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Improving the field of organic photovoltaics through the development of new active layer materials with unique photophysical properties

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Improving the Field of Organic Photovoltaics through the Development of
New Active Layer Materials with Unique Photophysical Properties

by

Denise Femia

A Dissertation Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the Requirements for the Degree
DOCTOR OF PHILOSOPHY
(CHEMISTRY)

December 2016

ii

Dedication

To my family and friends,
and my husband, who never gave up on me.

iii

Acknowledgements

I would like to extend my deepest gratitude to Prof. Mark E. Thompson for giving me
guidance throughout my graduate career and supporting my academic research. I have grown as
a scientist as well as a person by constantly pushing against my comfort zone. I would like to
thank Prof. Richard L. Brutchey for sparking my interest in inorganic chemistry. Thank you to
the members of my thesis committee, Prof. Barry Thompson and Prof. Michelle Povinelli, as
well as Prof. Smaranda Marinescu and Prof. Alex Benderskii for serving on my qualifying
committee.
Thank you to Prof. Nancy I. Totah at Syracuse University for taking me on as an NSF
Undergraduate researcher where I had my first research experience and Prof. David K. Johnson
at SUNY Geneseo. Under his guidance I completed my honors thesis research, and his support
as well as the support and encouragement of the chemistry faculty at SUNY Geneseo led me on
the path to graduate school.
The support of my family has been invaluable. Thank you to my parents JoAnn and
Dennis Femia and my sister Gina Femia for encouraging my curiosity as a young child. Thank
you for listening to me talk about my research and doing your best to understand it.
My co-workers in the MET research group have played an incredible role in my life
during the past five years. Dr. Sarah Conron, Dr. Eric McAnally and Dr. Patrick Erwin all took
the time to help me get my bearings in the lab, aid in the use of the chamber and thoroughly
explain the operating principles of OPVs. Dr. Slava Diev, Dr. Lincoln Hall and John Chen were
great mentors in the synthetic lab. Thank you to newer members of the group Jessica Golden,
Narcisse Ukwitegetse and Savannah Kapper; you have helped me see how much I have evolved
and developed as a scientist. The extraordinary effort of Judy Fong, our administrative assistant,

iv

keeps the lab running smoothly. The group has been my extended family during my time in Los
Angeles and it is surprisingly difficult to leave them behind.
Finally, I would like to thank my husband, Dr. Robert E. Parker IV, for his love and
support during our simultaneous pursuit of advanced degrees on opposite coasts. You believe in
me when I doubt myself, and I wouldn’t have made it as far as I have without your
encouragement. I couldn’t ask for a better life partner.

v

Table of Contents
Page
Dedication ...................................................................................................................................... ii
Acknowledgements ........................................................................................................................ iii
Table of Contents .............................................................................................................................v
List of Tables ................................................................................................................................. ix
List of Figures ..................................................................................................................................x
List of Schemes ............................................................................................................................ xiii
Abstract ........................................................................................................................................ xiv
Chapter 1. Introduction ....................................................................................................................1
1.1 Energy Sources ..............................................................................................................1
1.1.1 The problems associated with traditional energy sources .....................................1
1.1.2 Alternative energy sources ....................................................................................4
1.1.3 Photovoltaic Technology ......................................................................................5
1.2 Introduction to Organic Photovoltaics (OPVs) ..............................................................7
1.2.1 Structure of an OPV ..............................................................................................8
1.2.1.1 Material Components of an OPV ...............................................................10
1.2.2 Basic Operating Principles of an OPV................................................................11
1.2.3 Limitations of OPVs ...........................................................................................14
1.3 Characterization of OPVs ............................................................................................15
1.3.1 Open Circuit Voltage ..........................................................................................15
1.3.2 Short Circuit Current...........................................................................................16
1.3.3 Fill Factor ............................................................................................................17
1.3.4 Efficiency ............................................................................................................18
1.4 Overview of Work .......................................................................................................18

vi

Chapter 1 References .........................................................................................................20
Chapter 2. Synthesis and Characterization of a Family of Perylene Compounds .........................24
2.1 Introduction ..................................................................................................................24
2.2 Synthesis of Intermediates and Target Compounds.....................................................26
2.2.1Anthraquinone-1,5-dicarbaldehyde .....................................................................27
2.2.2 1,5-diamino-4,8-dibromoanthraquinone .............................................................27
2.2.3 1,5-diamino-4,8-diphenylanthraquinone .............................................................28
2.2.4 1,5-diiodo-4,8-diphenylanthraquinone ...............................................................28
2.2.5 1,5-diphenyl-4,8-distyrenylanthraquinone ..........................................................29
2.2.6 4,8-diphenylanthraquinone-1,5-dicarbaldehyde .................................................29
2.2.7 6,12-diphenyl-1,2,7,8-tetracyanoperylene ..........................................................30
2.3 Results and Discussion ................................................................................................31
2.3.1 Calculated Energies and Geometries ..................................................................33
2.3.2 Photophysical Properties .....................................................................................35
2.4 Conclusions ..................................................................................................................37
Chapter 2 References .........................................................................................................38
Chapter 3. Tetracyanoperylene: Device Performance and Singlet Fission Efficiency ..................40
3.1 Singlet Fission ..............................................................................................................40
3.1.1 Singlet Fission in OPVs ......................................................................................42
3.1.2 Singlet Fission of Perylene and its Derivatives ..................................................43
3.2 Tetracyanoperylene ......................................................................................................45
3.2.1 Experimental Section ..........................................................................................45
3.3 Results and Discussion ................................................................................................47
3.3.1 Steady-state Thin Film Photophysical Characterization .....................................47
3.3.2 Time-resolved Photophysics ...............................................................................48

vii

3.3.2.1 Solution State Dynamics ............................................................................49
3.3.2.2 Solid State Dynamics .................................................................................50
3.3.3 Crystal Structure .................................................................................................53
3.3.4 Device Performance ............................................................................................54
3.4 Conclusions ..................................................................................................................58
Chapter 3 References .........................................................................................................59
Chapter 4. Design and Synthesis of a Series of Metal-bis-o-phenazine (MBOP) Complexes as
Symmetry-Breaking Charge Transfer Chromophores ...................................................................62
4.1 Introduction ..................................................................................................................62
4.1.1 Open Circuit Voltage and the Charge Transfer State .........................................62
4.1.2 Symmetry Breaking Charge Transfer .................................................................62
4.1.2.1 Device Applications of SBCT ...................................................................63
4.1.2.2 Metal-bis-o-phenazine ...............................................................................64
4.2 Experimental Section ...................................................................................................64
4.2.1 1-hydroxyphenazine ............................................................................................65
4.2.2 Tert-butyl-1-methoxyphenazine .........................................................................66
4.2.3 T-butyl-1-hydroxyphenazine ..............................................................................66
4.2.4 General procedure for metal-o-bis-phenazine (MBOP) complexes ...................67
4.3 Results and Discussion ................................................................................................68
4.3.1 Synthesis of the target materials .........................................................................68
4.3.2 Photophysical Characterization of Methoxyphenazine and Hydroxyphenazine
Ligands .........................................................................................................................70
4.3.3 Photophysical Characterization of MBOP Complexes .......................................71
4.3.4 Photophysical Characterization of tbuMeOP......................................................73
4.3.5 Photophysical Characterization of ZntbuBOP ....................................................74
4.3.5.1 Comparison to Acridine Derivative .................................................................75

viii

4.4 Conclusions ..................................................................................................................78
Chapter 4 References .........................................................................................................79
Chapter 5. Metal bis-o-diazatetracene (MBOAT) Complexes Designed as Singlet Fission
Chromophores ................................................................................................................................81
5.1 Introduction ..................................................................................................................81
5.1.1 Singlet Fission in Thin Film ...............................................................................81
5.1.2 Intramolecular Singlet Fission ............................................................................81
5.1.2.1 Tetracene Dimers .......................................................................................82
5.1.2.2 Pentacene Dimers.......................................................................................83
5.1.3 Proposed System .................................................................................................83
5.2 Experimental Section ...................................................................................................84
5.2.1 1-hydroxy-2,9-diazatetracene .............................................................................84
5.2.2 Metal-bis-o-diazatetracene (MBOAT) General Procedure .................................85
5.3 Results and Discussion ................................................................................................85
5.3.1 Synthesis of the Ligand and Metal Complexes...................................................85
5.3.2 Characterization of the Free Ligand ...................................................................85
5.3.3 Characterization of Metal Complexes ................................................................87
5.4 Conclusions and Future Outlook .................................................................................89
Chapter 5 References .........................................................................................................90

ix

List of Tables
Table 2.1 Key parameters of perylene, TCP, and DPTCP. ................................................................. 35
Table 3.1 Device architectures and key parameter of devices in this study. ....................................... 56
Table 4.1 Comparison of absorption maxima, reduction potentials, and calculated LUMO values for
the MBOP complexes. ......................................................................................................................... 73

x

List of Figures
Figure 1.1 Predicted temperature of the Earth's surface in the year 2100. Adapted from
http://climate.nana.gov ........................................................................................................................... 2
Figure 1.2 Present-day coral reef locations and predicted aragonite saturation levels as a result of
increased oceanic CO2 concentration. Note that by 2050, most locations will have marginal to
extremely marginal conditions. Adapted from http://www.globalchange.gov/. .................................... 3
Figure 1.3 A look at the total energy reserves, both finite and renewable, compared to global energy
use in 2015. ............................................................................................................................................ 4
Figure 1.4 Forty years of photovoltaic efficiency certified by the National Renewable Energy
Laboratory. Note that highest-performance PV technologies involve multi-junction cells. Adapted
from http:// http://www.nrel.gov/ncpv/ ................................................................................................. 5
Figure 1.5 Top: solar photon flux at ASTM G173-03 AM1.5 G as a function of wavelength. Middle:
the molar extinction coefficients of GaAs, c-Si and the organic D/A, in this instance CuPC and C60.
Bottom: percentage of incident photons captured by a device of each type with 100 nm thick active
layer. Adapted from ref. 7. .................................................................................................................... 6
Figure 1.6 Basic structure of OPVs. The left figure a) represents a lamellar, or planar, heterojunction
while b) depicts a bulk heterojunction. Adapted from ref. 13 .............................................................. 8
Figure 1.7 Examples of common OPV materials mentioned within the text. ...................................... 9
Figure 1.8 Events in an OPV leading to charge collection. 1) Absorption of a photon of light with
energy hv generates a bound electron-hole pair. 2) Exciton migration to the D/A interface. 3) CT
state formation, with coulombically bound electron and hole localized to the acceptor and donor,
respectively. 4) Charge separation into free carriers. 5) Charge migration and collection at the
appropriate electrodes. Note the relative energy levels of the frontier orbitals of the donor and
acceptor materials. .............................................................................................................................. 12
Figure 1.9 Representative I-V curve of an OPV. MPP represents the maximum power output from
the device, while JMPP and VMPP represent the current and the voltage, respectively, at that power. FF,
VOC, and JSC are discussed in the text. Adapted from Ref. 27. ........................................................... 16
Figure 2.1 Left: parent structire of perylene and numbering convention. Middle: 1,2,7,8-
tetracyanoperylene (TCP). Right: 6,12-diphenyl-1,2,7,8-tetracyanoperylene (DPTCP). ................... 24
Figure 2.2 Retrosynthetic schemes demonstrating the different approaches to substituting the TCP
core while maintaining the last step of the reaction. Top represents the synthons from the literature
procedure; middle depicts the attempted pathway to a 5,11-DPTCP; bottom shows the synthesis that
was successfully implemented to obtain a 6,12-substituted TCP. ...................................................... 32

xi

Figure 2.3 Lowest energy geometries of perylene (left), TCP (middle) and DPTCP (right) from DFT
calculations, showing edge-on view as a depiction of the increasing dihedral angle. Face-on view is
included for comparison. Carbon atoms are in black, hydrogen atoms are white, and nitrogen atoms
are blue. ............................................................................................................................................... 34
Figure 2.4 Absorption (top) and emission (bottom) spectra of perylene (black trace), TCP (red trace)
and DPTCP (blue trace) in dichloromethane. ..................................................................................... 36
Figure 3.1 Illustration of the transitions involved in the singlet fission process. Note the overall
singlet character of the ME state, as the spin states on both chromophores are oriented to cancel one
another out. ......................................................................................................................................... 41
Figure 3.2 Relative rates of formation of the ME state, SF, and CT to C60 in pentacene/C60 (left) and
tetracene/C60 (right). ............................................................................................................................ 42
Figure 3.3 A) Device architecture and B) external quantum efficiency of the first reported OPV
device with EQE over 100%. .............................................................................................................. 43
Figure 3.4 Depiction of the different known crystalline phases of perylene. .................................... 44
Figure 3.5 Absorption (top) and emission (bottom) of TCP in solution and thin film. See legend for
details. ................................................................................................................................................. 47
Figure 3.6 Time-correlated single photon counting (TCSPC) traces of TCP in solution (black trace)
and in a thin film (red trace). ............................................................................................................... 48
Figure 3.7 Top: Transient absorption data of TCP in solution. Bottom, left: Global analysis fits for
singlet and excimer state absorbances. Bottom, right: Decay time of the singlet population (black
trace) and rise time of the excimer population (red trace). ................................................................. 49
Figure 3.8 Top: Transient absorption data of TCP in a neat thin film. Bottom, left: Global
analysis fits for singlet and triplet state absorbance profiles. Bottom, right: Decay time of the
singlet population (black trace) and rise time of the triplet population (red trace). Note the
rapid decay of the triplet population, suggesting that the average population begins to decay
before the overall yield is achieved. ..............................................................................................51
Figure 3.9 Crystal structure of TCP. Top: view along the b-axis. Note that chromophores in the top
layer are perfectly eclipsing chromophores in the bottom layer in the bottom layer along the b-axis.
Middle: view along the c-axis. Bottom: Space-filling model of the unit cell. .................................... 52
Figure 3.10 AFM image of a neat thin film of TCP. .......................................................................... 54
Figure 3.11 Left: Device stack for the architectures discussed. Right: I-V curves of the three device
architectures listed in Table 3.1. ......................................................................................................... 55
Figure 3.12 External Quantum efficiency (EQE) (blue trace) overlaid with thin film absorption of
TCP (red trace) and CuPC (blue trace). .............................................................................................. 56

