowly, while the solution was stirred. The reaction
mixture was heated to 70
o
C and stirred under N
2
overnight. Once the reaction mixture was
cooled to room temperature, ~200 mL of water were added and the precipitate was collected by
filtration. Product was recrystallized from ethanol to give white platelettes. Yield: 79 %.
1
H
NMR (400 MHz, CDCl3, 25
○
C): δ = 6.86 (s, 4H), 3.90 (t, J = 6.6 Hz, 4H), 1.75 (q, J = 6.6 Hz,
4H), 1.48-1.41 (m, 4H), 1.36-1.30 (m, 8H), 0.93-0.88 (m, 6H) ppm.
1,4-diiodo-2,5-hexyloxybenzene: para-(hexyloxy)benzene (2.20 g, 7.9 mmol), iodine
(1.72 g, 13.6 mmol), H
5
IO
6
(0.861 g, 3.8 mmol), acetic acid (26 mL), concentrated sulfuric acid
(1.2 mL), water (5.0 mL), and CH
2
Cl
2
(4.0 mL) were all loaded into a round bottom flask
equipped with a stirring bar and a condenser and the reaction mixture was heated to 70
o
C
overnight. The reaction mixture was poured into ~200 mL of water and extracted with CH
2
Cl
2

197

three times. The combined organic layer was dried with Na
2
SO
4
and the solvent was removed in
vacuo. The product was passed through a small silica plug using hexanes:CH
2
Cl
2
8:2 as the
eluent. The solvent was removed in vacuo and the product was recrystallized from ethanol to
give white flaky platelettes. Yield: 62 %.
1
H NMR (400 MHz, CDCl
3
, 25
○
C): δ = 7.17 (s, 2H),
3.93 (t, J = 6.5 Hz, 4H), 1.80 (q, J = 6.5 Hz, 4H), 1.54-1.46 (m, 4H), 1.38-1.33 (m, 8H),
0.94-0.88 (m, 6H) ppm.
1,4-(hexyloxy)-2,5-(TMS-ethynyl)benzene: 1,4-diiodo-2,5-hexyloxybenzene (2.001 g,
3.8 mmol) was loaded into a Schlenk flask equipped with a stirring bar and pumped/purged with
N
2
three times. The flask was filled with N
2
and the dihalide was dissolved in 12.0 mL of THF
and 9.0 mL of Et
3
N under inert atmosphere, and sparged with N
2
for 10 minutes. Ph(PPh
3
)
2
Cl
2

(0.142 g, 0.20 mmol) and CuI (0.090 g, 0.47 mmol) were added to the flask against the positive
flow of N
2
and the resultant reaction mixture was sparged for additional 10 minutes. The solution
was cooled to 0
o
C and TMS-acetylene (1.19 mL) was added drop-wise under an inert
atmosphere. The reaction mixture was warmed to room temperature and stirred for three hours in
the dark, under N
2
atmosphere. The reaction mixture was then poured into a saturated NH
4
Cl
solution and extracted with CH
2
Cl
2
. The combined organic layer was dried with Na
2
SO
4
, and the
solvent was removed in vacuo. The crude product was purified by column chromatorgraphy on
silica gel using hexanes as the eluent. The product was used in the next step without further
purification. Yield: 92 %.
1
H NMR (400 MHz, CDCl
3
, 25
○
C): δ = 6.89 (s, 2H), 3.94
(t, J = 6.5 Hz, 4H), 1.78 (q, J = 6.5 Hz, 4H), 1.54-1.46 (m, 4H), 1.36-1.30 (m, 8H), 0.93-0.87 (m,
6H), 0.25 (s, 18H) ppm.
1,4-ethynyl-2,5-hexyloxybenzene: 1,4-(hexyloxy)-2,5-(TMS-ethynyl)benzene (0.730 g,
1.6 mmol), K
2
CO
3
(0.870 g, 6.3 mmol), THF (10.0 mL), methanol (4.0 mL), and a few drops of
198

