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Utilizing n-heterocyclic chromophores for solar energy harvesting.

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Utilizing N-Heterocyclic Chromophores for Solar Energy Harvesting.
by
Austin R. Mencke

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)

August 2023

Copyright 2023 Austin R. Mencke
ii

Epigraph
“3
And God said, “Let there be light,” and there was light.
4
God saw that the light was good...”
Genesis 1:3-4

iii
Acknowledgements
First and foremost, I would like to thank my mom, Michelle Ann Mencke. My mom was a
middle school history teacher who instilled a deep importance of higher education in me. She was
also an incredibly strong and loving woman, who fought for me every step of the way, and
provided a home in the mist of extreme adversity. Without her there is no doubt that I wouldn’t be
where I am right now. I miss her every day.
Secondly, I want to acknowledge my aunt Melissa McAlister, who provided a home for
me after my mother’s passing. My aunt Melissa had a very difficult job of nurturing a devastated
and broken child and through her love pushed me to excel in high school despite my circumstance,
a role that was unexpected, but a role she took on, nonetheless. Her push propelled me into an
excellent college and graduate program. To her I am eternally thankful.
I would like to thank my girlfriend, Amanda Clark. On top of a wonderful two years,
providing emotional support through a stressful life stage, Amanda managed to proof and edit my
thesis, despite not knowing most of the words or context, demonstrating a great proficiency for the
spelling and grammar. Her favorite part was a sentence that says, “Illuminating the solution results
in a steady color change from green to blue” because “[she] understood every word.”
To my brother Aaron McAlister, I owe many great adventures, laughs, and deep personal
growth. Despite being far from home, a home that is constantly shifting and changing while I’m
away, Aaron has made it feel not so far through many hours of phone calls and video games.
Becoming brothers was an unexpected twist to our lives, but I wouldn’t trade it for anything else
and it has been a great privilege. You are a great dad and I’m extremely proud of you.
iv
To my grandpa Merlyn Robert Mencke, with whom I share a middle name, I owe my love
of science. My grandfather was an engineer who worked at with McDonald-Douglas on the Gemini
I and II program, as well as at Idaho National Laboratory. He used science and engineering around
the house often to invent his own solutions to problems, and would frequently employ my help,
building in me a strong problem-solving toolbox. More importantly my grandpa lived my years
with both my mom and me, as well as with my aunt, taking care of both of them during hard times.
Being able to complete a fellowship with Brookhaven National Laboratory allowed me to continue
walking in my grandpa’s footsteps.
To Casey and Conner Campbell, I’d like to thank you for many holidays together, in
Seattle, Vegas, and Florida. Coming home, playing cards, and watching sports is always a
welcomed rest from a very stressful Los Angeles. I’d also like to thank Casey for giving me advice
whenever I call. I’m excited to come visit in Alabama.
To Mark Thompson, I’d like to thank him for never ending patience in teaching organic
synthesis and photophysics. The optical properties of molecules are something that is touched on
briefly in undergraduate classes, but for me, watching solutions change color is what got me
interested in chemistry. Being able to spend six-year understanding why materials are the color
they are and more importantly, how to use those optical properties for bettering the worlds energy
future has been wonderful. There might be arguments for people who are equal to Mark in terms
of expertise, but there is no one who is better than Mark. Plus, those guys didn’t give me a spot in
their labs, nor did they find endless opportunities for my own professional development like Mark
did.
To Peter Djurovich, I’d like to thank him for endless hours in his office talking through
research concepts. Peter Djurovich gave me the first full syntheses that worked, and then helped
v
me through collecting and interpreting data, and gave me the photophysical legs to stand on to
stride for more knowledge. Peter always has his door open and is ready to talk both science and
life.
I’d like to thank Matthew Bird, who welcomed me into his lab at Brookhaven National
Laboratory for four months to collected high-level spectroelectrochemistry data. Being able to
come see how a national lab functions was a fantastic learning experience and being given a paid
opportunity to live in New York and explore the east coast was wonderful. Furthermore, he was
fantastic company talking about guitars, places we’ve lived, and life while collecting data till two
in the morning. I consider him a friend and I’m glad our paths crossed.
I’d like to like Judy Fong for all of her help navigating the University of Southern
California these few years. USC is a very large university with many different systems in place for
payroll, ordering, reimbursables, and travel. Having someone who in an expert to turn to has saved
a countless amount of time towards my degree. Plus, Judy cuts cake better than anyone else in the
lab.
I’d like to thank all of my chemistry teachers and professors along the way, Mrs. Addie
Smith, Dr. Karisa Pierce, Dr. Kevin Bartlett, Dr. Daniel Schofield, and most importantly, Dr.
Samantha Robinson, my undergraduate principal investigator. Each one of these people
demonstrated a deep love and passion for chemistry, and being able to feed off that energy lead
me down the road I’m on now.
I’d like to thank all of my lab members and colleagues that last six years. Earnestly
enjoying coming in to work and spending time with your co-workers makes six years of graduate
school fly by. Principally I’d like to thank Konstantin Mallon for all of his help with organic
vi
synthesis, a skillset I am underdeveloped in, Dr. Savannah Kapper, the Volmer to my Stern, for all
the hours we spent teaching ourselves photo-redox chemistry, Dr. Jessica Golden for help getting
the bodipy synthesis off the ground, Dr. Abigail Tadle for teaching me how to use most of the
instruments in the lab, Dr. Daniel Sylvinson for help learning computation chemistry, Dr. Eric
McClure and James Fortwinger for help with MATLAB coding, Dr. Laura Estergreen, Dr. Michael
Kellogg, Dr. Fabiola Carduso-Delgado, Dr. Matthew Bain, and Thabassum Kallungal for their
help learning and performing transient absorption spectrometry, as well as their PI Dr. Stephen
Bradforth, as well as Arian Villicana, Ariel Ramirez, and Angel Lima, three undergraduates who
worked with me over various summers for their hard work, showcased in chapter 3 (Villicana,
Ramirez) and chapter 4 (Lima). I’d also like to thank anyone I’ve inevitably missed.
Lastly, I’d like to thank God for everything. My life hasn’t been easy, and there are many
reasons I shouldn’t be where I am, but thankfully there is exactly one reason that I am. The Lord
has made sure my path, going before me, behind me, and with me. As I step out from the University
of Southern California, the first time in my life I haven’t known what’s next, I know everything is
going to work out according to his plan.

vii
Table of Contents

Epigraph .......................................................................................................................................... ii
Acknowledgements ........................................................................................................................ iii
List of Tables ...................................................................................................................................x
List of Figures ................................................................................................................................ xi
Abstract ..........................................................................................................................................xv
Chapter 1: Light, the Universe, and Somethings. ............................................................................1
The Absorption of Light .............................................................................................................. 3
Photochemical Work ................................................................................................................... 7
Basic OPV Device Architecture .................................................................................................. 8
The Five Steps of Photocurrent Generation .............................................................................. 10
Step One: Light Absorption ...................................................................................................... 10
Step Two: Exciton Migration .................................................................................................... 11
Steps Three and Four: Exciplex Formation and Separation ...................................................... 12
Step Five: Charge Conduction .................................................................................................. 15
State of the Art Electron Acceptors ........................................................................................... 16
References ................................................................................................................................. 18
Chapter 2: Eliminating Decay Pathways in Ternary Solar Cells ...................................................22
Intro ........................................................................................................................................... 22
Results ....................................................................................................................................... 23
Conclusion ................................................................................................................................. 32
Acknowledgement ..................................................................................................................... 33
Synthesis .................................................................................................................................... 33
Anhydrous Non-exchange of Donor groups between NMe2PhIC and Benzaldehyde. ............. 35
Hydrous Exchange of Endcaps ................................................................................................. 35
Anhydrous Non-exchange of Endcaps ...................................................................................... 36
Exchange of Endcaps with 1 molar equivalent of water. .......................................................... 36
Anhydrous Non-exchange of Donor groups between NMe2PhIC and Benzaldehyde. ............. 36
Anhydrous exchange of acceptor groups between NMe2PhIC and 1,3-dimethylbarbituic
acid. ........................................................................................................................................... 37
Materials. ................................................................................................................................... 37
Device Fabrication. ................................................................................................................... 37
viii
Solar Cell Characterization. ...................................................................................................... 38
Device Stability Measurements. ................................................................................................ 38
Supporting Figures. ................................................................................................................... 38
References ................................................................................................................................. 41
Chapter 3: High Voltage Organic Photovoltaics based on an Acceptor-Donor-Acceptor
Molecule Utilizing a Bodipy Core. ................................................................................................43
Introduction ............................................................................................................................... 43
Results and Discussion .............................................................................................................. 45
Computational Chemistry ......................................................................................................... 54
Electrochemistry ........................................................................................................................ 58
Devices ...................................................................................................................................... 59
Conclusions ............................................................................................................................... 61
Acknowledgements ................................................................................................................... 63
Synthesis .................................................................................................................................... 63
Methods ..................................................................................................................................... 68
Nuclear Magnetic Resonance Spectra ....................................................................................... 70
Crystal Structures of BPB, BPIO, and BPIC-Fused .................................................................. 82
Absorption and Emission Spectra of BPM, BPB, BPIO, and BPT ........................................... 83
Electrochemistry ........................................................................................................................ 86
Computation Data ..................................................................................................................... 90
References ................................................................................................................................. 93
Chapter 4: Molar Absorptivity of Unstable Symmetry Breaking Charge Transfer Dimer via
Pulse Radiolysis. ............................................................................................................................97
Intro ........................................................................................................................................... 97
General Structure ....................................................................................................................... 99
Pulse Radiolysis ...................................................................................................................... 103
Transient Absorption ............................................................................................................... 113
Acknowledgement ................................................................................................................... 116
Experimental ........................................................................................................................... 117
References ............................................................................................................................... 119
Chapter 5: Understanding Intra- and Inter-Molecular Charge Transfer in
Carbene-Metal-Amide Complexes. .............................................................................................122
Introduction ............................................................................................................................. 122
ix
Results and Discussion ............................................................................................................ 125
Steady State Photophysics ....................................................................................................... 125
Spectroelectrochemistry .......................................................................................................... 127
Determining rate of Intersystem Crossing............................................................................... 132
Excited State Recreation ......................................................................................................... 138
Intermolecular CT via nsTA. .................................................................................................. 141
Conclusion ............................................................................................................................... 146
Acknowledgement ................................................................................................................... 148
Supporting Figures .................................................................................................................. 149
Experimental ........................................................................................................................... 153
References ............................................................................................................................... 158

x
List of Tables
Table 3.1. Photophysical properties of 1, BPM, BPB, BPIO, BPT, and BPIC in toluene and
acetonitrile.
a
Measured using a Horiba Fluorohub TCSPC with a 405 nm laser source
(<300 ps IRF).
b
Measured with ps-TCSPC. ..................................................................................50
Table 3.2. Temperature Dependent Steady-State Photophoysical Properties of Bodipy Dyes
in 2-MeTHF, which has a ET(30) of 36.5 .
a
Measured using a Horiba Fluorohub TCSPC with
a
405 nm laser source (<300 ps IRF).
b
Room temperature values of ΦPL were experimentally
measured using a Hamatsu Integrating Sphere. .............................................................................53
Table 3.3. Calculated Excited State Energetics and HOMO-LUMO overlap. .............................57
Table 3.4. Red./Ox. Potentials and Corresponding LUMO HOMO Levels. ................................59
Figure 3.8. (a) J-V characteristics of the OPV devices using either PCE-10:BPT (1:2 w/w) or
PCE-10:BPIC (1:1 w/w) with or without annealing, under the illumination of AM1.5G,
100 mW/cm
2
. (b) The EQE spectra for the OPV devices processed under theremal annealing
conditions. ......................................................................................................................................60
Table 3.5. Photovoltaic Performance Parameters of PCE-10:BPT and PCE-10:BPIC OPVs
under illumination of AM1.5G, 100 mW/cm
2
. .............................................................................60
Table 3.7. Photophysical properties of 1, BPM, BPB, BPIO, BPT, and BPIC in
dichloromethane and 2-methyltetrahydrofuran.
a
Measured using a Horiba Fluorohub TCSPC
with a 405 nm laser source (<300 ps IRF). ....................................................................................85
Table 5.1. ps-TCSPC decay trace amplitudes, time constants, and derived rates. ......................134
Table 5.2. ps-TCSPC decay trace amplitudes as well as time constants, and derived rates
from joint ps-TCPSC and ps-TA studies. ....................................................................................138

xi
List of Figures
Figure 1.1. Example of an orbital perturbation of the s orbital caused by the electric field
(grey) of photon resulting in the time average formation of a p orbital, which is an allowed
transition. .........................................................................................................................................4
Figure 1.2. Orbital diagram of frontier orbitals of H2. The ionization energy (IE) is given by
the red arrow on the left, while the electron affinity (EA) is given by the blue arrow in the
middle. The green arrow on the left shows the energy of excited state. Notice that green arrow
is smaller than the vector sum of IE and EA. ..................................................................................6
Figure 1.3. Orbital diagram of HOMO* and LUMO*. The IE* is given by the red arrow,
while the EA* is given by the blue arrow. Attention is called to the difference in magnitude
to the two arrows compared to their equivalents if figure 2. On the right, a cartoon schematic
of a voltmeter is drawn to demonstrate that a potential capable of work exists between these
two states as it does between two-half cells of a galvanic cell. .......................................................7
Figure 1.5. Frontier orbitals for the excited donor and ground state acceptor materials on the
active layer. Left) The offset between the Donor LUMO* and the Acceptor LUMO is large
and therefore the exciplex is easily split, and JSC is increased. However, EDA (black arrow) is
small and therefore VOC is minimized. Right) The LUMO*-LUMO gap is small, and therefore
the exciplex has difficulty being split, minimizing JSC. However, EDA (in red) is larger than in
the previous picture (black) and therefore VOC is increased. .........................................................13
Figure 2.1. Left) Device architecture for determining the effects of cold and hot solution
conditions on JSC, VOC, PCE, and FF. Middle) Device architecture for determining the effects
of cold vs hot solution on device lifetime. Right) Molecular structure of BTIC family of A-
D-A molecules. ..............................................................................................................................24
Figure 2.2. Plots of VOC (top left), FF (top right), JSC (bottom left), and PCE (bottom right) as
a function of reaction time in the dark at 65 °C. ............................................................................25
Figure 2.3. Normalized PCE (top left), VOC (top right), JSC (bottom left), and FF (bottom
right) vs aging time under 10 sun light illumination......................................................................26
Figure 2.4. Left)Absorption spectra of a solution of BTIC and BTIC-4F under 1 sun
illumination over 2.5 hours. Right)
19
F-NMR spectra of (1) BTIC-4F, (2) BTIC-2F, (3)A
solution of BTIC and BTIC-4F kept in the dark at 65 °C for 72 hours (4) A film of BTIC and
BTIC-4F kept in the dark at 65 °C for 72 hours. (5) A solution of BTIC and BTIC-4F
illuminated under one sung intensity at 48 hours. (6) A film of BTIC and BTIC-4F illuminated
under one sung intensity at 48 hours. .............................................................................................28
Figure 2.5.
1
H-NMR peaks of PhIC (1), NMe2PhBarb (2), NMe2PhIC (3), PhBarb (4), the
crude reaction mixture between PhIC and NMe2PhBarb in wet chlorobenzene (5) and the
crude reaction mixture between PhIC and NMe2PhBarb in dry chlorobenzene (6). The
numbers supper imposed on spectra 5 and 6 corrolated to that peak originating from spectra
1-4. .................................................................................................................................................30
Scheme 2.1. General chain reaction mechanism for the water catalyzed endcap exchange. ........31
xii
Scheme 2.2 Synthesis of PhIC, NO2PhIC, and NMe2PhIC ...........................................................33
Scheme 2.3 Synthesis of PhBarb, NO2PhBarb, NMe2PhBarb. .....................................................34
Scheme 2.4. Reaction between NMe2PhIC and Benzaldehyde. ....................................................35
Scheme 2.5. Reaction between NMe2PhIC and PhBarb. ..............................................................35
Figure S2.1.
1
H-NMR of the crude reaction mixture between PhIC and NMe2PhBarb in dry
chlorobenzene with 0.7 L water added (3), PhBarb (2) and NMe2PhBarb (1). ...........................39
Figure S2.2.
1
H-NMR of the crude reaction mixture between benzaldehyde and NMe2IC in
dry chlorobenzene (3), PhIC (2) and NMe2PhIC (1). ....................................................................40
Figure S2.3.
1
H-NMR of the crude reaction mixture between 1,3-dimethylbarbituic acid and
NMe2IC in dry chlorobenzene (4), 1,3-dicyanovinylindanone (3), NMe2PhBarb (2) and
NMe2PhIC (1). ...............................................................................................................................40
Scheme 3.1. General Synthetic Scheme of 1-3, BPM, BPB, BPT, BPIC. (i) 1,3-
dimethylbarbituric acid or 1,3-diethylthiobarbituric acid, pyridine, 110 °C, 24 hr. (ii)
indanedione or dicyanovinylindanone, BF3OEt2, acetic anhydride, RT, 30 min. (iii)
malonitrile, TiCl4, pyridine, RT. ....................................................................................................47
Figure 3.1. Molar absorption spectra of compounds 1, BPM, BPB, BPT, BPIO and BPIC at
room temperature (a) Molar absorption spectra of 1, BPM, BPB, BPT, and BPIC in toluene.
(b) Molar absorption spectra of BPT in toluene, dichloromethane, 2Me-THF, and acetonitrile.
........................................................................................................................................................49
Figure 3.2. Emission spectra of compounds 1, BPM, BPB, BPT, and BPIC in fluid solution
at room temperature (a) Emission spectra of 1, BPM, BPB, BPT, BPIO, and BPIC in toluene.
(b) Emission spectra of BPIC in toluene, dichloromethane, 2Me-THF, and acetonitrile. ...........50
Figure 3.3. Emission Spectra of BPM and BPT in 2-MeTHF at 77 K (solid) and 298 K
(dashed). .........................................................................................................................................52
Figure 3.4. Picosecond transient absorption spectra of BPIC in MeCN. ......................................54
Figure 3.5. LUMO (top, mesh) and HOMO (bottom, solid) of 1 (left) and BPIC (right). ...........56
Figure 3.6. (a) Hole and (b) Electron NTO of BPIC. *B3LYP/6-31G** .....................................56
Figure 3.7. Cyclic voltammograms for the first oxidation and reduction waves of BPB, BPT,
BPIO and BPIC collected in 0.1 M n-Bu4NPF6 in CH2Cl2. Potentials were measured against
the Fc/Fc
+
redox couple. Cyclic voltammograms of all the oxidation and reduction waves of
these compounds are provided in the SI. .......................................................................................58
Figure 3.21. Crystal structures of BPB, BPIO, and BPIC-Fused. Thermal ellipsoids are
shown at the 50% probability. All hydrogens are omitted for clarity. The atom colors are: B
(pink), C (grey), N (blue), O (red), and F (green). ........................................................................82
Figure 3.23. Absorption and emission spectra of BPB in various solvents. .................................83
Figure 3.24. Absorption and emission spectra of BPIO in various solvents. ...............................84
Figure 3.25. Absorption and emission spectra of BPT in various solvents. .................................84
xiii
Figure 3.26. Cyclic voltammograms of BPB though (solid) the full solvent window and
(dotted) the first reduction and oxidation waves collected in 0.1 M n-Bu4NPF6 in CH2Cl2.
Potentials were measured against the Fc/Fc
+
redox couple. ..........................................................86
Figure 3.28. Cyclic voltammograms of BPIO though (solid) the full solvent window and
(dotted) the first reduction and oxidation waves collected in 0.1 M n-Bu4NPF6 in CH2Cl2.
Potentials were measured against the Fc/Fc+ redox couple. .........................................................87
Figure 3.29. Differential Pulse Voltammogram of BPIO (left) with Ir(PPy)3 and (right)
without. ..........................................................................................................................................87
Figure 3.30. Cyclic voltammograms of BPT though (solid) the full solvent window and
(dotted) the first reduction and oxidation waves collected in 0.1 M n-Bu4NPF6 in CH2Cl2.
Potentials were measured against the Fc/Fc+ redox couple. .........................................................88
Figure 3.31. Differential Pulse Voltammogram of BPT (left) with Ir(PPy)3 and (right)
without. ..........................................................................................................................................88
Figure 3.32. Cyclic voltammograms of BPT though (solid) the full solvent window and
(dotted) the first reduction and oxidation waves collected in 0.1 M n-Bu4NPF6 in CH2Cl2.
Potentials were measured against the Fc/Fc+ redox couple. .........................................................89
Figure 3.33. Differential Pulse Voltammogram of BPIC (left) with Ir(PPy)3 and (right)
without. ..........................................................................................................................................89
Figure 3.34. Frontier Orbitals for BPM. .......................................................................................90
Figure 3.35. Natural Transition Orbitals for the S1 of BPM. ........................................................90
Figure 3.36. Frontier Orbitals for BPB. ........................................................................................91
Figure 3.37. Natural Transition Orbitals for the S1 of BPB. .........................................................91
Figure 3.38. Frontier Orbitals for BPT. ........................................................................................92
Figure 3.39. Natural Transition Orbitals for the S1 of BPT. ........................................................92
Scheme 4.1. Structure of dipyrrin and dipyridylmethene chromophores. ...................................100
Scheme 4.2. Traditional synthetic route of meso-fused bis-bodipy compounds. ........................101
Scheme 4.3. Reaction proposed by Wang et A. (top) versus observed products (bottom). .........102
Figure 4.1. Normalized absorption spectra of the S1 bands of the compounds presented in this
chapter in dichloromethane. .........................................................................................................103
Figure 4.4. Cation molar absorptivity spectra SBCT complexes in a solution of 20 mM
triphenylamine in dichloroethane. ...............................................................................................111
Figure 4.5. Molar difference absorptivity spectra of (red) the m8B anion in a solution of 10
mM TBAPF and 20 mM biphenyl in THF or (blue) the m8B cation in a solution of 20 mM
triphenylamine in dichloroethane. ...............................................................................................112
Figure 4.6. The sum of the cation and anion molar absorptivity difference spectra (black)
overlaid with the 100ps MA-TA spectrum (red.) for zDIP2 (top left), bis-DIPYR (top right),
and m8B (bottom). .......................................................................................................................115
xiv
Figure 5.1. Cartoon schematic of a PS transferring charge to EC ..............................................123
Figure 4.2. Left) General structures of the cMa complexes studied here ...................................126
Figure 5.4 – (Left) Excited state absorptivity spectra of 10 mM 𝐶𝑢𝐶𝑧𝑀𝐴𝐶 , 𝐶𝑢𝐵𝐶𝑧𝑀𝐴𝐶 , and
𝐶𝑢𝑃 ℎ𝐶𝑧𝑀𝐴𝐶 in a solution of 20 mM triphenylamine in o-xylene in air 5 ns after electron
pulse. (Right) Excited state absorptivity spectra of 10 mM 𝐶𝑢𝐶𝑧𝑀𝐴𝐶 , 𝐶 𝑢 𝐶𝑁𝐶𝑧𝐷𝐴𝐶 , and
𝐶𝑢𝐶𝑧𝐶𝐴𝐴𝐶 in o-xylene under Ar 10 ms after electron pulse ......................................................129
Figure 5.5. The bulk electrolysis spectra of 𝐶𝑢𝐵𝐶𝑧𝑀𝐴𝐶 (left) and A𝑢𝐵𝐶𝑧𝑀𝐴𝐶 (right) in
THF under neutral (0 V vs Ag, black) oxidative (0.9 V vs Ag, red) or reductive conditions (-
2.1 V vs Ag, blue). .......................................................................................................................130
Figure 5.6. (a) Cation molar absorptivity spectra of 10 mM cMa complexes in a 20 mM
solution of triphenylamine in o-xylene. (b) Anion molar absorptivity spectra of 10 mM cMa
complexes in 10 mM TADPF in THF. ........................................................................................131
Figure 5.7. The psTA spectra of 𝐶𝑢𝐵𝐶𝑧𝑀𝐴𝐶 (left) and 𝐴𝑢𝐵𝐶𝑧𝑀𝐴𝐶 (right) after excitation
with a 405 nm pump source (solid lines). ....................................................................................137
Figure 5.8. The sum of the PR molar absorptivity plots (black) compared to the S1 state (blue)
and the T1 state (red) from SADS analysis .................................................................................140
Figure 5.9. 𝐴𝑢𝐵𝐶𝑧𝑀𝐴𝐶 nsTA spectra ........................................................................................142
Figure 5.10. Triplet absorption spectra of 𝐶𝑢𝐶𝑧𝐶𝐴𝐴𝐶 (red) and 𝐶𝑢𝐶𝑁𝐶𝑧𝐷𝐴𝐶 (black) in
aerated o-xylene, before O2 quenching of the triplet state and cation sensitization can occur
(<5ns) ...........................................................................................................................................149
Figure 5.11. Cation molar absorptivity spectra of 10 mM 𝐶𝑢𝐶𝑧𝐶𝐴𝐴𝐶 in 20 mM solution of
triphenylamine in o-xylene. .........................................................................................................149
Figure 5.12. Cation absorptivity of 10 mM 𝐶𝑢𝐶𝑁𝐶𝑧𝐷𝐴𝐶 in benzonitirile. ...............................150
Figure 5.13. nsTA spectra of 𝐶𝑢𝐶𝑧𝑀𝐴𝐶 in THF (left) and toluene (right). ..............................150
Figure 5.14. nsTA spectra of 𝐴𝑢𝐶𝑧𝑀𝐴𝐶 in THF (left) and toluene (right). ..............................150
Figure 5.15. nsTA spectra of 𝐶𝑢𝐵𝐶𝑧𝑀𝐴𝐶 in THF (left) and toluene (right). ...........................151
Figure 5.16. nsTA spectra of 𝐴𝑢𝐵𝐶𝑧𝑀𝐴𝐶 in THF (left) and toluene (right). ...........................151
Figure 5.19. nsTA spectra of 𝐶𝑢𝑃 ℎ𝐶𝑧𝑀𝐴𝐶 in THF (left) and toluene (right). .........................151
Figure 5.20. nsTA spectra of 𝐶𝑢𝐶𝑁𝐶𝑧𝐷𝐴𝐶 in THF (left) and toluene (right). Degradation of
𝐶𝑢𝐶𝑁𝐶𝑧𝐷𝐴𝐶 occurs with illumination in THF...........................................................................152
Figure 5.21. nsTA spectra of 𝐶𝑢𝐶𝑧𝐶𝐴𝐴𝐶 in THF. ....................................................................152
Figure 5.22. nsTA spectra of 𝐴𝑢𝐶𝑧𝐶𝐴𝐴𝐶 in THF (left) and toluene (right). ............................152
Figure 5.23. nsTA spectra of 46 M 𝐴𝑢𝐶𝑧𝑀𝐴𝐶 and 7 mM MePI in THF. ...............................153
Figure 5.24. nsTA spectra of 75 M 𝐴𝑢𝐵𝐶𝑧𝑀𝐴𝐶 and 30 mM MePI in THF............................153

xv
Abstract
Organic photovoltaic (OPVs) have seen a recent surge in photoconversion
efficiencies (PCE) from 12.5% to almost 20% due to the adoption of strong electron donating cores
flanked on either side by strong electron acceptors, called an A-D-A motif, in their electron
acceptor layers . This meteoric rise promises to enable the development of thinner and transparent
solar panels to supplement a renewable energy infrastructure based on Si devices. However,
despite this vast improvement in the technology, there is still more work to be done to realize
device commercialization.
Current A-D-A molecules utilize bulky donor cores that are synthetically complex and
therefore costly, and a simplification of this core is desired to increase market feasibility. Similarly,
materials are required to ultrapure in order to reach high PCEs and maintain long device lifetimes.
However, even with ultrapure materials, we have found a degradation reaction between OPV
materials which impacts these parameters. In this thesis we will first look at understanding and
mitigating degradation with ultrapure materials, followed by showcasing new synthetically
simplified materials for OPVs. Lastly, we will examine bypassing OPV devices altogether to
directly harvest photo-potential to drive the production of solar fuels.