xii

Figure 4.1 a.) Device architecture of the I-V curves in b.). ............................................................... 63
Figure 4.2 Different MtbuBOP isomers. ............................................................................................ 69
Figure 4.3 Absorption of MeOP (black trace) and HOP (blue trace). Emission of MeOP
(red trace). ........................................................................................................................................... 70
Figure 4.4 Absorption of MBOP complexes. ..................................................................................... 71
Figure 4.5 NiBOP absorption in CH2Cl2 (black) and tetrahydrofuran (blue). ................................... 72
Figure 4.6 Absorption and emission of tbuMeOP. ............................................................................ 74
Figure 4.7 Absorption and emission of ZntbuBOP in polar (THF, blue traces) and non-polar
(cyclohexane, black trace) solvents. ................................................................................................... 75
Figure 4.8 Absorption and emission of 1-ethoxyacridine in dichloromethane. Inset: structure of 1-
ethoxyacridine. .................................................................................................................................... 76
Figure 4.9 Emission of zinc bis-o-acridine in different solvents. Inset: structure of zinc-bis-o-
acridine. ............................................................................................................................................... 77
Figure 5.1 a.) Diphenyltetracene b.) bis-alkynyltetracene c.) metal-bis-o-diazatetracene (MBOAT),
M = Zn, Cu, Ni. .................................................................................................................................... 82
Figure 5.2 Absorption (solid line) and emission (dashed line) of MeOAT ligand. ............................ 86
Figure 5.3 Absorption of a thin film of MeOAT. ............................................................................... 87
Figure 5.4 Absorption spectra of ZnBOAT (black trace), NiBOAT (red trace) and CuBOAT (blue
trace). ................................................................................................................................................... 88

xiii

List of Schemes
Scheme 2.1 Synthesis of TCP. The intermediate structure prior to Knoevenagel condensation is
displayed in brackets. ........................................................................................................................... 25
Scheme 2.2 Total synthetic scheme for DPTCP. ................................................................................. 26
Scheme 4.1 Synthesis of MBOP complex. M = Zn, Ni, Cu. ............................................................... 65
Scheme 4.2 Synthesis of t-bu-1-methoxyphenazine. ........................................................................... 68
Scheme 5.1 Synthesis of ligand and metal complex. M = Zn, Ni, Cu. ............................................... 84

xiv

Abstract
Problems associated with consumption of the limited resource of fossil fuels, inclusive of
climate change, pollution and acidification of the ocean due to increased atmospheric levels of
carbon dioxide, have driven scientists to seek out alternative sources of energy. Solar energy in
the form of photovoltaics presents as an attractive option because of the abundance of energy
supplied to the earth by the sun. An efficient method of capturing the energy for use in
applications that are currently fueled by non-renewable resources is of interest. Organic
photovoltaics (OPVs) embody the third generation of solar cell technology that has made strides
in efficiency and stability toward commercial applications. However, as a recent technology,
there are improvements to be made in the field before the technology is on par with silicon
photovoltaics. This work seeks to utilize the excitonic excited state in OPVs to achieve carrier
multiplication through singlet fission and to decrease the energetic cost of separating the exciton
into free carriers through symmetry-breaking charge transfer. Chapter 2 introduces novel
materials that can act as singlet fission acceptors within the active layer to replace the
commonly-used fullerenes. Chapter 3 examines one of the candidates in terms of singlet fission
efficiency as well as device performance. The concept of symmetry-breaking charge transfer is
introduced in Chapter 4, along with the synthesis and characterization of materials that were
proposed for this purpose. Chapter 5 examines the utility of using a metal center to hold two
singlet fission chromophores in communication, ideally to achieve an increase in singlet fission
yield compared to the free chromophores.

1

Chapter 1 Introduction
1.1 Energy Sources
1.1.1 The problems associated with traditional energy sources
The consumption of fossil fuels to meet the energy demands of modern society has had
dire consequences on the environment. Although a non-renewable source of energy, the earth’s
supply of fossil fuels is sufficient to meet global energy demands for the next hundred years.
However, the negative impact that the location, transportation and consumption of these
materials has on the atmosphere and ecosystems of the planet highlights the need to turn to
alternative, clean sources of energy to continue to supply power to the earth and its inhabitants.
Since the industrial revolution, human demand of energy has increased exponentially.
1
As a
result, in order to meet the energetic demand, an increasing amount of materials such as coal, oil
and gasoline have been harvested from the earth and combusted, leading to pollution, issues from
fracking, and a release of carbon dioxide into the earth’s atmosphere.
When carbon dioxide is released into the atmosphere, it acts as a heat trap. This trapped
heat has led to the increase in the average temperature of the earth’s surface. Correlating this
value to the amount of carbon dioxide in the atmosphere has allowed for scientists to extrapolate
the effect that such “global warming” will have in the coming century. If the concentration of
this greenhouse gas continues to increase exponentially with every passing year, the average
temperature of the Earth’s surface could reach values as high as 42 °C by the year 2100
(Figure 1.1), as predicted by NASA. The warming of the surface has an effect on the average
temperature of the ocean as well, causing polar ice caps to melt, which can have a catastrophic
impact on arctic environments. This also leads to dramatic weather events.
2

Equally troubling from an environmental standpoint is the acidification of the oceans
leading to a depletion in the abundance of available carbonate ions. As the concentration of
atmospheric carbon dioxide increases, it dissolves into bodies of water and has been linked to a
decrease in the pH of the ocean by 0.1 during the 20
th
century.
2
The decrease in pH is a result of
carbon dioxide reacting with carbonate ions to form bicarbonate, decreasing the abundance of
aragonite. This mineral is used by sealife including coral reefs and exoplankton, incorporating
the carbonate ion into the structure of their exoskeletons and contributing to their vividly colorful
appearance.
3
Without it, their skeleton systems are considerably weakened and an overall bleach
in their appearance is observed. Figure 1.2 depicts the present day location of coral reefs, as
well as the pH of the ocean. It is predicted that within the next fifty to one hundred years, if
atmospheric CO
2
continues to rise according to the observed trend, the ocean will no longer be a
Figure 1.1 Predicted temperature of the Earth's surface in the year 2100. Adapted from
http://climate.nana.gov
3

viable environment for coral reefs, which will negatively impact the tropical fish and animals
that live within these systems.

Figure 1.2 Present-day coral reef locations and predicted aragonite saturation levels as a result of
increased oceanic CO
2
concentration. Note that by 2050, most locations will have marginal to
extremely marginal conditions. Adapted from http://www.globalchange.gov/.
4

Figure 1.3 A look at the total energy reserves, both finite and renewable, compared to global
energy use in 2015.
4

1.1.2 Alternative energy sources
For these reasons, it is imperative that humans find a carbon-neutral renewable source of
energy to lessen and potentially alleviate some of the ill effects that have arisen from the
consumption and use of fossil fuels as the primary source of energy. Such alternative energy
sources include geothermal, wind, hydro, biomass, and solar. It is apparent from Figure 1.3 that
the most naturally abundant source of renewable energy that exists is solar energy.
4
In fact,
enough sunlight reaches the Earth’s surface in just over an hour to meet global energy demands
for an entire year.
5
Demands for energy were at 18.5 TWy
-1
in 2015, an increase of 2.5 TWy
-1

from 2009. Energy from the sun that hits landmasses and could potentially be used is at
14,900TWh
-1
, where harvesting even 1% of that energy would meet current global demands.
6

The challenge that scientists face is devising an efficient method of capturing the sun’s energy to
5

meet global demands in a cost-effective manner for solar energy to become a feasible
replacement to fossil fuels.
1.1.3 Photovoltaic Technology
Figure 1.4 outlines the history of photovoltaic materials with advances to the different
technologies and current state-of-the-art values. The first generation of photovoltaic technology
involves silicon solar cells. Highly pure single crystalline silicon (c-Si) exhibits the best
performance. Silicon can be doped to generate positive (p-type) carriers or negative (n-type)
carriers, and bringing the two materials into contact with each other creates a p-n junction. At
this p-n interface, there is an implicit driving force that leads to efficient carrier separation,
collection and overall device performance. Production has been streamlined to make silicon PVs
increasingly cost competitive with fossil fuels. Due to its indirect bandgap, silicon has weak
Figure 1.4 Forty years of photovoltaic efficiency certified by the National Renewable Energy
Laboratory. Note that highest-performance PV technologies involve multi-junction cells.
Adapted from http:// http://www.nrel.gov/ncpv/
6

absorption in the visible region of the solar spectrum out to its band edge of 1.1 eV
(Figure 1.5).
7
Fortunately, it has efficient carrier diffusion length on the order of 100-300 μm.
8

Therefore a thick layer of c-Si can be used to absorb a greater percentage of photons in the
visible region. However, this thick layer of crystalline material affects the physical properties of
the c-Si photovoltaic, resulting in a rigid, heavy device.
Second generation photovoltaic technology includes III-V semiconductors such as GaAs,
CdS and CdSe. GaAs is a direct bandgap material and, as a result, has a higher extinction
coefficient to its band edge. Consequently, GaAs cells achieve high PCE and are lightweight,
however the material cost is prohibitively high and applications are limited to aerospace
Figure 1.5 Top: solar photon flux at ASTM G173-03 AM1.5 G as a function of wavelength.
Middle: the molar extinction coefficients of GaAs, c-Si and the organic D/A, in this instance
CuPC and C60. Bottom: percentage of incident photons captured by a device of each type
with 100 nm thick active layer. Adapted from ref. 7.
7

technologies. The third generation technology encompasses organic photovoltaics and dye-
sensitized solar cell. The efficiency of dye-sensitized solar cells has plateaued around 10% in
recent years, and the requirement of a liquid electrolyte complicates the practical application of
this technology. A newly emerging technology, perovskites, has recently gained interest in the
community.
9
Although the efficiency of perovskites climbed rapidly in a short time span,
limitations in stability and materials have prevented these materials from reaching the level of
commercialization.
1.2 Introduction to Organic Photovoltaics (OPVs)
In 1975, the first organic photovoltaic was reported. Tang and co-workers deposited
chlorophyll-a, the compound responsible for the green coloration of plants and the primary
absorber in photosynthesis, in between two metal electrodes. The efficiency of this single-
junction solar cell was 0.001% upon illumination.
10
The poor performance could be attributed to
the nature of the excited state in organic materials, as opposed to inorganic materials. This will
be discussed in detail in the following sections. As previously mentioned, a p-n interface
encourages the directional separation of charges for efficient charge collection at the appropriate
electrode. With this knowledge, Tang employed copper phthalocyanine (CuPC) and perylene-
3,4,9,10-bis-benzimidazole (PTCDI) to generate the first donor-acceptor OPV, achieving almost
1% efficiency.
11
Improvements from there have led to the current state-of-the-art efficiencies
approaching 11%. Despite the low efficiency compared to silicon solar cells, certain intrinsic
qualities of OPVs can lead to specialized applications, such as efficiency at low-light conditions
and potential for lightweight, flexible units. The materials themselves can be solution-deposited
via inkjet printing, a less energy-intensive process than that which is required to fabricate c-Si
PVs. In fact, a solar park has been established in Denmark, polymer OPVs printed on plastic
8

substrates with the capacity to generate 6 kW/h energy, which demonstrates the utility of this
technology.
12
Furthermore, with tunable absorption, OPVs have the potential for efficient semi-
transparent cells.
1.2.1 Structure of an OPV
Organic solar cells can take on a variety of structures utilizing different materials, though
certain similarities are constant and remain consistent across the board. A simple OPV stack is
outlined below and displayed in Figure 1.6.
13
The top electrode, typically a transparent
conducting oxide (TCO), is on top of a transparent substrate which can be glass or plastic.
Commonly used TCOs are indium-doped tin oxide (ITO) or fluorine-doped tin oxide (FTO),
though the latter has a considerably more rough surface and typically needs further processing in
order to yield good performance. Treatment of PEDOT:PSS with phosphoric acid has been
proven to work to generate a flexible all-plastic solar cell.
14
There has also been interest in the
use of modified graphene
15
or silver nanowires as a transparent electrode material.
16

The active layer performs the absorption that generates the photocurrent of the device.
Both small molecules and polymers are the main absorbing materials in OPVs. Some small
molecules may include a metal center, however the nature of the absorption of these materials is
excitonic and therefore these organometallics are classified as OPV materials. A few examples
Figure 1.6 Basic structure of OPVs. The left figure a) represents a lamellar, or planar,
heterojunction while b) depicts a bulk heterojunction. Adapted from ref. 13
9

of materials are displayed in Figure 1.7. The active layer consists of the more easily oxidized, p-
type material, or the donor, and the more easily reduced, n-type material, or acceptor. These
materials can be deposited in a lamellar structure, such that the materials are deposited
separately, or there can be an induced intermixing to create a bulk heterojunction. The benefit of
the bulk heterojunction comes from the fact that organic materials have short exciton diffusion
lengths, where charge separation can only occur at a donor-acceptor (D/A) interface. The
trouble, however, is characterization of the blend structure of an OPV, prevention of pathways
Figure 1.7 Examples of common OPV materials mentioned within the text.
10

between electrodes of one type of material that would inhibit device performance, as well as
greater batch-to-batch variability. Vapor deposition of BHJ affords greater control over the
structure of the blend, and can afford enhanced reproducibility. Planar heterojunctions can be
controllably and reproducibly characterized, which make them better candidates for study in an
academic setting. Planar heterojunctions are most frequently fabricated through vapor
deposition, though in some cases the donor will be solution-processed and the acceptor vapor-
deposited on top, if the decomposition temperature is below the sublimation temperature.
The third required layer in the structure of an OPV is the top electrode. Typically
aluminum is used, as it has an appropriate work function to accept electrons from the acceptor
material to complete the circuit and generate current, although gold and silver are more
expensive alternatives. Additional layers have been utilized to enhance device performance. For
example, a transparent layer of PEDOT:PSS, ZnO, or MoO
3
will be used to change the work
function of the ITO and act as an electron blocking layer. Bathocuproine (BCP) is usually
utilized as a buffer layer between the acceptor and the top metal electrode. Without this buffer
layer, the hot deposition temperature of the metal will damage the active layer below. Although
BCP is traditionally the most prevalently used material, it has been shown that other buffer
materials are as effective, and a blend of BCP:C
60
enhances device performance compared to
BCP alone.
17