water were all combined in a round bottom flask equipped with a stir bar, and stirred for 2 hours.
The reaction mixture was then poured into a saturated NH
4
Cl solution and extracted with
CH
2
Cl
2
. The combined organic layer was dried with Na
2
SO
4
and solvent was removed in vacuo.
The product was used in the next step without further purification. Yield: 95 %.
1
H NMR
(400 MHz, CDCl
3
, 25
○
C): δ = 6.95 (s, 2H), 3.97 (t, J = 6.5 Hz, 4H), 3.33 (s, 2H),
1.80 (q, J = 6.5 Hz, 4H), 1.51-1.43 (m, 4H), 1.38-1.30 (m, 8H), 0.93-0.87 (m, 6H) ppm.
1,4-(ethylhexyl)benzene: Initially, the Grignard reagent of ethylhexyl was prepared by
reacting freshly ground magnesium (1.403 g, 57.7 mmol) with bromoethylhexyl (10.0 mL,
56.2 mmol) in 75 mL of freshly distilled THF under an inert N
2
atmosphere for an hour upon the
beginning of the exothermic reaction. In another Schlenk flask 1,4-dichlorobenzene (3.201 g,
21.8 mmol) and Ni(dppp)Cl
2
(0.030 g, 0.06 mmol) were dissolved with THF under inert N
2

atmosphere. The solution was cannula transferred to the flask containing the Grignard solution
and the resultant reaction mixture was heated to 50
o
C and allowed to react for 48 hours under an
inert N
2
atmosphere. Once cooled to room temperature, the reaction mixture was poured into
water and quenched with a dilute HCl solution. The product was extracted with CH
2
Cl
2
,
combined organic layer was dried with Na
2
SO
4
and the solvent was removed in vacuo. The
product was purified by flash chromatography on silica gel using hexanes as the eluent.
Colorless oil. Yield: 66 %.
1
H NMR (400 MHz, CDCl
3
, 25
○
C): δ = 7.04 (s, 4H),
2.49 (d, J = 7.0 Hz, 4H), 1.56-1.49 (m, 2H), 1.32-1.18 (m, 16H), 0.91-0.82 (m, 12H) ppm.
1,4-(ethylhexyl)-2,5-diiodobenzene: 1,4-(ethylhexyl)benzene (2.386 g, mmol), iodine (1.726 g,
13.6 mmol), H
5
IO
6
(0.862 g, 3.8 mmol), acetic acid (25 mL), concentrated sulfuric acid
(1.3 mL), water (5.0 mL), and CH
2
Cl
2
(5.0 mL) were all loaded into a round bottom flask
equipped with a stirring bar and a condenser and the reaction mixture was heated to 70
o
C
199

overnight. The reaction mixture was cooled to 0
o
C and quenched with a potassium carbonate
solution. The mixture was poured into ~200 mL of water and extracted with CH
2
Cl
2
three times.
The combined organic layer was dried with Na
2
SO
4
and the solvent was removed in vacuo, the
product was used in the next step without further purification. Colorless oil. Yield: 95 %.
1
H
NMR (400 MHz, CDCl
3
, 25
○
C): δ = 7.55 (s, 2H), 2.54 (d, J = 7.1 Hz, 4H), 1.72-1.63 (m, 2H),
1.35-1.20 (m, 16H), 0.91-0.83 (m, 12H) ppm.
1,4-(ethylhexyl)-2,5-(TMS-ethynyl)benzene: 1,4-(ethylhexyl)-2,5-diiodobenzene (2.84 g,
5.1 mmol) was dissolved in freshly distilled THF (25 mL) under the inert N
2
atmosphere,
Pd(PPh
3
)
4
(0.200 g, 0.17 mmol) and CuI (0.048 g, 0.25 mmol) were added to the reaction flask
against the positive flow of N
2
, and the reaction mixture was sparged with N
2
for 15 minutes.
The flask was heated to 60
o
C and a solution of TMS-acetylene (1.75 mL, 12.6 mmol) in Et
3
N
(23 mL). The reaction mixture was stirred under N
2
overnight at 60
o
C, then cooled to room
temperature. The reaction mixture was poured into a saturated NH
4
Cl solution and extracted with
CH
2
Cl
2
. The combined organic layer was dried with Na
2
SO
4
and the product was purified by
flash chromatography on silica gel using hexanes as the eluent. Colorless oil. Yield: 59 %.
1
H
NMR (400 MHz, CDCl
3
, 25
○
C): δ = 7.20 (s, 2H), 2.62 (d, J = 7.1 Hz, 4H), 1.75-1.67 (m, 2H),
1.33-1.21 (m, 16H), 0.91-0.87 (m, 12H), 0.25 (s, 18H) ppm.
1,4-(ethylhexyl)-2,5-ethynylbenzene: 1,4-(ethylhexyl)-2,5-(TMS-ethynyl)benzene (1.603 g,
3.2 mmol), K
2
CO
3
(1.903 g, 13.8 mmol), THF (15.0 mL), methanol (5.0 mL) were all combined
in a round bottom flask equipped with a stir bar, and stirred for overnight. The solvent was then
removed in vacuo, and the product was passed through a silica plug using hexanes and CH
2
Cl
2
.
The solvent was removed in vacuo, and the product was used in the next step without further
200