1

Chapter 1: Light, the Universe, and Somethings.
The Industrial Revolution has had a profound and generally beneficial impact on humanity
as a whole. Improvements to the steam engine brought on first by Thomas Newcomen and then
James Watt,
1
lead to improvements in coal mining,
1
textile manufacturing,
2
the automation of the
blast furnace,
3
and the invention of the paddle steamer
4
and steam locomotoive.
2
In turn, increases
of coal supply and automated blast furnace lead to improvements in the quality and quantity of
iron manufactured, followed by the commercial production of steel.
3
Improved textile
manufacturing demanded larger supplies of input goods, which lead to the development of
synthetic bleaching agents and dyes, and therefore a booming chemical industry.
5-7
Meanwhile,
improvements to transportation enabled international trade, increased the movement of people,
and contributed need to great engineering works like the Suez and Panama canals. This period
lead to drastic booms in world population, gross domestic product per capita, personal wealth, the
standard of living,
8
access to food, literacy, and urbanization, and would lead to social revolutions
including the increased civil rights for workers, women, children, and minorities, and the rise of
capitalism, communism, and liberalism.
However, these advantages for mankind have come at an environmental cost to the planet.
The primary fuel source from the commercialization of the steam engine until now have been
hydrocarbon fossil fuel, i.e coal, oil, and natural gas. Not only are reserves of fossil fuels limited,
9

emissions from the burning of fossil fuels are the underlying cause of anthropomorphic climate
change leading to a rise in average global temperature and with it desert expansion, more frequent
fires, heat waves, storms, flooding and droughts, melting of the polar ice caps, and destruction of
coral reefs, all off which risk mass extinction events and lead to severe economic and health
impacts on humans.
10, 11
The most recent report by the International Panel on Climate Change
2

(IPCC) has stated that an increase in the average global temperature of 1.5 °C compared to
pre-industrial levels is a guarantee by 2030, and a rise of 2 °C can only be staved off by a drastic
decrease in emission.
10

Failure to cut emissions rapidly is complicated by society’s increasing need for energy,
predicted to rise 48% globally by 2050.
12
In order to meet emission goals, the development and
implementation of carbon-neutral or carbon-negative renewable energy sources are required.
While several sources exist, none are more promising than solar energy. The total solar flux
incident on the earth at any given moment is 173,000 terawatts (TW)
13
or 23,000 terawatt-years
(TWyr). For comparison the total world energy use for 2015 was 18.5 terawatt-years
14
and
therefore estimated to be roughly 27 TWyr in 2050, or 0.1% of total incident solar energy.
Reasonable realization of this energy source, given 20% photoconversion efficiency, PCE (defined
later), and assuming an upper limit of 6% of the landmass towards photovoltaic harvesting, gives
a total energy generation of 8,300 TWyr over 30 years (TWyr30), compared to an energy
consumption of 660 TWyr30. That means if we implement only 0.4% of the earths total land mass
towards photovoltaic usage, we would meet global energy demands for 2050. In comparison, wind
energy possesses a TWyr30 value of 1,500, while hydroelectric accounts for 90 TWyr30.
For solar energy harvesting, silicon-based technologies reign supreme and are likely to
remain that way. Photovoltaic systems based on either mono-crystalline or poly-crystalline Si
account for over 80% of the photovoltaic (PV) market share, with thin film technologies such as
CIGS and CdTe making up the rest.
15, 16
Silicon owes its dominance to a number of factors. First
Shockley and Queisser calculated the maximum PCE for a single p-n junction PV for an ideal
material to be 30%,
17
though more recent studies pin that number to 33%.
18
PVs based on Si have
a maximum calculated efficiency of 30%,
19
with lab grade devices realizing 26%,
20
and
3

commercial devices reaching 20%.
16
Secondly the natural abundance of Si is high, and the reliance
of it in the computer industry has led to a large processing infrastructure.
21

In addition to Si based PV, organic photovoltaics (OPV) could serve to supplement the PV
infrastructure. High molar absorptivity of organic molecules allow for the construction of devices
on the 100s of nanometer scale, producing devices that are compact, light-weight, and flexible.
Another key advantage of OPVs discussed in chapter 3, is that unlike Si and other band-gap
absorbers, organic molecules can be synthesized that are optically transparent, absorbing photons
only in the near-infrared, NIR. This allows for devices that human can look through. Such devices
have application as replacements of high-efficiency windows which typically have a coating for
absorbing NIR light to keep interiors cool, but ultimately waste the energy stored in these photons.
The rest of this chapter and majority of this thesis will discuss such applications.
The Absorption of Light
In order to understand the working dynamics of a photovoltaic, we must first understand
why and how materials absorb light in the first place. Regardless of the material, its basic building
block is a collection of atoms, bonded together via molecular orbitals (MOs), a linear combination
of each atom’s individual atomic orbitals (AOs) that participates in the bond. An exception is made
for noble gases, which don’t bond, and therefore are single atoms possessing only atomic orbitals.
Each of these MOs (AOs) has an energy associated with its ability to stabilize an electron versus
vacuum. A MO (AO) diagram can be constructed by ranking these orbitals in order of ascending
energy from the bottom up. (orbital energies are generally negative, so this diagram moves from
most negative towards zero). These orbitals are then filled with the electrons present in the system,
also from the bottom up (or aufbau). The highest state to be filled with is called the highest
occupied molecular orbital (HOMO) if it is doubly filed or the singly occupied molecular orbital
4

(SOMO) if it is singularly filled and the lowest unfilled state is called the lowest unoccupied
molecular orbital (LUMO). The remaining MOs can referenced in relation to these key orbitals,
i.e. the orbital one above the LUMO is called the LUMO+1.
Optical transitions generally occur as response of the electric field of an occupied orbital
(the unoccupied orbitals have no electron, and hence no field) to the electric field of a photon,
which causes a perturbation of the orbital geometry. If this perturbation is in line with a series of
selection rules,
22
the transition is said to be allowed and readily occurs, and if not it is “forbidden”
(but still possesses a fractional chance of happening). In the terms of purely atomic transitions,
like those found in neon, the selection rules typically require a change in angular moment (i.e. s-p
transitions are allowed while s-s transitions are forbidden), while MO requires a change in MO
symmetry (i.e. g-u or u-g are allowed, but not g-g or u-u). A picture of a s-p transition is given in
figure 1.

Figure 1.1. Example of an orbital perturbation of the s orbital caused by the electric field (grey) of photon resulting
in the time average formation of a p orbital, which is an allowed transition.

Additionally, the perturbation cannot simply result in an orbital transformation, the
perturbation must contain enough energy to populate an electron from the starting MO (AO) to
one with the same geometry of perturbation. Therefore, in small molecules, only photons with
energies that match the change in energy are absorbed. Because the resulting state has an increased
5

energy, it is commonly referred to as the excited state, while the original state is known as the
ground state. Traditionally, we assign the optical transitions with a naming convention that informs
us first of where the electron was, and then where it went, i.e. a transition from the HOMO to the
LUMO is called a HOMO-LUMO transition. However, this convention is a bit of a misnomer.
Optical transitions are generally not single electron processes, though a large portion of the time
they are dominated by one electron doing the majority (>95%) of moving.
Furthermore, while the originating and destination orbitals resemble the HOMO and the
LUMO, they are not the HOMO and the LUMO, especially not in terms of energy. Remember that
the HOMO energy is defined as the energy required to remove an electron from the system to
vacuum (aka the ionization energy, IE) and the LUMO is the energy required to add an electron
from vacuum to the system (aka the electron affinity, EA). In both these cases, the energy is
affected by the other electrons in the systems providing electron-electron repulsion. For example,
in H2, the liberation of an electron is assisted by the other electron pushing it away, and the addition
of an electron is resisted by the two current electrons also pushing it away. In the case of the optical
HOMO-LUMO transition in H2, when the transitional electron ends up in the “LUMO like orbital”
(LUMO*), only one electron is resisting its occupation on the atom, not two. Additionally, the
stationary electron in the “HOMO like orbital” (HOMO*) is relaxed by not having a second
electron to which it must be paired, but also destabilized by the decrease in bond order. Lastly, the
negatively charge electron is coulombically attracted to, and often in small molecules
electronically coupled, to the positively charge hole it has left behind, again perturbing the
energetic landscape of the transition. Therefore, while the HOMO-LUMO gap is a rough estimate
of the HOMO-LUMO transition energy, it is only an estimate. While the transition energy is
typically lower than the estimate, there are cases where this is not the case, as in the
6

symmetry-breaking charge transfer molecules presented in chapter 4. This principle is represented
in figure 2.

Figure 1.2. Orbital diagram of frontier orbitals of H 2. The ionization energy (IE) is given by the red arrow on the left,
while the electron affinity (EA) is given by the blue arrow in the middle. The green arrow on the left shows the energy
of excited state. Notice that green arrow is smaller than the vector sum of IE and EA.
While the description given above represents a singular molecule, typically in the gas or
liquid state, it can be generalized to polymers and extended solids as well. Remember that a band
diagram is an extension of a bond diagram that describes a large (>10
20
) collection of orbitals.
Because nature resists degeneracy, the energetics of the individual bonds broaden, giving rise to
the range of allowed energies seen in a band. Optical transitions still occur as a result of
perturbation of the electron cloud from being in the configuration of one band to another. However,
because the electron is being now being repealed by >10
20
electrons minimal energy difference
exist between what is now the band gap (what was the HOMO-LUMO gap) and the transition
energy, where it is being repealed by >10
20
minus one, and they can be considered equal.
Therefore, both are called the band gap.

7

Photochemical Work

Figure 1.3. Orbital diagram of HOMO* and LUMO*. The IE* is given by the red arrow, while the EA* is given by
the blue arrow. Attention is called to the difference in magnitude to the two arrows compared to their equivalents if
figure 2. On the right, a cartoon schematic of a voltmeter is drawn to demonstrate that a potential capable of work
exists between these two states as it does between two-half cells of a galvanic cell.
Going back to figure 2, notice that HOMO* is missing an electron, while LUMO* contains
a destabilized electron. If one was to imagine now bringing in an electron from vacuum into the
MO stack of this excited state, by the aufbau principle the electron would not fall down into
LUMO* but instead into HOMO*, and therefore the excited state EA, EA*, would be greatly
increased, and therefore is it is easier to reduce the molecule (Figure 3). Likewise, if we imagine
removing an electron from the excited state, the electron in LUMO* would be removed first, and
therefore the excited state IE, IE*, would be greatly diminished, that is it is easier to oxidize the
molecule. Rehm and Weller quantified the relationship between EA and EA* as well as IE and
IE* in equations 1 and 2 respectively, where E00 is the transition energy and wr is an electrostatic
work term.
23, 24

𝐸 𝐴 ∗
= 𝐸𝐴 + 𝐸 00
+ 𝑤 𝑟 (eq. 1)
𝐼𝐸
∗
= 𝐼𝐸 − 𝐸 00
+ 𝑤 𝑟 (eq. 2)
8

By applying a Hess’s law type analysis between the ground state and excited state, one can
quickly deduce that photochemical energy can be obtained by ionization of the excited state
followed by oxidation of the cationic state back to the ground state, under the approximation that
EA of the cationic state is simply the negative of IE for the ground state. If instead of removing
and replacing the charge to and from vacuum, a wire could be attached to the involved orbitals,
the photochemical potential could instead be utilized to power an electronic device. In essence this
is all a PV is, a means to move an electron from the LUMO* to HOMO in a way such that it does
work, much in the same way a galvanic cell carries an electron from the anode to the cathode.
However, in practice charge separation and extraction are more complicated.
Basic OPV Device Architecture

Figure 4. Left) A general schematic for how an organic photovoltaic is ordered spatially, with each layer on top of
one another and example thicknesses given. Right) A energy diagram of the HOMO (bottom of rectangle) and LUMO
(top of rectangle) for each component. The transfer of electron and holes is given at the bottom and top of the figure
respectively.
Organic photovoltaics can be constructed with a few as three components, or layers, but
generally are fabricated with at least five. These layers are ordered in the device as the anode, hole
transport layer (HTL), active layer, electron transport layer (ETL), and the cathode, with the active
9

layer being of principle concern to this body of work. Moving in order of this stack, IE and EA
decrease in value. It is important that each newly encountered material has both lower IE and EA
then the previous layer so that charges can free move from one electrode to the next. This
architecture with relative IE and EA levels is given in figure 4.
The simplest layers to describe are the anode and cathode. In addition to serving as
conductive metal contacts that extract charge, the difference in work function between the two
metals also serves to create an internal electric field to guide the electron and hole to the cathode
and anode respectively. The electron and hole transport layers are optional layers that serve to
passivate surface defects of the electrodes, improve their work functions, and decrease the barrier
to charge transfer between the electrodes and active layer.
25
The active layer functions as the
absorber of light, and is the only layer where all five processes important to photovoltaic charge
generation occurs: photon absorption, exciton diffusion, exciplex formation and dissociation, and
charge conduction.
While single component active layers are known,
26, 27
it’s much more common for devices
to incorporate binary
28-33
or ternary architectures,
34, 35
where the active layer is comprised of at
least one material that functions as an electron acceptor (A) and one that functions as an electron
donor (D). These materials can also be called p-type and n-type materials respectively. A
difference in EA and IE between the two materials drives charge transfer from the excited state of
one material to the ground state of the other. The components of the active layer can be layered
themselves, termed bi-layer and tri-layer as appropriate, however as the vast majority of OPVs are
solution processed its more simple to avoid this architecture, as the solvents required to process
each new layer can dissolve the last. Instead, most OPVs employ an active layer structure called a
bulk heterojunction (BHJ), where the two or three components of the active layer form a network
10

of interwoven micro-crystallin domains on the order of 10s of nanometers is size. From a device
processing standpoint this is advantage as it allows the components of the active layer to be mixed
together in a single solution. Fortunately, this alternative solution also results in higher preforming
devices so the layering motif of the active layer can be abandoned altogether. To understand why
the adoption of BHJ results in better performing devices, it is now important for us to understand
the five steps of charge generation in OPVs.
The Five Steps of Photocurrent Generation
Again, there are five key processes to the harvesting of solar energy with a PV: photon
absorption, exciton diffusion, exciplex formation and dissociation, and charge conduction. While
these five steps are common to all PV technologies, they will be framed here in the context of
OPVs. Charge conduction is the only process common to all layers of the OPV. The other four
occur solely in the active layer and are hence why that layer is termed that. Below is given a brief
overview of the total process, in order to give the reader context for the following extended
sections.
Step One: Light Absorption
As previously discussed, the absorption of light results in a destabilized electron in
LUMO* and the absence of an electron, or a hole, in HOMO*. This electron-hole pair is also
known as the exciton. Ideally both D and A are strong chromophores capable of absorbing light,
and in doing so enter into their excited states D* or A*, respectively. Remember, small molecule
excitation energies tend to be smaller than the HOMO-LUMO gap, and therefore the exciton is
not capable of transferring just a single charge to an adjacent identical ground state molecule.
Instead, the exciton undergoes a random hopping mechanism (the exciton is an electron-hole pair,
11

and thus has no net charge and is not influenced by the intrinsic electric field of the device),
transferring energy from site to site. Should the exciton (either D* or A*) diffuse to the D/A
interface, the D* can transfer an electron to A, or A* a hole to D.
In doing so, an exciplex is formed, a heterogenous bimolecular excited state. In organic
media, this exciplex experiences significant coulombic attraction which must be paid in order to
separate the exciplex, contributing to a voltage loss. Nevertheless, separation occurs from the
electron and hole now being able to hop away from each other to neighboring A and D molecules
respectively. This hopping continues, now influenced by the electric field and no longer random,
across the A and D domains, then into the ETL and HTL, followed by the cathode and anode, and
out of the device to be used in an external circuit.
36

Step Two: Exciton Migration
As we’ve previously discussed molecular absorption and the energetics of the excited state
or exciton in depth, will skip forward to step two of the photocurrent generation process, exciton
migration. In organic molecules, the electron and hole of an exciton are localized particles,
typically on two wavefunctions that share considerable spatial overlap. As a result, these charges
experience a large degree of coulombic attraction and coupling between the wavefunctions, and
therefore form excitons with binding energies between 0.5-1 eV, exhibit poor exciton mobilities,
and without undergoing an electron spin-flip, rapidly recombine on the order of 0.1-10 ns. These
types of excitons are referred to as Frenkel excitons.
37

For contrast, extended solid materials such as Si, where the number of electrons is >10
20
and the dielectric large, the electron and hole of the exciton delocalize over several lattice sites and
experience minimal coulombic attraction or coupling to one another. These types of excitons,
12

referred to as a Wannier-Mott exciton, therefore exhibit minimal binding energy between the two
charges, on the order of 0.1 eV or less, large exciton mobility, and lifetimes of microseconds.
The low mobility of the Frenkel excitons, combined with their short lifetimes, means that
this type of exciton can only randomly hop up to 5-10 nm.
38
This presents a problem, as active
layers tend to be 100-200 nm thick in order to absorb the majority of photons incident on the
device. If we were to employ a bi-layer motif for the active layer, where the two layers were of
equal thickness, only those excitons formed within 10 nm of the D/A interface, which amounts to
10-20% of the layer thickness, would have the potential to be harvest. Herein lies the genius of the
BHJ motif. By forming a heterogenous solid with interwoven D and A domains on the order of
10s on nanometers, the average distance between an exciton and the D/A interface can be
drastically reduced, and more excitons reach it.
Steps Three and Four: Exciplex Formation and Separation
While two separate steps, exciplex formation and separation are strongly correlated, being
governed by the same thermodynamic design principles, and both impact the short-circuit current,
JSC, and the open-circuit voltage, VOC, in similar ways, and therefore will be discussed together.
In a working OPV, the device energetics are such that the LUMO* of D is above the LUMO
of A and electron transfer from the former to the latter is possible (Figure 5). A similar picture
can be drawn for the HOMO* of A and the HOMO of D for hole transfer. Rates of transfer of these
particles are govern by Marcus theory, and proportional to exp(-DG/RT), where DG is the Gibbs
free energy change associated with the transfer of charge.
13

Figure 1.5. Frontier orbitals for the excited donor and ground state acceptor materials on the active layer. Left) The
offset between the Donor LUMO* and the Acceptor LUMO is large and therefore the exciplex is easily split, and J SC
is increased. However, E DA (black arrow) is small and therefore V OC is minimized. Right) The LUMO*-LUMO gap
is small, and therefore the exciplex has difficulty being split, minimizing J SC. However, E DA (in red) is larger than in
the previous picture (black) and therefore V OC is increased.
After charge transfer from the exciton to ground state, a delocalized D-A* complex is
formed, called an exciplex. While this exciplex show minimal overlap of electron and hole
wavefunctions, and thus is only minimally coupled, the charges are still spatial close, ~2 nm, and
thus coulombically bound. The coulombic potential energy between the two states is roughly on
the order of 10 kbT at room temperature, where kb is Boltzmann’s constant, and thermal energy
alone is not sufficient for drive exciplex separation. Instead, many mechanisms play a role in
efficient separation of this state, such as excess exciton energy, intermolecular charge
delocalization, interface disorder, and differences in material work function.
39, 40

However, it’s important to point out that neither exciplex formation nor exciplex separation
are irreversible reactions, and instead an equilibrium exists between the exciton, exciplex, and
geminal recombination of free carriers in any given device. The rates of backwards charge transfer
between the free carriers to the exciplex, and the exciplex to the exciton, corresponding to moving
backwards in the photocurrent generation process, are greatly diminished as the offset between the
14

LUMO* of the donor and the LUMO of the acceptor is increased, and like wise the HOMO-
HOMO* of set for D and A* respectively. Therefore, increasing these offsets has a positive impact
on the generation of free carriers, and therefore increases the maximum current the device can
provide. Generally, the benchmark for device current is the current provided while the device is
under no load, also known as the short-circuit current, ISC. However, the distinction between
current and current density is often ignored in OPV literature, and that same ignorance will be
applied here, with JSC being the benchmark for device current instead.
However, while increasing the LUMO*-LUMO or HOMO-HOMO* offset is great for
maximizing JSC, remember that the goal of a PV is to supply power, and J SC is only one side of
that coin. When the LUMO* of D transfers its electron to the LUMO of A, that electrochemical
potential difference is lost, and now the maximum potential that can be realized is dependent on
between the HOMO of D and the LUMO of A, DEDA. There exists a balancing act between
maximizing JSC and DEDA.
It worth noting here that realizing DEDA is, as the reader may have guess, improbable
because many intrinsic loss pathways exist, such as geminate and vicinal recombination, material
resistivity, injection barriers between layers, energetic penalties for charge separation, and so
forth.
28, 36, 39, 40
As such, the standard metric for potential we used to bench mark devices is the
maximum voltage which can be supplied before current is reversed, or the open-circuit voltage,
Voc, which is proportional to DEDA, and can be improved by eliminating the loss pathways.

15

The maximum power, Pmax, that a photovoltaic cell can realize is given by equation 3, where
FF is the fill factor, an ideality ratio between the JV performance of an ideal semiconductor versus
the measured device. It is calculated by dividing the product of Jmax and Vmax, the current and
voltage at Pmax respectively, by JSC and VOC, shown in equation 4, and isessentially a measure of
voltage and charge loss once the photocurrent has been generated. Lastly, the photoconversion
efficiency,h, of the cell is measured by taking the quotient of Pmax divided by the incident power,
Pin. As one can see, both Pmax and therefore the efficiency of the cell are dependent on JSC and VOC,
and such a middle ground must be reached in terms of DEDA. Unfortunately, that middle ground
is dependent on each material and necessitates many iterations of device architecture to perfect.
𝑃 𝑚𝑎𝑥
= 𝐽 𝑆𝐶
∗ 𝑉 𝑂𝐶
∗ 𝐹𝐹 (Eq. 3)
𝐹𝐹 =
𝐽 𝑚𝑎𝑥 × 𝑉 𝑚𝑎𝑥 𝐽 𝑆𝐶
∗𝑉 𝑂𝐶
(Eq. 4)
𝜂 =
𝑃 𝑚𝑎𝑥 𝑃 𝑖𝑛
=
𝐽 𝑆𝐶
∗𝑉 𝑂𝐶
∗𝐹𝐹
𝑃 𝑖𝑛
(Eq. 5)
Step Five: Charge Conduction
After the exciplex is separated, the resulting electron and hole can be treated as isolated,
non-coulombically bound as localized charges on A and D respectively. Under the presence of the
internal electric field the charges hop away from each other and towards their respective transport
layers and electrodes. While some groups have applied a localized charge hopping mechanism
following a derivation from non-adiabatic Marcus theory,
41, 42
others have argued that the charges
delocalize over several molecules (or monomer units in the case of polymer materials) and a
general Mullkian-Hush treatment is better suited.
39, 40, 43
The second case better describes the
dominance of polymer and planer materials in OPVs. Regardless, charge transfer continues until
16

either the charge reaches an electrode, encounters a trap state, or encounters the opposite charge
at the D/A interface leading to recombination. The latter events result in charge and voltage losses
for the device, and are mitigated with increasing device purity and increasing charge transfer,
respectively.
State of the Art Electron Acceptors
Now that the working fundamentals of an organic photovoltaic have been layout, we can
turn to focus on what types of materials are utilized as electron donors and acceptors. In the highest
performing devices, the electron donor are exclusively polymer donors comprised of their own
donor and acceptor subgroups, forming what is know as a D-A structure, while electron acceptors
are exclusively large polyaromatic electron donating cores flanked on either side by strong electron
accepting groups, forming an A-D-A structure. Devices utilizing these two materials have reached
18 and 19% PCE for binary and ternary devices respectively.
34, 35, 44-46
While optimization of the
donor material is important, breakthroughs in PCE over the last six years have been lead from the
adoption and improvement of the A-D-A acceptor material, and is where the focus of this thesis
are.
Figure 1.6. Chemical structures of ITIC and Y6, examples of A-D-A electron acceptor materials. Ar and Ak are
aryl- and alkyl- chains respectively.
17

The current A-D-A archetype of the electron acceptors were adopted over fullerene-based
acceptors in 2017, with the utilization of ITIC, and as of 2023 is now dominated by the Y-series
of molecules, with one of the most well known examples, Y6, given in figure 6. The vast majority
of cases, donor cores are comprised of pyrrole and thiophene rings in varying configuration. On
the other hand, without fail, the electron acceptor core has been dicyanovinyl-indanone in all of
the record holding materials. The intramolecular charge transfer that occurs upon excitation of
these materials serves to greatly facilitate exciplex formation by lowering the binding energy of
the material by separating the electron and hole wavefunctions. Furthermore, the planner structures
allow for great face-face pi stacking in the microcrystalline acceptor domains, serving the
delocalize the electron over several repeat units, facilitating exciplex separation and charge
conduction. Both of these functions contribute to the dominance of this archetype.
This thesis will focus on the continued development of these materials, with concepts
outlined here continuing to be developed. Chapter 2 focuses on an interesting inter-molecular
exchange mechanism between two different acceptor subgroup which effects device performance
and lifetimes. Chapter 3 looks at simplification of the donor core which take up to nine steps to
synthesize, with a bodipy chromophore requiring only three to address issues concerning scale up
and commercialization. Chapter 4 lays the ground work for studying charge localization and
transfer in a series of symmetry break charge transfer chromophores, which serves as an alternative
motif to the current A-D-A one, presenting its own unique advantages and challenges. Lastly
chapter 5 will deviate away from the OPV and seek to study light induced intermolecular charge
transfer reactions in photosensitizers in order to harvest the photochemical potential of the excited
state to drive synthetic reactions.

18

References
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22

Chapter 2: Eliminating Decay Pathways in Ternary Solar Cells
Intro
In the previous chapter we talked about charge transfer dynamics in simple binary organic
photovoltaics (OPVs). However, a ternary architecture, comprised of either two donors and one
acceptor or two acceptor and one donor are also commonplace. Ternary OPVs offer advantages
over their more simple binary devices. By adding a third component to the active layer of devices,
more control can be exercised over controlling morphology, orbital offset, and charge mobilities,
all of which affect device performance.
1
These strategies have enable ternary devices to reach
photoconversion efficiencies, PCE, of close to 20%.
2, 3

However, an equally important component is material purity. Impurities can have an
impact on device lifetime and photostability by providing reaction sites for active layer
degradation,
4, 5
as well as impacting the extraction of charges by providing trap states.
6, 7
However,
research is generally concerned with extrinsic impurities, i.e. contaminate introduced via improper
purification of starting materials. Some research has been conducted on degradation at the
organic-inorganic interface,
8, 9
little has been studied about organic-organic interactions. In related
organic light emitting diodes, photoreactions between the dopant and host lead to decreased device
lifetimes, especially for blue emitting devices.
10
As such understanding organic-organic reactions
are immediately important to understanding OPV device lifetime, especially for ternary devices,
where six possible unique interactions are found.
In this chapter, we find that during device fabrication under elevated conditions lead to the
formation of impurities from reactions between the organic substitutes of the active layer. In
particular, we find that the A-D-A acceptor molecules are subject to exchange of the acceptor
23

endcaps between the two acceptor materials, and formation of up to six A-D-A archetype
molecules in the active layer. These impurities lead to decreases in open-circuit voltage, VOC,
short-circuit current, JSC, PCE, and fill factor, FF, both in the freshly fabricated device and
overtime, leading to reduced device lifetimes. Further diving into the mechanism of end-cap
exchange, we find that the vinyl bonds of the A-D-A molecules are able to undergo
retro-Knoevenagel condensation, which allows for a scrambling of A and D when the forward
reaction occurs.
Results
Electron acceptors based on the di(thienocyclopenta)thienobenzothiophene (BT) core have
resulted in some of the highest preforming semitransparent OPVs.
11
Opaque devices utilizing an
active layer comprised of BTIC, BT flanked on both sides with dicyanovinylindanone (IC), have
realized photo conversion efficiencies (PCE) of 8.3% using a 100 nm Ag cathode when paired
with PCE-10 as an electron donor. Opaque devices fabricated using the tetrachloronated analogue
BTIC-4Cl reached PCE of 11.2% using the same Ag cathode thickness. By reducing the Ag
cathode thickness to 10 nm in the PCE-10:BTIC-4Cl device, Yongxi Li et Al. were able to realize
semitransparent devices with a transmission over 40% while maintaining PCE of 7.1%. As such,
ternary blends of PCE-10:BTIC:BT-4IC are attractive for OPVs.
24

Figure 2.1. Left) Device architecture for determining the effects of cold and hot solution conditions on J SC, V OC, PCE,
and FF. Middle) Device architecture for determining the effects of cold vs hot solution on device lifetime.
Right) Molecular structure of BTIC family of A-D-A molecules.
Working in collaboration with Stephen Forrest’s group at the University of Michigan
(where Li belongs), our two groups studied the effect of thermal annealing ternary devices based
on PCE-10:BTIC:BT-4C films. Li fabricated ternary devices with an overall device architecture
of ITO/ZnO (30 nm)/ PCE-10:BTIC/BTIC-4Cl (1:0.75:0.75, w/w/w, 80 nm) / MoOx (10 nm)/ Ag
(100 nm), as shown in Figure 1, where the bulk hetero junction (BHJ) active layer was solution
processed from chlorobenzene (CB). In order to maximize the effects of temperature on the
composition of the active layer, devices were fabricated using fresh (maintained at 25 °C and
fabricated withing 0.5 hours) or aged (maintained at 65 °C for between 3-24 hours, then returned
to 25 °C for processing) CB solutions. The effect of aging on the devices PCE, Jsc, Voc, and FF are
also presented in Figure 2. Heating the solution before solution processing results in a broader
range of observed Voc and Jsc for a given batch compared to the fresh control batch. Furthermore,
as the aging time is increased the fill factor and PCE decrease.