1.2.1.1 Material Components of an OPV
Certain considerations have to be made when screening materials for OPV applications.
The most prevalent acceptor material is C
60
which, until recently, was unchallenged as the
highest performance material. A more recent material, ITIC (see Figure 1.7) has been certified
at almost 11% efficiency when paired with a polymer donor.
18
This achievement was
11

accomplished in part by pairing the acceptor with a material whose absorption complimented
rather than overlapped its own, to obtain broadband absorption of the solar spectrum.
There is a much more diverse library of high-performing donor materials. Though some
of the champion devices utilize a polymer donor, this work focuses on small molecule materials,
and polymers are excluded from this discussion. One of the most-commonly studied donors is
CuPC, though it achieves modest efficiencies. Other materials include BODIPYs,
19

porphyrins,
20
squaraines,
21
and acenes. These materials have high extinction coefficient
(ɛ ~10
5
M
-1
cm
-1
), complimentary absorption to C
60
, and appropriate energy levels to act as an
electron donor. These details will be discussed more in depth in the following sections and the
connection that the materials have to the overall device properties will be made clear.
1.2.2 Basic Operating Principles of an OPV
There are a series of events that occur within an OPV under illumination (Figure 1.8). A
thorough understanding of these events will lead to an overall picture of the key factors
necessary for a high-performing device. Upon illumination, a photon is absorbed, generating a
Coulombically bound electron-hole pair known as an exciton in the active material (depicted as
the donor in Figure 1.8). This exciton has to migrate to the D/A interface before it can
dissociate into free charges. Exciton migration depends on the diffusion length (L
D
) of the
material, dictated by the following equation:
𝐿 𝐷 = √𝐷 𝜏 0
,
where D is the diffusivity of the material and τ
0
represents the lifetime of the excited state. The
longer-lived the excited state, the longer the diffusion length and the higher the chances that it
will reach a D/A interface to separate into free charges. Most organic materials exhibit L
D
on the
12

order of 5-30 nm.
22
In order to absorb sufficient light to maximize device efficiency, donor and
acceptor layer thicknesses should be on the order of 50-200 nm, but at this thickness, most
absorbed photons would not reach a D/A interface before relaxing to the ground state. BHJ
Figure 1.8 Events in an OPV leading to charge collection. 1) Absorption of a photon of
light with energy hv generates a bound electron-hole pair. 2) Exciton migration to the D/A
interface. 3) CT state formation, with coulombically bound electron and hole localized to
the acceptor and donor, respectively. 4) Charge separation into free carriers. 5) Charge
migration and collection at the appropriate electrodes. Note the relative energy levels of the
frontier orbitals of the donor and acceptor materials.
13

structures are useful to decrease the spatial distance that an exciton must travel before reaching a
D/A interface to improve overall device performance.
Two mechanisms of energy transfer dictate the transport of an excited state through a
material. Forster resonance energy transfer (FRET) is the transfer of energy from one
chromophore to the next through dipole-dipole coupling.
23
The rate of this process is dictated by
the following equation:
𝑘 𝐹𝑅𝐸𝑇 =
8.8 × 10
−28
𝑛 4
𝜏 0
𝑟 6
𝑗 𝐾 2
𝑚𝑜𝑙
where n is the index of refraction of the material, τ
o
is the lifetime of the excited state, r is the
intermolecular distance between the molecules, j relates to the spatial overlap integral, and K
relates to the orientation of the molecules.
7
Through this equation, it is apparent that k
FRET
drops
off rapidly with increasing spatial distance, and is best for energy transfer within 5-10 nm. This
distance is considerably longer than that covered by the second energy transfer mechanism.
Dexter energy transfer (DET) is also referred to as electron exchange energy transfer.
Via DET, a ground-state molecule exchanges an electron with an excited-state molecule through
the overlap of the wavefunctions of the respective molecule.
24
Because of the nature of the
exchange, the two molecules must be in close proximity to each other such that there is overlap
between their orbitals to allow the exchange to occur. Long-lived excited states such as triplets
are more likely to migrate through DET.
At the D/A interface, a charge transfer (CT) state is formed. The electron from the
exciton on the donor is transferred to the acceptor molecule, but the oppositely-charged particles
are still Coulombically bound. The intermolecular CT state dissociates into free charges in the
charge separation step, which is driven by the energetic offset between the frontier orbitals of the
14

two materials. These charges are then transported to the appropriate electrodes to complete the
circuit and generate charge that can be collected from the device.
Charge transport is driven by the bias across the device, directing the correct charged
particles to the appropriate electrodes. Organic materials have poor charge transport properties
compared to inorganic materials, since the molecules have discreet orbitals instead of a
continuous conduction band. Carrier mobility can be improved through device engineering, such
as thermal or solvent annealing to induce crystallization within the material layers.
25
Crystalline
domains exhibit improved charge transport when the π systems between molecules are in good
communication. However, if a device is too crystalline, this may lead to “islands” of aggregated
material, which would negatively impact the film morphology and overall device performance.
Materials themselves can be engineered to favor crystallization and induce favorable domains
within a thin film in a device.
26

1.2.3 Limitations of OPVs
Shockley-Queissar performed a detailed analysis on the maximum possible efficiency
from a silicon solar cell.
27
The bandgap must be taken into consideration, as a material only
absorbs photons to its band edge. If the bandgap is too high, most photons will pass through the
material. If the bandgap is too small, a large fraction of photons will be absorbed; however,
those photons significantly greater than the bandgap will excite the electron to a higher energy
state, then thermalize to the band edge, giving off the excess energy as heat. A balance between
these two loss mechanisms must be achieved, and in fact the ideal bandgap was calculated as
identical to that of silicon to achieve the maximum theoretical efficiency of 33%.
28

A few assumptions that work for inorganic semiconductors have to be adjusted when
considering organic material systems. The Shockley-Queisser limit assumes that all photons
15

greater than the bandgap are absorbed, and that free charges are generated upon excitation. Due
to the excitonic nature of the excited state, a revised analysis was developed for OPVs that
determined a maximum theoretical efficiency of 22-27%.
29
This lower value results from the
energy that is necessary to separate the Coulombically-bound electron-hole pair at the D/A
interface, an energy that has been calculated at 300-500 meV.
1.3 Characterization of OPVs
In order to determine the operating parameters of an OPV, the current-voltage (I-V)
dependence is characterized both in the dark and in the light. A typical I-V curve is represented
in Figure 1.9.
30
The dark trace displays diode behavior, with low current at negative bias that
rectifies at a certain voltage. In the light, the current increase is related photocurrent generation,
which is discussed in detail in Section 1.3.2.
1.3.1 Open Circuit Voltage
The open circuit voltage (V
OC
) of an OPV relates to the value of the voltage at which
injection of the minority carrier is equal to charge generation in the device. This is also the point
at which no current is generated by the device (J = 0). The maximum theoretical V
OC
for a given
material set is dictated by the energetic offset between the highest occupied molecular orbital
(HOMO) of the donor material and the lowest unoccupied molecular orbital (LUMO) of the
acceptor material, called ΔE
DA
. In reality, V
OC
is affected by formation of the CT state and the
necessity of 300-500 meV energy to separate the exciton into free charges.
31
Measures can be
taken to reduce the exciton binding energy, increasing the CT state energy and improving the
V
OC
of the device. For example, employing materials that exhibit a phenomenon known as
symmetry-breaking charge transfer form an intramolecular CT state that leads to improved V
OC

when employed in a device. This technique will be discussed in greater detail in Chapter 4.
16

1.3.2 Short Circuit Current
The short circuit current (J
SC
) represents the photocurrent generated by the device at zero
applied voltage. This relates to the current output of the device under normal operating
conditions. J
SC
depends on the absorption of the materials utilized. Unlike inorganic materials
that absorb all light above the bandgap, organic materials have discreet, relatively narrow
absorption profiles. In this regard, it is of interest to expand the absorption bands of these
materials to cover a larger area of the solar spectrum, since only photons of the appropriate
wavelengths will be absorbed and potentially converted to electricity within the OPV. Another
technique is to choose donor and acceptor materials that absorb in different regions of the solar
spectrum to achieve broadband absorption. Care must be taken, however, to find a balance
Figure 1.9 Representative I-V curve of an OPV. MPP represents the
maximum power output from the device, while J
MPP
and V
MPP
represent the
current and the voltage, respectively, at that power. FF, V
OC
, and J
SC
are
discussed in the text. Adapted from Ref. 27.
17

between maximizing absorption while still maintaining a large ΔE
DA
to yield sufficient V
OC.

The external quantum efficiency (EQE) of a device is characterized by scanning the
current output at each wavelength of the solar spectrum. It reveals the percentage of incident
photons that are converted to electricity at given wavelengths. The integrated area under the
EQE curve correlates to the J
sc
obtained from the I-V curve.
J
SC
can suffer from a number of factors. Short L
D
of the material will limit how thick the
active layers can be, so the ideal thickness of the material needs to maximize light absorption as
well as the number of excitons that reach the D/A interface to charge separate. Furthermore,
charges can experience geminate or non-geminate recombination mechanisms and relax back to
the ground state.
32
Due to a charge build-up at the D/A interface, bimolecular recombination is
reduced at low light, since fewer free charges are generated within the material and therefore are
less likely to quench and result in an energetic loss.
1.3.3 Fill Factor
The fill factor relates to how efficiently generated charges migrate through a device. This
term is defined by the following equation:
𝐹𝐹 =
𝑃 𝑀𝐴𝑋 𝑉 𝑂𝐶
× 𝐽 𝑆𝐶

where P
MAX
represents the maximum power point of the device, or the point on the curve where
the product of J and V is maximized. The fill factor relates to the “squareness” of the I-V curve,
as a fill factor of 100% would have V
OC
and J
SC

as P
MAX
, and the area of the red box would equal
the area of the blue box (Figure 1.9). Fill factor is negatively affected by bimolecular
recombination. The photocurrent should be voltage-independent prior to rectification, so voltage
dependent photocurrent indicates poor carrier mobility within the device, resulting in a poor fill
18

factor. Additionally, an “S-shaped” curve is a result of an energetic barrier within the device or
poor carrier extraction, and again would be reflected in the fill factor.
33

1.3.4 Efficiency
The previously-mentioned characterization parameters all contribute to the power
conversion efficiency (PCE or η) of the device. Simply put, it is the ratio between the power
generated by the device over the power incident on the device. The overall expression is
displayed below:
𝑃𝐶𝐸 =
𝑃 𝑜𝑢𝑡 𝑃 𝑖𝑛

Filling in the key parameters that dictate output, the expression can be written as:
𝑃𝐶𝐸 =
𝑉 𝑂𝐶
× 𝐽 𝑆𝐶
× 𝐹𝐹
𝑃 𝑖𝑛

Achieving maximum efficiency out of a device requires maximizing all three terms, related to
absorption without compromising the ΔE
DA
, and ensuring that the materials are capable of
transporting charge that will be collected at the electrodes. Materials can be designed with
certain photophysical properties for the purpose of maximizing J
SC
or V
OC
, and this work focuses
on the design, synthesis and characterization of materials for that purpose.
1.4 Overview of Work
A selection of materials were proposed, synthesized and studied as potential photoactive
materials in OPVs. Chapter 2 explores the synthesis of a family of perylene compounds that are
energetically sufficient to replace C
60
as the acceptor chromophore in OPVs. Chapter 3
introduces the phenomenon of singlet fission as a pathway to exceed the Shockley-Queissar
19

limit. Tetracyanoperylene is further explored as a singlet fission material and its device
performance is tested. Chapter 4 introduces a concept known as symmetry breaking charge
transfer (SBCT) which can reduce the energy loss from separating the Coulombically-bound
electron-hole pair known as an excimer by reducing the energy required to separate the charges
through a charge-transfer intermediate. A series of metal bis-o-phenazine (MBOP) complexes
were synthesized as a family of compounds that can be characterized for that purpose. In
Chapter 5, a series of metal bis-o-diazatetracene (MBOAT) complexes were synthesized and
characterized. It is believed that these compounds will successfully perform singlet fission, as
the energetics are appropriately aligned for the phenomenon to occur. The metal complex
broadens and extends the absorption further into the red region of the solar spectrum, allowing
for a larger percentage of photons from the sun to be collected and captured as usable energy.

20

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Monfray, P.; Mouchet, A.; Najjar, R. G.; Plattner, G.-K.; Rodgers, K. B.; Sabine, C. L.;
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Functional Materials for Transistor and Solar Cell Applications. Journal of the American
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Tanaka, H.; Nakamura, E., Columnar Structure in Bulk Heterojunction in Solution-Processable
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103 (5), 053306.