purification. Yield: 91 %.
1
H NMR (400 MHz, CDCl
3
, 25
○
C): δ = 7.26 (s, 2H), 3.27 (s, 2H),
2.64 (d, J = 7.2 Hz, 4H), 1.73-1.64 (m, 2H), 1.34-1.19 (m, 16H), 0.91-0.88 (m, 12H) ppm.
1,2,4,5-tetra(TMS-ethynyl)benzene: 1,2,4,5-tetrabromobenzene (0.335 g, 0.85 mmol),
Pd(PPh
3
)
4
(0.103 g, 0.09 mmol) and CuI (0.047 g, 0.25 mmol) were loaded into a Schlenk flask
equipped with a stirring bar. The flask was vacuumed and refilled with N
2
three times. Freshly
distilled THF (6.0 mL) were added to the flask and the solution was sparged with N
2
for ten
minutes. An air free solution of TMS-acetylene (1.0 mL, 7.2 mmol) in Et
3
N (6.0 mL) was added
to the reaction flask dropwise, while the reaction mixture was stirred and heated to 60
o
C. The
reaction was stirred over night under an inert atmosphere. Once cooled to room temperature, the
reaction mixture was poured into a saturated NH
4
Cl solution and extracted with CH
2
Cl
2
. The
combined organic layers were dried with Na
2
SO
4
and the product was purified by flash
chromatography on silica gel using hexanes. White solids. Yield: 97 %.
1
H NMR (400 MHz,
CDCl
3
, 25
○
C): δ = 7.56 (s, 2H), 0.25 (s, 36H) ppm.
1,2,4,5-tetraethynylbenzene: 1,2,4,5-tetra(TMS-ethynyl)benzene (0.153 g, 0.33 mmol), K
2
CO
3

(0.173 g, 1.3 mmol), THF (3.0 mL), methanol (1.8 mL) and three drops of water were all
combined in a round bottom flask equipped with a stir bar, and stirred for three hours. The
solvent was then removed in vacuo, and the product was passed through a silica plug using
hexanes and CH
2
Cl
2
. Due to the brown color of the product, additional purification by flash
chromatography on silica gel using hexanes and CH
2
Cl
2
was performed. The solvent was
removed in vacuo, and the product was used in the next step without further purification. Once
the solvent dries up, the product turns a gold color. Yield: 98 %.
1
H NMR (400 MHz, CDCl
3
, 25
○
C): δ = 7.64 (s, 2H), 3.42 (s, 4H) ppm.
201

ortho-bis(TMS-ethynyl)benzene: A solution of 1,2-diiodobenzene (0.30 mL, 2.3 mmol) in THF
(8.0 mL) and Et
3
N (7.0 mL) was sparged with N
2
for 10 minutes. The catalysts, Pd(PPh
3
)
2
Cl
2

(0.095 g, 0.08 mmol) and CuI (0.052 g, 0.27 mmol) were added to the reaction flask against the
positive flow of N
2
, and the reaction mixture was sparged with N
2
for additional 15 minutes. The
reaction mixture was cooled to 0
o
C, and TMS-acetylene (0.78 mL, 5.6 mmol) was added
dropwise to the stirred reaction mixture. The ice bath was removed and the reaction mixture was
stirred under N
2
, in the dark, overnight. The reaction mixture was poured into a saturated NH
4
Cl
solution and extracted with CH
2
Cl
2
. The combined organic layers were dried with Na
2
SO
4
and
the product was purified by flash chromatography on silica gel using hexanes. White solids.
Yield: 83 %
1
H NMR (400 MHz, CDCl
3
, 25
○
C): δ = 7.47 -7.44 (m, 2H), 7.25-7.22 (m, 2H),
0.27 (s, 18H) ppm.
ortho-bisethynylbenzene: ortho-bis(TMS-ethynyl)benzene (0.034 g, 0.13 mmol), K
2
CO
3