25

0 3 6 12 24
0.74
0.75
Reaction Time (hr)
V
OC
(V)
0 3 6 12 24
0.6
0.65
0.7
Reaction Time (hr)
FF

0 3 6 12 24
19
20
21
Reaction Time (hr)
J
SC
(mA/cm
2
)
0 3 6 12 24
9
9.5
10
10.5
Reaction Time (hr)
PCE (%)

Figure 2.2. Plots of V OC (top left), FF (top right), J SC (bottom left), and PCE (bottom right) as a function of reaction
time in the dark at 65 °C.
In order to study the effects of thermal aging on device lifetime, both fresh and aged
devices were illuminated under a 10 suns intensity white light source for 265 hours (figure 3). To
prevent chemical degradation between the BHJ and the charge transport layers, the layers were
isolated with C70 (2 nm) and IC-SAM layer between MoOx and ZnOx layers respectively.
9, 12
Over
the course of 265 hours, the fresh device displays a PCE 85% of that of its original value. Over the
same timeframe, the aged device displays a PCE less than 65% of its base. Mirroring these changes
26

are sharp decreases in Jsc and FF, though the relative change in FF eventually converges with the
fresh device.
0 50 100 150 200 250 300
60
80
100
Time (hr)
PCE (%)
Cold
Hot
0 50 100 150 200 250 300
98
99
100
Time (hr)
V
OC
(%)
Cold
Hot
0 50 100 150 200 250 300
60
80
100
J
SC
(%)
Time (hr)
Cold
Hot
0 50 100 150 200 250 300
80
90
100
FF (%)
Time (hr)
Cold
Hot

Figure 2.3. Normalized PCE (top left), V OC (top right), J SC (bottom left), and FF (bottom right) vs aging time under
10 sun light illumination.
In order to elucidate the identity of the impurities, BTIC and BTIC-4Cl were dissolved in
CB in a 1:1 ratio and heated at 65 °C under a N2 environment for 15 hours. The crude mixture was
then purified via column chromatography to give three fractions each analyzed by MALDI and
1
H-NMR spectroscopy (Fig x). The MALDI spectrum shows three significant sets of peaks, with
maximum masses of 1685, 1754, and 1823. The maximum masses of the first and last sets of peaks
agree well with calculated masses of 1684 and 1822 calculated for BTIC and BTIC-4Cl, while the
27

intermediate mass seems consistent with the BTIC analogue containing only 2 chlorine groups.
The position of the 2 chlorine groups can be confirmed via
1
H-NMR. The
1
H-NMR spectrum
shows that peak splittings and chemical shifts of the unknown product are similar to both the IC
and dichlorinated IC withdrawing groups in BTIC and BTIC-4Cl, respectively, but with only half
intensity compared to the rest of the signals coming from the donor core. Solving the
1
H-NMR
spectrum shows that the formed species is BTIC-2Cl, which has one of each withdrawing group.
This result indicates that the acceptor end caps exchange in solution, consistent with the reactivity
of C=C bonds over C-H and C-X bonds, where X is a halide. To validate that an equilibrium is
formed via the reaction, a solution of BTIC-2Cl was stirred under identical conditions and yielded
BTIC and BTIC-4Cl as products.
Due to the overlapping peak signals coming from the withdrawing groups in BTIC,
BTIC-2Cl, and BTIC-4Cl, tracking the kinetics of the crude reaction mixture via
1
H-NMR is
impractical. Instead, the analogous molecules BTIC-2F, and BTIC-4F possess easily discernible
peaks via
19
F-NMR and were used to track reaction kinetics instead. Solutions of BTIC and
BTIC-4F (1:1 w/w) were heated at temperatures ranging from 0 °C to 100 °C for 24 hr. The
reaction quotient increases with increasing temperatures, from Q = 0 at 0 °C to Q = 2.86 at 100 °C.
Interestingly Q has a value of 0.11 at 25 °C, showing that even at room temperature, processing
conditions end cap exchange occurs. To understand if this was entirely a solution phenomenon, or
if it also exists in processed devices, a film of BTIC:BTIC-4F was spin-coated from CB, prepared
in the same manner as the fresh devices (vide supra). Heating the film to 65 °C for 72 hours
resulted in no detection of BTIC-2F via
1
H-NMR.
In order to check whether the reaction proceeds via a light activated [2+2] cycloaddition
followed by bond cleavage (Scheme 1), the progress of the reaction was monitored for a solution
28

under a one sun intensity AM1.5G lamp at 0 °C for 2.5 hr. Illuminating the solution results in a
steady color change from green to blue. Recording the spectrum via UV-Vis spectrometry shows
a bleaching of the band at ~750 nm attributed to loss of BTIC and BTIC-4F and formation of a
new band ~600 nm (Figure 4).
19
F-NMR spectra of this solution reveals two new fluorine doublets
not attributed to either BTIC-4F or BTIC-2F. Photodegradation products of the related ITIC
molecule have been reported wherein the IC moiety undergoes a ring-closing reaction with the
ITDD core, resulting in a blue shift of the absorption band.
13
It is possible that our observed
photodegradation product is analogous. Illumination of a film of BTIC:BTIC-4F under the same
conditions for 48 hr results in no change in the
19
F-NMR spectrum. Additionally, the reaction was
performed in the dark in CB at 65 °C for 40 hr and the
19
F-NMR spectrum shows formation of
BTIC-2F. As [2+2] cycloadditions are thermally forbidden but photochemically allowed
transitions in accordance with Woodward-Hoffmann rules, the mechanism proposed in Scheme 1
can be ruled out.
14

400 500 600 700 800 900
0
0.25
0.5
0.75
1
Normalized Absorption
Wavelength (nm)
Fresh
1 hr
1.5 hr
2 hr
2.5 hr

Figure 2.4. Left)Absorption spectra of a solution of BTIC and BTIC-4F under 1 sun illumination over 2.5 hours.
Right)
19
F-NMR spectra of (1) BTIC-4F, (2) BTIC-2F, (3)A solution of BTIC and BTIC-4F kept in the dark at 65 °C
for 72 hours (4) A film of BTIC and BTIC-4F kept in the dark at 65 °C for 72 hours. (5) A solution of BTIC and
BTIC-4F illuminated under one sung intensity at 48 hours. (6) A film of BTIC and BTIC-4F illuminated under one
sung intensity at 48 hours.

29

In order to further test the mechanism of endcap exchange, more readily synthesized
model materials with more diagnostic
1
H-NMR handles are preferred. Vinyl bonded D-A systems
where D is a benzene-based core and A is either IC or 1,3-dimethylbarbituate are attractive due to
the availability of starting materials that allow for one-step reactions. By incorporating strong
electron withdrawing groups (EWG) or electron donating groups (EDG) directly onto the benzene
core, para to the vinyl bond, the chemical shift of the two resulting benzene doublets can be moved
away from the chemical shifts on the benzene backbone of IC. When the EWG dimethylammonia
is used the chemical shifts of the benzene ring move upfield to ~6.5 ppm. To these ends, PhIC,
NMe2PhIC, PhBarb, and NMe2PhBarb were each synthesized via a Knoevenagel condensation
between either benzaldehyde, 4-dimethylaminobenzaldehyde and IC or 1,3-dimethylbarbituric
acid. The chemical shifts of each molecule are given the synthetic information.
Refluxing PhIC and NMe2PhBarb in CB overnight results in formation of peaks belonging
to PhBarb and NMe2PhBarb with a Keq of 0.44 (Figure 5). Interestingly, peaks in the aldehyde
region of the spectra are observed at 10.02 and 9.73 ppm, corresponding to the
4-dimethylaminobenzalde and benzaldehyde, respectively. This result suggests that a retro-
Knoevenagel condensation is taking place, in which dissolved water hydrolyzes the C=C bond to
form the benzaldehyde and respective free doubly protonated acceptor molecule. It’s important to
stress that the observation of only the signal corresponding to the aldehyde proton the
benzaldehydes being observed is not determinantal to the conclusion that those products are
present. Aldehyde signals in
1
H-NMR present as narrow peaks which allow them to stand above
the baseline in comparison to the broader signals coming from other protons in the molecule.

30

Figure 2.5.
1
H-NMR peaks of PhIC (1), NMe 2PhBarb (2), NMe 2PhIC (3), PhBarb (4), the crude reaction mixture
between PhIC and NMe 2PhBarb in wet chlorobenzene (5) and the crude reaction mixture between PhIC and
NMe 2PhBarb in dry chlorobenzene (6). The numbers supper imposed on spectra 5 and 6 corrolated to that peak
originating from spectra 1-4.
To verify that the end cap exchange proceeds through a retro-Knoevenagel condensation
reaction, the reaction of PhIC and NMe2PhBarb was repeated with anhydrous CB that was further
dried over 3 Å molecular sieves. Contrary to the experiment conducted in as-obtained CB, no color
change was observed and no observable peaks of products are found in the
1
H NMR spectra
(Figure 5). However, introducing 1 M equivalent water into the mixture and refluxing for the same
duration results in NMR peaks corresponding to trace amounts of products, as well as those of
31

benzaldehyde (Figure S1). This confirms that the endcap exchange process does undergoes a H2O-
catalyzed dissociative retro-Knoevenagel condensation reaction to generate the aldehyde and free
methylene compounds, which then reform either the original benzaldehydes, or exchange to form
new products. This conclusion is also supported by the reaction between two archetype NFAs
BTIC and BTIC-4F. We also examined reactions of NMe2PhIC with both benzaldehyde and
1,3-dimethylbarbituric acid in anhydrous CB to determine whether free donors or acceptors can
directly exchange with a D-A molecule (Figure S2-3). Interestingly, reacting NMe2PhIC with 1,3-
dimethylbarbituric acid results in the formation of NMe2Barb and free IC, while the reaction with
benzaldehyde failed to exchange with the NMe2PhIC. This can be rationalized by the acidic C-H
bond in 1,3-dimethylbarbituric acid being able to undergo a 1,2-addition across the vinyl bond,
followed by elimination of a proton and the IC group to give the observed products.

Scheme 2.1. General chain reaction mechanism for the water catalyzed endcap exchange.

The proposed mechanism of the retro-Knoevenagel reaction is given in scheme 2. After
hydrolysis of the C=C bond (initiate) the generic formed activated methylene AH2 is able to add
32

to the vinyl bond of a D’-A’ molecule, giving a sp
3
transition state carbon. The intermediate then
undergoes 1,2-elimination to form D’-A, producing A’H2 in the process and the reaction
propagates indefinitely. Reaction termination occurs when a free activated methylene finds an aryl
aldehyde. The exchange reaction that results from both D-A and D’-A’ hydrolyzing to aryl
aldehyde and free activated methylene, followed by those free intermediates mixing cannot be
ruled out as a coinciding reaction, however since the population of the aryl aldehyde and activate
methylene is low, it can be ruled out as a significant contributor to endcap exchange.
Conclusion
In summary, we find that ternary OPV comprised of two ADA type electron acceptors and
a generic donor undergo significant enough degradation when subjected to heat in the solution
phase, such as when the device is being thermally anneals. This degradation leads to immediate
decreases in FF and PCE compared to cold processed devices, as well as further decreases to PCE
and JSC during operation leading to reduced LD80 device lifetimes. The mechanism of this
degradation was elucidated to be initiated by a retro-Knoevenagel condensation of the vinyl bond
between the D-A subgroup in the presence of water. This results in the formation of an aryl
aldehyde D and an activated methylene A. A is then able to perform a 1,2 addition over the vinyl
bond of D’-A’, where in the resulting intermediate undergoes a 1,2 elimination to form D’-A and
A’ which propagates the reaction. More striking is the ability of deliberately added A to initiate
this process by adding to the D’-A’ even in the absence of water. While all reactions examined
here examined the processing conditions of ternary cells, there is no reason that A could not attack
D-A, resulting in intermediates and by products that effect the efficiency of binary devices as well.
These result serves to highlight the extreme importance of purity, both in processing solvents and
materials, in the overall performance and lifetime of OPVs.
33

Acknowledgement
I would like to thank Yongxi Li for his work on the BTIC related studies and device work.
Experimental
Synthesis

Scheme 2.2 Synthesis of PhIC, NO 2PhIC, and NMe 2PhIC
Synthesis of 2-(2-benzylidine-3-oxo-2,3-dihydo-1H-indene-1-ylidine)malononitrile. (PhIC).
The compound was synthesized based on a modified literature procedure. Under an inert
atmosphere of nitrogen using standard Schlenk techniques, 1-(Dicyanomethylene)-3-indanone
(183.2 mg, 943.39 mmol) was dissolved in 20 mL of toluene. Benzaldehyde (0.09 mL, 0.9 g, 900
mmol) and 3 drops of pyridine as a catalyst was added and the reaction was heated to reflux for 4
hr. The solvent was removed in vacuo to give a black solid. It was washed 3x with sat. K2CO3(aq),
3x with methanol, and 3x with ether, affording a pure green powder (63.4 mg, 224.6 mmol, 25.4%
yield). .
Synthesis of 2-(2-(4-(dimethylamino)benzylidene)-3-oxo-2,3-dihydro-1H-inden-1-
ylidene)malononitrile (NMe2PhIC).
1-(Dicyanomethylene)-3-indanone (100.6 mg, 518.04 mmol) and (74.5 mg, 499.35 mg)
and 4-dimethylaminobenzaldehyde were dissolved in 5 mL ethanol. The solution quickly turned
from grey to purple. 10 mL piperdine was added and the solution turned red. The solution was
then refluxed for 4 h. The solvent was removed in vacuo to give an orange solid. The solid was
34

washed 3x with cold ethanol and 2x with ether, affording a dark purple powder (101.2 mg,
311.03 mmol, 62.3 %).

Scheme 2.3 Synthesis of PhBarb, NO 2PhBarb, NMe 2PhBarb.
Synthesis of 5-benzylidene-1,3-dimethylpyrimidine-2,4,6(1H,3H,5H)-trione (PhBarb).
NN’-dimethylbarbituric acid (641.9 mg, 4.11 mmol) was dissolved in 25 mL H2O.
Benzaldehyde (0.5 mL, 5 mg, 5 mmol) was added and the solution was left to stir for 2 h in which
time a precipitate was formed. The precipitate was washed 3x with ethanol and 3x with ether to
afford a white powder (977 mg, 3.40 mmol, 69.4%).
1
H NMR (δ)8.58 (s, 1H), 8.11 – 8.00 (m, 2H),
7.58 – 7.43 (m, 3H), 3.40 (d, J = 19.5 Hz, 6H).
Synthesis of 5-(4-(dimethylamino)benzylidene)-1,3-dimethylpyrimidine-2,4,6-(1H,3H,5H)-
trione (NMe2PhBarb).
NN’-dimethylbarbituric acid (641.9 mg, 4.11 mmol) and 4-dimethylbenzaldehyde
(493.6 mg, 3.31 mmol) were dissolved in 10 mL ethanol. The solution immediately turned
orange and precipitate began to form (confirmed by dissolving the two materials in ethanol
individually which formed translucent solutions and then adding them together to generate an
orange solution which began precipitating). The solution was refluxed for 6 h. The solution was
then cooled in an ice bath and washed 3x with cold ethanol and 2x with ether to afford a salmon-
colored powder (870.5 mg, 3.03 mmol, 91.5%).
1
H NMR (δ) 8.44 (s, 1H), 8.43 – 8.36 (m, 2H),
6.77 – 6.65 (m, 2H), 3.40 (d, J = 3.3 Hz, 6H), 3.15 (s, 6H).

35

Anhydrous Non-exchange of Donor groups between NMe2PhIC and Benzaldehyde.

Scheme 2.4. Reaction between NMe 2PhIC and Benzaldehyde.
Under an inert atmosphere of nitrogen using standard Schlenk techniques, NMe2PhIC (10.3 mg,
31.7 mmol) and benzaldehyde (0.05 mL, 52.0 mg, 490 mmol) were added to 10 mL of dry
chlorobenzene taken form an anhydrous sealed bottled and further dried over 3 Å molecular sieves
to make a purple solution. After refluxing for 48 hours, the solution remained purple. The solvent
was removed in vacuo.
1
H-NMR spectroscopy revealed only starting material.

Hydrous Exchange of Endcaps

Scheme 2.5. Reaction between NMe 2PhIC and PhBarb.
Under an inert atmosphere of nitrogen using standard Schlenk techniques, PhIC (10.6 mg,
43.40 mmol) and NMe2PhBarb (10.7 mg, 32.89 mmol) were added to 10 mL of hydrous
chlorobenzene to form an orange-yellow solution. After refluxing 16 hours, the solution turned
dark red. The solvent was removed in vacuo.
1
H-NMR spectroscopy revealed a mixture of product
and reactants in a 1:1.44 ratio, as well as peaks at 10.02 and 9.73 belonging to the aldehyde proton
of benzaldehyde and 4-methylamino-benzaldehyde respectively. Only the aldehyde peaks were
observed due to the sharpness of these peaks granting them the ability to stand above the baseline.
36

Anhydrous Non-exchange of Endcaps
Under an inert atmosphere of nitrogen using standard Schlenk techniques , PhIC
(10.02 mg, 41.02 mmol) and NMe2PhBarb (10.15 mg, 31.20 mmol) were added to 10 mL of dry
chlorobenzene taken from an anhydrous sealed bottle and further dried over 3 Å molecular sieves
to make an orange-yellow solution. After refluxing 16 hours, the solution remained orange-yellow.
The solvent was removed in vacuo.
1
H-NMR spectroscopy revealed only starting material.
Exchange of Endcaps with 1 molar equivalent of water.
Under an inert atmosphere of nitrogen using standard Schlenk techniques , PhIC
(10.52 mg, 43.07 mmol) and NMe2PhBarb (10.73 mg, 32.98 mmol). were added to 10 mL of dry
chlorobenzene taken from an anhydrous sealed bottle and further dried over 3 Å molecular sieves
to make an orange-yellow solution. DI water (0.7 mL, 700 mg, 38.86 mmol) was added via
micropipette. After refluxing 16 hours, the solution turned brown. The solvent was removed in
vacuo.
1
H-NMR spectroscopy revealed traces of product in the baseline of the NMR. The ratio of
product to starting material was 0.07:1.00. Peaks at 10.02 and 9.73 corresponding to the aldehyde
peaks of benzaldehyde and 4-methylamino-benzaldehyde were also observed.
Anhydrous Non-exchange of Donor groups between NMe2PhIC and Benzaldehyde.
Under an inert atmosphere of nitrogen using standard Schlenk techniques, NMe2PhIC (XX mg,
xx mmol) and benzaldehyde (0.05 mL, 490 mmol) were added to 10 mL of dry chlorobenzene
taken form an anhydrous sealed bottled and further dried over 3 Å molecular sieves to make a
purple solution. After refluxing for 48 hours, the solution remained purple. The solvent was
removed in vacuo.
1
H-NMR spectroscopy revealed only starting material.
37

Anhydrous exchange of acceptor groups between NMe2PhIC and 1,3-dimethylbarbituic
acid.

Under an inert atmosphere of nitrogen using standard Schlenk techniques, NMe2PhIC (XX mg,
XX mmol) and 1,3-dimethylbarbiutate (xx mg, xx mmol) were added to 10 mL of dry
chlorobenzene taken form an anhydrous sealed bottled and further dries over 3 Å molecular sieves
to make a purple solution. After refluxing for 48 hours, the solution turned deep red. The solvent
was removed in vacuo.
1
H-NMR spectroscopy revealed a mixture of products and reactants in a
1:1 ratio. Furthermore, no trace of aldehyde protons were observed.
Materials. The cathodic buffer material IC-SAM, and ADA materials BTIC
11
, BTIC-4Cl
15
,
ITIC
16
, ITIC-4F
12
, & ITIC-DM
17
were synthesized according to previous reports. PCE-10, Y16,
and Y16-4F were purchased via 1-Materials. In air and anhydrous chlorobenzene were purchased
from Sigma-Aldrich.
Device Fabrication. Device fabrication has been described previously.
15
Briefly, pre-patterned
ITO-coated glass substrates were cleaned with detergent, acetone, and isopropanol, followed by
CO2 snow cleaning and exposure to UV-ozone for 15 minutes. The ZnO layer was spin coated
from ZnO precursor solution and annealed at 150 °C for 30 minutes. The IC-SAM was dissolved
in methanol and spin coated on top of the ZnO layer. This layer was then rinsed with methanol to
remove any residue. The hot ternary blend was prepared at 20 mg/mL PCE-10:BTIC:BTIC-4Cl
(1:0.75:0.75, w/w/w) in CB at 65 °C for 15 hr, while cold solution was prepared the same way
minus heating. The solution was allowed to cool to room temperature, filtered through a
0.45 micron PTFE syringe filter and spin coated onto the substrate. The substrate was transferred
38

into a vacuum deposition chamber where the C70, MoO3 and Al films were deposited at 0.2 Å /s in
a high vacuum chamber with a base pressure of 10
-7
torr. The deposition rates and thicknesses
were measured using quartz crystal monitors and calibrated post-growth by variable-angle
spectroscopic ellipsometry. Device areas of 2 mm × 2 mm were defined by the overlap of the ITO
anode and the Al cathode with a 50 µm thick shadow mask.
Solar Cell Characterization. The current density-voltage (J-V) characteristics and external
quantum efficiencies (EQE) of the cells were measured in a glovebox under N2. The EQE
measurements were performed with via a Xe lamp. The current output from the devices, as well
as from a reference National Institute of Standards and Technology (NIST)-traceable Si detector,
were recorded using a lock-in amplifier. Light from the Xe lamp was filtered to simulated AM
1.5G spectrum used as the source for J-V measurements. The lamp profile is varied using a neutral
density filter and was calibrated by a Si reference cell certified by NREL. Each cell was measured
with six light intensities from 0.001 to 1 sun. J SC was calculated from the EQE spectra and fell
withing 7 % of the relative value from that measured via JV characteristics.
Device Stability Measurements. All devices were encapsulated by sealing a glass cover slide on
top of the substrate under N2 in a glove box. All aging of the devices were preformed under air
under 10 suns intensity light. To avoid excess heating, the cells were actively cooled to 35 °C.
Illumination from LEDs operating at various intensities to produce a photocurrent equivalent to
AM1.5G illumination equivalent to 1-sun intensity was used. To avoid excess heating at 10
Supporting Figures.
39

Figure S2.1.
1
H-NMR of the crude reaction mixture between PhIC and NMe 2PhBarb in dry chlorobenzene with 0.7
mL water added (3), PhBarb (2) and NMe 2PhBarb (1).

40

Figure S2.2.
1
H-NMR of the crude reaction mixture between benzaldehyde and NMe 2IC in dry chlorobenzene (3),
PhIC (2) and NMe 2PhIC (1).

Figure S2.3.
1
H-NMR of the crude reaction mixture between 1,3-dimethylbarbituic acid and NMe 2IC in dry
chlorobenzene (4), 1,3-dicyanovinylindanone (3), NMe 2PhBarb (2) and NMe 2PhIC (1).

41

References
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of Highly Efficient Ternary Organic Photovoltaics: From Morphology and Energy Loss to
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Shi, M.; Zuo, L.; Chen, H., Desired open-circuit voltage increase enables efficiencies approaching
19% in symmetric-asymmetric molecule ternary organic photovoltaics. Joule 2022, 6 (3), 662-
675.
3. He, C.; Pan, Y.; Ouyang, Y.; Shen, Q.; Gao, Y.; Yan, K.; Fang, J.; Chen, Y.; Ma, C.-
Q.; Min, J.; Zhang, C.; Zuo, L.; Chen, H., Manipulating the D:A interfacial energetics and
intermolecular packing for 19.2% efficiency organic photovoltaics. Energy & Environmental
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4. Mateker, W. R.; Douglas, J. D.; Cabanetos, C.; Sachs-Quintana, I. T.; Bartelt, J. A.;
Hoke, E. T.; El Labban, A.; Beaujuge, P. M.; Fréchet, J. M. J.; McGehee, M. D., Improving the
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Materials Chemistry C 2018, 6 (2), 219-225.
6. Liu, R.-L.; Tang, C.; Huang, W., Overdamping Carrier Transport and Quantitative
Thermodynamic Analyses of Traps in Organic Semiconductor NPB. The Journal of Physical
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M., Role of Nonfullerene Acceptor Impurities and Purification on the Efficiency and Stability of
Organic Photovoltaics. Solar RRL 2023.
8. Ding, K.; Li, Y.; Forrest, S. R., Characterizing and Improving the Thermal Stability of
Organic Photovoltaics Based on Halogen-Rich Non-Fullerene Acceptors. ACS Appl Mater
Interfaces 2022, 14 (4), 5692-5698.
9. Li, Y.; Huang, X.; Ding, K.; Sheriff, H. K. M., Jr.; Ye, L.; Liu, H.; Li, C. Z.; Ade, H.;
Forrest, S. R., Non-fullerene acceptor organic photovoltaics with intrinsic operational lifetimes
over 30 years. Nat Commun 2021, 12 (1), 5419.
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G.; Noh, C.; Kim, S.; You, Y., Degradation of blue-phosphorescent organic light-emitting devices
involves exciton-induced generation of polaron pair within emitting layers. Nat Commun 2018, 9
(1), 1211.
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11. Li, Y.; Zhong, L.; Gautam, B.; Bin, H.-J.; Lin, J.-D.; Wu, F.-P.; Zhang, Z.; Jiang, Z.-
Q.; Zhang, Z.-G.; Gundogdu, K.; Li, Y.; Liao, L.-S., A near-infrared non-fullerene electron
acceptor for high performance polymer solar cells. Energy & Environmental Science 2017, 10 (7),
1610-1620.
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Optimization Enables over 13% Efficiency in Organic Solar Cells. J Am Chem Soc 2017, 139 (21),
7148-7151.
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Photodegradation of A-D-A Acceptors for Organic Photovoltaics*. Angew Chem Int Ed Engl
2021, 60 (47), 24833-24837.
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Solar Cells. Adv Mater 2016, 28 (42), 9423-9429.

43

Chapter 3: High Voltage Organic Photovoltaics based on an Acceptor-Donor-Acceptor
Molecule Utilizing a Bodipy Core.
Introduction
Extended solid photovoltaic systems (PV) based on either mono- or poly-crystalline silicon
currently hold 80-90% of the photovoltaic market share, while thin film technologies (i.e CIGS,
CdTe, amorphous-Si) make up the remainder.
1-2
The dominance of Si based PV is well earned,
owing to the large natural abundance of Si and high commercial photoconversion efficiencies
(PCE) of 15-20%.
2
However, the aforementioned technologies all rely on bandgap absorbers,
which by definition absorb all light above their bandgap, resulting in opaque devices. Organic and
organometallic systems, on the other hand, have discrete gaussian like absorptions which only
absorb certain parts of the light spectrum. As such, near-infrared (NIR) chromophores have
attracted considerable interest for semitransparent organic photovoltaic (ST-OPVs) as a way of
producing optically clear devices for use in “smart”-windows for buildings, reaching 6-8% PCE
when using a binary bulk heterojuction (BHJ) active layer.
3-5
The PCE of these ST-OPVs are close
to one third of the 18% PCE measured for the highest preforming binary BHJ non-transparent
organic photovoltaic (OPVs),
6-7
with a large portion of the decreased PCE coming from
transmitted photons in the ST-OPVs.
One limitation of using organic molecules is the nature of the formed excitation upon the
absorption of light. Unlike extended solids which form delocalized Wannier-Mott excitons with
binding energies (BE) under 0.1 eV, the local excited (LE) states in organic molecules form tightly
bound Frenkel excitons, with BE between 0.5-1 eV.
8
Splitting this exciton serves as a large source
of open circuit voltage (VOC) losses in OPVs. In order to get around this limitation, it is common
to use molecules which have charge transfer (CT) excited states, which feature less tightly bound
44

excitons with BE between the two regimes. In the past our group has studied a number of boron
dipyrrin (bodipy) dimers or zinc bis-dipyrin complexes (zDIP) capable of undergoing symmetry
breaking charge transfer (SBCT) with this aim in mind.
9-13
Utilizing a perchlorinated zDIP
complex, ZCl, we demonstrated an OPV device with a VOC of 1.33 V strongly contrasting the C60
control device that exhibited a VOC of 0.86 V, despite an identical driving force for charge
separation and extraction.
11

Another technique is to evoke intramolecular charge transfer (ICT) state by linking an
electron donor subunit (D) and acceptor subunit (A) together in an alternating pattern. This
technique is commonly employed for both polymers donor
14-16
and small molecule acceptor
acceptors in OPVs.
17-18
In particular adoption of an A-D-A motif for small molecule acceptors is
what has let OPVs reach the aforementioned 19% PCE. However, the donor cores seen in these
materials are often synthetically complex, and utilize expensive starting materials, and involve
coupling reactions requiring heavy metal catalysts. For example, the donor core of Y6 alone
requires 7 overall steps with a total yield of less than 10 %.
In this light, we have examined if a bodipy core may be suitable as a donor in the A-D-A
motif. Bodipy dyes are chemically and photochemically stable, strongly absorbing (e = 10
4
– 10
5

M
-1
cm
-1
), and tunable chromophores. Additionally they can be synthesized from simple pyrroles
and aryl aldehydes in one pot in high yields (30-60%).
19-21
In this paper, we have synthesized and
characterized 5 new A-D-A small molecule acceptors utilizing 1,3,5,7-tetramethyl-8-phenyl-
bodipy (1) as the donor core, each end capped with a unique A, including 3-dicyanovinylindanone
(IC). The acceptors are strategically installed at the 2- and 6- position of the parent molecule, where
LUMO density resides. This results in a bathochromic shift in the transition energy of the
compounds, with compounds possessing maximum wavelengths of absorption (lmax) out to
45

623 nm. Furthermore, evidence of the formation of CT states is observed, and the presence of
strong electron withdrawing group results in more negative LUMO energies (-3.54 – -3.77 eV),
making these compounds suitable of OPVs. OPV cells where fabricated using two of the
compounds paired with PCE-10 as a donor polymer
14
and achieve high VOC of 0.67 - 0.72 V.
Results and Discussion
The tetramethyl-bodipy core (1) was synthesized by double condensation of
2,4-dimethylpyrrole on benzaldehyde followed by reduction with dicholorodicyanoquinone
(DDQ), deprotonation with diisopropylethylamine (DIPEA) and borylation with boron trifloride
ethyl etherate (BF3OEt2) in a modification of the traditional one pot synthesis reported in
literature.
19, 22-23
1 was produced this way in 33% yield as an orange-green metallic solid. Of note,
the selection of THF over CH2Cl2 as the reaction solvent allowed for this reaction to be carried out
on the gram scale due to the increased solubility of DDQ. Formylation of the core occurs via
stepwise Vilsmeier-Haack formylations of 1 with N,N-dimethylformamide (DMF) and
phosphorous(V) oxychloride to yield 2-formyl-tetramethyl-bodipy (2) and
2,6-diformyl-tetramethyl-bodipy (3) as red solids in 81% and 94% yield respectively. BPB, and
BPT were synthesized through a heated base catalyzed Knoevenagel condensation onto 3 at
110 °C hr utilizing pyridine as a catalyst.
24
BPB was isolated as a matte brown solid in 71% yield
while BPT was isolated as a matte green solid in 38% yield. Attempts to synthesize BPM or
BPIO utilizing a base driven approach resulted in marginal yields, while attempts to synthesize
BPIC resulted in a ring-closed product BPIC-FUSED in 2% yield. The methyl groups of the
bodipy core are known to be acidic.
25
After condensation of IC onto 3, pyridine deprotonates one
of the methyl groups, which then attacks the dicyanovinyl double bond forming a seven membered
ring. The single crystal structure of BPIC-FUSED is given below. While the product isolated
46

results from attack by the methyl groups in the 1- and 7- position, deprotonation and ring closure
is equally probable from all methyl groups, as well is intermolecular nucleophilic attack. The crude
reaction mixture most likely contains many isomers and oligomers, reflected in the low yield of
BPIC-FUSED.
Recently, a report of numerous ADA molecules being synthesized through a BF3OEt2
catalyzed reaction with acid anhydrides and the requisite methylene has been published.
26

Following this reference, BPIO was synthesized as a metallic brown solid in 93% yield and BPIC
was synthesized as a matte green solid in 73% yield. Important to this reaction is the ability for
BF3 to coordinate to both the formyl groups of 3, in order to react with the acid anhydride to form
a diacetal, and the keto oxygen of indanedione and dicyanovilylindanoe, stabilizing its enol form,
and activating the substrate towards condensation. With the lack of the keto group in malonitrile,
the condensation doesn’t occur. Instead, BPM was synthesized via a modification of the reaction
using the oxophile TiCl4 to activated 3 and pyridine to deprotonate malonitrile, yielding a metallic
green solid in 13.4% yield. This overall reaction scheme is represented in Scheme 1. The overall
reaction yield for BPIO is 23 % over 3 steps, which is a marked improvement over Y6, which has
an overall yield of <10% over 7 steps.
47

Structural Data
Single crystals of BPB, and BPIO were grown via layering of pentane and a saturated
solution in CDCl3 in an NMR tube, while single crystals of BPIC-Fused were grown via slow
evaporation of ethyl acetate, and each was analyzed via X-ray diffraction (Figure SI1).
BPIC-Fused presents as a largely planer structure, with a dihedral angle of 10° between the core
and IC moieties. While the 7 membered ring formed is also largely planer, each of the sp
3
carbons
bearing the two cyano groups are puckered out of the plane of the ring and point in opposite
directions of each other. Conversely, BPB and BPIO possess large dihedral angles between the
core and methylene unit of 40° and 42° respectively. This deviates away from the traditional planer

Scheme 3.1. General Synthetic Scheme of 1-3, BPM, BPB, BPT, BPIC. (i) 1,3-dimethylbarbituric acid or
1,3-diethylthiobarbituric acid, pyridine, 110 °C, 24 hr. (ii) indanedione or dicyanovinylindanone, BF 3OEt 2, acetic
anhydride, RT, 30 min. (iii) malonitrile, TiCl 4, pyridine, RT.
48

nature of conventional ADA molecules, which will impact coupling of the A and D units
13
and
effect device morphology.
Photophysical Properties
The molar absorptivity and emission spectra of these dyes in solvents of various polarity
are shown in figure 2, 3, and S14-17 and tabulated in Table 1, S1. The large value of molar
absorptivity (ε) displayed by each compound is indicative that the large oscillator strength of the
parent compound 1 is able to be preserved through conjugation of electron acceptors in the 2- and
6- position. These dyes display values of e in toluene between 6.95 × 10
4
M
-1
cm
-1
for BPT to 13.4
× 10
4
M
-1
cm
-1
for BPIO. The series feature bathochromic shifts relative to 1 of up to 128 nm in
toluene, out to 627 nm in BPIC, trending with an increase in electron acceptor strength of the
installed substitutes.
Increasing the electron withdrawing strength of the acceptor groups also increases the
solvatochromic character of the dyes. The parent compound 1 displays a solvatochromic shift of
240 cm
-1
between toluene and acetonitrile for its lowest energy transition, while BPIC displays a
shift of 630 cm
-1
. Additionally, BPIC shows a change in its full width at half max (DFWHM) for
this transition of 20% between the two solvents, compared to the parent compound’s DFWHM of
8%. Interestingly BPT, which displays a similar solvatochromic shift of 610 cm
-1
shows a
DFWHM of 50%. The relatively large change for BPT compared to BPIC stems from the latter’s
already wide absorption manifold in toluene (1800 cm
-1
).