24

Chapter 2. Synthesis and Characterization of a Family of Perylene Compounds
2.1 Introduction
Perylene is a polycyclic aromatic hydrocarbon compound consisting of two naphthalene
molecules bonded together at the 1,8-positions (Figure 2.1). It belongs to a class of compounds
known as rylene dyes. Perylene derivatives have been utilized as dyes,
1
in organic lasers,
2

fluorescent organic light-emitting diode materials,
3
organic field-effect transistor materials,
4
and
OPV applications.
5
In OPVs, the first donor-acceptor system used a perylene diimide (PDI)
derivative as the electron acceptor to a copper phthalocyanine (CuPC) donor. A modest PCE of
0.95% was achieved.
6
Subsequently, C
60
has been favored as an electron acceptor material, as its
high level of symmetry affords excellent stabilizing of the negative charge through delocalization
of the electron and allows for three-dimensional charge transport, where a planar aromatic
molecule might exhibit directional charge transport based on the molecular orientation.
7
Most of
the highest performing state-of-the-art solar cells utilize a fullerene derivative, whether it is C
60
,
C
70
which has a broader absorption profile, or the soluble analogue PCBM, which is utilized in
spin-cast devices. Only recently has a non-fullerene acceptor paired with a polymer achieved
over 10% PCE.
8

Problems associated with C
60
arise from its costly synthesis and purification. It is also an
Figure 2.1 Left: parent structire of perylene and numbering convention. Middle: 1,2,7,8-
tetracyanoperylene (TCP). Right: 6,12-diphenyl-1,2,7,8-tetracyanoperylene (DPTCP).
25

air-sensitive material and will rapidly oxidize in a thin film when exposed to air.
9
C
60
has also
been found to self-oligomerize within the thin film, decreasing the excited state lifetime and
negatively impacting the stability of the device efficiency over time.
10
Furthermore, the
absorption profile of C
60
is limited to the blue region of the spectrum, necessitating the use of
donor materials that absorb past 500 nm to ensure good spectral coverage. For these reasons,
there is interest to find an alternative acceptor material to C
60
in the hopes of improving overall
OPV cost-effectiveness, lifetime, and efficiency.
Recent interest has returned to perylene derivatives as promising acceptor materials.
While PDI derivatives are the most prevalent in the OPV literature,
5b
a perylene modified with
four electron-withdrawing cyano groups was reported to have a LUMO energy level comparable
to C
60
.
11
Another reason that a perylene derivative was identified as a target molecule was due to
its propensity toward singlet fission, which will be covered in greater detail in Chapter 3. The
structure of perylene is displayed in Figure 2.1, along with the structures of the target molecules
detailed below.
Scheme 2.1 Synthesis of TCP. The intermediate structure prior to Knoevenagel
condensation is displayed in brackets.
26

2.2 Synthesis of Intermediates and Target Compounds
General: Reagents were purchased from Sigma Aldrich and used as received. Dry solvent was
recovered from the SDS system. The reported procedures from reference 11 were followed to
synthesize 1,5-diiodoanthraquinone, 1,5-distyrenylanthraquinone, and the final 1,2,7,8-
tetracyanoperylene. A different route was used to synthesize anthraquinone-1,5-dicarbaldehyde
and is reported below. Synthesis of TCP and DPTCP are outlined in Scheme 2.1 and Scheme
2.2. UV–visible spectra were recorded on a Hewlett-Packard 4853 diode array
spectrophotometer. The steady-state emission at room temperature and 77 K was measured with
Photon Technology International QuantaMaster QM-400 spectrofluorometer. NMR
Scheme 2.2 Total synthetic scheme for DPTCP.
27

characterization was performed with a Varian Mercury 400 MHz spectrometer. Geometry
optimization calculations were performed using Titan, employing B3LYP 6-316* as a basis set.

2.2.1 Anthraquinone-1,5-dicarbaldehyde
To a clean flask containing 1,5-distyrenylanthraquinone (700 mg, 1.70 mmol) was added 28 mL
water and 28 mL acetonitrile. NaIO
4
(1.75 g, 7.5 mmol) was added along with RuCl
3
•H
2
O
(24.5 mg, 0.12 mmol). The reaction was stirred overnight. Water was added and the stirring was
halted. The solid brown product was collected via filtration and dried under vacuum.
Yield: 380 mg, 85%.
1
H NMR (400 MHz, Chloroform-d) δ 10.84 (s, 1H), 8.54 (d, J = 7.8 Hz,
1H), 8.17 (d, J = 7.6 Hz, 1H), 7.96 (t, J = 7.7 Hz, 1H).

2.2.2 1,5-diamino-4,8-dibromoanthraquinone
To a round bottom flask, 1,5-diaminoanthraquinone (5.0 g, 21 mmol) was added. After
dissolving in 250 mL CH
2
Cl
2
, N-bromosuccinimide (7.5 g, 42 mmol) was added and the reaction
was stirred overnight for 18 hours after which the reaction was poured over water to cause
precipitation of some product. Filtration isolated crystalline product. The filtrate was washed
with water then with brine, dried with magnesium sulfate, filtered, and the solvent was removed
in vacuo. The crude product was loaded onto silica gel and purified by column chromatography
and subsequently recrystallized from dichloromethane layered with hexanes. Total yield: 7.2 g,
90%.
1
H NMR: (400 MHz, Chloroform-d) δ 7.80 (d, J = 8.2 Hz, 1H), 7.51 (d, J = 8.1 Hz, 1H).

28

2.2.3 1,5-diamino-4,8-diphenylanthraquinone
To a clean two-neck round bottom flask with a condenser on the middle neck and a rupper
septum on the other neck, 1,5-diamino-4,8-dibromoanthraquinone (2.5 g, 6.3 mmol) was added.
Phenylboronic acid (1.5 g, 12.6 mmol) was added and the solid was dissolved in 250 mL toluene
and 50 mL ethanol. A solution of cesium chloride (20 g, 0.50 mmol) in 50 mL water was added
and the mixture was sparged with nitrogen gas for 15 minutes. Pd(PPh
3
)
4
(360 mg, 0.25 mmol)
was added and the solution was sparged an additional five minutes. The reaction was heated to
95 °C under N
2(g)
and stirred overnight. After cooling to room temperature, the reaction was
filtered to collect crystalline product. The bright red filtrate was washed with water and brine,
dried with magnesium sulfate, filtered, and the solvent was removed in vacuo. The crude
product was loaded onto silica gel and purified by column chromatography and subsequently
recrystallized from dichloromethane layered with hexanes. Total recovered gold-red crystalline
product 2.0 g, 5.0 mmol, 80%.
1
H NMR: (400 MHz, Chloroform-d) δ 7.71 (d, J = 7.6 Hz, 1H),
7.56 – 7.44 (m, 6H), 7.42 (d, J = 7.6 Hz, 1H).

2.2.4 1,5-diiodo-4,8-diphenylanthraquinone
To a clean Erlenmeyer flask under air, 1,5-diamino-4,8-diphenylanthraquinone (5.0 g,
12.8 mmol) was added and dissolved in 50 mL water. An equal amount of sulfuric acid was
added after cooling the flask in an ice bath. The slow addition of NaNO
2
(2.4 g, 34.8 mmol)
evolved gas, and the reaction was stirred at room temperature for three hours until evolution of
gas ceased. Potassium iodide (4.5 g, 27 mmol) was slowly added over five minutes, and the
mixture was stirred an additional two hours. The reaction was filtered to collect the crude brown
product, which was recrystallized from boiling water, then ethanol to yield an orange solid 7.0 g,
29

11.5 mmol, 90%.
1
H NMR: (400 MHz, Chloroform-d) δ 7.69 (d, J = 7.6 Hz, 1H), 7.55 – 7.42
(m, 5H), 7.40 (d, J = 7.7 Hz, 1H).
13
C NMR: (101 MHz, CDCl
3
) δ 181.56, 143.35, 136.33,
133.79, 132.07, 128.73, 102.69.

2.2.5 1,5-diphenyl-4,8-distyrenylanthraquinone
To a clean dry flask under nitrogen was added 1,5-diiodo-4,8-diphenylanthraquinone (1.0 g,
1.6 mmol). The middle neck of a two-neck round bottom flask was capped with a condenser
cooled with water. Addition of Bu
4
NBr (1.0 g, 3.2 mmol) and (MeCN)
2
PdCl
2
(100 mg,
0.39 mmol)

as solids was performed, and the flask was evacuated and backfilled with N
2(g)
. The
solids were dissolved in 20 mL toluene. Triethylamine (1.5 mL, 20 mmol) was added, as well as
excess styrene (1 mL, 8.2 mmol). The reaction was heated to 90 °C and stirred for 15 hrs. Upon
completion, the reaction flask was cooled to room temperature. Washing with sodium
bicarbonate in water followed by water and brine, drying the combined organic extracts with
magnesium sulfate, filtering the solid and removing the solvent via rotary evaporation yielded
the crude brown product. The solid was loaded onto silica gel and purified via column
chromatography to produce the pure orange product 780 mg, 1.38 mmol, 85%.
1
H NMR: (400
MHz, Chloroform-d) δ 8.31 (d, J = 8.0 Hz, 1H), 7.82 – 7.75 (m, 2H), 7.38 (d, J = 4.4 Hz, 4H),
7.29 (d, J = 4.7 Hz, 6H), 6.04 (d, J = 16.5 Hz, 1H).
13
C NMR: (101 MHz, CDCl
3
) δ 185.30,
140.30, 137.24, 136.25, 133.48, 133.12, 132.85, 129.22, 128.74, 128.54, 128.13, 127.23, 127.08.

2.2.6 4,8-diphenylanthraquinone-1,5-dicarbaldehyde
1,5-diphenyl-4,8-distyrenylanthraquinone (520 mg, 0.935 mmol) was added to a clean flask
under atmosphere and dissolved in 40 mL acetonitrile. Sodium periodate (1.6 g, 7.5 mmol) in
30

40 mL water was added to this flask and stirred. Upon addition of RuCl
3
•H
2
O (40 mg,
0.18 mmol), reaction mixture turned brown and was stirred overnight. The brown product was
collected by vacuum filtration 362 mg, 93%.
1
H NMR: (400 MHz, Chloroform-d) δ 10.70 (s,
1H), 8.40 (d, J = 8.0 Hz, 1H), 7.85 (d, J = 8.0 Hz, 1H), 7.44 (d, J = 8.7 Hz, 2H), 7.38 – 7.30 (m,
3H).

2.2.7 6,12-diphenylperylene-1,2,7,8-tetracyanoperylene
Fumaronitrile (156 mg, 2.0 mmol) was added to a clean dry schlenk flask under nitrogen and
dissolved in 2 mL dry dichloromethane. The flask was cooled to 0 °C in an ice bath. A solution
of triethylphosphine in hexanes (1 M, 2.0 mL, 2.0 mmol) was added slowly and stirred for five
minutes to generate the Wittig reagent. In a separate clean, dry round bottom flask, 1,5-
diphenyl-4,8-distyrenylanthraquinone (334 mg, 0.8 mmol) was dissolved in 20 mL dry
dichloromethane and added slowly via syringe to the cold flask containing the Wittig reagent.
The reaction was stirred for four hours at room temperature. After cooling the flask again,
0.5 mL DBU was added and the reaction stirred for an additional three hours. Water was added
to quench the reaction, which was extracted with dichloromethane (3 x 100 mL), washed with
water (2 x 50 mL) and brine (50 mL), then dried with anhydrous magnesium sulfate, filtered, and
concentrated via rotary evaporation. The crude material was loaded onto silica gel and purified
through column chromatography. Fluorescent green fractions were combined to yield the
product as an orange powder.
1
H NMR: (400 MHz, Chloroform-d) δ 9.32 (d, J = 8.0 Hz, 1H),
8.42 (s, 1H), 7.96 (d, J = 8.1 Hz, 1H), 7.62 (t, J = 6.7 Hz, 4H), 7.59 – 7.41 (m, 8H).

31

2.3 Results and Discussion
TCP was synthesized according to a modified literature procedure. The oxidation of an
alkene to an aldehyde via ozonolysis was altered, however, due to limited access to an ozone
generator. An alternate method using a catalytic amount of ruthenium trichloride in the presence
of sodium periodate was utilized to achieve oxidation of the alkene to an aldehyde functional
group. For the final reaction, the Wittig reagent was generated in-situ from the highly
electronegative fumaronitrile reacting with triethylphosphine in solution. Slow addition of the
quinone dicarbaldehyde ensured the temperature of the reaction did not climb, as this would
affect the overall yield of the synthesis. Care was taken to remove oxygen from the DBU base
before addition for the Knoevenagel condensation, and through these steps a respectable yield of
36%, comparable to the reported value, was obtained.
Interest in the synthesis of a more soluble form of TCP arose when the material was
found to aggregate in solution as well as form highly crystalline thin films. By substituting the
perylene core of TCP with alkane groups or with phenyl substituents, the goal was to break up
the aggregation without altering the HOMO or LUMO levels of the material. In order to
preserve the final double Wittig coupling plus Knoevenagel condensation to generate the
perylene core, several synthetic pathways were identified to introduce the desired alkyl
substituents at accessible positions. Substituting the core symmetrically was also considered the
most feasible considering synthetic restraints. Initially, the 4,10 or 5,11 positions were target, as
substitution in the 6,12 positions was predicted to inhibit the final ring-closing reaction by
creating steric interference between the substituent and the cyano groups at the 1,7 positions
(Figure 2.2).

32

With no literature precedence for a 1,2,5,6-tetra substituted anthraquinone, a
retrosynthetic scheme was devised starting with 2,7-diaminoanthraquinone to target a 3,8-
diphenylanthraquinone-1,5-dicarbaldehyde. This was transformed to 2,7-diiodoanthraquinone
via the same Sandmeyer conditions used on the 1,5-diaminoanthraquinone. A Suzuki coupling
with phenylboronic acid yielded the 2,7-diphenylanthraquinone. When treated with Vilsmeyer
formylation conditions, however, the phenyl group was substituted in the 4 position, rather than
formylating the anthraquinone core. It was realized that the anthraquinone ring was too electron-
poor to be the target of the formylation reaction, and a different approach was necessary to yield
a substituted perylene core.
Figure 2.2 Retrosynthetic schemes demonstrating the different approaches to substituting the
TCP core while maintaining the last step of the reaction. Top represents the synthons from the
literature procedure; middle depicts the attempted pathway to a 5,11-DPTCP; bottom shows the
synthesis that was successfully implemented to obtain a 6,12-substituted TCP.
33

Bromination with NBS was proposed and employed to add the halide directly across the
amine groups of the original 1,5-diaminoanthraquinone starting material. A Suzuki coupling
reaction was then used to install the phenyl groups, leaving the amine groups available to utilize
the same synthetic steps in the synthesis of TCP. As predicted, however, the final Knoevenagel
condensation was inhibited by the steric interference between the phenyl groups and the cyano
substituents, resulting in a yield less than 1% regardless of reaction time. The temperature was
increased to encourage the reaction to progress, but the yield did not improve. For this reason,
though the target compound was synthesized, only simple photophysical characterization could
be performed on this derivative.