(0.068 g, 0.49 mmol), and methanol (1.0 mL) were all combined in a round bottom flask
equipped with a stir bar, and stirred overnight at room temperature. The reaction mixture was
poured into a saturated NH
4
Cl solution and extracted with CH
2
Cl
2
. The combined organic layer
was dried with Na
2
SO
4
and the solvent was removed in vacuo. The product was used in the next
step without further purification. Colorless oil.
1
H NMR (400 MHz, CDCl
3
, 25
○
C): δ =
7.53-7.51 (m, 2H), 7.32-7.30 (m, 2H), 3.33 (s, 2H) ppm.
A2.4 Crystallographic information
The single crystal X-ray diffraction data were collected on a Bruker SMART APEX DUO
3-circle platform diffractometer with the -axis fixed at 54.74°. The diffractometer was equipped
with an APEX II CCD detector and an Oxford Cryosystems Cryostream 700 apparatus for low-
temperature data collection. Mo K

radiation (TRIUMPH curved-crystal monochromator) from a
202

fine-focus tube was used to collect the data for C
46
H
26
and Cu K

radiation from an I S
microsource with Quazar multilayer focusing optics was used for C
63
H
50
O•C
7
H
8
. A complete
hemisphere of data was scanned on omega and phi at a detector resolution of 512 x 512 pixels
using the BIS software package.
6
The frames were integrated using the SAINT algorithm to give
the hkl files corrected for Lp/decay.
7
The absorption correction was performed using the
SADABS program.
8
The structures were solved by intrinsic phasing and refined on F2 using the
Bruker SHELXTL Software Package.
9-12
All non-hydrogen atoms were refined anisotropically.
Single crystals of o-BETB were grown by slow evaporation of THF under an inert
atmosphere at room temperature for several weeks, to produce burgundy prisms. A suitable
crystal was mounted into the diffractometer using paraffin oil and a nylon loop. The data was
collected at a temperature of 100 K. Complete data (100.0 %, 3626 reflections) was collected
with R(gt) = 4.18 % and R(all) = 5.04 % after absorption correction (T
max
= 0.990 and T
min
=
0.870).
Single crystals of BETX were grown by slow evaporation of toluene under an inert
atmosphere at room temperature for several weeks, to produce red needles. A suitable crystal
was mounted into the diffractometer using paraffin oil and nylon loop. The data was collected
at a temperature of 100 K. Complete data (97.8 %, 17479 reflections – 17878 calculated, 17479
observed) was collected with R(gt) = 6.95 % and R(all) = 8.38 % after absorption correction
(T
max
= 0.940 and T
min
= 0.680).

203

Table A2.1. Crystallographic parameters for BET-B and BETX crystals.
Parameter o-BETB BETX
Molecular formula C
46
H
26
C
63
H
50
O, C
7
H
8

MW 578.67 915.16
Lattice type Monoclinic Triclinic
Space group C 2/c P -1
a 20.5497(14) 11.3862(2)
b 13.1892(9) 21.3855(3)
c 14.0762(10) 21.4813(3)
α 90 82.2140(10)
β 130.1720(8) 82.9820(10)
γ 90 78.2240(10)
V (Å
3
) 2915.2 (4) 5049.24(14)
Z value 4 4
Density (g cm
-3
) 1.319 1.204
T 100 (2) 100 (2)
GOF 1.006 1.026
R
1
0.0418 0.0838
wR
2
0.1341 0.1966

A2.5 References
1. Roberts, S. T.; McAnally, R. E.; Mastron, J. N.; Webber, D. H.; Whited, M. T.; Brutchey, R.
L.; Thompson, M. E.; Bradforth, S. E., Journal of the American Chemical Society 2012, 134
(14), 6388-6400.
2. Johnson, J. M.; Chen, R.; Chen, X.; Moskun, A. C.; Zhang, X.; Hogen-Esch, T. E.;
Bradforth, S. E., The Journal of Physical Chemistry B 2008, 112 (51), 16367-16381.
3. Pramanik, C.; Miller, G. P., Molecules 2012, 17 (4), 4625.
4. Müller, A. M.; Avlasevich, Y. S.; Schoeller, W. W.; Müllen, K.; Bardeen, C. J., Journal of
the American Chemical Society 2007, 129 (46), 14240-14250.
5. Moon, H.; Zeis, R.; Borkent, E.-J.; Besnard, C.; Lovinger, A. J.; Siegrist, T.; Kloc, C.; Bao,
Z., Journal of the American Chemical Society 2004, 126 (47), 15322-15323.
6. BIS.
7. SAINT+8.34A.
8. SADABS.
9. Sheldrick, G., Acta Crystallographica Section C 2015, 71 (1), 3-8.
204

10. Sheldrick, G., Acta Crystallographica Section A 2008, 64 (1), 112-122.
11. SHELXL.
12. SHELXTL.
205

The end.