49

400 500 600 700
0
0.2
0.4
0.6
0.8
1
Wavelength (nm)
BPIC
BPT
BPIO
BPB
BPM
1
Normalized Absorption
500 600 700
0
2
4
6
8
10
Toluene
CH
2
Cl
2
2Me-THF
MeCN
Molar Absorptivity (10
4
M
-1
cm
-1
)
Wavelength (nm)

Figure 3.1. Molar absorption spectra of compounds 1, BPM, BPB, BPT, BPIO and BPIC at room temperature (a)
Molar absorption spectra of 1, BPM, BPB, BPT, and BPIC in toluene. (b) Molar absorption spectra of BPT in toluene,
dichloromethane, 2Me-THF, and acetonitrile.

Each compound emits between 548-627 nm in toluene. The emission manifold for each
dye is broad and featureless across solvents of all polarities, resembling the line shape of the parent
compound. Compared to 1, which displays a negative solvatochromic shift of 270 cm
-1
moving
from toluene to acetonitrile, BPT displays a larger hypsochromic shift of 580 cm
-1
for these same
solvents, while the reddest emitter BPIC exhibits a reduced shift of 140 cm
-1
. The magnitude of
the Stokes shift in each dye is largely solvent independent. BPM demonstrates the smallest shift
of 400 cm
-1
in both toluene and acetonitrile, while BPT demonstrates a shift of 1100 cm
-1
for each.
BPIC displays the only solvent dependent Stokes shift of 850 cm
-1
in toluene and 1350 cm
-1
in
acetonitrile. The increasing solvatochromic character in these dyes for both absorption and
emission stems from the presence of a newly formed close lying CT state which the LE transition
found in the parent compound is able to couple too.
27
This CT state is formed by coupling the
bodipy core with the acceptor groups. The lack of solvent dependance on the Stokes shifts for the
majority of the dyes suggests that this CT state lacks a permanent dipole change in the excited
state, and the transfer of charge is shared equally between the two wings.
50

500 600 700 800
0
0.2
0.4
0.6
0.8
1
Wavelength / nm
BPIC
BPT
BPIO
BPB
BPM
1
Normalized Emission
600 650 700 750 800 850 900
0
0.2
0.4
0.6
0.8
1
Toluene
CH
2
Cl
2
2-MeTHF
MeCN
Normalized Emission
Wavelength (nm)

Figure 3.2. Emission spectra of compounds 1, BPM, BPB, BPT, and BPIC in fluid solution at room temperature (a)
Emission spectra of 1, BPM, BPB, BPT, BPIO, and BPIC in toluene. (b) Emission spectra of BPIC in toluene,
dichloromethane, 2Me-THF, and acetonitrile.

lAbs λEm
ΦPL
τ kr knr
(nm) (nm) (ns) (10
8
s
-1
) (10
8
s
-1
)
Toluene
1 503 515 62% 3.3
a
1.9 1.1
BPM 548 576 83% 3.4
a
2.5 0.50
BPB 574 596 21% 1.4
a
1.5 5.8
BPIO 582 603 32% 1.34
a
2.4 5.1
BPT 598 639 11% 0.78
a
1.4 11
BPIC 627 664 <1%
<0.02 [99%]
b

1.1 [1%]
- -
Acetonitrile
1 497 508 62% 3.5
a
1.8 1.1
BPM 536 562 68% 3.4
a
2.0 0.93
BPB 558 583 8.5% 0.55
b
1.6 17
BPIO 566 593 15% 0.71
a
2.11 12
BPT 577 616 1.3% 0.13
b
1.0 76
BPIC 604 658 <1%
<0.02
b
[93%]
2.5 [7%]
- -
Table 3.1. Photophysical properties of 1, BPM, BPB, BPIO, BPT, and BPIC in toluene and acetonitrile.
a
Measured
using a Horiba Fluorohub TCSPC with a 405 nm laser source (<300 ps IRF). bMeasured with ps-TCSPC.

51

All dyes but BPIC are brightly luminescent in non-polar solvents, with photoluminescent
quantum yields, ΦPL, ranging from 11 - 83 % in toluene. Decreases in ΦPL trend with the
increasing electron withdrawing strength of the acceptor moiety and are mirrored with a decrease
in the measured excited state lifetimes (τ). BPM possess similar values of t and FPL in both polar
and nonpolar solvents, however the other dyes show marked decreases for both when moving to
polar solvents. The radiative decay rates, kr, for the dyes are largely unchanged between polar and
nonpolar solvents. Instead, the principal origin of the decrease in ΦPL stems from a marked increase
of the non-radiative decay rate, knr, in polar solvents. CT states are known to increase knr by
facilitating charge recombination directly to the ground state. The increase in knr in polar solvents
is a clear departure from the parent bodipy 1 which has values of knr in both non-polar and polar
solvents. Unfortunately, the values of kr and knr cannot be determined for BPIC due to its value
of ΦPL falling below the limits of detection for our integrating sphere.
In order to further elucidate the nature of the non-radiative decay pathway, emission
properties were measured for the dyes in a 2-MeTHF glass at 77 K, and are given in figure 4 and
table 2. Upon cooling, the fluorescence spectrum of each compound undergoes a rigidochromic
blue shift and an increase in height of the 0-1 shoulder is observed. Looking at effects of cooling
on the lifetime of the compounds, BPM shows no observable change, while the lifetime of BPIC
goes from being below the measurable limit of our cryogenic setup (<500 ps) to 2.79 ns. By
freezing the solvent molecules into a glass, the molecules are unable to reorganize around the new
electronic configuration of the excited state, leading to a destabilization of the CT and removing
its influence on the transition energy and excited state lifetime.
52

550 600 650 700 750 800
0
0.5
1
0
0.5
1
0
0.5
1
0
0.5
1
550 600 650 700 750 800
0
0.5
1
Normalized Emission (AU)

2Me-THF RT
2Me-THF 77 K
BPM
BPB
BPIO
BPT
BPIC

Wavelength (nm)

Figure 3.3. Emission Spectra of BPM and BPT in 2-MeTHF at 77 K (solid) and 298 K (dashed).

53

λEm,
RT
τRT
a
ΦPL, RT

kr, RT knr, RT
λEm,
77K
τ77K
Rigidochromic
Shift
(nm) (ns) (10
8
s
-1
) (10
8
s
-1
) (nm) (ns) (cm
-1
)
BPM 576 3.28 77% 1.05 0.32 568 3.15
245
BPB 593 1.23 24% 1.94 6.19 583 2.83
289
BPIO 602 1.17 27% 2.31 6.24 592 2.72
281
BPT 624 0.54 8.3% 1.52 16.7 606 2.62
476
BPIC 670 < 0.5 < 2% - - 648 2.69
507
Table 3.2. Temperature Dependent Steady-State Photophoysical Properties of Bodipy Dyes in 2-MeTHF, which has
a ET(30) of 36.5 .
a
Measured using a Horiba Fluorohub TCSPC with a 405 nm laser source (<300 ps IRF).
b
Room
temperature values of ΦPL were experimentally measured using a Hamatsu Integrating Sphere.

In order to observe internal conversion from LE → CT, picosecond transient absorption
spectra were recorded for BPIC in acetonitrile (Figure 5). At 300 fs, the transient trace in
acetonitrile possesses a excited state absorption (ESA) located about 470 nm, as well as a ground
state bleach (GSB) and stimulated emission (SE) located about 600 and 670 nm respectively,
correlating well to the steady-state absorption and emission spectra. However, by 1 ps the intensity
of a small region of the SE from ~650-700 nm recovers and by 5 ps rises above the baseline. This
feature is reminiscent of an ICT state observed in related bis-bodipy and zinc dipyrrin molecules.
11,
13
In those systems, two tethered bodipy (BDP) or zinc dipyrrin (zDIP) chromophores are held
close to one another and form a weakly coupled electronic interaction. Upon excitation of one
ligand, symmetry breaking charge transfer rapidly occurs on the ps timescale, shuttling charge to
the other ligand and forming a BDP
+
-BDP
-
or zDIP
+
-zDIP
-
ICT state. In the molecules presented
here, it is likely that the ICT state takes on a A
d-
-BDP
+
-A
d-
motif. As such, it is expected to find
similar features to the SBCT systems in the psTA spectra.
54

Figure 3.4. Picosecond transient absorption spectra of BPIC in MeCN.

Computational Chemistry
In order to gain insight into the electronic structure of reported dyes, density functional
theory (DFT) was employed at the B3LPY/6-31G
**
level in order to calculate ground state
geometries, while time-dependent density functional theory (TD-DFT) at the
CAM-B3LYP/6-31G** level to was used to calculate excited state transitions. (Figure 6,
S26,28,30). All dyes display a twisted structure, with dihedral angles between the acceptor and
core moieties falling between 29° 41°, in agreement with the collected X-ray data. The lack of
planarity between these subunits reduces the degree of conjugation across the methylene bridge.
This reduces the ability of the electron withdrawing group to red shift the transition, but also
prevents strong coupling of the donor to the acceptor and inhibits delocalization of particles over
the entire molecule.
The HOMO of each dye is located principally on the bodipy core, with only marginal
density on either wing, while the LUMO spreads itself more evenly across the closer atoms of the
ICT State
ICT State
Stimulated Emission
(SE)
55

wing moieties. In both cases, the electron density present on the core moiety mirrors that found in
the parent dye. The calculated natural transition orbitals (NTOs) of the principal S1 feature for each
dye display a hole occupying the HOMO and an electron occupying the LUMO, both residing
primarily on the bodipy core (Figure 7, S27,29,31). Counter to the charge transfer characteristics
observed in the emission data, the S1 NTOs displays a local excited state (LE) character, mirroring
the parent compound. Overlap between the hole and electron NTOs was calculated according to
prior literature reports.
28
Moving from the parent complex to BPIC, overlap decreases only slightly
from 69% to 64% (Table 3).
56

Figure 3.5. LUMO (top, mesh) and HOMO (bottom, solid) of 1 (left) and BPIC (right).

Figure 3.6. (a) Hole and (b) Electron NTO of BPIC. *B3LYP/6-31G**

57

S1
Oscillator
Strength
HOMO-LUMO
overlap (eV)
BPM 2.34 1.12 0.68
BPB 2.44 1.70 0.66
BPT 2.35 2.00 0.64
BPIC 2.05 1.55 0.64
1 2.57 0.54 0.69
Table 3.3. Calculated Excited State Energetics and HOMO-LUMO overlap.

58

Electrochemistry
The electrochemical properties in DCM of the five dyes were evaluated using cyclic
voltammetry (CV) (Figure 8, SI18, 20, 22, 24) and differential pulse voltammetry (DPV)
(Figure S19, 21, 23, 25) in a three-electrode setup, measured against the ferrocene redox couple
or the Ir(ppy)3 redox couple vs Fc. HUMO and LUMO values were calculated based on literature
and are given in table 4.
29
Compared to 1, all five newly synthesized dyes show more positive
oxidation potentials and less negative reduction potentials,. In the case of BPB and BPIO, the
addition of the respective end caps imparts reversibility to the 1
st
reduction wave not found in 1.
The shift in oxidation and reduction potentials leads to deeper HOMO and LUMO respectively. In
the case of BPIC, the LUMO energy is stabilized by over 800 meV compared to the parent
compound. This leads to BPIC possessing a LUMO energy comparable to ITIC (-3.77 eV vs -3.83
eV), making it attractive as a low-cost electron acceptor layer for organic photovoltaics.
30

-1.5 -1 -0.5 0 0.5 1 1.5
BPB BPT BPIO BPIC
Potential (V vs Fc/Fc
+
)

Figure 3.7. Cyclic voltammograms for the first oxidation and reduction waves of BPB, BPT, BPIO and BPIC
collected in 0.1 M n-Bu 4NPF 6 in CH 2Cl 2. Potentials were measured against the Fc/Fc
+
redox couple. Cyclic
voltammograms of all the oxidation and reduction waves of these compounds are provided in the SI.
59

Ox. Red. LUMO HOMO
Parent 0.76 -1.57 -2.98 -5.66
BPB
0.98 -1.09 -3.54 -5.92
BPT
1.00 -1.04 -3.60 -5.94
BPIO 1.04 -1.09 -3.54 -5.99
BPIC 1.07 -0.9 -3.77 -6.02

Table 3.4. Red./Ox. Potentials and Corresponding LUMO HOMO Levels.
Electrochemical values are measured in V vs. Fc/Fc+ while HOMO and LUMO values are in eV. Redox potentials
were acquired via differential pulse voltammetry. HOMO and LUMO levels were calculated from redox potentials.
29

Devices
To understand the suitability of bodily based ADA materials as electron acceptors, solution
processed bulk hetero junction (BHJ) devices utilizing either BPT or BPIC as acceptors and the
polymer PCE-10 as the donor were fabricated with a structure of
ITO/ZnO (30 nm)/donor:acceptor (90 nm, 1:1-1.5 w/w)/MoOx (15 nm)/Ag (100 nm).
31
in ITO is
indium tin oxide in this instance. The device architectures illuminated current density-voltage
(J-V) curves and external quantum efficiency (EQE) spectra are given for BPT and BPIC (Figure
9). The values for open circuit voltage (VOC) , short circuit current (JSC), fill factor (FF), and PCE
of the devices are given in table 5. The active layer for the optimal PCE-10:BPT device was spin
coated from chlorobenzene with a donor/acceptor (D/A) ratio of (1:2 w/w) while the active layer
for the PCE-10:BPIC device was spin coated from chloroform with a D/A ratio of (1:1 w/w).
Devices were prepared both as-cast and with thermal annealing at 100 °C for the BPT based
devices and 120 °C for the BPIC based devices for a period of 10 minutes though minimal
differences were observed.
60

-0.5 -0.25 0 0.25 0.5 0.75 1
-15
-10
-5
0
5
10
Voltage (V)
PCE-10:BPT as Cast
PCE-10:BPT Annealed (100 °C)
PCE-10:BPIC as Cast
PCE-10:BPIC Annealed (120 °C)
Current Density (mA/cm
2
)
400 500 600 700 800 900
0
20
40
60
EQE (%)
wavelength (nm)
PCE-10:BPIC as Cast
PCE-10:BPIC Annealed (120 C)

Figure 3.8. (a) J-V characteristics of the OPV devices using either PCE-10:BPT (1:2 w/w) or PCE-10:BPIC (1:1 w/w)
with or without annealing, under the illumination of AM1.5G, 100 mW/cm
2
. (b) The EQE spectra for the OPV devices
processed under theremal annealing conditions.

Devices
D:A
(w/w)
VOC
(V)
JSC
(mA/cm
2
) FF
PCE
(%)
PCE-
10:BPT
a
1:2 0.70 4.5 0.36 1.1
PCE-
10:BPT
b
1:2 0.72 4.8 0.35 1.2
PCE-
10:BPIC
a
1:1 0.67 7.6 0.39 2.0
PCE-
10:BPIC
b
1:1 0.68 8.2 0.42 2.3
Table 3.5. Photovoltaic Performance Parameters of PCE-10:BPT and PCE-10:BPIC OPVs under illumination of
AM1.5G, 100 mW/cm
2
.
a
As-cast film.
b
With thermal annealing at 100 °C for 10 minutes.

The PCE-10:BPT and PCE-10:BPIC devices demonstrate high values of VOC between
0.67-0.72 V, comparable to voltages initially observed for PCE-10:ITIC devices. In the case of the
PCE-10:BPT device, it reaches a comparable VOC by increasing the offset between the LUMO of
the acceptor and the HOMO of the donor by 230 meV compared to the ITIC based device.
However, the PCE-10:BPIC device reaches a similar voltage with only a 60 meV increase in the
offset compared to the ITIC device. This serves to demonstrate the influence of the close lying CT
61

state on device energetics and is consistent with the dominance of dicyanovinylindanone based
acceptor groups in ADA acceptor materials.
On the other hand, both bodipy based devices suffer from PCE between 1.1 and 2.3 % and
poor EQE curves, stemming from low JSC and FF. Despite using highly absorbing components,
the JSC for the annealed PCE-10:BPT device is a marginal 4.8 mA/cm
2
while the value for the
annealed PCE-10:BPIC device reaches 8.2 mA/cm
2
, with FF of 0.35 and 0.42 respectively. Based
on the J-V curves, both devices suffer from low shunt resistance leading to high rates of charge
recombination between generated electron and hole pairs. In contrast to the ITIC, BTIC, and Y6
families which crystalize in planer structures
32-33
, the gas phase calculations of BPT and BPIC
and crystal structures of BPB and BPIO have a large torsion angle between the donor core and the
acceptor wing. While this large torsion helps prevent delocalization of e
-
and h
+
across the entire
molecule, it inhibits crystallization of the molecules during device fabrication, impacting charge
extraction during device operation.
Conclusions
The bodipy based ADA molecules presented here represent a class of synthetically simple
electron acceptor materials for use in organic photovoltaics. In contrast to Y6 which requires 7
steps and has an overall reaction yield less than 10%, these materials require only 3 steps, and in
the best cases, have overall reaction yields between 19-24 %. This improvement is achieved based
on the fact that the core can be synthesized on the gram scale in a one pot synthesis. Improvements
to synthetic ease can help decrease the cost of OPVs and lead to easy commercialization.
These materials are strong absorbers of light due the transition strength of the bodipy core
being preserved. Substitutions of strong withdrawing groups on the 2- and 6- positions results in a
bathochromic shift from 503 nm out to 627 nm. Pushing optical transitions out past 700 nm to the
62

NIR such that materials are transparent in the visible region of the spectrum is necessary for the
fabrication of ST-OPVs. Additionally, despite the large oscillator strength of the parent molecule
being preserved, it is apparent that a close lying CT state now exists, evidenced by changes in t,
FPL, and knr with increasing acceptor strength and solvent polarity. The influence of a CT is
desirable as it serves to decouple the e
-
and h
+
of the excited state, lowering the transition’s BE,
and improving Voc. Further bathochromic shifts could be realized by truncating the methyl groups
and allowing the acceptor to come into plane, providing for better conjugation between the
respective set of orbitals. However, care must be exercised not to delocalize the particles over the
entirety of the molecule, removing the CT state.
OPV devices were fabricated using BPT and BPIC as electron acceptors paired with PCE-
10 as an electron donor. High values for VOC were observed for the devices, comparable to devices
based on a PCE-10:ITIC active layer, demonstrating that these materials can be utilized to extend
the A-D-A framework to much smaller donor cores. On the other hand, devices suffered from low
values of JSC, FF, PCE, and EQE. Looking at the J-V curves, the poor performance of these devices
seemed to stem from a high frequency of charge recombination during device operation. Unlike
high performance ADA materials, the molecules represent here crystalize as a twisted structure
due to steric hindrance of the methyl groups present on the bodipy core. Again, truncation of these
methyl groups could serve to benefit future materials.
In conclusion these bodipy based ADA material present exciting new prospect for the
commercialization of OPVs. By coupling highly absorbing transitions to CT transitions with low
BE, simple polycyclic aromatic chromophores can be utilized in place of complex donor cores
currently seen in the literature, while maintaining high VOC. High JSC have already been correlated
to active layer crystallinity, and better crystallinity can be imparted to simple ADA materials by
63

minimizing steric bulk or utilizing other simple cores. By simplifying the overall synthetic scheme,
OPVs can been produced in a cheaper and more readily available manner.
Acknowledgements
Xinjing Huang and Yongxi Lir are thanked for their work fabricating BPIC and BPT
devices respectively. Thabassum Ahammad is thanked for transient absorption studies. Jonas
Schaab is thanked for collecting x-ray diffraction data.
Experimental
Synthesis
All reagents were purchased from commercial were used without further purification. Ethyl
acetoacetate and phosphorus oxychloride were purchased from Acros Organics. Zinc powder (325
mesh), 1,3-indadione, and potassium carbonate were purchased from Oakwood Chemicals.
Sodium nitrite, ethylene glycol, N,N-dimethylformamide, triphenylphosphine,
1,3-dimethylbarbituric acid, 1,3-diethyl-2-thiobarbituric acid, and malonic acid were purchased
from Sigma-Aldrich. Acetic acid was purchased from Merck. Sodium hydroxide, chloroform,
toluene, and acetonitrile were purchased from EMD Millipore. 2,4-dimethylpyrrole was purchased
from Ambeed as a slightly brown solution and used without further purification.
1-(dicyanovinyl)-3-indanone
34
were synthesized according to prior literature procedure, synthetic
details are provided in the SI. All reactions were carried out under nitrogen using standard Schlenk
line techniques unless otherwise noted.
1,3,5,7-Tetramethyl-8-phenyl-4,4-difluoroboradiazaindacene(1) was synthesized from
combined literature methods.
19, 22-23
2,4-dimethylpyrrole (4.0 mL, 3.7 g, 11 mmol, 2 equiv.),
benzaldehyde (2 mL, 2.0 g, 19.60 mmol, 1 equiv.) and catalytic trifluoroacetic acid (0.1 mL) were
64

added to 200 mL dry THF in a Schlenk flask with a stir bar. The reaction was allowed to stir in the
dark for 24 hr, yielding a pink solution. 2,3-dichloro-5,6-dicyano-1,4-beznoquinone (4.39 g,
18.99 mmol, 1 eq.) was dissolved in 50 mL of dry THF and cannula transferred into the reaction
mixture, to give a red solution. The solution was allowed to further stir 3 hours.
Diisopropylethylamine (21 mL, 15 g, 120 mmol, 6 equiv.) was then added, giving a yellow-orange
solution. The reaction was allowed to stir for 1 hr, upon which boron trifluoro diethyl etherate
(21 mL, 24 g, 170 mmol, 9 equiv.) was added, producing a purple solution. The reaction was
allowed to stir overnight. The reaction was then filtered through a pad of celite and the filtrate was
then washed with sat. K2CO3(aq) (3 x 200 mL). The organic layer was dried with MgSO4 and
evaporated in vacuo. The crude compound was further purified by column chromatography using
hexanes:DCM (50:50), yielding 1 as a metallic orange-green solid (2.06 g, 6.35 mmol, 33% yield.)
.
1
H-NMR (400 MHz, Chloroform-d) δ 7.50-7.45 (m, 3H), 7.29-7.26 (m, 2H), 5.97 (s, 2H), 2.55
(s, 6H), 1.37 (s, 6H).
2-formyl-1,3,5,7-Tetramethyl-8-phenyl-4,4-difluoroboradiazaindacene(2) was synthesized
from combined synthetic methods.
35-36
. 1 (0.994 g, 3.07 mmol) was added to an oven dried Schlenk
flask and then dissolved by adding 50 mL of anhydrous chloroform. In a separate Schlenk flask
placed inside of an ice bath, phosphoryl chloride (10 mL, 16.40 g, 106.96 mmol, 35 equiv.) was
added dropwise to N,N-dimethylformamide (10 mL, 9.44 g, 129.15 mmol, 42 equiv.) to give solid
Vilsmeier reagent. The first solution of 1 was cannula transferred into the vessel containing the
Vilsmeier reagent, before removing the ice bath and being left to stir at 60° C for 3 hr. The reaction
was cooled to room temperature, quenched with an aqueous solution of saturated K 2CO3 (3x
100 mL) and extracted with DCM (3x 100 mL).. The organic layer was dried with MgSO4 and
evaporated in vacuo, yielding 2 as a red solid (0.879 g, 2.50 mmol, 81.4% yield).
1
H-NMR (400
65

MHz, Chloroform-d) δ 10.01 (s, 1H), 7.59 – 7.49 (m, 3H), 7.32 – 7.27 (m, 2H), 6.15 (s, 1H), 2.82
(s, 3H), 2.62 (s, 3H), 1.65 (s, 3H), 1.42 (s, 3H).
2,6-diformyl-1,3,5,7-Tetramethyl-8-phenyl-4,4-difluoroboradiazaindacene (3) was
synthesized following a modified synthetic route for 2.
35-36
2 (0.972 g, 2.78 mmol) was added to
an oven dried Schlenk flask and then dissolved by adding 50 mL of anhydrous chloroform. In a
separate Schlenk flask inside of an ice bath, phosphoryl chloride (10 mL, 16.40 g, 106.96 mmol,
38 equiv.) was added dropwise to N,N-dimethylformamide (10 mL, 9.44 g, 129.15 mmol,
46 equiv.) to give solid Vilsmeier reagent. The first solution of 2 was cannula transferred to the
Vilsmeier reagent, before removing the ice bath and being left to stir at 60° C for 6 hr. The reaction
was cooled to room temperature, quenched with an aqueous solution of saturated K2CO3 (100 mL)
and extracted with DCM (3x 100 mL). The organic layer was then washed with brine (3x 100 mL)
and with DI water (100 mL) The organic layer was dried with MgSO4 and evaporated in vacuo,
yielding 3 as a red solid (0.991 g, 2.63 mmol 94.5% yield).
1
H-NMR (400 MHz, Chloroform-d) δ
10.06 (s, 2H), 7.62 – 7.56 (m, 3H), 7.30 (dd, J = 6.8, 2.9 Hz, 2H), 2.88 (s, 6H), 1.71 (s, 6H).
2,6-bis-(dicyanovinyl)-1,3,5,7-Tetramethyl-8-phenyl-4,4-difluoroboradiazaindacene (BPM)
was synthesized following a modified literature procedure.
24
3 (98.69 mg, 396.26 mol) and
malanonitrile (71.05 mg, 1.08 mmol, 4.1 equiv.) were added to a Schlenk flask and dissolved in
10 mL of anhydrous CH2Cl2. The solution was then chilled in an ice bath. TiCl4 (0.50 mL, 860
mg, 17 equiv.) and pyridine (1 mL, 980 mg, 48 equiv.) were added. The reaction was left to stir
for 5 hr, over which time the solution turned orange. The solvent was removed in vacuo to give a
purple solid. Solid was redissolved in CH2Cl2 and washed with 0.1 M HCl(aq) (3x 100 mL). The
organic layer was filtered through a pad of celite, the filtrate collected, and the solvent removed in
66

vacuo, yielding BPM as purple solid (16.6 mg, 34.8 mmol, 16.6% yield).
1
H-NMR (400 MHz,
Chloroform-d) δ 7.68 (s, 2H), 7.63 – 7.56 (m, 3H), 7.36 – 7.27 (m, 2H), 2.68 (s, 6H), 1.49 (s, 6H).
5,5'-((5,5-difluoro-1,3,7,9-tetramethyl-10-phenyl-3H,5H-5l4-dipyrrolo[1,2-c:2',1'-
f][1,3,2]diazaborinine-2,8-diyl)bis(methaneylylidene))bis(1,3-dimethylpyrimidine-
2,4,6(1H,3H,5H)-trione), BPB, was synthesized following a modified literature procedure.
24
3
(149.67 mg, 393.66 mol), 1,3-diethyl-2-thio-barbituic acid (309.54 mg, 1.98 mM, 5 equiv.)
triphenylphosphine (5.18 mg, 19.75 mol, 0.05 equiv.) and 50 mL dry toluene were added to a
Schlenk flask. The reaction was heated to 110 °C and left to reflux for 48 hr. The reaction was
allowed to cool and washed with a saturated K2CO3(aq) (3x 100 mL). The organic layer was dried
with MgSO4 and evaporated in vacuo. The crude product was recrystallized from layering
hexane:CH2Cl2 yielding BPB as a matte brown solid (183.20 mg, 278.22 μmol, 70.89% yield).
Single crystals were grown from CDCl3 and Pentane in a 1:2 layering method in an NMR tube.
1H NMR (400 MHz, Chloroform-d) δ 8.33 (s, 2H), 7.57 – 7.53 (m, 3H), 7.36 (dd, J = 7.5, 2.1 Hz,
2H), 3.40 (s, 6H), 3.33 (s, 6H), 2.55 (s, 6H), 1.36 (s, 6H).
13
C-NMR (100 MHz, Chloroform-d) δ
161.87, 159.55, 151.23, 149.02, 129.80, 129.73, 127.73, 118.29, 28.87, 28.41, 14.94, 14.32.