2.3.1 Calculated Energies and Geometries
The effect of substitution on the energy levels of perylene were of interest to determine
whether the materials would be sufficient as electron acceptors. Calculations were performed to
correlate to the known values of the frontier orbitals of perylene, as well as to observe how the
phenyl substituents would affect the energies of DPTCP compared to TCP. Geometry
optimization was performed to determine the lowest energy conformer of the molecules, and
initially to determine whether the steric hinderance between the phenyl and cyano substituents in
the bay position of DPTCP would inhibit formation of a stable molecule.
From Figure 2.3 it is apparent that as the number of substituents in the bay position on
perylene is increased, the dihedral angle of the perylene core is also increased. Interestingly,
both TCP and DPTCP are oriented such that the molecules have C
2
symmetry with all cyano
substituents pointing in the same direction, rather than yielding a molecule with C
i
symmetry that
would result from two cyano substituents pointing up and two substituents pointing down.
34

Values of the dihedral angles are listed in Table 2.1, as well as the calculated HOMO and
LUMO energy levels. The addition of the four cyano substituents lowers the LUMO 1.5 eV
relative to perylene, while the HOMO is decreased by the same amount, leaving the HOMO-
LUMO gap of the two perylene derivatives nearly identical, although the absorption spectra
suggest the optical bandgaps of the two materials are dissimilar. There is a smaller difference
between the energy levels of DPTCP compared to TCP. The LUMO of DPTCP is 200 meV
higher compared to TCP, due in part to the electron-donating effects of the phenyl substituents.
The HOMO is increased by over 300 meV, resulting in DPTCP having a narrower band gap than
the other two derivatives. This is reflected in the photophysical properties of the material, which
will be addresses in the following section. From cyclic voltammetry measurements, the
reduction potentials of TCP and DPTCP were -0.61 V and -1.1 V, respectively. Calculating the
Figure 2.3 Lowest energy geometries of perylene (left), TCP (middle) and DPTCP (right)
from DFT calculations, showing edge-on view as a depiction of the increasing dihedral
angle. Face-on view is included for comparison. Carbon atoms are in black, hydrogen
atoms are white, and nitrogen atoms are blue.
35

Table 2.1 Key parameters of perylene, TCP, and DPTCP.
Molecule Abs max
(nm)
PL
(nm)
QY
(%)
E
LUMO

(DFT)
E
HOMO

(DFT)
Dihedral
angle
Perylene 438 443 95 -1.90 -4.95 0.0 °
TCP 457 472 90 -3.52 -6.46 19.4 °
DPTCP 480 510 85 -3.32 -6.08 32.3 °

LUMO results in a LUMO energy of -4.0 eV for TCP and a LUMO value of -3.5 eV for DPTCP,
calculated according to literature method.
12
Though TCP has a higher reduction potential than
C
60
, the LUMO level of C
60
from different methods has been calculated at -4.0 eV. Therefore,
the energetic alignment of both cyano-substituted perylene derivatives are within the range to use
these materials as an electron acceptor material in OPVs. The device performance of TCP as an
electron acceptor in OPVs will be discussed further in Chapter 3.

2.3.2 Photophysical Properties
The solution absorption and emission spectra of TCP and DPTCP as they compare to the
parent perylene molecule are displayed in Figure 2.4. Perylene, a rigid planar aromatic
molecule, has absorption and emission profiles that are well-defined with clear vibronic
progression that follow the Franck-Condon progressions. The photoluminescence quantum yield
(PLQY) is high (90%) as there are few non-radiative decay pathways to quench the excited state
prior to emission.
As the planarity of the aromatic ring is distorted, two trends are noticeable. First, the
absorption spectrum loses the defined vibronic progression that is well-defined in the
unsubstituted parent molecule. The absorption peak also red-shifts by 19 nm from perylene to
36

TCP and by an additional 23 nm for DPTCP due to distortion of the ground state and excited
state energy wells as the aromatic backbone of the molecule is twisted. A similar trend is
observed in the emission profiles, as the emission peak red-shifts and the overall line shape
becomes broadened with an increasing dihedral angle. Furthermore, the quantum yield is not
350 400 450 500 550
0.0
0.2
0.4
0.6
0.8
1.0
Absorbance (au)
TCP
Perylene
DPTCP
Wavelength (nm)
450 500 550 600 650
0.0
0.2
0.4
0.6
0.8
1.0
Perylene
TCP
DPTCP
Emission (au)
Wavelength (nm)
Figure 2.4 Absorption (top) and emission (bottom) spectra of perylene (black
trace), TCP (red trace) and DPTCP (blue trace) in dichloromethane.
37

appreciably affected, though it too follows the same trend of decreasing with increasing dihedral
angle. These results are summarized in Table 2.1. The band edge of perylene is consistent with
the calculated HOMO-LUMO gap of 3.05 eV. The band edge of TCP suggests it has an optical
bandgap of 2.58 eV, which is 360 meV narrower than the calculated HOMO-LUMO bandgap.
The optical bandgap of DPTCP is also narrower than that obtained through DFT calculations at
2.38 eV, a difference of 380 meV.

2.4 Conclusions
Two tetracyanoperylene derivatives were synthesized as potential acceptor materials for
use in OPVs. The yield of DPTCP is low due to steric inhibition of the final ring-closing step.
The photophysical properties of these two materials were compared to perylene and a decrease in
the energy gap due to an increase in the dihedral angle, correlating to a distortion of the perylene
aromatic system was observed. The electron-donating substituents further decreased the
HOMO-LUMO gap of DPTCP relative to TCP. The energy levels of both cyano-substituted
perylene derivatives are appropriate for application as acceptor materials in OPVs.

38

Chapter 2 References

1. Gsänger, M.; Bialas, D.; Huang, L.; Stolte, M.; Würthner, F., Organic Semiconductors
based on Dyes and Color Pigments. Advanced Materials 2016, 28 (19), 3615-3645.
2. Sadrai, M.; Bird, G. R., A new laser dye with potential for high stability and a broad band
of lasing action: Perylene-3,4,9,10-tetracarboxylic acid-bis-N,N′(2′,6′ xylidyl)diimide. Optics
Communications 1984, 51 (1), 62-64.
3. Armstrong, N. R.; Wang, W.; Alloway, D. M.; Placencia, D.; Ratcliff, E.; Brumbach, M.,
Organic/Organic′ Heterojunctions: Organic Light Emitting Diodes and Organic Photovoltaic
Devices. Macromolecular Rapid Communications 2009, 30 (9-10), 717-731.
4. (a) Zhan, X.; Tan, Z. a.; Domercq, B.; An, Z.; Zhang, X.; Barlow, S.; Li, Y.; Zhu, D.;
Kippelen, B.; Marder, S. R., A High-Mobility Electron-Transport Polymer with Broad
Absorption and Its Use in Field-Effect Transistors and All-Polymer Solar Cells. Journal of the
American Chemical Society 2007, 129 (23), 7246-7247; (b) Schön, J. H.; Kloc, C.; Batlogg, B.,
Perylene: A promising organic field-effect transistor material. Applied Physics Letters 2000, 77
(23), 3776-3778.
5. (a) Nielsen, C. B.; Holliday, S.; Chen, H.-Y.; Cryer, S. J.; McCulloch, I., Non-Fullerene
Electron Acceptors for Use in Organic Solar Cells. Accounts of Chemical Research 2015, 48
(11), 2803-2812; (b) Kozma, E.; Catellani, M., Perylene diimides based materials for organic
solar cells. Dyes and Pigments 2013, 98 (1), 160-179.
6. Tang, C. W., Two ‐layer organic photovoltaic cell. Applied Physics Letters 1986, 48 (2),
183-185.
7. McAfee, S. M.; Topple, J. M.; Hill, I. G.; Welch, G. C., Key components to the recent
performance increases of solution processed non-fullerene small molecule acceptors. Journal of
Materials Chemistry A 2015, 3 (32), 16393-16408.
8. Zhao, W.; Qian, D.; Zhang, S.; Li, S.; Inganäs, O.; Gao, F.; Hou, J., Fullerene-Free
Polymer Solar Cells with over 11% Efficiency and Excellent Thermal Stability. Advanced
Materials 2016, 28 (23), 4734-4739.
9. Sauvé, G.; Fernando, R., Beyond Fullerenes: Designing Alternative Molecular Electron
Acceptors for Solution-Processable Bulk Heterojunction Organic Photovoltaics. The Journal of
Physical Chemistry Letters 2015, 6 (18), 3770-3780.
39

10. Burlingame, Q.; Tong, X.; Hankett, J.; Slootsky, M.; Chen, Z.; Forrest, S. R.,
Photochemical origins of burn-in degradation in small molecular weight organic photovoltaic
cells. Energy & Environmental Science 2015, 8 (3), 1005-1010.
11. Bhargava Rao, B.; Wei, J.-R.; Lin, C.-H., New Synthetic Routes to Z-Shape
Functionalized Perylenes. Organic Letters 2012, 14 (14), 3640-3643.
12. Djurovich, P. I.; Mayo, E. I.; Forrest, S. R.; Thompson, M. E., Measurement of the lowest
unoccupied molecular orbital energies of molecular organic semiconductors. Organic
Electronics 2009, 10 (3), 515-520.

40

Chapter 3. Tetracyanoperylene: Device Performance and Singlet Fission Efficiency
3.1 Singlet Fission
To put the following work into context, the phenomenon known as singlet fission must
first be introduced and its relationship to enhancing the theoretical maximum efficiency of OPVs
established. Singlet fission was originally discovered in 1965 in a single crystal of anthracene.
1

Via this process, a material excited by one photon of light into a singlet state shares that energy
with a neighboring chromophore to generate two triplet states. Materials known to perform this
process include acenes such as tetracene
2
and pentacene,
3
carotenoids,
4
1,3-
diphenylisobenzofuran,
5
and rylene dyes such as perylene.
6

There are a few requirements that need to be met in order for singlet fission to occur.
First, the energy of the singlet state (S
1
) must be at least twice that of the triplet (T
1
) state in
order to meet the energetic requirements of turning one singlet state into two triplets.
7
Most
organic systems have a T
1
very close in energy to the S
1
state, therefore singlet fission is an
uncommon phenomenon. If the energy of S
1
is significantly greater than T
1
an enhancement in
the rate of singlet fission is observed, as is the case with pentacene in which singlet fission has
been observed on the order of 100 fs.
3
Systems where the reaction is slightly endergonic will
perform at a slower rate, such as with tetracene, with a singlet fission rate equal to 7 ps.
2

A second factor that affects the occurrence of singlet fission dictates that there must be a
minimum of two chromophores in communication, where one remains in its ground state upon
an excitation event.
7
The orientation of the neighboring chromophores to that initial excited
molecule affects the efficiency of singlet fission. This is why in solution, one excited
chromophore will fluoresce to the ground state. However, in a solid state such as in a single
41

crystal or thin film, there is a site upon which to form the delocalized correlated triplet pair, with
an overall spin state equal to 1 (Figure 3.1).
8
This correlated triplet pair intermediate state leads
to the two triplet states that are localized onto individual chromophores. Because of the overall
singlet character of the intermediate state, singlet fission yields triplets on a rapid time scale (fs-
ps), in contrast to a process such as intersystem crossing which is spin-forbidden and occurs
slowly for organic molecules with no heavy atoms to induce spin-orbit coupling, in the
microsecond regime.
For efficient singlet fission, the system of interest must have no fast decay pathways that
would quench the excited state prior to singlet fission. Chromophores with high fluorescence
quantum yields in solution are promising candidates, as fluorescence occurs on a relatively slow
(~ ns) time scale. Furthermore, if the fluorescence is quenched in the solid crystalline or thin
film state, it indicates an intermolecular process that is forming a new state, and is a further
indication that the material may perform singlet fission, as long as that material also meets the
energetic requirements outlined above. Time-correlated photophysical experiments such as
transient absorption are used to study the nature of the excited states and verify whether it is
singlet fission or a different intermolecular process that is quenching emission.

Figure 3.1 Illustration of the transitions involved in the singlet fission process. Note the
overall singlet character of the ME state, as the spin states on both chromophores are oriented
to cancel one another out.
42

3.1.1 Singlet Fission in OPVs
Although the phenomenon has been known to exist for over 50 years, interest in singlet
fission was renewed when a potential application was identified. The idea of converting one
absorbed photon of light into two free carriers could be used to achieve carrier multiplication
within an OPV, since one absorbed photon could generate two pairs of free carriers within a
device if singlet fission occurs with 100% efficiency and both triplet excitons are able to diffuse
to the D/A interface to charge separate.
9
Assuming these requirements are met, Nozik
reevaluated the Shockley-Queisser limit to determine that replacing one of the active materials in
an OPV could increase the maximum device efficiency from 33% up to 45%.
10

Accomplishing this task is more challenging than it may appear, as the relative rates
within a system must align. In order to take advantage of singlet fission within an OPV, k
SF

must be faster than k
CT
between the materials; otherwise the singlet exciton with charge transfer
at the interface before forming the two triplet excitons (Figure 3.2).
8
The rate of singlet fission
in pentacene is competitive with k
CT
with C
60
since the process is energetically downhill;
however, k
SF
is much slower in tetracene and therefore significantly slower than k
CT
.
11
Another
point to consider is the fact that most singlet fission materials have a high energy band edge and
therefore absorb in the same region as C
60
. Utilizing a secondary absorber to capture photons
Figure 3.2 Relative rates of formation of the ME state, SF, and CT to C
60
in pentacene/C
60

(left) and tetracene/C
60
(right).
43

below the bandgap of tetracene, Baldo and co-workers were able to boost OPV efficiency to
1.27% with a tetracene/CuPC/C
60
device stack compared to a modest 0.5% when only
tetracene/C
60
were the active layer materials.
12
Through further device engineering and the use
of pentacene as the singlet fission material, the same group was able to achieve a device with an
external quantum efficiency greater than 100% (Figure 3.3), demonstrating an achievement of
singlet fission within the device contributing to the photocurrent.
13
It should be noted that
optical modifications were utilized such that the incident light had multiple passes through the
material and therefore increased probability of absorption.