Abstract (if available)

Abstract Organic photovoltaics (OPVs) are a cheaper, sustainable alternative to their inorganic counterparts, but their low power conversion efficiencies have hampered their utilization. Recently Nozik and Michl proposed incorporating singlet fission (SF) materials into OPVs to harvest the high energy photons and convert them into pairs of low energy triplet excitons to reduce thermal relaxation losses. Singlet fission is an excited state process which occurs in organic materials whose S₁ state energy is twice that of the T₁ state. During this process, the S₁ exciton which was generated by a single photon absorption rapidly converts into a pair of triplet excitons located on adjacent chromophores, and coupled into an overall singlet spin state. SF was discovered in crystalline anthracene and tetracene in the 1960s, however, the potential uses of SF materials in enhancing the performance of organic electronics has spurred renewed interested among the scientific community. The field of singlet fission has experienced rapid growth in the past decade as researchers have explored new materials for SF and attempted to answer important questions such as which factors govern the rate of singlet fission in crystalline and amorphous materials, which other electronic states facilitate SF, how should the chromophores be oriented next to each other for optimal SF, and which factors influence the separation of the triplet excitons from the correlated triplet pair. ❧ This thesis presents the design strategies, synthesis, and photophysical properties of a series of covalently‐linked tetracene arrays, which allowed us to learn about the effects of relative chromophore orientation and electronic coupling on SF dynamics, the separation of the triplet excitons from the strongly coupled correlated triplet pair state, and the role of exciton delocalization in driving SF in tetracene systems. ❧ We observed that in ethynyltetracene SF proceeded much faster than in unsubstituted tetracene, and we initially constructed covalently linked dimers of ethynyltetracenes. First, the effects of π orbital overlap in two cofacial dimers with different degrees of π overlap—bis(ethynyltetracenyl)‐xanthene, and ortho‐bis(ethynyltetracenyl)‐benzene, BETX and o‐BETB, respectively. It was found that too much π overlap in BETX results in excited state decay via excimer, while some π overlap was ideal for rapid (< 10 ps) relaxation into the ¹(T₁T₁) state in o‐BETB. The triplet excitons can be separated from the correlated triplet pair via energy transfer to neighboring chromophores in bulk or in a diphenyltetracene host. We then explored the effects of through‐bond chromophore coupling on SF efficiencies in meta‐, and para‐bis(ethynyltetracenyl)benzenes. It was found that the conjugating para‐diethynylbenzene linkers facilitate rapid singlet fission, while the meta‐diethynylbenzen bridged tetracenes primarily relax via fluorescence from the S₁ state. Furthermore, in solution, the para‐ dimers exhibited spectral signatures of the monomeric T₁ state absorption, which suggests that the rotational flexibility about the linker axis in the para‐ dimers resulted in decoupling of the tetracenes in the excited state. Lastly, the effect of exciton delocalization on SF rate was explored in a chemically stabilized, phenylated o‐BETB‐dimer (Ph‐o‐BETB) and a structurally analogous tetramer (Ph‐TETB). It was found that exciton delocalization in the tetramer resulted in a SF rate that was ten times faster than that in the dimer. ❧ The discoveries of SF dynamics in these systems are inspiring the development of next generation of optimized SF materials for use in OPVs and other organic electronic applications.

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Asset Metadata

Creator Korovina, Nadezhda V. (author)

Core Title Singlet fission in covalently-linked tetracenes

School College of Letters, Arts and Sciences

Degree Doctor of Philosophy

Degree Program Chemistry

Publication Date 05/22/2017

Defense Date 10/17/2016

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag acenes,dimers,dyes,excitons,materials,OAI-PMH Harvest,organic,photophysical,photovoltaics,singlet fission,solar energy,spectroscopy

Format application/pdf (imt)

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Thompson, Mark E. (committee chair), Bradforth, Stephen E. (committee member), Kresin, Vitaly (committee member)

Creator Email korovina@usc.edu,nadia.korovina@gmail.com

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c40-324438

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Rights Korovina, Nadezhda V.

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