5,5'-((5,5-difluoro-1,3,7,9-tetramethyl-10-phenyl-3H,5H-5l4-
dipyrrolo[1,2-c:2',1'-f][1,3,2]diazaborinine-2,8-diyl)bis(methaneylylidene))bis(1,3-diethyl-2-
thioxodihydropyrimidine-4,6(1H,5H)-dione), BPT, was synthesized following a modified
literature procedure.
24
3 (804 mg, 2.11 mmol), 1,3-diethyl-2-thio-barbituic acid (2.12 g, 10.59
mmol, 5 equiv.) triphenylphosphine (27.2 mg, 10.59 mmol, 0.05 equiv.) and 250 mL dry toluene
were added to a Schlenk flask with a stir bar. The reaction was heated to 110 °C and left to reflux
for 72 hr. The reaction was allowed to cool and washed with a saturated K2CO3(aq) (3x 100 mL).
The organic layer was dried with MgSO4 and evaporated in vacuo. The crude product was
67

recrystallized via chilling a layered solution of hexane:CH2Cl2 (604 mg, 805 μmol, 38.10% yield).
1H NMR (400 MHz, Chloroform-d) δ 8.33 (s, 2H), 7.57 – 7.53 (m, 3H), 7.36 (dd, J = 7.5, 2.1 Hz,
2H), 3.40 (s, 6H), 3.33 (s, 6H), 2.55 (s, 6H), 1.36 (s, 6H).
13
C-NMR (100 MHz, cdcl3) δ 178.68,
160.06, 157.74, 149.79, 133.82, 129.93, 129.77, 127.70, 118.51, 43.99, 43.52, 15.13, 14.39, 12.61,
12.40.
2,2'-((4,4-difluoro-1,3,5,7-tetramethyl-8-phenyl-3a,4-dihydro-4l4-
dicyclopenta[b,e]borinine-2,6-diyl)bis(methaneylylidene))bis(1H-indene-1,3(2H)-dione),
BPIO, was synthesized following a modified literature procedure
26
. 3 (249 mg, 660.1 mmol) and
1,3-indandione (206.9 mg, 1.42 mmol) was added to a Schlenk flask and dissolved with 30 mL
toluene. BF3OEt2 (0.81 mL, 930 mg, 10 equiv.) was added, after which the solution turned brown.
Acetic anhydride (0.67 mL, 720 mg, 10 equiv.) was added, after which to solution immediately
turned green before slowly turning purple. The solution was stirred for 1 hr. The solution was then
washed with saturated K2CO3(aq) (3x 100 mL). The solvent was then removed in vacuo, yielding a
purple solid. This solid was then washed three times with K2CO3, three times with methanol, and
three times with ether, yielding 391 mg (93.5 % yield) of product. Single crystals were grown from
CDCl3 and pentane in a 1:2 layering approach in an NMR tube.
1
H-NMR (400 MHz, cdcl3) δ 8.04
– 7.96 (m, 2H), 7.97 – 7.89 (m, 2H), 7.82 – 7.77 (m, 4H), 7.76 (s, 2H), 7.63 – 7.49 (m, 3H), 7.47
– 7.37 (m, 2H), 2.66 (s, 6H), 1.43 (s, 6H).
13
C-NMR (100 MHz, cdcl3) δ 189.60, 142.58, 140.26,
135.37, 135.23, 130.14, 129.77, 127.81, 123.33, 123.13, 14.49.
2,2'-(((4,4-difluoro-1,3,5,7-tetramethyl-8-phenyl-3a,4-dihydro-4l4-
dicyclopenta[b,e]borinine-2,6-diyl)bis(methaneylylidene))bis(3-oxo-2,3-dihydro-1H-indene-
2,1-diylidene))dimalononitrile, BPIC, was synthesized following a modified literature
procedure
26
. 3 (249 mg, 660.1 mmol) and 1-(Dicyanomethylene)-3-indanone (206.9 mg,
68

1.42 mmol) was added to a Schlenk flask and dissolved with 30 mL toluene. BF3OEt2 (0.81 mL,
930 mg, 10 equiv.) was added, after which the solution turned red. Acetic anhydride (0.67 mL,
720 mg, 10 equiv.) was added, after which to solution immediately turned green before slowly
turning blue. The solution was stirred for 1 hr. The solution was then washed with saturated
K2CO3(aq) (3x 100 mL). The solvent was then removed in vacuo, yielding a 353 mg of a green solid
(73.3 % yield).
1
H-NMR (400 MHz, cdcl3) δ 8.74 (dt, J = 7.9, 1.0 Hz, 2H), 8.45 (s, 2H), 7.95 (ddd,
J = 7.2, 1.6, 0.7 Hz, 2H), 7.88 – 7.78 (m, 4H), 7.66 – 7.59 (m, 3H), 7.48 – 7.44 (m, 2H), 2.69 (s,
6H), 1.49 (s, 6H).
13
C-NMR (100 MHz, cdcl3) δ 185.47, 160.09, 139.46, 137.51, 137.01, 135.53,
134.99, 129.91, 129.85, 129.46, 127.71, 125.46, 124.30, 113.95, 113.93, 15.27, 14.83, 14.52.
Methods
Electrochemistry
Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were performed through use
of an EG&G potentiostat/galvanostat model 283. The electrolyte solution was composed of 0.1 M
tetra-n-butylammonium hexafluorophosphate in anhydrous dichloromethane. The measurements
were taken under an inert atmosphere. The working, counter, and pseudoreference electrodes were
composed of glassy carbon, platinum wire, and silver wire respectively. The ferrocene redox
couple or the Ir(ppy)3 redox couple versus ferrocene were used as the internal references for all
electrochemistry measurements.
Photophyiscal Characterization
Molar absorptivity spectra were measured using a UV-VIS Agilent 4853 diode array spectrometer.
Photoluminescence quantum yields were recorded using a Hamatsu C9920 integrating sphere
equipped with a xenon lamp. Lifetimes were measured using time-correlated single photon
69

counting (TCSPC) with a Horiba Fluorohub A+ equipped with either a 405 nm laser (<300 ps IRF)
source. Steady-state excitation and emission spectra were obtained using a Photon Technology
International QuantaMaster phosphorescence/fluorescence spectrofluorimeter. Emission,
photoluminescence quantum yield, and lifetime measurements were acquired from solutions at
maximum optical densities between 0.1 and 0.2 to minimize effects of solute-solute interactions.
Room-temperature photophysical measurements were recorded in toluene, dichloromethane, and
acetonitrile. 77K photophysical measurements were recorded in 2-methyltetrahydrofuran. NMR
spectra were recorded on a Varian 400 NMR spectrometer and referenced to the residual proton
resonance of the relevant solvent.
Computational Methods.
All calculations were performed using Q-CHEM 5.1. software package. Gas-phase geometry
optimizations and time-dependent density functional theory (TD-DFT) calculations were
preformed using the B3LYP and CAM-B3LYP functional respectively. The 6-31G** basis set as
implemented in Q-CHEM was used for both.

70

Nuclear Magnetic Resonance Spectra
Figure 3.9.
1
H-NMR (400 MHz, Chloroform-d) of 1,3,5,7-tetramethyl-8-phenyl-bodipy (1).

71

Figure 3.10.
1
H-NMR (400 MHz, Chloroform-d) of 2-formyl-1,3,5,7-tetramethyl-8-phenyl-bodipy (2).

72

Figure S3.11.
1
H-NMR (400 MHz, Chloroform-d) of 2,6-diformyl-1,3,5,7-tetramethyl-8-phenyl-bodipy (3).

73

Figure S3.12.
1
H-NMR (400 MHz, Chloroform-d) of BPM.

74

Figure S3.13.
1
H-NMR (400 MHz, Chloroform-d) of BPB.

75

Figure 3.14.
13
C-NMR (400 MHz, Chloroform-d) of BPB.

76

Figure 3.15.
1
H-NMR (400 MHz, Chloroform-d) of BPT.

77

Figure 3.16.
13
C-NMR (400 MHz, Chloroform-d) of BPT.

78

Figure 3.17.
1
H-NMR (400 MHz, Chloroform-d) of BPIO.

79

Figure 3.18.
13
C-NMR (400 MHz, Chloroform-d) of BPIO.

80

Figure 3.19.
1
H-NMR (400 MHz, Chloroform-d) of BPIC.

81

Figure 3.20.
13
C-NMR (400 MHz, Chloroform-d) of BPIC.

82

Crystal Structures of BPB, BPIO, and BPIC-Fused

Figure 3.21. Crystal structures of BPB, BPIO, and BPIC-Fused. Thermal ellipsoids are shown at the 50%
probability. All hydrogens are omitted for clarity. The atom colors are: B (pink), C (grey), N (blue), O (red), and F
(green).
83

Absorption and Emission Spectra of BPM, BPB, BPIO, and BPT
300 400 500 600 700
0
1
2
3
4
5
Wavelength (nm)
Toluene
CH
2
Cl
2
2MeTHF
MeCN
Molar Absorptivity (10
4
M
-1
cm
-1
)
500 550 600 650 700
0
0.2
0.4
0.6
0.8
1
Wavelength (nm)
Toluene
CH
2
Cl
2
2MeTHF
MeCN
Normalized Emission
Figure 3.22. Absorption and emission spectra of BPM in various solvents.

300 400 500 600 700
0
2.5
5
7.5
10
Wavelength (nm)
Toluene
CH
2
Cl
2
Norm. 2MeTHF
MeCN
Molar Absorptivity (10
4
M
-1
cm
-1
)
550 600 650 700
0
0.2
0.4
0.6
0.8
1
Wavelength (nm)
Toluene
CH
2
Cl
2
2MeTHF
MeCN
Normalized Emission

Figure 3.23. Absorption and emission spectra of BPB in various solvents.

84

400 500 600 700
0
2
4
6
8
10
12
14
Toluene
CH
2
Cl
2
2-MeTHF
MeCN
Molar Absorptivity (10
4
M
-1
cm
-1
)
Wavelength (nm)
550 600 650 700 750
0
0.2
0.4
0.6
0.8
1
Wavelength (nm)
Toluene
CH
2
Cl
2
2MeTHF
MeCN
Normalized Emission

Figure 3.24. Absorption and emission spectra of BPIO in various solvents.

300 400 500 600
0
1
2
3
4
5
6
7
8
Wavelength (nm)
Toluene
CH
2
Cl
2
MeCN
Molar Absorptivity / 10
4
M
-1
cm
-1
500 550 600 650 700 750
0
0.2
0.4
0.6
0.8
1
Wavelength (nm)
Toluene
CH
2
Cl
2
2MeTHF
MeCN
Normalized Emission

Figure 3.25. Absorption and emission spectra of BPT in various solvents.

85

lAbs λEm
ΦPL
τ kr knr
(nm) (nm) (ns) (10
8
s
-1
) (10
8
s
-1
)
Dichloromethane
1 501 512 61% 3.5
a
1.7 1.1
BPM 544 566 80% 3.2
a
2.5 0.62
BPB 570 593 24% 0.89
a
1.5 9.7
BPIO 576 600 26% 1.1
a
2.4 6.8
BPT 593 625 14% 0.92
a
1.5 9.3
BPIC 618 661 <2% - - -
2-Methyltetrahydrofuran
1 500 510 44% 3.2
a
1.4 1.8
BPM 541 564 68% 3.4
a
2.4 0.70
BPB 566 587 24% 1.2
b
2.0 6.4
BPIO 574 606 27% 1.2
a
2.3 6.2
BPT - 613 8.3% 0.55
b
1.5 17
BPIC 623 623 <2% - - -
Table 3.7. Photophysical properties of 1, BPM, BPB, BPIO, BPT, and BPIC in dichloromethane and
2-methyltetrahydrofuran.
a
Measured using a Horiba Fluorohub TCSPC with a 405 nm laser source (<300 ps IRF).

86

Electrochemistry
-3 -2 -1 0 1 2
-60
-40
-20
0
20
40
60
80
Current ( A)
Potential (V vs Fc/Fc
+
)

Figure 3.26. Cyclic voltammograms of BPB though (solid) the full solvent window and (dotted) the first reduction
and oxidation waves collected in 0.1 M n-Bu 4NPF 6 in CH 2Cl 2. Potentials were measured against the Fc/Fc
+
redox
couple.

-2 -1 0 1 2
-30
-20
-10
0
10
20
Current ( A)
Potential (V vs Fc/Fc
+
)
IrPpy3
-2 -1 0 1 2
-40
-30
-20
-10
0
10
20
30
40
Current ( A)
Potential (V vs Fc/Fc
+
)

Figure 3.27. Differential Pulse Voltammogram of BPB (left) with Ir(PPy) 3 and (right) without.
87

-3 -2 -1 0 1 2
-100
-50
0
50
100
Current ( A)
Potential (V vs Fc/Fc
+
)

Figure 3.28. Cyclic voltammograms of BPIO though (solid) the full solvent window and (dotted) the first reduction
and oxidation waves collected in 0.1 M n-Bu4NPF6 in CH 2Cl 2. Potentials were measured against the Fc/Fc+ redox
couple.

-3 -2 -1 0 1 2
-40
-30
-20
-10
0
10
20
30
40
50
Current / A
Potential / V vs Fc/Fc
+
Ir(PPy)
3
Oxidation
Reduction
-3 -2 -1 0 1 2
-60
-50
-40
-30
-20
-10
0
10
20
30
40
Current ( A)
Potential (V vs Fc/Fc
+
)
Oxidation
Reduction

Figure 3.29. Differential Pulse Voltammogram of BPIO (left) with Ir(PPy)3 and (right) without.
88

-3 -2 -1 0 1 2
-80
-60
-40
-20
0
20
40
60
Current ( A)
Potential (V vs Fc/Fc
+
)

Figure 3.30. Cyclic voltammograms of BPT though (solid) the full solvent window and (dotted) the first reduction
and oxidation waves collected in 0.1 M n-Bu4NPF6 in CH2Cl2. Potentials were measured against the Fc/Fc+ redox
couple.
-3 -2 -1 0 1
-30
-20
-10
0
10
20
30
40
50
Reduction
Oxidation
Current / A
Potential / V vs Fc/Fc
+
Ir(PPy)
3
-3 -2 -1 0 1 2
-60
-50
-40
-30
-20
-10
0
10
20
30
Reduction
Oxidation
Current ( A)
Potential (V vs Fc/Fc
+
)

Figure 3.31. Differential Pulse Voltammogram of BPT (left) with Ir(PPy)3 and (right) without.
89

-3 -2 -1 0 1 2
-100
-50
0
50
100
Current ( A)
Potential (V vs Fc/Fc
+
)

Figure 3.32. Cyclic voltammograms of BPT though (solid) the full solvent window and (dotted) the first reduction
and oxidation waves collected in 0.1 M n-Bu4NPF6 in CH2Cl2. Potentials were measured against the Fc/Fc+ redox
couple.

-3 -2 -1 0 1 2
-70
-60
-50
-40
-30
-20
-10
0
10
20
30
40
50
Oxidation
Reduction
Current ( A)
Potential (V vs Fc/Fc
+
)
IrPpy3
-3 -2 -1 0 1 2
-60
-50
-40
-30
-20
-10
0
10
20
30
40
Current ( A)
Potential (V vs Fc/Fc
+
)
Oxidation
Reduction

Figure 3.33. Differential Pulse Voltammogram of BPIC (left) with Ir(PPy)3 and (right) without.

90

Computation Data

HOMO LUMO
Figure 3.34. Frontier Orbitals for BPM.

Hole Particle
Figure 3.35. Natural Transition Orbitals for the S 1 of BPM.
91

HOMO LUMO
Figure 3.36. Frontier Orbitals for BPB.

Hole Particle
Figure 3.37. Natural Transition Orbitals for the S 1 of BPB.

92

HOMO LUMO
Figure 3.38. Frontier Orbitals for BPT.

Hole Particle
Figure 3.39. Natural Transition Orbitals for the S1 of BPT.

93

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97

Chapter 4: Molar Absorptivity of Unstable Symmetry Breaking Charge Transfer Dimer
via Pulse Radiolysis.
Intro
In the previous chapter we examined utilizing an acceptor-donor-acceptor (ADA) motif to
reduce the binding energy of bodipy based devices and improve the open circuit voltage (Voc) in
organic photovoltaics (OPVs). In these ADA motifs, the BE between the hole, h
+
, and the electron,
e
-
, is decreased because the two charges are spatially separated and share minimal orbital overlap,
and therefore are only weakly coupled together, operating in an in-between range between being
a Frankel exciton and a Wariner-Mott exciton. Another strategy for creating weakly coupled
excitons is to employ a symmetry breaking charge transfer (SBCT) motif.
SBCT is a well-studied process due to its importance in enabling photosynthesis in the
photoreaction centers of purple bacteria.
1, 2
In these photoreaction centers, two bacteriochlorophyll
(BChl), called the special pair, are held co-facial in C2 symmetry. Upon the absorption of
850 nm light by one of the two BChl sites, the locally excited, LE, BChl*-BChl dimer rapidly
splits into BChl
+
-BChl
-
, breaking the C2 symmetry element. The electron on BChl
-
is then
transferred down a chain of reaction centers to drive the production of hydrogen sulfide.
2
Notable,
is that the transfer of charge occurs across two identical chromophores, which by definition of
identical, have the same HOMO and LUMO energies, and therefore no enthalpic driving force for
charge transfer. Despite this limitation, this process is incredible efficient, reaching 100% quantum
yield for charge transfer, all while using 1.4 eV photons.
3

As such, transfer of charge without a driving force is particular interesting for organic
electronic application, such as OPVs for maximizing VOC. Previously our group has studied a
98

number of simplified dimers based on bis-dipyrrin or bis-dipyridylmethenes capable of undergoing
SBCT.
4-8
Importantly, we’ve demonstrated that by using a perchlorinated zinc bis-dipyrrin called
ZCl paired tetraphenyldibenzoperyflantrene (DBP), we could realize a V OC of 1.33 V.
6
This is a
marketed improvement over an analogus C60:DPB device that had a similar driving force to
charge separation.
Study of charge transfer in these dyads is important for the increased performance of
devices. While we’ve studied the kinetics and thermodynamics of charge transfer in these
molecules, we’ve always been blind to the charge being transferred. While there is no
thermodynamic difference in transferring either an e
-
or a h
+
, orbital coupling between the two
chromophores will impart a kinetic difference, and therefore preference of one type of charge being
transferred over the other. For example, we have recently reported that coupling between the two
LUMOs in meso-bridge bis-bodipy is at maximum when the two chromophores are held planer to
one another, and conversely HOMO-HOMO coupling is maximum when the two chromophores
are held orthogonally to one another. As this family of molecules is held orthogonal due to steric
hindrance, it reasonable to hypothesize that h
+
transfer occurs more often than e
-
transfer.
In order to probe transfer of one charge over the other, Laura Estergreen has collected a
series of anisotropic transient absorption (TA) studies in order to follow this process. Typically,
TA measurements are conducted with the pump and probe polarities offset by the “magic angle”
of 54.74°. Doing so samples all absorption transitions of an excited state, regardless of the direction
of the transition dipole vector with regards to the pump polarity. By deviating from the magic
angle, anisotropic TA can selectively probe transitions aligned either with the pump pulse, by
keeping the probe parallel to the pump, or those perpendicular to the pump by keeping the probe
perpendicular.
9
Doing so allows for us to selectively probe the two orthogonal ligands.
99

However, before we are able to decipher the anisotropic TA data, we need to better
understand the excited state absorption, ESA, bands associated with each. Because the h
+
and e
-
in
SBCT state of these dimers have only weak to no coupling between them due to their orthogonal
nature, they two chromophores can largely be treated as the independent cation and anion,
respectively. Therefore, the anisotropic TA spectra of the SBCT state will ideally be comprised of
cationic transitions on one half of the SBCT state, and anionic transitions on the other, assuming
100% selective transfer of charge.
In order to assign those ionic transition, pulse radiolysis experiments were conducted to
measure the cationic and anionic absorption spectra of several dimers belonging to these SBCT
families as well as related monomers. In this chapter we will detail the process of collecting molar
absorption spectra for ions via pulse radiolysis, relevant as well to the following chapter, as well
as initial insights the spectra for the SBCT dimer and monomers give us concerning particle
localization. Then we will validate the assumption that the MA-TA ESA bands step from the
cationic transitions on one dimer half and the anionic transitions on the other, followed by future
work needed to be done to understand preferential charge transfer.
General Structure
The general structure of the dipyrrin and dipyridylmethene compounds discussed in this
chapter are given in scheme 1. Ph-bodipy has been discussed in the previous chapter. H-bodipy
is formed via a substituting a proton in the meso position of Ph-bodipy, while m8B is formed via
substitution of a second tetramethyl-bodipy chromophore in the meso position instead. zDIP2 is
formed via chelating two tetramethyl-dipyrrinato ligands to Zn. The chromophores in both m8B
and zDIP2 are held orthogonal to each other. In m8B the orthogonality is due to steric hindrance
from the methyl groups, while in zDIP2 it come from the tetrahedral field of the Zn metal center.
100

DIPYR result from a substitution of the pyrrole rings of bodipy with pyridine
10
, while bis-DIPYR
is the result of linking two such chromophores via their meso-position. Bis-DIPYR has more
torsional freedom about its meso-linkage than in either of the dipyrrin based dimers, however it
still cannot planarize dues to steric hindrance from the C-H groups.

Scheme 4.1. Structure of dipyrrin and dipyridylmethene chromophores.
The syntheses of these dyes have been previously reported and are generally facile.
4, 7, 8, 10-
16
An exception exists with the synthesis of m8B. The conventional route for a generic alkyl
subsitituded meso-bridged bodipy is laid out in scheme 2. If involves the condensation of pyrrole
onto acetoxyacetyl chloride followed by deprotonation and borylation with DIPEA and BF3OEt2
to yield generic structure 1. While Cosa et Al. reported high yields of 75% for this step with
2,4-dimethylpyrrole,
17
Sitkowska et Al. reported 47% yield for the same reaction.
18
This result is
much more in line with the traditional ~30% yield on Ph-bodipy. Similarly, our group in Whited
101

et Al. reported much poorer yield of 11% when 2-methylpyrrole was used instead,
4
and Strasser et
Al. reported 40% yield when 2,4-dimethyl-3-ethyl-pyrrole was used.
19

Regardless, 1 is hydrolyzed to 2 via either an acid catalyzed route
4, 20
through HCl in yields
of 23-71% or via based catalyzed routes with LiOH
21, 22
or NaOH
23, 24
with yields between 36-98%
and 63-92% respectively. The alcohol group of 2 is oxidized via Dess-Martin periodinane
17, 25
with
yields of 57-86% to form 3, followed by second condensation of pyrrole, oxidation with DDQ,
deprotonation by DIPEA, and borylation with BF3OEt2 to give the generic meso-fused bis-bodipy
4 with low yields
4, 16
between 9-38%. Overall, the yield of the entire process ranges from 0.1% to
25% and requires four separate reactions. As such either a simplified pathway, a higher yielding
pathway, or both is desirable.

Scheme 4.2. Traditional synthetic route of meso-fused bis-bodipy compounds. Solvent, reaction times, and order of
additions is omitted as many options exist in the literature. i)DIPEA, BF 3OEt 2. Yields range between 11-75%. ii) HCl,
yield 23-71% or LiOH/NaOH, yield 36-98%. iii) Dess-Martin periodinae, yield 57-86%. iv) 2 Eq. R-pyrrole, DIPEA,
BF 3OEt 2, yield 9-38%
To this end, Wang et Al. have published a report of synthesizing m8B directly from
2,4-dimethyl pyrrole and oxalyl chloride in 10% yield. This seems like a logical choice of substrate
as oxalyl chloride is a “dimer” of the active site in acetoxyacetyl chloride. However, despite best
efforts by several chemists in our group, we’ve never been able to replicate their results, while a
collaborator was able to produce a meager 10 mg used in this study, though the sample is not pure
(yield unknown.) Purifying the reaction between 2,4-dimethylpyrrole and oxalyl chloride via
column chromatography produced 1,2,2-tris(2,4-dimethylpyrroly)-ethan-1-one as a major product
102

and 1,2-bis(2,4-dimethylpyrroly)-ethan-1,2-dione as minor product (scheme 3). No bis-dipyrrin
product is detected. This is consistent with other literature reports.
26, 27
Ketones are much less
active towards nucleophilic attack by pyrroles then aldehydes due to increased steric hindrance.
Nucleophilic attack by pyrroles in such conditions is normally preceded by reduction of the ketone
group to an alcohol or transmutation of the oxygen with chlorine,
11
and to this end these isolated
products may be useful in reducing the synthetic complexity of bis-bodipy but that is beyond the
scope of this chapter.

Scheme 4.3. Reaction proposed by Wang et A. (top) versus observed products (bottom).

The photophysical characteristics of bodipy and DIPYR monomer and dimer dyes are well
studied
4, 7, 8, 10-15
and their absorption spectra in dichloroethane are presented in figure 1. The
bodipy family of dyes have strong absorption manifolds, with the two monomers presented here
103

possessing molar absorptivity, e, of 6.7 × 10
4
M
-1
cm
-1
for Ph-bodipy and 10 × 10
4
M
-1
cm
-1
for
H-bodipy due to the reduced mass from removing the phenyl group. zDIP2 possess a value of e,
14 × 10
4
M
-1
cm
-1
, roughly twice the magnitude of Ph-bodipy stemming from the presence of
two chromophore units. Unfortunately, due to the impurity in m8B a molar absorptivity cannot be
calculated. Values of e for both Bis-DIPYR
7
and DIPYR
10
have been reported to be 4.1 × 10
4
M
-
1
cm
-1
and 2.9 × 10
4
M
-1
cm
-1
, respectively. Again Bis-DIPYR demonstrates an increase in e due
to having two chromophore units.
400 450 500 550 600
0
0.2
0.4
0.6
0.8
1
Wavelength (nm)
Ph-bodipy
H-bodipy
zDIP2
m8B
Normalized Absoprtivity
300 350 400 450 500 550
0
0.2
0.4
0.6
0.8
1
Wavelength (nm)
Bis-DIPYR
Normalized Absoprtivity

Figure 4.1. Normalized absorption spectra of the S 1 bands of the compounds presented in this chapter in
dichloromethane.
Pulse Radiolysis
Due to the orthogonal nature between the two chromophores in each of the dimers, the two
moieties demonstrate weak to no coupling.
8
In this case, the SBCT state can largely be treated as
two individual ions, one positive, one negative, spatially tethered together. As such, when a
molecule that transfers a hole from the excited chromophore to the adjacent ground state copy,
probing the initially excited portion via TA spectroscopy should reveal signals belonging to the
anion, while probing the orthogonal initially unperturbed half should produce signals for the
104

cation. The opposite picture should be true in the case of electron transfer. Therefore, collecting
the molar absorption spectra of each ion a requisite for properly studying selective charge transfer
and determining the ratio of charges transferred.
For the compounds presented here, each possess only quasi-reversible reduction and
oxidation waves, precluding the use of chemical titration to measure the molar absorptivity
spectrum of each ion. Instead, pulse radiolysis, a time resolved spectroscopy which affords the
measuring of ion concentration, presents itself as a means to collecting these spectra.
Pulse radiolysis works by utilizing a pulse of high energy electrons (> 1 MeV per electron)
to input energy into a volume of anaerobic solvent (Figure 2). In the experiments presented here,
that input of energy is roughly ~2 meV per electron, with an initial energy per electron of ~9 meV.
This high input of energy causes the solvent molecules to spontaneously release electrons creating
solvent cations, Solv
+
, and solvated electrons, esolv
-
. If a perfectly non-polar solvent is used, the
esolv
-
are unable to escape the large Onsager radii and recombines with Solv
+
, creating solvent
singlet, triplet, and ground states. These excited states can then be utilized to sensitize a given
dissolved analyte. Due to the short singlet lifetime of most organic solvents (picoseconds), on the
time scale of diffusion (nanoseconds), only solvent triplets are lefts to sensitize the analyte.
On the other hand, if the solvent is perfectly polar, the Onsager radii approaches zero, and
therefore results in every esolv
-
escaping from Solv
+
. These free species are then able to reduce or
oxidize dissolved analyte to produce analyte anions and cations respectively. By selecting solvents
with highly unstable cations (i.e. THF), Solv
+
will decompose before it has time to oxidize analyte,
resulting in a solution of only analyte anions. Similarly, by introducing oxygen into the cuvette,
esolv
+
can be quenching out, leaving only Solv
+
to produce a solution of only analyte cations. In
reality, solvents exist on a polarity spectrum and both pathways are observed. In order to get around
105

this, solvents close to the extremes are used, where the trace amounts of unwanted species can be
discounted.
Figure 4.2. Description of pulse radiolysis, Left) A pulse of high energy electrons (b
-
radiation) inputs 2MeV of
energy per electron into the solvent. Top) For low polarity solvents, the ejected e
-
is recaptured by the solvent, yielding
solvent excited states. Bottom) For high polarity solvents, the ejected e
-
escapes creates a solvated electron and solvent
cation. Right) In either case, the created species are able to sensitize analyte present in the solution.