3.1.2 Singlet Fission of Perylene and its Derivatives
The parent structure perylene has been shown to exhibit singlet fission in the single
crystalline form. With a singlet energy of 2.85 eV and T
1
equal to 1.52 eV, the process is
energetically uphill by about 200 meV. Two polymorphs of perylene are known and have been
studied (Figure 3.4).
14
One polymorph, β-perylene, crystallizes in a herringbone structure with
Figure 3 A) Device architecture and B) external quantum efficiency of the first reported
OPV device with EQE over 100%.
44

an intermolecular distance of 5.84 Å between parallel chromophores. This form exhibits singlet
fission when exposed to a magnetic field with energy equal to twice the triplet state of perylene.
On the contrary, α-perylene has a much closer intermolecular distance of 3.9, and when exposed
to a magnetic field, requires more energy input before triplet formation correlated to singlet
fission is observed. Interestingly, it was concluded that singlet fission was slower than excimer
formation, or a lower-energy excited state in the α-perylene crystal. No overall triplet yield was
reported for these perylene systems. Recently, a group observed triplet formation in an α-
perylene single crystal by exciting the material with a laser pulse at 240 nm, twice the energy of
T
1
.
15

Perylene diimide (PDI) derivatives have been studied for their singlet fission properties.
Wasilewski et al. studied N,N-bis(n-octyl)-2,5,8,11-tetraphenyl-PDI, which crystallizes in a slip-
stacked orientation, and observed a triplet rise time of 180 ps with a triplet yield of 140 ± 20%.
16

Figure 3.4 Depiction of the different known crystalline phases of perylene.
45

Around the same time, peropyrene, a derivative of perylene that is benzannulated at the 3,4 and
9,10 positions, was found to exhibit rapid excimer formation into a lower energy state, causing
singlet fission to be too energetically uphill to access.
17
A more recent study examined the effect
of bay-substitution on PDI derivatives to increase the dihedral angle of the perylene core.
Interestingly, they determined that a twisted core enhanced the singlet fission triplet yield from
79% to 105%.
18
These materials all have a slip-stacked orientation in the crystal structure, which
has been suggested as imperative to yield efficient singlet fission.
7

3.2 Tetracyanoperylene
As previously discussed in Chapter 2, tetracyanoperylene (TCP) is a derivative of the
parent compound perylene that has four electron-withdrawing substituents that lower the HOMO
and LUMO levels of the molecule to -6.6 eV and -4.0 eV respectively. This material is
energetically suited to replace C
60
as an acceptor material in OPVs and its utility in this
application was characterized. Furthermore, as perylene compounds have been observed to
perform singlet fission, the time-dependent photophysical characteristics of TCP were of interest,
especially in the thin film, to determine what effect the addition of the four cyano groups had on
singlet fission efficiency. For these reasons, we were motivated to look at 1,2,7,8-
tetracyanoperylene as a singlet fission acceptor material in OPVs.

3.2.1 Experimental Section
Tetracyanoperylene was synthesized according to the procedure discussed in Chapter 2.
19

The photophysical characteristics of the material were characterized both in solution and in the
thin film solid state by uV-Vis absorption measurements, emission and excitation taken with a
46

fluorimeter, and quantum yield calculated through use of an integrating sphere. Energy levels of
the material were calculated using Titan (B3LYP 6-31*) and correlated with electrochemical
measurements. Thin films of TCP were fabricated using the Organic Vapor Phase Deposition
system (OVPD). Material was deposited at a rate of 1.5-2.5 Å s
-1
onto quartz substrates cleaned
with 1% solution of tergitol in water and rinsed with isopropanol. The temperature of the
substrate holder was kept at -40 °C during deposition. Time-correlated single-photon counting
(TCSPC) and transient absorption (TA) measurements were performed by Dr. Saptaparna Das
and Jimmy Joy in the Bradforth group to characterize time-resolved photophysical properties of
the material. Samples were dissolved in THF in a nitrogen glove box and transferred to a
1 mm x 1 cm quartz cuvette at a concentration with O.D. ~0.3 for solution measurements. Thin
film samples were epoxy sealed between two quartz slides in a nitrogen glove box.
Device fabrication was carried out in an Angstrom deposition chamber. C
60
, BCP, CuPC,
and TCP were purified by vacuum thermal gradient sublimation in a tube furnace prior to use in
the chamber. Aluminum pellets were used as purchased. Glass patterned ITO-coated substrates
were cleaned following a standard procedure with 1% solution of tergitol in water, boiled in
tetrachloroethane, acetone and ethanol for ten minutes each, and baked in a UV-ozone furnace
for ten minutes to get rid of the native oxide layer. Materials in the vacuum chamber were
deposited at rates of 0.2-2.0 Å s
-1
. Crystals of TCP were grown via vacuum thermal gradient
sublimation suitable for x-ray crystal structure determination.

47

3.3 Results and Discussion
3.3.1 Steady-state Thin Film Photophysical Characterization
Figure 3.5 displays the absorption and emission spectra of a thin film of TCP as it
compares to the solution state spectra. The broadened, mostly featureless absorption spectrum is
indicative of strong intermolecular coupling. The shoulder at 487 nm is indicative of the
formation of J-aggregation in the thin film. The emission is similarly featureless and red-shifted
by 170 nm to 642 nm. Such emission is usually indicative of excimer formation, which is also
an intermolecular process but typically lower energetically than the triplet state of a molecule.
350 400 450 500 550
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Absorbance (au)
Wavelength (nm)
Thin Film
Soln (THF)
450 500 550 600 650 700 750 800
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Emission
Wavelength (nm)
Thin Film
Soln (THF)
Figure 3.5 Absorption (top) and emission (bottom) of TCP in solution and thin film. See
legend for details.
48

However, emission efficiency from this state is only 3%, indicating that an additional non-
emissive state is being accessed that is quenching the emission that was observed in solution.
The absorption, emission and PLQY of a thin film of 5% wt. TCP in
poly(methylmethacrylate) (PMMA) was studied. Though the line shape of the absorption and
emission is maintained, the PLQY decreases to 40%, which is indicative of aggregation even at
low concentrations. The tendency toward aggregation motivated the synthesis of a derivative
with bulky substituents to inhibit aggregation, though the low overall synthetic yield of that
molecule omitted it from this study. The molar absorptivity of TCP is 12,000 M
-1
cm
-1
.

3.3.2 Time-resolved Photophysics
Time-resolved solution state experiments were performed on samples dissolved in
tetrahydrofuran by collaborators in the Bradforth group. These experiments determine the nature
of the excited state to provide insight into the new state that is formed in thin film that is not
5 10 15 20 25 30
1
10
100
1000
PL Counts
Time, ns
Solution
Film
Figure 3.6 Time-correlated single photon counting (TCSPC) traces of TCP in solution (black
trace) and in a thin film (red trace).
49

present in solution. From the TCSPC data (Figure 3.6), it is predicted that the new state is the
triplet state accessed via singlet fission, for the reasons that will be discussed below. A solution
of TCP has a single exponential decay of 6 ns, while the thin film has a multi-exponential decay
curve. The fast component decays in 260 ps, while the slower decay was fit at 10 ns.

3.3.2.1 Solution State Dynamics
Transient absorption of a solution of TCP in THF yielded unexpected results. Based on
the single exponential decay of 6 ns, the TA data was expected to show the ground state bleach
Figure 3.7 Top: Transient absorption data of TCP in solution. Bottom, left: Global analysis fits
for singlet and excimer state absorbances. Bottom, right: Decay time of the singlet population
(black trace) and rise time of the excimer population (red trace).
50

along with S
1
S
n
absorbance traces at short time delays that decay to zero at longer time delays.
However, what is observed is an increase in the ground state bleach with increasing time delays,
as well as new absorption features growing in (Figure 3.7, top). This indicates that with passing
time, more molecules are excited out of the ground state and brought into a different excited
state.
It is predicted that this new state is an excimer formation (Figure 3.7, bottom). Due to its
low solubility, it is possible that the concentration of TCP in THF that is necessary to yield
sufficient TA data is high enough to induce the formation of small aggregates in solution, rather
than isolated molecules. Since a 1 mm cuvette is used to collect TA data, the sample must be
concentrated enough (O.D. ~0.3) to yield good signal-to-noise, where a sample of the same
optical density in a 1 cm cuvette, such as that used to collect TCSPC data, is much less
concentrated. As a result, the new state is attributed to excimer formation, which is long-lived
on the time scale of the experiment.

3.3.2.2 Solid State Dynamics
The thin film excited state decay has two time components: a very fast decay (0.26 ns)
followed by a slow decay. The fast decay is attributed to singlet fission, as it is a rapid decay
pathway, while the slow decay is a result of triplet-triplet annihilation causing delayed
fluorescence. The transient absorption data confirm the occurrence of singlet fission
(Figure 3.8). The decay of the singlet absorption features gives rise to a new feature at 480 nm,
which is attributed to T
1
T
n
absorption based on literature values for perylene.
20
The rise time
of this state is calculated at 9 ps, too fast to be attributed to intersystem crossing. Furthermore,
through global analysis, the triplet yield is 105%, and it is only though singlet fission that the
51

number of triplet states in a sample can exceed the singlet population. For these reasons, it is
concluded that TCP can be listed as a singlet fission chromophore.

It should be noted that, in an attempt for thorough characterization, a film of perylene
was fabricated in the OVPD with the intention of analyzing the time-dependent photophysical
properties, which are not available in the literature. However, due to its low sublimation
Figure 3.8 Top: Transient absorption data of TCP in a neat thin film. Bottom, left: Global
analysis fits for singlet and triplet state absorbance profiles. Bottom, right: Decay time of the
singlet population (black trace) and rise time of the triplet population (red trace). Note the
rapid decay of the triplet population, suggesting that the average population begins to decay
before the overall yield is achieved.
52

temperature (180 °C), any thin film of perylene fabricated at a low temperature of the substrate
holder (-40 °C) immediately crystallized upon warming to room temperature, and caused too
much scattering to yield meaningful data.
Figure 3.9 Crystal structure of TCP. Top: view along the b-axis. Note that chromophores
in the top layer are perfectly eclipsing chromophores in the bottom layer in the bottom layer
along the b-axis. Middle: view along the c-axis. Bottom: Space-filling model of the unit
cell.
53

3.3.3 Crystal Structure
The crystal structure of TCP is an interesting deviation from reported crystal structures of
perylene and PDI derivatives (Figure 3.9). Unlike the slip-stacked geometry that is predicted to
be imperative to efficient singlet fission which has been observed in perylene and PDI
derivatives,
16, 18
TCP crystals have perfectly-overlapping molecules along the b-axis of the
crystal. There are four TCP molecules per unit cell in the Pca21 space group. It occupies an
orthorhombic unit cell, and does not exhibit the herringbone stacking that is common with other
perylene derivatives. TCP-TCP spacing is 3.742 Å between chromophores that eclipse each
other. Along the ab plane of the crystal, the molecules are oriented alternating 90° twisted from
one another to point the cyano groups on one molecule away from those on the nearest
neighboring molecules. The intermolecular face-to-face spacing is 3.7 Å, which is closer to the
structure of α-perylene, though the orientation is neither herringbone-like nor slip-stacked. The
dihedral angle of the perylene framework is 20°, comparable to the angle calculated and
discussed in Chapter 2. Interspatial distance between the nitrogen on the cyano group and the
hydrogen on a neighboring perylene molecule lies in the range of 2.6-2.7 Å, which suggests
hydrogen bonding interactions between chromophores. The C-C bond lengths of the perylene
core vary from 1.37-1.43 Å which are expected for sp
2
hybridized carbon bond lengths.
Although the crystal structure of a single crystal of TCP exhibits perfectly aligned
molecules along the b-axis, this does not necessarily correlate to the preferred orientation within
a polycrystalline thin film. It is possible that the close-packed overlapping TCP chromophores
give rise to excimer formation and lower the overall singlet fission efficiency. However, it is
also possible that TCP forms an ordered, highly crystalline orientation. Atomic force
microscopy (AFM) characterization of the surface morphology of a neat thin film of TCP
54

suggests the latter (Figure 3.10). It would be interesting to characterize the neat film diffraction
pattern to determine the degree of crystallinity within the material.