To carry out pulse radiolysis experiments, we utilized the linear electron accelerator facility
(LEAF) at Brookhaven National Laboratory (BNL).
28
LEAF generates an electron pulse by twice
frequency doubling a 800 nm pulse from a Tsunami Ti:S laser to 200 nm. The 200 nm pulse then
strikes a Mg photocathode, ejecting electrons. The electrons are accelerated via a Klystron RF
generator to accelerate the electron through vacuum to a kinetic energy of 9MeV per electron,
before being passed thought the solvent container, in our case a 0.2 cm pathlength cuvette. Because
the electron pulse is triggered via a laser source, the entire process can be time-correlated to a
probe pulse from a xenon lamp, allowing for the collection of transient absorption spectra.
106

LEAF is equipped with a faraday cup for counting the total number of electrons passed
through the solvent cuvette (it’s important to stress that only energy is imputed into the solvent,
the total electron flux remains the same before and after the cuvette.) By utilizing an analyte with
a known ion molar absorptivity, the number of ions generated per faraday passed can be calculated.
Since the calculated ion concentration is 0.1-1 mM while the analyte concentration is many mM
in these experiments, it is assumed that all esolv
-
or Solv
+
transfers charge to the analyte and that
they do not recombine with each other due to the lower frequency of collision. Likewise, after
reduction (oxidation) of the analyte has concluded, the population of the analyte anion (cation) is
also in the mM regime, and therefore recovery of the neutral state via reaction between the analyte
anion (cation) and Solv
+
(esolv
-
) takes place on the ms timescale.
Due to the fact that esolv
-
(Solv
+
) is highly unstable, the reduction (oxidation) potential between
it and any given analyte is high enough that the apparent rate constant of charge transfer, kCT, is
diffusion limited and dependent on solvent choice alone. Secondly, because the driving force of
charge recombination between the analyte and esolv
-
(Solv
+
) is also sufficiently large, the rate of
recombination, kCR, is also diffusion limited. This fact holds for most analytes, allowing for an
external standard method to be used to calculate the concentration of an ion of interest.
We will employ a slight modification of the external standard method by leaving the standard
in the cuvette when measuring the analyte ion spectrum. As with the external standard method, we
will begin by first measuring the transient spectra for a cuvette of 20mM standard. However,
because the ion of the standard persists over the ms time scale, a second species, this time our
analyte, can be added to the cuvette at a concentration of 10 mM for a second run. Again, kCT from
esolv
-
is the same for both the standard and the analyte, and therefore it can be assumed that we will
make ion in a ratio of 2:1 standard:analyte. By selecting a standard with a higher reduction
107

(oxidation) potential then that of the analyte, the higher concentration of standard anions (cations)
will eventually reduce (oxidize) the analyte over the course of 100s of nanoseconds, resulting in
only analyte anions (cations) at long time scales. Molar absorptivity spectra are then generated by
adding back in the neutral spectrum bleach calculated from the change in neutral population and
the neutral molar absorption spectrum.
Due to the fact the rates of charge recombination with Solv
+
(esolv
-
) are miniscule compared to
charge transfer between the standard anion (cation) and analyte, the final population of the analyte
anion (cation) at later times is the same as the standard in the first cuvette, and in fact the
concentration of total anions (cations) in the second cell is the same as the concentration of
standard anions (cations) in the first cell. The advantage of doing the experiment this way is
two-fold. One, observing the evolution of the spectra for the standard and the analyte ions can be
useful in validating that reduction or oxidation of the analyte preceded as expected. Secondly, the
molecules presented in this study have extremely high values of e (2-14 × 10
4
M
-1
cm
-1
). By
reducing the concentration of these compounds in the cuvette, we allow more light to pass the
sample cuvette in the region of 500 nm.
The anion molar absorptivity spectra for the H-bodipy, Ph-bodipy, zDIP2, and bis-DIPYR
in 10mM TBAPF 20 mM biphenyl THF are given in figure 3. TBAPF is present as an electrolyte
to increase the lifetime of the esolv
-
anion. The anionic spectra for both H-bodipy and Ph-bodipy is
characterized by a high energy peak at 340 nm with a value of e between 1.5-2.5 × 10
4
M
-1
cm
-1

as well as a weaker transition at ~560 nm with a molar absorptivity of 1.0-1.5 × 10
4
M
-1
cm
-1
.
These features are replicated in the anionic spectra of zDIP2 as well with similar values of
e. However, the spectrum is dominated by a feature at 492 nm with a e of 10 × 10
4
M
-1
cm
-1
,
coincident with the neutral complex’s absorption spectra with half of its intensity (the neutral state
108

has a e of 18 × 10
4
M
-1
cm
-1
in THF). We’ve interpreted the presence of this strong transition to
stem from localization of the anion on only one of the two chromophores. Looking at the LUMO
of zDIP2, the Zn atom creates a node between the two chromophore units, creating two separate
electron wavefunctions. By reducing only one of the two chromophores, the other unit is still
present to absorb light, and therefore contributes its half of the neutral state’s absorption to the
anion’s spectrum. To observe solely the molar absorptivity of the reduced half, only half of the
calculated neutral bleach was added back in yielding the blue trace. This trace is consistent with
results for Ph-bodipy and H-bodipy, which possess only one chromophore unit.
Looking at bis-DIPYR, the absorptivity bears a striking resemblance to the ground state
absorption with a high lying broad absorption between 350 nm and 450 nm. Similarly to zDIP2
the LUMO density of bis-DIPYR presents nodal character between the two transitions.
10

Employing a similar strategy to zDIP2, only half of the neutral bleach is added back to the
difference spectrum, yielding the blue trace. As such, it seems like the anionic transition of
bis-DIPYR presents as a broad absorption between 300-550 nm with a molar absorptivity similar
to one neutral chromophore unit. Studies are currently in progress to measure the DIPYR anion
spectrum to verify this.

109

300 400 500 600 700 800 9001000
-5
0
5
10
15
20
25
Wavelength (nm)
Molar Absorptivity (10
3
M
-1
cm
-1
)
300 400 500 600 700 800 9001000
-5
0
5
10
15
20
25
Wavelength (nm)
Molar Absorptivity (10
3
M
-1
cm
-1
)

300 400 500 600 700
-20
0
20
40
60
80
100
120
Anionic Molar Absorptivity
Localized Absorptivity
Molar Absorptivity (10
3
M
-1
cm
-1
)
Wavelength (nm)
350 400 450 500 550 600 650
0
10
20
30
40
50
Anionic Molar Absorptivity
Localized Absorptivity
Molar Absorptivity (10
3

M
-1
cm
-1
)
Wavelength (nm)
Figure 4.3. Anion molar absorptivity spectra of SBCT complexes in a solution of 10 mM TADPF and 20 mM biphenyl
in THF. Data collected is displayed as symbols and the lines are interpolations between data.

110

The cationic spectra of the H-bodipy, Ph-bodipy, zDIP2, and bis-DIPYR collected in a
solution of 20 mM triphenylamine in dichloroethane are given in figure 4. The cationic spectra for
both H-bodipy and Ph-bodipy is characterized by a large absorption at 400 nm with a value
between of ~4 × 10
4
M
-1
cm
-1
. H-bodipy displays a second broad absorption from 500-900 nm.
Moving to zDIP2, the cationic spectra once again display similar peaks to Ph-bodipy, with the
addition of a strong absorption with a value of e of 6.5 × 10
4
M
-1
cm
-1
coincident with the neutral
species absorption and equal to roughly half its strength. Again, the HOMO density displays a
node where the Zn atom lays,
5
and a similar charge localization argument to the one above is
drawn. On the contrary, the HOMO of bis-DIPYR possess no node between the two
chromophores.
7
Coupled with the absence a feature reminiscent of the neutral absorption in the
cationic spectra, we concluded that the positive charge of the cation delocalizes over the entirety
of the molecule.

111

300 400 500 600 700 800 9001000
-5
0
5
10
15
20
25
30
35
40
45
Wavelength (nm)
Molar Absorptivity (10
3
M
-1
cm
-1
)
300 400 500 600 700 800 9001000
-15
-10
-5
0
5
10
15
20
25
30
35
40
45
Wavelength (nm)
Molar Absorptivity (10
3
M
-1
cm
-1
)

300 400 500 600 700 800 900
-40
-20
0
20
40
60
80
Cationic Molar Absorptivity
Localized Absorptivity
Molar Absorptivity (10
3
M
-1
cm
-1
)
Wavelength (nm)
300 400 500 600 700 800 900
-20
-10
0
10
20
30
40
50
Wavelength (nm)
Molar Absorptivity (10
3
M
-1
cm
-1
)

Figure 4.4. Cation molar absorptivity spectra SBCT complexes in a solution of 20 mM triphenylamine in
dichloroethane. Data collected is displayed as symbols and the lines are interpolations between data.
Without a pure sample of m8B, we are unable to record molar absorptivity spectra for it,
nor are any reported to our knowledge, and therefore calculating molar absorptivity spectra of
either ion isn’t possible. However, we can calculate the molar absorptivity difference spectra
between the ground state absorption and cation or anion absorption by dividing the difference
spectra from pulse radiolysis by the concentration of ions produced and the pathlength of the
cuvette (Figure 5). Outside the region of neutral state bleach, both spectra display features akin to
those found in the ionic molar absorptivity spectra of the dipyrrin based compounds. In quantifying
112

the neutral state bleach, the anion displays a De of -14 × 10
4
M
-1
cm
-1
, which is equal to
magnitude to the neutral e of zDIP2, which should have roughly the same molar absorptivity of
m8B. Looking at the LUMO density of m8B,
8
the molecule lacks a large node between the two
chromophores, allowing the negative charge to delocalized over the entire molecule. On the other
hand, the cation spectrum only displays a value of De in this region of -4 × 10
4
M
-1
cm
-1
. This
value is less than a quarter of the zDIP2’s molar absorptivity and defies being simply characterized
as localization of charge. More analysis on this state is underway but will require the synthesis of
pure m8B.
300 400 500 600 700 800 9001000
-16
-14
-12
-10
-8
-6
-4
-2
0
2
4
m8B
Wavelength (nm)
Anion (THF)
Cation (PhCN)
Molar DAbsorptivity (10
4
M
-1
cm
-1
)

Figure 4.5. Molar difference absorptivity spectra of (red) the m8B anion in a solution of 10 mM TBAPF and 20 mM
biphenyl in THF or (blue) the m8B cation in a solution of 20 mM triphenylamine in dichloroethane.

113

Transient Absorption
Each transition, whether it comes from the ground state, excited state, or ionic state, possess
a transition dipole moment, represented as a vector 𝑇 ̂
. In three-dimensional space, the vector can
be broken down between three orthogonal vectors, 𝑇 ̂
𝑥 , 𝑇 ̂
𝑦 , and 𝑇 ̂
𝑧 . These transitions likewise
maximumly interact with light polarized along their transition axis. For steady-state absorption
and pulse radiolysis, molecules exist in an even distribution, such that any given angle of polarized
light will encounter the same number of molecules orientated to interact with it. In transient
absorption spectroscopy, where your pump source is also polarized, this isn’t the case. The pump
source selectively excites molecular transitions aligned with its polarization, disrupting the
uniformity of the sample. The probe pulse then measures the absorptive properties of the
anisotropic distribution of excited state with respect its angle. In order to maximumly sample all
three transition vectors, the probe source is polarized at an angle of 54.73°, the so-called magic
angle, which is the root second-order Legendre polynomial, which describes three orthogonal
systems. Put more simply, it is also the angle between the any given face of a cube and the diagonal
between two corners, which is non-parallel with respect to all edges of the cube.
9

Magic angle transient absorption (MA-TA) data was collected for each of the above
compounds and has been reported previously.
8
MA-TA data is reproduced here, overlayed and
normalized to the sum of the molar absorptivity difference spectra for both the cation and anion of
a each of the above dimers (Figure 7). In the case of each dimer, the sum of the two ionic molar
absorptivity difference spectra well replicates the spectrum observed via MA-TA, validating the
hypothesis that the SBCT state can be thought of two minimally coupled ions held spatially close.
In the case of zDIP2, there exists minimal difference between the two. Bis-DIPYR and m8B
114

display a deviation about 560 nm and 600 nm, respectively, coming from stimulated emission of
the sample.
Additionally bis-DIPYR displays another high energy difference exists above 370 nm
which can be attributed from already poor light levels from the xenon probe source for the pulse
radiolysis experiment being further attenuated by the neutral sample absorption and an increase in
noise. There is also a vibronic mis-match between the neutral state bleach in the pulse radiolysis
spectrum and the ground state bleach in the MA-TA. A root cause of this is in progress of being
studied.

115

300 400 500 600 700 800 9001000
-15
-10
-5
0
5
Wavelength (nm)
Sum of Ionic
Molar DAbsorptivity
100ps MA-TA (norm.)
Molar DAbsorptivity (10
4
M
-1
cm
-1
)
300 400 500 600 700
-5
-2.5
0
2.5
5
Wavelength (nm)
Sum of Ionic
Molar DAbsorptivity
100 ps MA-TA (norm.)
Molar DAbsorptivity (10
4
M
-1
cm
-1
)
SE

300 400 500 600 700 800 9001000
-20
-15
-10
-5
0
5
Wavelength (nm)
Sum of Ionic
Molar DAbsorptivity
100 ps MA-TA
Molar Absorptivity (10
4
M
-1
cm
-1
)
SE

Figure 4.6. The sum of the cation and anion molar absorptivity difference spectra (black) overlaid with the 100ps
MA-TA spectrum (red.) for zDIP2 (top left), bis-DIPYR (top right), and m8B (bottom). The MA-TA spectra were
recorded in THF, the anion difference spectra were recorded in 10mM TBAPF 20 mM biphenyl THF, and the cation
difference spectra were recorded in 20 mM triphenylamine in dichloroethane.

Future Directions and Conclusions
MA-TA studies coupled with information gleamed via PR demonstrate that the SBCT state
of the dimer molecules can properly be considered as two minimally coupled molecular ions held
spatially close. This validates the usefulness of the PR data for understanding non-magic angel TA
in order to determine preferential charge transfer in the SBCT dimers. Laura Estergreen has already
collected anisotropy TA data for the dimer listed in the chapter. However, as previously discussed,
PR experiments records isotropic absorption. These means that while the spectra measured here
116

are immediately useful for understanding the isotropic MA-TA data, the ionic molar absorptivity
spectra need to be deconstructed as a series of gaussians, such that each transition can
independently scaled to account for the different transition dipole direction relative to the probe
pulse in anisotropic TA measurements to draw conclusions from that data set. After that is done,
preferential transfer of one charge can be furhter studied.
However, important information about these compounds from the pulse radiolysis data
alone can still be obtained. The presence of a transition akin to the neutral absorption in several of
the compounds combined with orbital analysis reveals that several of these compounds localize
charge one only one side of the molecule. Understanding charge localization or delocalization is
incredibly important to organic photovoltaics due to its effect on short circuit current and open
circuit voltage. Typically speaking, delocalized charge result in higher charge mobilities, which
diminishes geminal recombination leading to a higher number of carriers and reducing voltage
losses to separating charge.
29, 30
SBCT molecules that delocalize the charge the carry in the OPV
will result in better performing molecules, i.e. the bis-DIPYR dye is a great electron donor
material, while the m8B complex is a great electron acceptor. The localization of both hole and
electron in zDIP2 will be detrimental its performance as either an electron donor or acceptor
material in OPVs.
Acknowledgement
I would like to thank Laura Estergreen for collection of MA-TA spectra. I would like to
thank Dr. Matthew Bird at Brookhaven Nation Lab for his help collecting pulse radiolysis data.

117

Experimental
Pulse Radiolysis
Pulse radiolysis (PR) was used to measure the molar absorptivities of the oxidized
and reduced forms of the cMa photosensitizers studied here, as well as the absorption spectra of
the same complexes in their triplet excited states. PR experiments were conducted at the 9 MeV
Linear Electron Accelerator Facility (LEAF) at Brookhaven Nation Laboratory (BNL),
28
using
pulses less than 50 ps in duration. The optical detection path consisted of a pulsed xenon arc lamp,
a 0.2 cm pathlength quartz optical cuvette fitted with an airtight Teflon valve, a selectable band
pass interference filter (~10 nm) and either a silicon (400-1000 nm) or a germanium (1000-
1500 nm) photodiode (2-3 ns response time). Optical measurements were collected orthogonal to
the direction of the electron pulse through the sample cuvette.
Cations of the cMa complexes were generated by irradiating aerobic dichloroethane with
b
-
radiation to generate solvent cations, solvated electrons, and solvent excited states. The
dissolved oxygen readily quenches solvent/solute excited states and solvated electrons, while the
solvent cations can be utilized to sensitize the cMa solute. Molar absorptivities were determined
via a hybrid internal standard method with triphenylamine, whose cation molar absorptivity
spectrum is known.
Anions of each complex were generated by irradiating THF with b
-
radiation to generate
solvent cations and solvated electrons. Solvent excited states are not appreciably generated in THF
via pulse radiolysis. The THF cation readily decomposes before it has time to pass charge to a
solute, leaving only solvated electrons available for sensitization. The lifetime of the solvated
electron is stabilized by the presence of the electrolyte tetrabutylammonium hexafluorophosphate,
TBAPF, in order to ensure diffusion and reduction of analyte. Molar absorbtivities were measured
118

via a hybrid internal standard method using biphenyl, whose anion molar absorptivity spectrum is
known.
Magic-Angle TA
Femtosecond magic angel transient absorption measurements as previously described.
8

Briefly, ~10% of the output of a Ti:S regenerative amplifier was utilized to pump a Type II optical
parametric amplifier, yielding tunable pulsed between 500-550 nm used to excited the S0 →S1
transition in the case of bis-DIPYR and zDIP2. A homebuilt noncollinear optical parametric
amplifier was used to pump m8B. Probe pulsed were generated by passing a small portion of the
amplifiers output through a CaF2 plate, yielding white light (320-950 nm). A spectrograph was
used to disperse the probe source after sample onto a 256-pixel silicon diode array. The sample
cuvette has a pathlength of 1 mm, and measurements were conducted anaerobically.

119

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122

Chapter 5: Understanding Intra- and Inter-Molecular Charge Transfer in
Carbene-Metal-Amide Complexes.
Introduction
The last several chapters have focused on organic photovoltaics (OPVs). While photovoltaics
are necessary for decreasing society’s consumption of fossil fuels, they require a mechanism to store solar
energy for nighttime use. While storage of solar energy in solid state batteries is possible,
1
the means to do
so is material heavy. Based on the enthalpy change for intercalation, Li ion batteries have an energy density
of 0.4-0.9 MJ/kg. At that density, mining enough Li could have series impacts on air, water, and soil
pollution.
2
Contrasting the low energy density of Li, octane, a major constituent of gasoline, has a much
greater energy density of 46 MJ/kg. While burning of hydrocarbons has led to the current climate crisis,
this is largely due to the fact that the process is not reversible. If we could find a mechanism to reduce CO 2
into useable fuels, we could make the process carbon neutral. For example, even just preforming a 2 e
-

reduction of CO 2 yields methanol, a fuel with a 20 MJ/kg energy density. Moving away from carbon
entirely, the reduction of water to H 2 results in a fuel with an energy density of 141 MJ/kg.
As such, researching means of transforming solar energy into “solar fuels” is advantageous.
3, 4

A common approach to producing solar fuels is use a photosensitizer (PS) in conjunction with
an electrocatalyst (EC). Upon absorbing light PS is promoted to its excited state, PS*. Because at the
simplest of definitions, excitation is a transfer of an electron from the HOMO to the LUMO, the result is
the presence of a destabilized hole and electron in these orbitals, respectively. As such, PS* is both a more
potent oxidant and reductant than PS is.
5
This enables it to transfer charge to EC, which then uses that
charge in the production of solar fuel. The ionized PS can then recover its missing charge from an electrode
or a sacrificial agent, SAC, and the photocatalytic cycle can repeat. This cycle is drawn in Figure 1. It is
worth mentioning that PS* can first interact with the sacrificial agent and then the EC to the same effect.
123

Figure 5.1. Cartoon schematic of a PS transferring charge to EC. The cycle begins with light excitation (hn) to form
PS*. PS* then donates charge to EC which is able to carry out the production of solar fuels. The neutral PS is
regenerated when PS
+
or PS
-
takes either an electron or a hole, respectively, from the sacrificial agent (SAC). From
here the cycle can repeat.

Currently, the predominant photosensitizers are metal complexes involving heavy metals, such
as Ru and Ir.
6
These metals feature large spin orbit coupling constants (SOC) which allow for rapid
intersystem crossing (ISC) to long live triplet states, ensuring PS* has ample time to diffuse to the other
reactant. However, these metals have a low abundance in the earth’s crust, and therefore a high expense,
making them an unsustainable to a large scale up. Complexes reliant on much more abundant copper have
also been shown to have large excited state reduction, E
+/*
, and oxidation, E
*/-
, potentials. However most
copper complexes are three- or four-coordinate and suffer from large excited state geometric
reorganizations which diminish excited state lifetimes.
In searching for new PS comprised of earth abundant elements, these compounds should have
strong tunable absorptions, a long excited state lifetime, photo- and electro-chemical stability, and large
excited state oxidation or reduction potentials. Recently, several groups, including ours, have reported
highly emissive two-coordinate carbene-metal-amide (cMa) compounds (where M = Cu, Ag, Au). These
complexes feature strong absorption bands spanning the visible spectrum (e > 10
3
M
-1
cm
-1
, l max = 350-
124

600 nm)
7-11
and excited state lifetimes between 0.2-3 ms, making them attractive candidates as PS. This
chapter will detail the photophysics and photochemistry involved with transferring charge from a subset of
these cMa complexes to a substrate.
The lowest energy excited state in these complexes is a ligand-ligand charge transfer state
(LLCT), characterized as donation of an electron from the carbazole to the carbene to form a c
-
-M-a
+
state.
In this case, the oxidation state of the metal does not change, so geometric distortions associated from
moving from d
10
to d
9
in metal ligand charge transfer states (MLCT) are avoided. The energy of the LLCT
state can be tuned by proper selection of each ligand. Due to the large distance between the carbene and the
carbazole, and therefore the electron and the hole in the LLCT, the singlet-triplet energy gap, DE ST, is small
(< 600 cm
-1
).
8
Therefore, a rapid equilibrium is established between the lowest singlet, S 1, and triplet
state¸T 1 , even with the poor SOC of Cu. It is by having the majority of the population in the triplet state
that the long excited state lifetimes are realized.
12, 13
Furthermore, utilizing an LLCT state places the carbene
and carbazole moieties into geometries akin to their reduced and oxidized forms respectful. This helps
lower the internal reorganization energy associated with charge transfer, and minimized energy losses
associated with intermolecular charge transfer.
14

In this chapter, we investigate the photophysical properties and timescale of the molecules as
they move from the S 1 to T 1 state via ps-TA and ps-TCSPC. Assignment of bands in the TA spectra are
corroborated with spectroelectrochemical (SEC) measurements. Afterwards we will follow intermolecular
charge transfer between these cMa complexes with an electron acceptor via nsTA. In doing so, we generate
a full picture of the charge transfer dynamics associated with the absorption of light and bimolecular transfer
of charge for these complexes.
8, 14-16

Disclaimer: This chapter covers data and studies already presented in a ChemRxiv pre-print
written mostly between Michael Kellogg and myself as co-first authors, with additional studies and text
generated by our fellow collaborators and principle investigators. Portions of this chapter are direct quotes
of paragraphs from that pre-print, with figure numbers updated and the term ICT changed for LLCT.
However, those quotations are portions of the original text generated by myself without written
125

contributions from my collaborators (but not necessarily without intellectual contributions.) All quotes are
from reference 17. Contribution to figures and data is given at the end of this chapter.
Results and Discussion
Steady State Photophysics
The structures for the cMa complexes are given in Figure 2. The complexes are linear
two-coordinate coinage metal complexes comprised of a carbene (blue) and a carbazole (red)
bonded to a central Cu or Au atom. While all possible combination of these elements are
synthesizable, the complexes studied here are: 𝐶 𝑢 𝐶𝑧
𝑀𝐴𝐶 , 𝐴 𝑢 𝐶𝑧
𝑀𝐴𝐶 𝐶 𝑢 𝐵𝐶𝑧 𝑀𝐴𝐶 , 𝐴 𝑢 𝐵𝐶𝑧 𝑀𝐴𝐶 , 𝐶 𝑢 𝑃 ℎ𝐶𝑧
𝑀𝐴𝐶 , 𝐶 𝑢 𝐶𝑧𝐶𝑁 𝐷𝐴𝐶 ,
𝐶 𝑢 𝐶𝑧
𝐶𝐴𝐴𝐶
and 𝐴 𝑢 𝐶𝑧
𝐶𝐴𝐴𝐶
.
“The photophysical properties of the cMa complexes have been previously reported.
8, 9
The
steady state absorption and emission spectra in THF or 2-MeTHF are displayed in Figure 3. Each
complex features structured absorptions from 300-380 nm attributed to carbazole,
9
as well as a
relatively strong (e = 5000-9000 M
-1
cm
-1
), broad CT transition between 375-550 nm. The energy
of the LLCT band can be easily controlled via careful selection of the carbene and carbazole, with
the LLCT transition having lower energy with increasing carbene electrophilicity
(DAC > MAC > CAAC) as well as increasing carbazole nucleophilicity
(Cz > PhCz > BCz >> CNCz). The emission spectra all present as structureless bands indicative
of LLCT states, with lmax values spanning a 220 nm range. The Stokes shifts in THF or 2-MeTHF
range from 550 to 900 meV, indicating large excited state relaxation. Each complex has a long
lifetime in polar solvents (t = 0.08-2 ms), comparable to the lifetimes of 3- and 4- coordinate
copper photosensitizers.
6
”
126

Figure 4.2. Left) General structures of the cMa complexes studied here. The carbene group is given in blue, while the
amide is displayed in red. Middle) Possible substitutions on the carbazole, as well as naming convention for each.
Right) Possible carbenes used with their names. Bottom) N-Methylphthalime (MePI) and N,N’-
dimethyldihydrobenzimidaozle (BIH) used in electron and hole transfer studies, respectively.

300 350 400 450 500 550
0
2
4
6
8
10
300 350 400 450 500 550
0
2
4
6
8
10

Molar Absorptivity (M
-1
cm
-1
)
Cu
CAAC
Cz
Au
CAAC
Cz
Cu
MAC
Cz
Au
MAC
Cz

Wavelength (nm)
Cu
MAC
BCz
Au
MAC
BCz
Cu
MAC
PhCz
Cu
DAC
Cz
300 400 500 600 700 800
0.2
0.4
0.6
0.8
1
300 400 500 600 700 800
0
0.2
0.4
0.6
0.8
1

Normalized Emisson
Cu
CAAC
Cz
Au
CAAC
Cz
Cu
MAC
Cz
Au
MAC
Cz

Wavelength (nm)
Cu
MAC
BCz
Au
MAC
BCz
Cu
MAC
PhCz
Cu
DAC
Cz

Figure 3. Left) Molar absorptivity spectra for complexes found in this study. All spectra are in THF with the
exception of 𝐶 𝑢 𝐶𝑧𝐶𝑁 𝐷𝐴 𝐶 which is in 2-MeTHF. Right) Steady state emission spectra for complexes found in this study.
All spectra are in 2Me-THF with the exception of 𝐴 𝑢 𝐵𝐶𝑧 𝑀𝐴𝐶 which is in THF.
127

Spectroelectrochemistry
Transient absorption spectroscopy (TA) is commonly utilized for determining excited state
dynamics. TA spectra are divided into three types of features. The easiest to analyze the presence
(or lack therefore) of are the ground state bleach (GSB) and stimulated emission (SE). The ground
state bleach results from depletion of the ground state population upon the molecule entering the
excite state and therefore less light being absorbed by that species. SE on the other hand results
from a dipole-dipole interaction of the excited state and pump wavelength causing instantaneous
radiation of light from the excited state which is then recorded by the detector and interpreted as
less light being absorbed by the sample. Both the GSB and SE are simple to identify, as being
inverses of the absorption and emission spectra respectively.
Harder to interpret is the excited state absorption (ESA). The ESA is a result of new vertical
transitions from the excited S1/T1 state to higher lying Sn/Tn state respectively. Recently,
McCusker et Al. has published a paper detailing that the ESA features of the metal ligand charge
transfer (MLCT) states in a family of group 8 tris-pyridyl complexes can be thought of as a sum
of the cation and anion spectra of the ground state molecule.
18
To this end, bulk electrolysis and
pulse radiolysis (see previous chapter) become useful tools for simulating the ESA bands of the
LLCT state in these cMa compounds.
“Pulse radiolysis is used to measure the triplet state absorption spectra of compounds by
utilizing b
-
radiation to generate solvent triplet states which are then used to sensitize the analyte.
19

All five Cu-based compounds’ triplet ESA were measured in aerobic o-xylene and recorded in the
early time frame (<5 ns) before competition by cation sensitization and oxygen quenching of the
excited state could occur (Figure 4a and Figure S1). The validity of this approach was confirmed
128

by measuring the spectra of three of the compounds in an anerobic environment where only triplet
sensitization occurs (Figure 4b). The resulting spectra are in good agreement (Figure S1). For
each of the cMa complexes studied here, the excited state spectra have a peak between
600-800 nm. In addition, the excited state spectrum of 𝐶 𝑢 𝐶𝑧
𝐶𝐴𝐴𝐶
displays a NIR absorption located
near 950 nm.”
129

“For compounds with fully reversible electrochemical oxidation and/or reduction, the
absorption spectra of their oxidized (cation) or reduced (anion) forms can be readily obtained by
measuring the spectra of the compound in solution as it is oxidized or reduced during bulk
electrolysis (BE).
18
” As such Nina Baluyot-Reyes collect BE data at NREL of 𝐶 𝑢 𝐵𝐶𝑧 𝑀𝐴𝐶 and 𝐴 𝑢 𝐵𝐶𝑧 𝑀𝐴𝐶 .
“Figure 4 displays the BE absorption spectra for 𝐶𝑢
𝐵𝐶𝑧 𝑀𝐴𝐶 and 𝐴 𝑢 𝐵𝐶𝑧 𝑀𝐴𝐶 at positive, negative, and
neutral applied voltages. We assign the spectra at positive applied voltages to the cation and
negative applied voltages to the anion. For (𝐶 𝑢 𝐵𝐶𝑧 𝑀𝐴𝐶 )
+
, the spectrum shows a vibronically
structured absorption with the principal 𝑆 1
(0-0) peak maximum occurring at 720 nm. The
absorption spectra bears striking similarities to free carbazolium
7, 9, 20, 21
which is consistent with
oxidation being a carbazole centered event.
7-9
For (𝐴𝑢
𝐵𝐶𝑧 𝑀𝐴𝐶 )
+
, the spectrum displays a 10 nm red
shift compared to (𝐶𝑢
𝐵𝐶𝑧 𝑀𝐴𝐶 )
+
, but has the same shape. The spectra recorded under reducing
conditions for both compounds present as a broad featureless transition from 400 to 900 nm. Due
to the timeframe of collecting a spectrum by bulk electrolysis (minutes), it is unclear whether these
500 600 700 800 900 1000
0
0.2
0.4
0.6
0.8
1
Wavelength (nm)
Cu
MAC
Cz
Cu
MAC
BCz
Cu
MAC
PhCz
DAbs (norm.)
500 600 700 800 900 1000
0
0.2
0.4
0.6
0.8
1
Wavelength (nm)
Cu
MAC
Cz
Cu
CAAC
Cz
Cu
DAC
CNCz
DAbs (norm.)

Figure 5.4 – (Left) Excited state absorptivity spectra of 10 mM 𝐶 𝑢 𝐶𝑧
𝑀𝐴𝐶 , 𝐶 𝑢 𝐵𝐶𝑧 𝑀𝐴𝐶 , and 𝐶 𝑢 𝑃 ℎ𝐶𝑧
𝑀𝐴𝐶 in a solution of 20 mM
triphenylamine in o-xylene in air 5 ns after electron pulse. (Right) Excited state absorptivity spectra of 10 mM
𝐶 𝑢 𝐶𝑧
𝑀𝐴𝐶 , 𝐶 𝑢 𝐶𝑁𝐶𝑧 𝐷𝐴𝐶 , and 𝐶 𝑢 𝐶𝑧
𝐶𝐴𝐴𝐶 in o-xylene under Ar 10 ms after electron pulse. Minor differences between the two
experimental conditions are observed in the spectra for 𝐶 𝑢 𝐶𝑧
𝑀𝐴𝐶 . Data collected is displayed as symbols and the
lines are interpolations between data.

Figure 5.4 – (Left) Excited state absorptivity spectra of 10 mM 𝐶 𝑢 𝐶𝑧
𝑀𝐴𝐶 , 𝐶 𝑢 𝐵𝐶𝑧 𝑀𝐴𝐶 , and 𝐶 𝑢 𝑃 ℎ𝐶𝑧
𝑀𝐴𝐶 in a solution of 20 mM
triphenylamine in o-xylene in air 5 ns after electron pulse. (Right) Excited state absorptivity spectra of 10 mM
𝐶 𝑢 𝐶𝑧
𝑀𝐴𝐶 , 𝐶 𝑢 𝐶𝑁𝐶𝑧 𝐷𝐴𝐶 , and 𝐶 𝑢 𝐶𝑧
𝐶𝐴𝐴𝐶 in o-xylene under Ar 10 ms after electron pulse. Minor differences between the two
experimental conditions are observed in the spectra for 𝐶 𝑢 𝐶𝑧
𝑀𝐴𝐶 . Data collected is displayed as symbols and the
lines are interpolations between data.
130

spectra represent the reduced molecular species or the formation of aggregates or colloids at the
electrode surface.”
400 500 600 700 800 900
0
0.5
1
1.5
2
2.5
3
Absorption (OD)
Wavelength (nm)
Neutral
Oxidative
Reductive
720 nm
Cu
MAC
BCz
655 nm
595 nm
400 500 600 700 800 900
0
0.2
0.4
0.6
0.8
1
Absorption (OD)
Wavelength (nm)
Neutral
Oxidative
Reductive
Au
MAC
BCz

Figure 5.5. The bulk electrolysis spectra of 𝐶 𝑢 𝐵𝐶𝑧 𝑀𝐴𝐶 (left) and A𝑢 𝐵𝐶𝑧 𝑀𝐴𝐶 (right) in THF under neutral (0 V vs Ag, black)
oxidative (0.9 V vs Ag, red) or reductive conditions (-2.1 V vs Ag, blue). A 7-point smooth was applied to the data
set to remove noise predominate below 500 nm.
“For compounds with non-reversible redox events, monitoring the absorption spectra
during a BE experiment is problematic due to competing rates of ion degradation. In this case, PR
is useful for measuring the spectra of ions before they decay (<10 ms). An additional benefit of
PR over BE is that the former gives the molar absorptivity spectra of the charged species” The
molar absorptivity spectra of various Cu-based cMa ions are represented in Figure 6 and Figure
S2.
“For each of the Cu cMa cations in aerobic o-xylene, the molar absorptivity spectra feature
a vibronically structured band, located between 600-800 nm in 𝐶𝑢
𝐵𝐶𝑧 𝑀𝐴𝐶 , 𝐶𝑢
𝐶𝑧
𝑀𝐴𝐶 , and 𝐶𝑢
𝐵𝐶𝑧 𝑀𝐴𝐶 , and
between 800-1000 nm for 𝐶𝑢
𝑃 ℎ𝐶𝑧
𝑀𝐴𝐶 . Incomplete oxidation in o-xylene is observed for 𝐶𝑢
𝐶𝑁 𝐶𝑧
𝐷𝐴𝐶 , likely
due to charge delocalization across multiple solvent molecules stabilizing the solvent cation,
requiring a switch to benzonitrile. In benzonitrile, 𝐶𝑢
𝐶𝑁𝐶𝑧 𝐷𝐴𝐶 presents as a structureless band between
131

800-1000 nm (Figure S3). The shift to a more polar solvent could be the cause for the loss of
structured absorption.”
500 600 700 800 900
0
2
4
6
8
10
Cations
Wavelength (nm)
Cu
MAC
Cz
Cu
MAC
BCz
Cu
MAC
PhCz
Molar Absorptivity (10
3
M
-1
cm
-1
)
600 800 1000 1200 1400 1600
0
1
2
3
Anions
Wavelength (nm)
Molar Absorptivity (10
3
M
-1
cm
-1
)
Cu
MAC
Cz
Cu
CAAC
Cz
Cu
DAC
CNCz

Figure 5.6. (a) Cation molar absorptivity spectra of 10 mM cMa complexes in a 20 mM solution of triphenylamine in
o-xylene. (b) Anion molar absorptivity spectra of 10 mM cMa complexes in 10 mM TADPF in THF. Data collected
is displayed as symbols and the lines are interpolations between data.
“Due to reduction of the various Cu cMa compounds being carbene centered,
7-9
𝐶𝑢
𝐶𝑧
𝑀𝐴𝐶 ,
𝐶𝑢
𝐶𝑁𝐶𝑧 𝐷𝐴𝐶 , and 𝐶𝑢
𝐶𝑧
𝐶𝐴𝐴𝐶
each possess unique anion molar absorptivity spectra. 𝐶𝑢
𝐶𝑧
𝑀𝐴𝐶 and 𝐶𝑢
𝐶𝑁𝐶𝑧 𝐷𝐴𝐶
possess broad featureless absorptions extending from <500 to 1000 nm and from <500 to 700 nm,
respectively. 𝐶𝑢
𝐶𝑧
𝐶𝐴𝐴𝐶
displays a large featureless absorption from 600-1000 nm, as well as a large,
132

well-defined peak centered at ~1300 nm. Wavelengths below 500 nm cannot be measured for
these compounds due to the presence of the substantial ground state absorption limiting the
detection of the charged species. Since the reduced species are present in a relatively low
concentration, being generated uniformly through the cuvette, and spectra are collected in less than
10 ms, the formation of colloids can be excluded. The similarity of the anionic 𝐶𝑢
𝐵𝐶𝑧 𝑀𝐴𝐶 spectra
generated via BE to the anionic spectra generated via PR suggests the formation of aggregates or
colloids in the BE experiment does not take place. Both the cationic and anionic absorption spectra
are useful for providing basis spectra for analyzing intra- and inter-molecular charge transfer
events via TA.”

Determining rate of Intersystem Crossing
Previous work out of the Thompson and Bradforth groups have estimated the rates of
intersystem crossing (ISC) for 𝑀 𝐶𝑧
𝐶𝐴𝐴𝐶
and 𝑀 𝐶𝑧
𝑀𝐴𝐶 , where M = Cu, Ag, and Au using TCSPC.
8
In
that paper, we utilized an Arrhenius like fit of variable temperature lifetime measurements to
access DEST, as well as the ratio between the endergonic rate of intersystem crossing from the S1
to the T1, 𝑘 𝐼𝑆𝐶 𝑒𝑛𝑑𝑒 and the exergonic rate of intersystem crossing from the T1 to the S1, 𝑘 𝐼𝑆𝐶 𝑒𝑥𝑒 .
However, we were unable to abstract those absolute rates alone, which would be useful for
comparing how ligand effect ISC, which would likely affect both 𝑘 𝐼𝑆𝐶 𝑒𝑛𝑑𝑒 and 𝑘 𝐼𝑆𝐶 𝑒𝑥𝑒 and therefore
have no net effect on the ratio. Additionally, collecting lifetime measurements from 4 – 300 K in
is time intensive, and cannot be automated, requiring a dedicated day for each sample. As such,
easier to implement techniques that yield more information is desirable.
133

To that end, we employ a kinetic fit of an exact solution to TADF from Adachi et al.
22

which gives equation 1 and equation 2 for the rate of change of the S1 and T1 populations
respectively,
𝑑 𝑑𝑡
𝑆 1
= −(𝑘 𝑟 𝑠 + 𝑘 𝑛𝑟
𝑠 + 𝑘 𝐼𝑆𝐶 𝑒𝑥𝑒 )[𝑆 1
] + 𝑘 𝐼𝑆𝐶 𝑒𝑛𝑑𝑒 [𝑇 1
] Eq. 1
𝑑 𝑑𝑡
𝑇 1
= −(𝑘 𝑟 𝑇 + 𝑘 𝑛𝑟
𝑇 + 𝑘 𝐼𝑆𝐶 𝑒𝑛𝑑𝑒 )[𝑇 1
] + 𝑘 𝐼𝑆𝐶 𝑒 𝑥𝑒
[𝑆 1
] Eq. 2
where 𝑘 𝑟 𝑆 (𝑘 𝑟 𝑇 ) and 𝑘 𝑛𝑟
𝑆 (𝑘 𝑛𝑟
𝑇 ) are the radiative and non-radiative decay rates for 𝑆 1
(𝑇 1
). In our
case 𝑘 𝑟 𝑇 and 𝑘 𝑛𝑟
𝑇 are assumed to be 0 based on low temperature phosphorescence lifetimes
determined previously.
8
Therefore depletion of the triplet only occurs via ISC back to the singlet,
and emission of the sample only occurs from the singlet state. As such TCSPC luminescent decay
curves can be treated as measurement of the singlet population alone.
The TCSPC luminescent decay curves obtained for the cMa complexes show two distinct
regions. The first with a picosecond time constant, tp, assigned as “prompt fluorescence”, and the
second with 100s of nanoseconds to microsecond time constant, tTADF, attributed to the TADF
process. The prompt fluorescence occurs as direct emission from the S1 state following excitation,
while TADF occurs after the S1-T1 equilibrium is reached, with a given excited molecule switching
between the two spins multiple time before emission. The majority of the emission amplitude is
prompt (Ap) while only less than 2% of the amplitude comes from delayed emission (ADF = 1-Ap),
though the integrated area for the TADF emission is much larger than for the prompt emission.
Because both radiative and non-radiative decay rates are small compared to 𝑘 𝐼𝑆𝐶 𝑒𝑥𝑒 and 𝑘 𝐼𝑆𝐶 𝑒𝑛𝑑𝑒 , the
above equations can be further simplified to equations 3 and 4.
𝑘 𝐼𝑆𝐶 𝑒𝑥𝑒 = 𝐴 𝑝 1
𝜏 𝑝 Eq. 3
134

𝑘 𝐼𝑆𝐶 𝑒𝑛𝑑𝑒 = (1 − 𝐴 𝑝 )
1
𝜏 𝑝 Eq. 4
Assuming the entropy change between the two spin states is negligible, the Boltzmann factor and
equilibrium constant with respect to singlet formation, KS, can be written as follows.
𝐾 𝑆 =
𝑘 𝐼𝑆𝐶 𝑒𝑛𝑑𝑒 𝑘 𝐼𝑆𝐶 𝑒𝑥𝑒 =
1−𝐴𝑝
𝐴𝑝
=
1
3
exp (−
∆𝐸 𝑆𝑇
𝑘 𝐵 𝑇 ) Eq. 5
Therefore, by measuring the relative luminescent amplitudes via TCSPC, we can extract DE ST,
𝑘 𝐼𝑆𝐶 𝑒𝑥𝑒 , and 𝑘 𝐼𝑆𝐶 𝑒𝑛𝑑𝑒 without the need for cryogenic studies and by using a technique which is much
faster. Fabiola Cardoso-Delgado and Thabassum Ahammad collected ps-TCSPC data for all Cu
complexes with the exception of 𝐶 𝑢 𝐶𝑧
𝐶𝐴𝐴𝐶
whose tP was shorter than the instrument response
function, IRF, value of 22ps. As such it is examined later via ps-TA where the IRF is 300 fs. A
summary of their results is tabulated in Table 1.

Ap
(%)
ATADF
(%)
tp
(ns)
tTADF
(ms)
krel
a

(10
12
s
-1
)
𝑘 𝐼𝑆𝐶 𝑒𝑥𝑒
(10
9
s
-1
)
𝑘 𝐼𝑆𝐶 𝑒𝑛𝑑𝑒
(10
9
s
-1
)

∆𝐸 𝑆𝑇

𝐶 𝑢 𝐶𝑧
𝑀𝐴𝐶 98.7 1.3 220 1.3 0.2 4.5 0.06 83

𝐶 𝑢 𝐵𝐶𝑧 𝑀𝐴𝐶 98.6 1.4 300 0.7 0.2 3.3 0.05
b
80

𝐶 𝑢 𝐵𝐶𝑧 𝑀𝐴𝐶 (THF) 98.7 1.3 350 0.36 0.2 2.8 0.04
b
83

𝐶 𝑢 𝑃 ℎ𝐶𝑧
𝑀𝐴𝐶 98.1 1.9 280 0.72 0.2 3.5 0.069
b
73
a

𝐶 𝑢 𝑃 ℎ𝐶𝑧
𝑀𝐴𝐶 (THF)
98.1 1.9 340 0.46 0.2 2.8 0.058 73

𝐶 𝑢 𝐶𝑧𝐶𝑁 𝐷𝐴𝐶 98.3 1.7 127 0.67 0.2 7.7 0.14

75

𝐶 𝑢 𝐶𝑧𝐶𝑁 𝐷𝐴𝐶
(THF)
99.0 0.1 141 0.05 0.2 7 0.09
b
84

Table 5.1. ps-TCSPC decay trace amplitudes, time constants, and derived rates.
a
described below.

b
𝑘 𝐼𝑆𝐶 𝑒𝑛𝑑𝑜 was
calculated using the full solution by Adachi et al. due to comparable rates of 𝑘 𝑟 𝑆 and 𝑘 𝑛𝑟
𝑆 .

“Looking now at the specific effects of ligand identity, for the series of MAC compounds,
the choice of carbazole has the smallest effect on the ISC rates compared to the metal or carbene.
Increasing the electron richness of the carbazole by installing peripheral units at the 3- and
135

6- positions only decreases 𝑘 𝐼𝑆𝐶 𝑒𝑥𝑒 by 30% moving from 4.5·10
9
s
-1
in

𝐶 𝑢 𝐶𝑧
𝑀𝐴𝐶 to 3.3·10
9
s
-1
in 𝐶 𝑢 𝐵𝐶𝑧 𝑀𝐴𝐶 .
Changing the carbene has a more substantial effect on ISC. Changing from MAC to the more
electron poor CAAC in the 𝐶𝑢
𝐶𝑧

systems, 𝑘 𝐼𝑆𝐶 𝑒𝑥𝑒 increases four-fold from 4.5·10
9
s
-1
to 20.2·10
9
s
-1
.
𝐶𝑢
𝐶𝑁𝐶𝑧 𝐷𝐴𝐶 displays an intermediate value of 𝑘 𝐼𝑆𝐶 𝑒𝑥𝑒 of 7.7·10
9
s
-1
, benefiting from an electron poor
carbene, while suffering from an electron rich carbazole. These results are consistent with our
previously reported work which demonstrated that increasing the amount of metal character, by
decreasing the availability of the ligand to contribute to the formed molecular orbitals, results in
higher rates of ISC.
8
” Important also to point out is the lack of solvent effect on the rates of ISC,
despite solvents effect for tTADF. This signifies that polar solvent serves as a means to mediate
non-radiative decay, without effecting ∆𝐸 𝑆𝑇
.
While the choice of ligand does influence 𝑘 𝐼𝑆𝐶 𝑒𝑥𝑒 and 𝑘 𝐼𝑆𝐶 𝑒𝑛𝑑𝑒 , the largest contribution to rates
of ISC should stem from increased spin orbit coupling (SOC) from moving from Cu to the heavier
element Au. This effect is readily seen by the fact that all three Au compounds presented in this
study have tP below the IRF of the ps-TCSPC instrument, requiring analysis via ps-TA.
As such, Michael Kellogg and Cardoso-Delgado collected ps-TA for all compounds
present in this study. Their figures for 𝐶 𝑢 𝐵𝐶𝑧 𝑀𝐴𝐶 and 𝐴 𝑢 𝐵𝐶𝑧 𝑀𝐴𝐶 are reproduced here (Figure 7). These
two molecules are representative of the family as a whole, with the same trends being observed.
For a more detailed analysis the reader is directed to reference
17
. Spectra reproduced here were
taken in toluene with an excitation wavelength of 405 nm, corresponding to the LLCT band of
these two complexes. Spectra collected in THF is again available in the previous reference.
As previously discussed, the TA spectra are comprised of three features, a broad ESA band
from 475 nm to 800 nm, a GSB from 400 to 450 nm, and a SE from 500 to 620 nm which overlaps
136

with the ESA. At 0.3 ps the ESA peaks for both 𝐶 𝑢 𝐵𝐶𝑧 𝑀𝐴𝐶 and 𝐴 𝑢 𝐵 𝐶𝑧
𝑀𝐴𝐶 at 690 nm with a 0-1 shoulder
at 620 nm. This band bears striking resemblance to the BE data collected for the cation as well as
the PR cation data collected for the 𝐶 𝑢 𝐶𝑧
𝑀𝐴𝐶 and 𝐶 𝑢 𝑃 ℎ𝐶𝑧
𝑀𝐴𝐶 .

137

As the spectra progress through time, the GSB of neither compound recovers over the
1.5 ns indicating that the ground state population does not recover over this timeframe, consistent
with the >100 ns lifetime for both complexes. Over the course of 10 ps, the ESA of
𝐶 𝑢 𝐵𝐶𝑧 𝑀𝐴𝐶 undergoes a 10 nm bathochromic shift and loss of signal intensity, while 𝐴 𝑢 𝐵𝐶𝑧 𝑀𝐴𝐶 undergoes
just a lost of intensity. This same feature is also observed in THF on twice as fast of a timescale.
17

Meech et Al have published a study of 𝐴 𝑢 𝐵𝐶𝑧 𝐶𝐴𝐴𝐶
in chlorobenzene. Using psTA and fs stimulated
Raman spectroscopy they attribute this time regime to vibrational relaxation of a hot singlet state.
Fitting the dataset of all cMa reveals that the rate of relaxation for this state, krel is on the order of
0.2 – 0.8 × 10
12
𝑠 −1
, with rates being ~2x as fast in THF as they are in toluene.
400 500 600 700 800 900
-9
-6
-3
0
3
6
9
12
DAbs (mOD)
Wavelength (nm)
0.3 ps 1000 ps
1 ps 1250 ps
10 ps Abs
100 ps Em
400 500 600 700 800 900
-8
-6
-4
-2
0
2
4
6
8
10
DAbs (mOD)
Wavelength (nm)
0.3 ps 100 ps
1 ps 1500 ps
4 ps Abs.
10 ps Em.

Figure 5.7. The psTA spectra of 𝐶 𝑢 𝐵𝐶𝑧 𝑀𝐴𝐶 (left) and 𝐴 𝑢 𝐵𝐶𝑧 𝑀𝐴𝐶 (right) after excitation with a 405 nm pump source (solid
lines). The inverted absorption (blue dashed) and emission (red dash) spectra are displayed. Regions at 400 and 800
nm are removed due to pump scatter. Note the difference in time for the spectra slices between the two plots.
For the Cu complex, the structure ESA band at 10 ps is replaced with a second structured
band at 650 nm over the course of 1 ns. This new ESA band is significantly broader than the
previous. Additionally, over this timescale, the SE band is lost. This same spectral evolution is
also observed in 𝐴 𝑢 𝐵𝐶𝑧 𝑀𝐴𝐶 , but over a much shorter 100 ps. In the Cu complexes, this spectral shift
and loss of SE is comparable to tP from TCSPC. As such, this process is assigned ISC of the S 1
state to the T1. With this assignment now made, we can extract rates of 𝑘 𝐼𝑆𝐶 𝑒𝑥𝑒 for the Au complexes.
138

However, because only <2% of the excited state population resides in the S 1 state after
equilibration, it is insensitive to determining 𝑘 𝐼𝑆𝐶 𝑒𝑛𝑑𝑒 . Therefore, the rate of 𝑘 𝐼𝑆𝐶 𝑒𝑥𝑒 from the psTA
measurements was used to fit TCSPC traces with IRF convolution to extra Ap and ATADF. Using
these metrics, Kellogg and Cardoso-Delago were able to extract values of 𝑘 𝐼𝑆𝐶 𝑒𝑛𝑑𝑒 and therefore
DEST. Through this analysis we find that the DE ST values for the gold complex are roughly 30%
larger than they are in the Cu complexes, which is striking as the distance between the carbene and
carbazole (and therefore the e
-
and h
+
separation) are larger in the Au complexes than in the Cu
analogous by almost 0.25 Å, which should lead to a smaller DEST.
8
However, recent studies by
Muniz et Al. found that gold atom is a large contributor to the HOMO and LUMO densities than
copper is, which contributes to the higher molar absorptivity observed for those compounds, but
also a larger DEST.
23
A summary of results is presented in Table 2.

Ap
(%)
ATADF
(%)
tp
(ns)
tTADF
(ms)
krel
a

(10
12
s
-1
)
𝑘 𝐼𝑆𝐶 𝑒𝑥𝑒
(10
9
s
-1
)
𝑘 𝐼𝑆𝐶 𝑒𝑛𝑑𝑒
(10
9
s
-1
)

∆𝐸 𝑆𝑇

CaacCuCz 99.2 0.8 50 2 0.2 20.2 0.162 96
MacAuCz 99.62 0.38 8 0.93 0.3 120 0.5 110
MacAuBCz 99.52 0.48 12 0.46 0.3 83 0.4 109
MacAuBCz
(THF)
99.5 0.41 12 0.25 0.8 77 0.35 113
CaacAuCz 99.44 0.55 9 1.2 0.5 110 0.6 105
Table 5.2. ps-TCSPC decay trace amplitudes as well as time constants, and derived rates from joint ps-TCPSC and
ps-TA studies.

Excited State Recreation
“In order to further analyze the ESA spectra gleamed from TA, we have simulated the ESA
of several of the cMa complexes as sums of their cation and anion spectra measured from SEC
Figure 8. The features present in the simulated 𝐶 𝑢 𝐶𝑧
𝐶𝐴𝐴𝐶
are in good agreement with the relative
placement of the two features present in the T1 spectra for this compound, while only the higher
energy feature of the simulated spectra is present in the S1 ESA. Similarly, the simulated spectra
139

for the rest of the cMa complexes are largely comprised of the carbazolium feature between 1.5-
1.7 eV and match well to the respective S1 and T1 ESA which are dominated by a feature between
1.6-1.9 eV. Despite the choice of carbazole varying the oxidation potential of the cMa widely, its
appearance in the ESA is qualitatively similar. In the case of 𝐶 𝑢 𝐶𝑧
𝐶𝐴𝐴𝐶
and 𝐶 𝑢 𝐶𝑧
𝑀𝐴𝐶 , the relative
intensity of the absorption peaks and peak vibronics seem well matched between the simulated
and real spectra while this isn’t the case for the other compounds.”
“Notably, there is an energy mismatch between the two simulated and measured spectra
for each complex with the exception of the simulated and T1 ESA for 𝐶 𝑢 𝐶𝑧
𝑀𝐴𝐶 . Both peaks of the
𝐶 𝑢 𝐶𝑧
𝐶𝐴𝐴𝐶
simulated spectra are ~300 meV lower in energy then their T1 ESA counterparts, while
the carbazolium features in the ESA of the MAC family are somewhere between 0-200 meV. This
discrepancy is not present in the paper by McCusker et. Al.
18
This energy mismatch could
potentially be due to a larger coulombic attraction present in the LLCT of our compounds in
comparison to what is present in the prior work, or to coupling of the S1 and T1 states to higher
lying Sn and Tn states respectively. Elucidating a direct cause of the energy mismatch will be
further studied by the Thompson group.”
140

1 1.5 2 2.5 3
-1
0
1
2
3
4
5
6
Energy (eV)
Cat + An Emission
S
1
(norm) T
1
(norm)
Molar Absorptivity (10
3
M
-1
cm
-1
)
Cu
CAAC
Cz
(a)
500 600 800 1100 1600
Wavelength (nm)
1 1.5 2 2.5 3
-2
0
2
4
Wavelength (nm)
Cat + An
S
1
(norm) Absorption
T
1
(norm) Emission
Molar Absorptivity (10
3
M
-1
cm
-1
)
Cu
MAC
Cz
(b)
500 600 800 1100 1600
Wavelength (nm)
1 1.5 2 2.5 3
-4
-2
0
2
4
6
8
Energy (eV)
Cat + An
S
1
(norm) Absorption
T
1
(norm) Emission
Molar Absorptivity (10
3
M
-1
cm
-1
)
Cu
MAC
BCz
(c)
500 600 800 1100 1600
Wavelength (nm)
1 1.5 2 2.5 3
-3
-2
-1
0
1
2
3
4
5
6
7
8
Energy (eV)
Cat + An
S
1
(norm) Absorption
T
1
(norm) Emission
Molar Absorptivity (10
3
M
-1
cm
-1
)
Cu
MAC
PhCz
(d)
500 600 800 1100 1600
Wavelength (nm)
1 1.5 2 2.5 3
-2
0
2
4
Energy (eV)
Cat + An
S
1
(norm) Absoprtion
T
1
(norm) Emission Molar Absorptivity (10
3
M
-1
cm
-1
)
(e)
500 600 800 1100 1600
Wavelength (nm)
Cu
DAC
CNCz

Figure 5.8. The sum of the PR molar absorptivity plots (black) compared to the S1 state (blue) and the T1 state (red)
from SADS analysis of (a) 𝐶 𝑢 𝐶𝑧
𝐶𝐴𝐴𝐶 , (b) 𝐶 𝑢 𝐶𝑧
𝑀𝐴𝐶 , (c) 𝐶 𝑢 𝐵𝐶𝑧 𝑀𝐴𝐶 , (d) 𝐶 𝑢 𝑃 ℎ𝐶𝑧
𝑀𝐴𝐶 , and (e) 𝐶 𝑢 𝐶𝑁𝐶𝑧 𝐷𝐴𝐶 .