3.3.4 Device Performance
The device performance of TCP as a replacement acceptor to C
60
was addressed. The
device architectures and key parameters are summarized in Table 3.1. It was found that a layer
of pure TCP formed a very rough film surface, which is undesirable for efficient device
performance. This was verified through AFM, which demonstrated an average surface
roughness RMS = 150 nm, about two orders of magnitude more rough than a typical device
surface (Figure 3.10). (For reference, most working devices have RMS ≈ 1 nm.) A device
utilizing CuPC as an electron donor with a neat layer of TCP as the acceptor did not exhibit
diode behavior in the dark, and had weak photocurrent with a strong voltage dependence and
therefore poor fill factor under illumination (Figure 3.11, black trace). It is believed that the

Figure 3.10 AFM image of a neat thin film of TCP.
55

crystallinity of the acceptor layer caused pinholes in the acceptor such that the aluminum
electrode contacted directly to the donor layer, which leaves a continuous pathway for holes to
travel through the donor layer to the incorrect electrode, resulting in poor device performance.
Because the fill factor is so poor, the J
SC
and V
OC
are artificially high, so PCE is inflated and
does not reflect the true device performance.
To disrupt the aggregation of TCP, the material was doped with a small amount of BCP
during deposition. This wide-gap material was chosen because it would not contribute to the
absorption of the material and generate another interface for charge separation. Instead, its sole
purpose was to inhibit crystallization of the TCP. By co-depositing TCP and BCP at a 10:1 rate
ratio, a more continuous film was fabricated, and the device performance was enhanced
(Figure 3.11, blue trace). The device exhibited diode behavior in the dark and photocurrent
generation in the light, although the photocurrent was still strongly dependent on the applied
Figure 3.11 Left: Device stack for the architectures discussed. Right: I-V curves of the
three device architectures listed in Table 3.1.
56

Table 3.1 Device architectures and key parameter of devices in this study.
Device Donor Thickness Acceptor (nm)
J
SC
(V)
V
OC
(mA/cm
2
) FF (%) PCE (%)
D1 40 nm TCP (35) 0.106 0.815 16 0.014
D2 12 nm TCP:BCP (22) 0.073 0.460 23 0.008
D3 40 nm TCP:BCP (11) 0.079 0.565 30 0.014

voltage. The external quantum efficiency (EQE) of the device indicates current generation from
both the donor CuPC and the acceptor TCP (Figure 3.12). A thinner layer of the doped
400 500 600 700 800
0.0
0.5
1.0
1.5
2.0
CuPC
TCP
Absorbance (au)
Wavelength (nm)
0
2
4
6
8
10
EQE
Quantum Efficiency (%)
Figure 3.12 External Quantum efficiency (EQE) (blue trace)
overlaid with thin film absorption of TCP (red trace) and CuPC
(blue trace).
57

TCP:BCP acceptor improved the fill factor further, yielding the best-performing device (Figure
3.11, red trace). Device engineering may continue to improve device performance.
Though TCP was proven to work as an electron acceptor, its performance pales in
comparison to C
60
paired with CuPC, which achieves PCE = 1.4% using the same device
architecture with 40 nm thick layer of C
60
. This is potentially due to several reasons. First, the
absorption coefficient of TCP is lower than that of C
60
in the same region of the solar spectrum.
Second, doping BCP into TCP dilutes the absorption of TCP, further reducing the absorption of
the material. Finally, optimal performance is achieved with a layer that is 75% thinner than the
thickness of C
60
as an acceptor. These factors explain why the EQE response of TCP is low. It
would be interesting to study the effects of doping C
60
rather than BCP into TCP to break
aggregation, to improve carrier transport in the acceptor layer as well as enhance the absorption.

58

3.4 Conclusions
Transient absorption data suggests that tetracyanoperylene performs singlet fission with
52.5% efficiency, yielding triplets on the order of 105% on a time scale too rapid for intersystem
crossing. This system appears unique, as the crystal structure suggests that the preferred
orientation of perylene involves perfectly eclipsed chromophores rather than the slip-stacked
orientation that literature demands as imperative to singlet fission. However, since the time-
dependent studies were performed on polycrystalline thin films, the preferred orientation for the
molecules to perform singlet fission may in fact be a slip-stacked one closer to β-perylene crystal
structure. TCP was proven to work as an acceptor material in OPVs, though efficiency could be
improved by further device optimization. The material also has a lower absorption coefficient
than C
60
and performs best with a layer that is ¼ as thick as optimized C
60
devices, so it is not an
ideal replacement to C
60
.

59

Chapter 3 References

1. Singh, S.; Jones, W. J.; Siebrand, W.; Stoicheff, B. P.; Schneider, W. G., Laser
Generation of Excitons and Fluorescence in Anthracene Crystals. The Journal of Chemical
Physics 1965, 42 (1), 330-342.
2. Swenberg, C. E.; Stacy, W. T., Bimolecular radiationless transitions in crystalline
tetracene. Chemical Physics Letters 1968, 2 (5), 327-328.
3. Sebastian, L.; Weiser, G.; Bässler, H., Charge transfer transitions in solid tetracene and
pentacene studied by electroabsorption. Chemical Physics 1981, 61 (1), 125-135.
4. Rademaker, H.; Hoff, A. J.; Van Grondelle, R.; Duysens, L. N. M., Carotenoid triplet
yields in normal and deuterated Rhodospirillum rubrum. Biochimica et Biophysica Acta (BBA) -
Bioenergetics 1980, 592 (2), 240-257.
5. (a) Johnson, J. C.; Nozik, A. J.; Michl, J., High Triplet Yield from Singlet Fission in a
Thin Film of 1,3-Diphenylisobenzofuran. Journal of the American Chemical Society 2010, 132
(46), 16302-16303; (b) Schrauben, J. N.; Ryerson, J. L.; Michl, J.; Johnson, J. C., Mechanism of
Singlet Fission in Thin Films of 1,3-Diphenylisobenzofuran. Journal of the American Chemical
Society 2014, 136 (20), 7363-7373.
6. (a) Albrecht, W. G.; Michel-Beyerle, M. E.; Yakhot, V., Exciton fission in excimer
forming crystal. Dynamics of an excimer build-up in α-perylene. Chemical Physics 1978, 35 (1),
193-200; (b) Albrecht, W. G.; Michel-Beyerle, M. E.; Yakhot, V., Exciton fission in excimer-
forming crystal dynamics of excimer build-up. Journal of Luminescence 1979, 20 (2), 147-149.
7. Smith, M. B.; Michl, J., Singlet Fission. Chemical Reviews 2010, 110 (11), 6891-6936.
8. Nelson, C. A.; Monahan, N. R.; Zhu, X. Y., Exceeding the Shockley-Queisser limit in
solar energy conversion. Energy & Environmental Science 2013, 6 (12), 3508-3519.
9. Lee, J.; Jadhav, P.; Reusswig, P. D.; Yost, S. R.; Thompson, N. J.; Congreve, D. N.;
Hontz, E.; Voorhis, T.; Baldo, M. A., Singlet Exciton Fission Photovoltaics. Accounts of
Chemical Research 2013, 46 (6), 1300-1311.
10. Hanna, M. C.; Nozik, A. J., Solar conversion efficiency of photovoltaic and
photoelectrolysis cells with carrier multiplication absorbers. Journal of Applied Physics 2006,
100 (7), 074510.
60

11. (a) Rao, A.; Wilson, M. W. B.; Hodgkiss, J. M.; Albert-Seifried, S.; Bässler, H.; Friend,
R. H., Exciton Fission and Charge Generation via Triplet Excitons in Pentacene/C60 Bilayers.
Journal of the American Chemical Society 2010, 132 (36), 12698-12703; (b) Tritsch, J. R.; Chan,
W.-L.; Wu, X.; Monahan, N. R.; Zhu, X. Y., Harvesting singlet fission for solar energy
conversion via triplet energy transfer. Nat Commun 2013, 4.
12. Jadhav, P. J.; Mohanty, A.; Sussman, J.; Lee, J.; Baldo, M. A., Singlet Exciton Fission in
Nanostructured Organic Solar Cells. Nano Letters 2011, 11 (4), 1495-1498.
13. Congreve, D. N.; Lee, J.; Thompson, N. J.; Hontz, E.; Yost, S. R.; Reusswig, P. D.;
Bahlke, M. E.; Reineke, S.; Van Voorhis, T.; Baldo, M. A., External Quantum Efficiency Above
100% in a Singlet-Exciton-Fission–Based Organic Photovoltaic Cell. Science 2013, 340 (6130),
334-337.
14. Pick, A.; Klues, M.; Rinn, A.; Harms, K.; Chatterjee, S.; Witte, G., Polymorph-Selective
Preparation and Structural Characterization of Perylene Single Crystals. Crystal Growth &
Design 2015.
15. Ma, L.; Tan, K. J.; Jiang, H.; Kloc, C.; Michel-Beyerle, M.-E.; Gurzadyan, G. G.,
Excited-State Dynamics in an α-Perylene Single Crystal: Two-Photon- and Consecutive Two-
Quantum-Induced Singlet Fission. The Journal of Physical Chemistry A 2014, 118 (5), 838-843.
16. Eaton, S. W.; Shoer, L. E.; Karlen, S. D.; Dyar, S. M.; Margulies, E. A.; Veldkamp, B.
S.; Ramanan, C.; Hartzler, D. A.; Savikhin, S.; Marks, T. J.; Wasielewski, M. R., Singlet Exciton
Fission in Polycrystalline Thin Films of a Slip-Stacked Perylenediimide. Journal of the
American Chemical Society 2013, 135 (39), 14701-14712.
17. Nichols, V. M.; Rodriguez, M. T.; Piland, G. B.; Tham, F.; Nesterov, V. N.; Youngblood,
J. W.; Bardeen, C. J., Assessing the Potential of Peropyrene as a Singlet Fission Material:
Photophysical Properties in Solution and the Solid State. The Journal of Physical Chemistry C
2013, 117 (33), 16802-16810.
18. Nagarajan, K.; Mallia, A. R.; Reddy, V. S.; Hariharan, M., Access to Triplet Excited
State in Core-Twisted Perylenediimide. The Journal of Physical Chemistry C 2016, 120 (16),
8443-8450.
19. Bhargava Rao, B.; Wei, J.-R.; Lin, C.-H., New Synthetic Routes to Z-Shape
Functionalized Perylenes. Organic Letters 2012, 14 (14), 3640-3643.
61

20. Parker, C. A.; Joyce, T. A., Formation efficiency and energy of the perylene triplet.
Chemical Communications (London) 1966, (4), 108b-109.

62

Chapter 4. Design and Synthesis of a Series of Metal-bis-o-phenazine (MBOP)
Complexes as Symmetry-Breaking Charge Transfer Chromophores

4.1 Introduction
4.1.1 Open Circuit Voltage and the Charge Transfer State
As has been discussed previously, the excited state of a small molecule differs from that
of an inorganic material in the Coulombic attraction between the electron-hole pair. In an
inorganic solid, the excited state is delocalized across several sites creating a high dielectric that
requires little added energetic input to separate the two charges and generate current within a
device. Due to low dielectric of an organic or organometallic medium, the material does not
shield the oppositely charged particles from each other, resulting in a more tightly-bound
electron-hole pair. The energy required to separate the pair into free charges negatively impacts
the V
OC
of the D/A material pair, resulting in a loss of 0.3-0.5 eV.
1
If the attraction between the
oppositely-charged species can be reduced, a higher-energy CT state can be accessed, leading to
fewer energetic losses and an overall higher V
OC
.

4.1.2 Symmetry Breaking Charge Transfer
The molecule bianthryl was found to exhibit solvent-dependent photophysical
properties.
2
The absorption was the same regardless of solvent; however, a trend emerged based
on the polarity of the solvent used for photophysical analysis. In highly non-polar solvent,
bianthryl exhibited a high PLQY. As the polarity of the solvent increased, PLQY decreased.
Examining the shape of the emission, the emission peak maximum shifted to lower energies with
increasing solvent polarity. It was found that polar solvents stabilize an intramolecular charge
63

transfer (CT) state. Upon excitation to the S
1
state, a localized excimer is formed on one of the
anthracene chromophores. In a polar medium, a CT state with a localized hole on one
anthracene chromophore and a localized electron on the other is stabilized. The more polar the
environment, the lower in energy the state, explaining the observed red-shift in the emission
spectrum. A non-polar solvent cannot stabilize the CT state, and therefore the state is too high in
energy to access, so emission from S
1
is observed. Because the two anthracene chromophores
are orthogonal in the excited state, the CT state is long-lived.
3

This process, referred to herein as symmetry-breaking charge transfer (SBCT) is utilized
by photosynthetic systems to prevent back-electron transfer and achieve rapid charge separation
after absorption.
4
The environment surrounding a “special pair” breaks the symmetry of the
system by stabilizing the positive charge on one chlorophyll moiety and the negative charge on
the second. Intermolecular SBCT systems are studied for efficient charge separation and
transport in biomimetic photosynthetic light harvesting systems.
5
Intramolecular SBCT is of
interest for its application toward OPVs. This phenomenon has been observed in BODIPY
dimers
6
as well as homoleptic zinc dipyrrin compounds.
7

4.1.2.1 Device Applications of SBCT
By spatially separating the electron and hole on separate chromophores within the same
molecule, the exciton binding energy is weakened. Therefore, less energy is necessary to form
the intermolecular CT state at the D/A interface in an OPV. This would reflect in a V
OC
closer to
the ΔE
DA
. A zinc dipyrrin acceptor with energy levels identical to C
60
was incorporated into a
device and, when paired with the same donor, achieved a V
OC
450 mV higher than the C
60

device of the same architecture (Figure 4.1).
8
It is a worthwhile pursuit to expand the library of
64

SBCT materials with strong absorption in the visible region to achieve high-performing OPVs
with fewer energetic losses that result from generating free charges from excitonic excited states.

4.1.2.2 Metal-bis-o-phenazine
A series of chromophores designed to perform SBCT are proposed, synthesized and
characterized. A diaza-anthracene analogue, phenazine, was proposed with a hydroxyl
substituent adjacent to the nitrogen. This chromophore could act as a bidentate LX-type ligand,
and two are able to couple to the same metal center to generate a M
2+
center. Using different
metals changes the geometry of the metal complex, and it would be possible to study the effect
of a tetrahedral configuration, where the two chromophores are orthogonal to one another, to a
square planar complex where the ligands lie in the same plane. These materials would be
suitable for application as donor chromophores in OPVs.