141

Intermolecular CT via nsTA.
Spectra collected via psTA provide an excellent opportunity to observe excitated state
relaxation and ISC of the S1 into the T1. However, the maximum collection time point, 1.5 ns,
makes it ineffective for studying bimolecular reactions, as they necessitate two molecular to
diffuse towards each other on the nano second timescale. In order to get around this limitation, we
utilized a nsTA spectrometer, with an IRF of 2 ns, and a maximum collection window of several
seconds to observe bimolecular CT.
“For all 5 cMa complexes in THF and toluene, the nsTA spectra display no noticeable
changes from 10 ns to 2 ms, when the traces return to baseline (Figures 9a-b, S4-13). However,
during the experimental run time, we find that in THF solution both 𝐶𝑢
𝐶𝑧
𝐶𝐴𝐴𝐶
and 𝐶𝑢
𝐶𝑁𝐶𝑧 𝐷𝐴𝐶
decompose while 𝐶𝑢
𝑃 ℎ𝐶𝑧
𝑀𝐴𝐶 decomposes on irradiation in both solvents. On the other hand,
degradation is not observed when these complexes are excited with either the 400 or 405 nm
sources described earlier, nor during the collection of photophysical data reported previously,
7-9

nor with photoreactor studies of 𝐶𝑢
𝑃 ℎ𝐶𝑧
𝑀𝐴 𝐶 where the complex is irritated with 460 nm LEDs for 90
minutes.
17
(Figure S59b). We therefore do not suspect inherent instability in the 𝑆 1
and 𝑇 1
LLCT
states, but rather because 355 nm leads to excitation into a higher lying ligand field state which
may be unstable. While we are currently limited to 355 nm laser as our excitation source, future
work will substitute direct excitation into the LLCT state for these nsTA studies.”

142

400 500 600 700 800 900
-4
-2
0
2
4
6
8
10
Wavelength (nm)
DAbsorption (mOD)
10 ns 200 ns
50 ns 300 ns
100 ns 500 ns
Absorption
(a)
400 500 600 700 800 900
-2
-1
0
1
2
3
4
5
DAbsorption (mOD)
Wavelength (nm)
10 ns 500 ns
100 ns 1000 ns
250 ns 2000 ns
Absorption
(b)

400 500 600 700 800 900
-2
-1
0
1
2
3
4
Wavelength (nm)
DAbsorption (mOD)
10 ns 100 ns
20 ns 250 ns
50 ns 500 ns
Absorption BE Cation
(c)
400 500 600 700 800 900
-1
0
1
2
3
Wavelength (nm)
DAbsorption (nm)
4 ns 100 ns
20 ns 150 ns
50 ns 300 ns
Absorption BE Cation
(d)

400 500 600 700 800 900
-1
-0.5
0
0.5
1
1.5
2
Wavelength (nm)
DAbs (mOD)
20 ns 200 ns
50 ns 400 ns
100 ns 800 ns
Absorption BE Anion
BIH Triplet ESA
(e)

Figure 5.9. 𝐴 𝑢 𝐵𝐶𝑧 𝑀𝐴𝐶 nsTA spectra as neat solutions in THF (a) and toluene (b), with MePI in THF (c) and in toluene (d),
and with BIH in THF (e). A carton schematic of charge tranfer between the excited cMa and Q (bottom right). Spectra
in (a), (b), and (d) were collected on a Magnitude enVISion, while spectra in (c) and (e) were collected on a Magnitude
enVISTA (see Experimental). All data was pumped with 355 nm except for (a) which was pumped with 420 nm.

Q
cMa
*
Q
+/-
cMa
-/+
143

“In order to study charge transfer dynamics from PS* to an electron acceptor…, the excited
states of various cMa complexes were used to reduce N-methylphthalimide, MePI.” “To avoid
photo-degradation issues, we chose to use a gold based cMa, i.e. 𝐴𝑢
𝐶𝑧
𝑀𝐴𝐶 , which is stable to 355
nm excitation.” 𝐴 𝑢 𝐵𝐶𝑧 𝑀𝐴𝐶 possess a E
+/
* of -2.43 V vs Fc, while MePI possess a E
0/-
of -1.92 V vs
Fc, well within the ability to be reduced by PS*.
23
“The addition of 7 mM MePI to 46 mM 𝐴𝑢
𝐶𝑧
𝑀𝐴𝐶
in THF results in a decreased lifetime of 240 ns, attributed to the transfer of either charge or energy
to MePI. However, despite the decreased lifetime, no new features are observed in the spectra
(Figure S41). Recording the ground state absorption of the sample cuvette pre- and
post-experiment reveals a large amount of sample degradation post-quenching. This degradation
is unsurprising as the cyclic voltammogram (CV) trace reveals a non-reversible oxidation wave
for the compound
9
, leading to an anion lifetime too short for the cation to recombine with MePI
-

to form the stable neutral state. As the oxidative non-reversibility in 𝐴𝑢
𝐶𝑧
𝑀𝐴𝐶 is attributed to
polymerization of the unsubstituted 3-, and 6- positions in the carbazole ligand, consistent with
irreversible oxidation of bare Cz,
9
substitutions at these locations with tert-butyl groups in 𝐴𝑢
𝐵𝐶𝑧 𝑀𝐴𝐶
should impart reversibility, which is observed in its CV.
23, 24
Still some degradation is observed
upon adding MePI to the solution. We believe this instability is due again to excitation of a ligand
field transition, this time of the cation, upon irradiation with 355 nm light.
“To circumvent the problems associated with 355 nm excitation in the nsTA system, a flow
cell apparatus with a reservoir volume of 100 mL was used to mitigate the slow degradation of the
complex. Upon the addition of 30 mM MePI to 75 mM 𝐴𝑢
𝐵𝐶𝑧 𝑀𝐴𝐶 in THF, a new feature matching
the cation peak is seen at ~720 nm which persists for ~110 ms (Figure S41-42). The PL is
extinguished with a 50 ns lifetime, indicating charge transfer with no recombination to the excited
state (Figure S42a). The 110 ms lifetime is more than sufficient for a sacrificial reductant to
144

regenerate the neutral 𝐴 𝑢 𝐵𝐶𝑧 𝑀𝐴𝐶 . Reducing the concentration of MePI to 6 mM in THF allows us to
clearly observe signal from the triplet alone at early time slices which slowly evolves into the
cation spectra over ~250 ns (Figure 9c). The quenching rate constant, kq, of 𝐴 𝑢 𝐵𝐶𝑧 𝑀𝐴𝐶 with MePI of
(1.4 ± 0.1) × 10
10
𝑀 −1
𝑠 −1
is in agreement with that obtained previously.
23
”
“Electron transfer from (𝐴 𝑢 𝐵𝐶𝑧 𝑀𝐴𝐶 )* to MePI was reexamined in toluene (Figure 9d). Using
280 mM MePI, results in the formation of the cation at the earliest experimental time delays
resolvable. Throughout the course of the experiment, the ratio between the triplet and cation peaks
remain unchanged and all kinetic traces have a lifetime of 120 ns, less than the lifetime of the
complex alone. Photoluminescence is also detected until the TA spectra returns to baseline. This
result suggests that while an electron is transferred from 𝐴 𝑢 𝐵𝐶𝑧 𝑀𝐴𝐶 to MePI in toluene, as expected,
the ion pair of (𝐴 𝑢 𝐵𝐶𝑧 𝑀𝐴𝐶 )
+
and MePI
-
is not able to escape from the nonpolar solvent cage. Inside
the cage, the charges then collide many times, recombining to either the ground or excited state of
𝐴 𝑢 𝐵𝐶𝑧 𝑀𝐴𝐶 . Attempts to repeat this experiment at a lower concentration of 6 mM MePI resulted in no
cation peaks being observed, mostly likely attributed to the increased driving force required to
transfer charge in non-polar solvents.
5
”
To study the transfer of a hole from the cMA excited state to an electron donor, we selected
to reduce NN’-dimethyl-dihydrophenylbenzimidazole, BIH, a well-known sacrificial reductant,
with an oxidation potential of 0.33 V vs SCE.
25
Its worth noting that converting electrochemical
potentials between Fc and SCE is difficult due to differences between the two electrodes. Fc serves
as an internal standard in the organic solution to calibrate a quasi-reference electrode, such as Ag
wire, which drifts as a function of solvent and surface quality. SCE on the other hand is a true
reference electrode, giving a consistent potential to measure redox couples against. However, SCE
employs an aqueous environment to do so. The interface between the aqueous and organic acts as
145

a resistive junction, which is dependent on the organic solvent, concentration of dissolved species
in both solvents, and surface area of the interface.
26-28
Conversion factors between the two vary
withing in a 200 meV window. Here, we employ the standard conversion factor of Fc
+/0
= 0.4 V
vs SCE, but add 100 meV error bars setting the oxidation potential of BIH to -0.07 ± 0.1 V vs Fc.
This places the -0.09 V vs Fc photo-reduction potential of 𝐴 𝑢 𝐵𝐶𝑧 𝑀𝐴𝐶 within this potential window,
and potentially too weak to oxidize BIH.
23
However, despite the meager driving force for charge
transfer, the oxidation of BIH is irreversible, as BIH promptly falls apart, preventing back-electron
transfer to BIH, which should serve to drive the reaction forward.
Unlike electron transfer to MePI, the reaction between (𝐴 𝑢 𝐵𝐶𝑧 𝑀𝐴𝐶 )* and BIH in THF results
in no degradation of the cMa over the course of the ns TA experiment. As such, use of the flow
cell was forgone. The reaction between (𝐴 𝑢 𝐵𝐶𝑧 𝑀𝐴𝐶 )* and BIH results in a depletion of the triplet band
and formation of a broad absorption between 450-900 nm. This band has great overlap with the
anion absorption between 600-900 nm, however the region between 450-600 displays a sharp
deviation (figure x). To verify whether hole transfer took place as intended or if we carried out
triplet energy transfer, Baluyot-Reyes measured the triplet spectra of BIH. While a qualitative
match is made between the last time slice of the nsTA experiment and anion spectra in the red
region, a complete match is made between the that last time slice and the triplet spectra of BIH,
confirming that we haven’t in fact reduced (𝐴 𝑢 𝐵𝐶𝑧 𝑀𝐴𝐶 )*, but transferred energy from it. Again, this
serves to highlight the difficulty in converting between potentials measured vs SCE and those
measured vs Fc. Future work in the Thompson group aims at using compounds more readily able
to undergo hole transfer from (𝐴 𝑢 𝐵𝐶𝑧 𝑀𝐴𝐶 )*, such as N,N,N’,N’,-tetramethylphenylenediamine
(E
0/-
= 0.04 V vs SCE),
29
N,N-dimethyl-2,4-dimethoxyaniline (0.14 V vs SCE),
30
and
146

Tetrakis-(dimethylamino)-ethylene (-0.78 V vs SCE).
31
All of these compounds have triplet
energies well above the 2.2 eV record for 𝐴 𝑢 𝐵𝐶𝑧 𝑀𝐴𝐶 .
Conclusion
The TADF complexes presented here represent a family of strong photo reductants for the
production of methanol and hydrogen gas from simple feedstocks such as CO2 and water
respectively. The molecules offer strong, tunable absorption manifolds, large excited state
reduction potentials, long lifetimes enables by dynamics equilibrium between the S1 and T1 states,
and electrochemical stability. The long equilibrium between the S1 and T1 states is afforded by a
small DEST (70-120 meV) which allows for rapid ISC between the two states, even without a heavy
atom. We demonstrated the ability to easily quantify absolute values of 𝑘 𝐼𝑆𝐶 𝑒𝑥𝑒 and 𝑘 𝐼𝑆𝐶 𝑒𝑛𝑑𝑒 instead of
a ratio of the two in a facile way through fitting ps-TCSPC. In the case of the Au complexes, where
𝑘 𝐼𝑆𝐶 𝑒𝑥𝑒 is faster than the IRF of the ps-TCSPC, ps-TA can be utilized to supplement the previous
data set and extract meaningful values. This is a marked improvement in data acquisition time
when compared to our prior thermal fitting.
8

Results from the ps-TCPSC experiments indicate the change in rates of ISC stem from
transmetalation from Cu to Au, unsurprising as the latter has a much large SOC. However, the
identity of the ligands also effected rate of ISC. Of the two groups, the identity of the carbene
played the larger role, with a 4.5 times increase when moving from MAC to CAAC, seeming from
the latter’s increase metal contribution in its orbital density. The identity of the carbazole had a
smaller 30% change moving from diphenylcarbazole to carbazole, owed to the weaker metal
contribution to the carbazole group.
The excited state dynamic of the complexes were monitored via ps-TA, which revealed
that upon irradiation with 405 nm light, the sample first forms a hot S1 state that vibronically
147

relaxes on the order of 2-5 ps. As the complex ISC from S1 to T1, the spectra blue shifts, and a lost
of SE is observed, this process does so with a rate consistent with data from the ps-TCSPC
experiments. The SADS are consistent with the T1 spectra from pulse radiolysis. The SADS
spectra for both the S1 and T1 states are well simulated as a sum of the cationic and anionic spectra
from PR and BE, though with a minor energy shift applied (0-300 meV). The cause of this shift is
unknown, as it isn’t observed in earlier work by McCusker et Al.
18
Potentially it stems from an
increased columbic potential energy between the two charges, or it steams from the coupling of
these LLCT based LLCT states with higher lying singlet and triplet MLCT states. As the DE ST is
much larger, it would affect the S1 and T1 states differently.
“In the ns regime, TA studies of 𝐴 𝑢 𝐵𝐶𝑧 𝑀𝐴𝐶 were used to spectroscopically capture the transfer
of either a hole to BIH or an electron to MePI in THF. Here, quenching produces the respective
cation and anion of the 𝐴 𝑢 𝐵𝐶𝑧 𝑀𝐴𝐶 with a diffusion limited charge transfer rate, with spectral signatures
in good agreement with the ionic spectra collected via SEC. The cation of 𝐴 𝑢 𝐵𝐶𝑧 𝑀𝐴𝐶 possesses a 110
ms lifetime before either charge recombination or degradation. The anion of 𝐴 𝑢 𝐵𝐶𝑧 𝑀𝐴𝐶 , formed in the
presence of a sacrificial reductant (BIH), has a lifetime of greater than 2 ms. These lifetimes are
more than sufficient for oxidation by separate source to recover the ground state for the
photocatalytic cycle to continue.”
“Moving from THF to toluene results in an equilibrium seen in the earliest time slices,
between (𝐴 𝑢 𝐵𝐶𝑧 𝑀𝐴𝐶 )* and the solvent caged (𝐴 𝑢 𝐵𝐶𝑧 𝑀𝐴𝐶 )
+
and MePI
-
. The same equilibrium behavior
is seen for 𝐶𝑢
𝐵𝐶𝑧 𝑀𝐴𝐶 . Such a cage mimics covalently linking the PS to an EC, removing the kinetic
rate limit stemming from the two diffusing together during the production of solar fuels.
32
By
using an inert covalent linker, this intermolecular charge separate ion pair can be rapidly realized,
increasing catalytic rates. While degradation was observed in nsTA experiments for some of the
148

other copper cMa complexes using a high energy excitation source (355 nm), no such degradation
was observed when the same molecules were irradiated by lower energy sources at 460 nm LEDs,
indicating that the PS family presented here will be stable to illumination by sunlight. Further
work to establish these materials as possible PS compounds will investigate the transfer of charges
to electrocatalysts of interest for the reduction of either CO2 or water splitting. Finally, work is
underway on the cMa class of complexes will explore covalently linking a cMa to a desired
electrocatalyst to remove the need for long time diffusion, facilitating the photocatalytic cycle.”
Acknowledgement
I would like to thank, in order of work displayed, Nina Baluyot-Reyes for collecting bulk
electrolysis spectra. I would like to thank Thabassum Ahammad and Fabiola Cardoso-Delgado for
collecting ps-TCPSC traces. I’d like to thank Michael Kellogg and F.C.D for collecting ps-TA
spectra. I’d like to thank Collin Muniz and M.K. for collecting the toluene 𝐴 𝑢 𝐶𝑧
𝑀𝐴𝐶 nsTA data in
Pennsylvania. The rest of the nsTA data was also collected in a team of two, with each of these
five people working with me to collect it. I’d like to thank Matthew Bird for his help assisting me
with collection of the pulse radiolysis data.

149

Supporting Figures
400 500 600 700 800 900 1000
-0.4
0
0.4
0.8
1.2
Cu
CAAC
Cz
Cu
DAC
CNCz
Wavelength (nm)
DAbsoprtion (norm.)

Figure 5.10. Triplet absorption spectra of 𝐶 𝑢 𝐶𝑧
𝐶𝐴𝐴𝐶 (red) and 𝐶 𝑢 𝐶𝑁𝐶𝑧 𝐷𝐴𝐶 (black) in aerated o-xylene, before O2 quenching
of the triplet state and cation sensitization can occur (<5ns). The negative feature observed in the 𝐶 𝑢 𝐶𝑧
𝐶𝐴𝐴𝐶 spectra is
attributed to Cherenkov radiation from the electrons slowing down as it passes into solution media. The circles are
data points and the line is the interpolation between the symbols.
500 550 600 650 700 750 800
0
1
2
3
Cu
CAAC
Cz
in o-xylene
with 20 mM triphenylamine
Wavelength (nm)
Molar Absorptivity (10
3
M
-1

cm
-1
)

Figure 5.11. Cation molar absorptivity spectra of 10 mM 𝐶 𝑢 𝐶𝑧
𝐶𝐴𝐴𝐶 in 20 mM solution of triphenylamine in o-xylene.
The circles are data points and the line is the interpolation between the symbols.
150

400 600 800 1000 1200 1400 1600
0.5
1
1.5
2
2.5
3
Wavelength (nm)
Absorptivity (mO.D.)
10 mM Cu
DAC
CNCz

in benzonitrile

Figure 5.12. Cation absorptivity of 10 mM 𝐶 𝑢 𝐶𝑁𝐶𝑧 𝐷𝐴𝐶 in benzonitirile. The circles are data points and the line is the
interpolation between the symbols.
400 500 600 700 800 900
-1
0
1
2
DAbs (mOD)
Wavelength (nm)
10 ns 500 ns
50 ns 1000 ns
200 ns 2000 ns
400 500 600 700 800 900
-1
0
1
2
DAbs (mOD)
Wavelength (nm)
20 ns 1000 ns
200 ns 1500 ns
500 ns 2500 ns

Figure 5.13. nsTA spectra of 𝐶𝑢
𝐶𝑧
𝑀𝐴𝐶 in THF (left) and toluene (right).
400 500 600 700 800 900
-0.5
0
0.5
1
DAbs (mOD)
Wavelength (nm)
20 ns 600 ns
100 ns 1000 ns
300 ns 2000 ns
400 500 600 700 800 900
-1
0
1
2
DAbs (mOD)
Wavelength (nm)
10 ns 500 ns
100 ns 1000 ns
250 ns 2000 ns

Figure 5.14. nsTA spectra of 𝐴𝑢
𝐶𝑧
𝑀𝐴𝐶 in THF (left) and toluene (right).
151

400 500 600 700 800 900
-1
0
1
2
Wavelength (nm)
10 ns 250 ns
50 ns 500 ns
100 ns 1000 ns
DAbs (mOD)
400 500 600 700 800 900
-1
0
1
2
DAbs (mOD)
Wavelength (nm)
10 ns 500 ns
50 ns 1000 ns
200 ns 2400 ns

Figure 5.15. nsTA spectra of 𝐶𝑢
𝐵𝐶𝑧 𝑀𝐴𝐶 in THF (left) and toluene (right).
400 500 600 700 800 900
-5
0
5
10
DAbs (mOD)
Wavelength (nm)
10 ns 200 ns
50 ns 300 ns
100 ns 500 ns
1.5 ns, psTA Absorption
400 500 600 700 800 900
-2
0
2
4
D Abs (mOD)
Wavelength (nm)
10 ns 500 ns
100 ns 1000 ns
250 ns 2000 ns
1.5 ns, psTA Absorption

Figure 5.16. nsTA spectra of 𝐴𝑢
𝐵𝐶𝑧 𝑀𝐴𝐶 in THF (left) and toluene (right). The psTA traces at 1.5 ns are displayed to
demonstrate the effectiveness of the PL subtraction method presented here (see below).
400 500 600 700 800 900
-1
0
1
2
3
4
DAbs (mOD)
Wavelength (nm)
20 ns 300 ns
100 ns 500 ns
200 ns 1000 ns
400 500 600 700 800 900
-1
0
1
2
DAbs (mOD)
Wavelength (nm)
10 ns 500 ns
50 ns 1000 ns
200 ns 1600 ns

Figure 5.19. nsTA spectra of 𝐶𝑢
𝑃 ℎ𝐶𝑧
𝑀𝐴𝐶 in THF (left) and toluene (right).
152

400 500 600 700 800 900
-0.2
0
0.2
0.4
Wavelength (nm)
10 ns 74 ns
24 ns 100 ns
50 ns 150 ns
DAbs (mOD)
400 500 600 700 800 900
-0.5
0
0.5
1
DAbs (mOD)
Wavelength (nm)
10 ns 500 ns
100 ns 1000 ns
250 ns 2000 ns

Figure 5.20. nsTA spectra of 𝐶𝑢
𝐶𝑁𝐶𝑧 𝐷𝐴𝐶 in THF (left) and toluene (right). Degradation of 𝐶 𝑢 𝐶𝑁𝐶𝑧 𝐷𝐴𝐶 occurs with
illumination in THF.
400 500 600 700 800 900
-1
0
1
2
3
DAbs (mOD)
Wavelength (nm)
100 ns 2000 ns
500 ns 4000 ns
1000 ns 7000 ns

Figure 5.21. nsTA spectra of 𝐶𝑢
𝐶𝑧
𝐶𝐴𝐴𝐶 in THF.
400 500 600 700 800 900
-1
0
1
2
3
DAbs (mOD)
Wavelength (nm)
20 ns 300 ns
100 ns 500 ns
200 ns 1000 ns
400 500 600 700 800 900
-1
0
1
2
3
DAbs (mOD)
Wavelength (nm)
10 ns 500 ns
100 ns 1000 ns
250 ns 2000 ns

Figure 5.22. nsTA spectra of 𝐴𝑢
𝐶𝑧
𝐶𝐴𝐴𝐶 in THF (left) and toluene (right).
153

600 650 700 750
-0.1
0
0.1
0.2
0.3
Wavelength (nm)
10 ns 500 ns
50 ns 1000 ns
100 ns 2000 ns
DAbs (mOD)

Figure 5.23. nsTA spectra of 46 M 𝐴𝑢
𝐶𝑧
𝑀𝐴𝐶 and 7 mM MePI in THF.
400 500 600 700 800 900
-4
-2
0
2
4
6
8
Wavelength (nm)
DAbs (mOD)
6 ns 100 ns
12 ns 200 ns
50 ns 2000 ns
Absorption PR Cation

Figure 5.24. nsTA spectra of 75 M 𝐴𝑢
𝐵𝐶𝑧 𝑀𝐴𝐶 and 30 mM MePI in THF. A peak attributed to the 𝐴𝑢
𝐵𝐶𝑧 𝑀𝐴𝐶 cation is
observed in the earliest time traces.
Experimental
Pulse Radiolysis
Pulse radiolysis (PR) was used to measure the molar absorptivities of the oxidized and
reduced forms of the cMa photosensitizers studied here, as well as the absorption spectra of the
same complexes in their triplet excited states. PR experiments were conducted at the 9 MeV
Linear Electron Accelerator Facility (LEAF) at Brookhaven Nation Laboratory (BNL),
33
using
pulses less than 50 ps in duration. The optical detection path consisted of a pulsed xenon arc lamp,
a 0.5 cm pathlength quartz optical cuvette fitted with an airtight Teflon valve, a selectable band
154

pass interference filter (~10 nm) and either a silicon (400-1000 nm) or a germanium (1000-
1500 nm) photodiode (2-3 ns response time). Optical measurements were collected orthogonal to
the direction of the electron pulse through the sample cuvette.

Triplet sensitization experiments were conducted in one of two methods. In the first method
o-xylene degassed with argon was irradiated with b
-
radiation to generate solvent cations, solvated
electrons, and solvent excited states. The excited state singlets decay many times faster than the
rate of diffusion in the solvent, such that only triplet state solvent molecules are available for
sensitization experiments. Since the population of the solvent triplet states is many times larger
than the solvent cation or solvated anion, it is assumed that the after sensitization of the solute, the
resulting spectrum is that of the triplet state of the analyte of interest. In the second method, the
solution of o-xylene was kept aerobic. Here the spectra were recorded in the first 10 ns, before
sufficient quenching from O2 has occurred to reduce the population of the excited state. A
comparison of the results of the two methods is made in the main body of the paper.
Cations of the cMa complexes were generated by irradiating aerobic o-xylene or
benzonitrile with b
-
radiation to generate solvent cations, solvated electrons, and solvent excited
states. The dissolved oxygen readily quenches solvent/solute excited states and solvated electrons,
while the solvent cations can be utilized to sensitize the cMa solute. Molar absorptivities were
determined via an internal standard method with triphenylamine, whose cation molar absorptivity
spectrum is known.
Anions of each complex were generated by irradiating THF with b
-
radiation to generate
solvent cations and solvated electrons. Solvent excited states are not appreciably generated in THF
via pulse radiolysis. The THF cation readily decomposes before it has time to pass charge to a
155

solute, leaving only solvated electrons available for sensitization. The lifetime of the solvated
electron is stabilized by the presence of the electrolyte tetrabutylammonium hexafluorophosphate,
TBAPF, in order to ensure diffusion and reduction of analyte. Molar absorbtivities were measured
via the internal standard method using biphenyl, whose anion molar absorptivity spectrum is
known.

.
Sample Preparation for nsTA
The samples were prepared in anhydrous toluene or THF further dried over alumina
sieves, through a dry solvent dispensing system. The nsTa experiments employed a custom build
air-free ~1 cm borosilicate cuvette, with an attached Schlenk valve which allows the sample to
be sparged with N2 and sealed under positive pressure.
23
The samples where sparged for 15
minutes prior to optical work.
Flow Cell Measurements
For flow cell experiments, a custom made 1 cm glass pathlength cuvette with two 3 mm
outer diameter sidearm inlet/outlet was used. The inlet sidearm was attached via a FEP lined Tygon
tube (ID 1/8” OD 1/4”) threaded through a 14/20 joint compression clamp thermometer adapter
into a 50 mL three-neck round bottom flask (RBF). The outlet sidearm was attached via the FEP
line Tygon tubing to a gear pump (Cole-Parmer No. 7144-05), which also fed back to the
three-neck RBF. The third neck of the RBF was fitted with an appropriately sized rubber septa that
was used to bubble degas the reservoir in operando. The solvent was allowed to circulate through
the system for five minutes while being bubble degassed before spectra were collected. The final
156

reservoir volume accounting for volume of the lines, cuvette, and pump amounts to ~100 mL and
the system has a flow rate of 40 mL/min.
nsTA
The nsTA experiments were performed on a series of nsTA instruments manufactured fom
Magnitue Instruments.
34
Most experiments were performed on an enVISTA instrument house at
USC unless otherwise noted. A 355 nm pump beam was generated from the third harmonic of a
pulse Nd:YAG laser house within the instrument. The pump was set to a 5 kHz repetition rate with
pulse energies between 40-75 mJ/cm
2
. A xenon lamp was utilized as the probe source. The
monochromator was equipped with slits of 1.2 mm, with a spectral resolution of 6.3 nm. The
oscilloscope was set to a 2 ns step size.
The quenching experiment between 𝐴 𝑢 𝐵𝐶𝑧 𝑀𝐴𝐶 and MePI in toluene were performed by
Michael Kellogg and Collin Muniz using an enVISion nsTA instrument at the Magnitude
Instruments facility in State College, PA. They used a similar set-up to what was described above,
though an external 355 nm 3
rd
harmonic Nd:YAG laser was used as the source, which may have
different pump energies. Lastly, neat 𝐴 𝑢 𝐵𝐶𝑧 𝑀𝐴𝐶 experiments in THF and quenching experiments
between 𝐴 𝑢 𝐵𝐶𝑧 𝑀𝐴𝐶 and 30 mM MePI in THF were performed on a separate enVISion instrument
equipped with an external optical parametric oscillator set to 420 nm with a repetition rate of
50 Hz. The pump beam had a spotsize of 8 mm diameter with a pulse fluence of 2.6 mJ/cm
2
. A
xenon probe source was used to collect spectra from 400-1000 nm, and a halogen lamp was used
to collect spectra from 900-2500 nm.

157

Bulk Electrolysis, ps-TCSPC, ps-TA
This work was performed by collaborators and readers are encourage to check out the
experimental section of ref. 17 for more details.

158

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Abstract (if available)

Abstract Organic photovoltaic (OPVs) have seen a recent surge in photoconversion efficiencies (PCE) from 12.5% to almost 20% due to the adoption of strong electron donating cores flanked on either side by strong electron acceptors, called an A D A motif, in their electron acceptor layers . This meteoric rise promises to enable the development of thinner and transparent solar panels to supplement a renewable energy infrastructure based on Si devices. However, despite this vast improvement in the technology, there is still more work to be done to realize device commercialization.

Current A D A molecules utilize bulky donor cores that are synthetically complex and therefore costly, and a simplification of this core is desired to increase market feasibility. Similarly, materials are required to ultrapure in order to reach high PCEs and maintain long device lifetimes. However, even with ultrapure materials, we have found a degradation reaction between OPV materials which impacts these parameters. In this thesis we will first look at understanding and mitigating degradation with ultrapure materials, followed by showcasing new synthetically simplified materials for OPVs. Lastly we will examine bypassing OPV devices altogether to directly harvest photo potential to drive the production of solar fuels.

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Asset Metadata

Creator Mencke, Austin Robert (author)

Core Title Utilizing n-heterocyclic chromophores for solar energy harvesting.

School College of Letters, Arts and Sciences

Degree Doctor of Philosophy

Degree Program Chemistry

Degree Conferral Date 2023-08

Publication Date 08/07/2023

Defense Date 04/24/2023

Publisher University of Southern California. Libraries (digital)

Tag OAI-PMH Harvest,organic optoelectronis,photovoltaics,solar cells,solar fuels

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Thompson, Mark (committee chair), Armani, Andrea (committee member), Thompson, Barry (committee member)

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