4.2 Experimental Section
Materials were purchased from Sigma Aldrich and used as received. Synthesis of 1-
methoxyphenazine was carried out according to the literature procedure.
Figure 4.1 a.) Device architecture of the I-V curves in b.).
65

4.2.1 1-hydroxyphenazine
The synthesized 1-methoxyphenazine (100 mg, 0.48 mmol) was added to a clean, dry round
bottom flask under nitrogen and dissolved in 5 mL dry dichloromethane. The flask was cooled
to -78 °C in dry ice/acetone bath. Four equivalents of boron tribromide in hexanes (1 M, 2.0 mL,
2.0 mmol) were slowly added via syringe. The cooling bath was removed and the reaction was
stirred for four hours. Excess BBr
3
was quenched by the slow addition of water until the fuming
ceased. The reaction mixture was poured over a beaker of NaHCO
3(aq)
and subsequently
extracted with dichloromethane (3 x 25 mL) until the water layer was almost colorless. The
combined organic extracts were washed with water, brine, dried over anhydrous magnesium
sulfate which was filtered, and concentrated via rotary evaporation. The crude material was
loaded onto silica gel and purified via column chromatography. The non-emissive 1-
methoxyphenazine eluted first, followed by emissive unreacted starting material. The product
was recrystallized from dichloromethane layered with methanol to obtain yellow crystals:
1
H
NMR: (400 MHz, Chloroform-d) δ 8.42 (dd, J = 8.4, 1.8 Hz, 1H), 8.26 (dd, J = 7.6, 2.5 Hz, 1H),
7.91 – 7.81 (m, 3H), 7.78 (dd, J = 8.9, 7.5 Hz, 1H), 7.10 (d, J = 8.0 Hz, 1H).

Scheme 4.1 Synthesis of MBOP complex. M = Zn, Ni, Cu.
66

4.2.2 Tert-butyl-1-methoxyphenazine
1-methoxycatechol (700 mg, 5.0 mmol) was dissolved in ether and cooled to -78 °C. O-chloranil
(1.23 g, 5.0 mmol) was slowly added and the reaction was stirred, maintaining temperature for 3
hours. The reaction was filtered to obtain 3-methoxy-o-quinone, which was dried on the filter
paper and used quickly. T-butylphenylenediamine (700 mg, 4.26 mmol) was dissolved in 8 mL
benzene to which was added 8 mL acetic acid. The quinone was added to this solution and the
reaction was left for twelve hours. The reaction mixture was carefully poured over a saturated
solution of aqueous sodium bicarbonate and extracted with dichloromethane. The combined
organic extracts were washed with brine, dried with magnesium sulfate, filtered and concentrated
via rotary evaporation. The crude product was loaded onto silica gel and purified by column
chromatography to afford a yellow oil, the mixture of the 6- and 8-t-butyl-1-methoxyphenazine
isomers, which was used without further purification.
1
H NMR (400 MHz, Chloroform-d) δ 8.32
(ddd, J = 9.2, 1.3, 0.6 Hz, 1H), 8.16 – 8.10 (m, 1H), 7.93 (ddd, J = 9.3, 2.2, 1.2 Hz, 1H), 7.81 (dt,
J = 8.8, 1.2 Hz, 1H), 7.73 (ddd, J = 8.8, 7.5, 1.2 Hz, 1H), 7.05 (dt, J = 7.5, 1.2 Hz, 1H), 4.17 (d, J
= 1.2 Hz, 3H), 1.48 (d, J = 1.2 Hz, 9H).
13
C NMR (101 MHz, CDCl
3
) δ 155.18, 154.27, 144.19,
143.58, 141.05, 136.50, 130.24, 129.99, 129.44, 123.64, 121.25, 106.09, 56.43, 35.51, 30.74.

4.2.3 T-butyl-1-hydroxyphenazine
The oil from the previous reaction was taken up in a clean dry round bottom flask and dissolved
in dry dichloromethane under nitrogen. The flask was cooled to -78 °C in a cooling bath of dry
ice and acetone. Four equivalents of 1 M boron tribromide in hexanes were slowly added and ice
bath was removed. The reaction was stirred at room temperature for four hours, after which time
excess BBr
3
was quenched with water. The reaction mixture was poured over a saturated
67

aqueous solution of sodium bicarbonate, extracted with dichloromethane, washed with water
then brine, dried with anhydrous magnesium sulfate, filtered and concentrated via rotary
evaporation. The crude product was loaded onto silica gel and purified via column
chromatography.
1
H NMR (400 MHz, Chloroform-d) δ 8.25 – 8.10 (m, 3H), 8.00 – 7.92 (m,
1H), 7.80 – 7.70 (m, 2H), 7.25 – 7.19 (m, 1H), 1.49 (d, J = 2.4 Hz, 9H).

4.2.4 General procedure for metal-bis-o-phenazine (MBOP) complexes
The appropriate hydroxyphenazine ligand (50 mg, 0.25 mmol 1-hydroxyphenazine; 0.94 mg,
_mmol t-butyl-1-hydroxyphenazine) was dissolved in ethanol in a clean round bottom flask.
Half an equivalent of the appropriate metal acetate hydrate (0.127 mmol) was dissolved in
ethanol and added to the ligand, affording an immediate color change. The reaction was stirred
overnight, and the product collected by filtration and purified through repeated recrystallizations
or precipitations.
ZnBOP pink solid precipitated from CH
2
Cl
2
layered with MeOH, yield: 65%.
1
H NMR (400
MHz, Chloroform-d) δ 9.24 (d, J = 8.8 Hz, 1H), 8.53 (d, J = 7.7 Hz, 1H), 8.17 – 8.08 (m, 1H),
7.98 – 7.89 (m, 1H), 7.81 – 7.74 (m, 1H), 7.72 – 7.64 (m, 2H), 7.56 (d, J = 8.8 Hz, 1H), 7.05 (d,
J = 7.6 Hz, 1H), 6.74 – 6.65 (m, 1H), 6.40 (dd, J = 8.7, 1.1 Hz, 1H), 6.14 (d, J = 7.6 Hz, 1H),
6.08 – 5.99 (m, 1H).
NiBOP purple solid recrystallized from Ch
2
Cl
2
layered with EtOH, yield: 45%
1
H NMR (400
MHz, Chloroform-d) δ 8.32 – 8.18 (m, 1H), 7.92 – 7.82 (m, 1H), 7.86 – 7.75 (m, 1H).
CuBOP blue solid precipitated from CH
2
Cl
2
layered with EtOH, yield: 76%
68

ZntbuBOP pink solid recrystallized from EtOH layered with H
2
O, yield: 68%
1
H NMR (400
MHz, Chloroform-d) δ 8.23 – 8.09 (m, 2H), 7.95 (t, J = 8.7 Hz, 1H), 7.74 (d, J = 10.8 Hz, 2H),
7.22 (d, J = 3.6 Hz, 1H), 1.49 (d, J = 2.4 Hz, 9H).

4.3 Results and Discussion
4.3.1 Synthesis of the target materials
Synthesis of methoxyphenazine was found in the literature and carried out successfully.
However, the reported transformation of the compound to the hydroxyphenazine derivative by
refluxing with HBr was unsuccessful. Initial attempts of deprotection of the methoxy group to
an alcohol was attempted with only one equivalent of BBr
3
to recover unreacted starting
material. The reaction was successful after the addition of four equivalents of BBr
3
though
complete conversion of functional group was never obtained. Longer reaction time lowered the
overall yield of the reaction, so since separation of the product and starting material was easily
achieved through column chromatography, the synthetic conditions were not optimized further.
The initial metal complex synthesized was the ZnBOP complex, which afforded a pink
solid. Care had to be taken during the work up of the complex to not expose it to acidic
conditions, as the ligands would become protonated and dissociate in the presence of acid. The
complex also did not run on basic alumina column, so purification was achieved by washing the
complex with ethanol to remove excess zinc acetate hydrate, and hexanes was used to wash
Scheme 4.2 Synthesis of t-bu-1-methoxyphenazine.
69

excess unbound ligand. CuBOP and NiBOP were synthesized to compare the photophysical
properties of the square planar complexes to the tetrahedral zinc complex and determine whether
the geometry had an effect on these properties.
In order to assess the affinity for SBCT of the metal complexes, a derivative was targeted
that would be reasonably soluble in non-polar solvents as well as polar solvents. To improve the
solubility of the molecule in non-polar solvents, non-polar t-butyl substituents were incorporated
into the structure of the phenazine ligand by using 4-t-butylphenalinediamine as the building
block for the phenazine ligand (Scheme 4.2). Due to the nature of the synthesis of t-butyl-1-
methoxyphenazine, there was no regioselectivity of where the t-butyl substituent would end up
with respect to the methoxy group. As a result, the reaction yielded two isomers. It was possible
to achieve some separation of these isomers via column chromatography; however, the t-butyl
substituent did not affect the photophysical properties of the ligand so it was deemed
unnecessary to separate the isomers, and the mixture was carried through subsequent reaction
conditions. As a result, there are three iterations of the final ZntbuBOP complex that are present
in the sample (Figure 4.2). As previously mentioned, the photophysical properties of these
derivatives are identical, so for the purpose of this study they can be considered as one complex.
Figure 4.2 Different MtbuBOP isomers.
70

4.3.2 Photophysical Characterization of Methoxyphenazine and Hydroxyphenazine Ligand
The solution absorption profiles of both phenazine derivatives are similar in shape to the
unsubstituted parent compound.
9
The spectra can be seen in Figure 4.3. The intense band is
attributed to the allowed π-π* transition, while the weaker band arises from a forbidden n-π*
transition.
10
In solution, 1-methoxyphenazine (MeOP) has an appreciable PLQY = 20% that is
diminished when converted to 1-hydroxyphenazine (HOP) (<1%). The quenching is a result of
n-π
*
CT from the lone electron pair on the oxygen to the phenazine ring, which decays non-
radiatively. The low fluorescence quantum yield could also be attributed to proton-exchange of
the hydroxyl moiety with the solvent, another non-radiative decay pathway. Weak vibronic
progression is observable in the high intensity band of both MeOP and HOP, similar to the
vibronic progression observed in the absorbance of anthracene.
The emission of MeOP has a broad, featureless structure, a reflection of the lower-

350 400 450 500 550 600 650
0.0
0.2
0.4
0.6
0.8
1.0
Intensity (au)
Wavelength (nm)
MeOP Em
MeOP Abs
HOP Abs
Figure 4.3 Absorption of MeOP (black trace) and HOP (blue trace). Emission of
MeOP (red trace).
71

intensity absorption band. This is also comparable to the emission of phenazine from the
literature. Emission from HOP was too weak for the fluorimeter to detect a signal.

4.3.3 Photophysical Characterization of MBOP Complexes
In Figure 4.4, the absorption profiles of the three metal-o-phenazine complexes in
dichloromethane are displayed. The intensity and maximum of the lowest-energy absorption
band affects the appearance of each of the complexes, resulting in ZnBOP having a pink
appearance, CuBOP appearing green, and NiBOP having a purple coloration. The lowest energy
band is attributed to either a MLCT or LL’CT between the two ligands. All three complexes
have low PLQY (<1%) in CH
2
Cl
2
, which potentially suggests the complexes perform SBCT as
designed. No emission signal was detected with the fluorimeter. However, the complexes are
insoluble in less polar solvents, so there is no point of comparison to determine whether PLQY
increases with decreasing solvent polarity.
Figure 4.4 Absorption of MBOP complexes.
72

Interestingly, NiBOP in tetrahydrofuran forms a deep blue solution, in contrast to the
purple hue when the compound is dissolved in dichloromethane or as a crystalline solid. This
color change is reflected in the solution state absorption spectrum (Figure 4.5). The intensity of
the third absorption band is increased substantially to the point where it is almost as strong as the
maximum intensity peak displayed. It is proposed that the solvent coordinates to an empty
coordination site on the square planar NiBOP complex, where a lone pair of electrons on the
oxygen in THF donates its electron density into an empty p orbital on the metal center to form a
pseudo square pyramidal 5-coordinate nickel complex.

400 500 600 700 800
0.0
0.2
0.4
0.6
0.8
1.0
Absorbance (au)
Wavelength (nm)
DCM
THF
Figure 4.5 NiBOP absorption in CH
2
Cl
2
(black) and
tetrahydrofuran (blue).
73

Table 4.1 Comparison of absorption maxima, reduction potentials, and calculated LUMO values
for the MBOP complexes.
Compound Abs max 2
nd
max E
red
LUMO (calc)
ZnBOP 379 nm 508 nm -1.545 V -2.94 eV
NiBOP 379 nm 514 nm -1.589 V -2.89
CuBOP 384 nm 556 nm --- ---

Cyclic voltammetry was used to approximate the energy levels of ZnBOP and NiBOP.
The LUMO was calculated from the reduction potential, and the results are summarized in
Table 4.1. The calculated LUMO values are appropriate to use these metal complexes as a
donor to C
60
, suitable for use in OPVs.

4.3.4 Photophysical Characterization of tbuMeOP
The absorption and emission spectra of the tbuMeOP chromophore are represented in
Figure 4.6. Interestingly, the π-π* absorption band is less structured than that of MeOP. The
absorption of the weaker band is identical to MeOP, as is the shape and maximum of the
emission. Emission intensity is weaker (PLQY = 10%), which could be a result of vibrations of
the t-butyl group contributing to non-radiative decay of the excited state.
74

4.3.5 Photophysical Characterization of ZntbuBOP
Due to the added solubility afforded by the t-butyl groups, it was possible to characterize
the photophysical properties of ZntbuBOP in both polar and non-polar solvents. From the
absorption spectrum in Figure 4.7, it is apparent that there is no significant solvent effect on the
absorption profile of the complex. Comparing the absorption of ZntbuBOP to its less soluble
counterpart, it is interesting to note that the intensity of the third absorption peak is much weaker
(~10% of the maximum absorption band, compared to ~40% in ZnBOP). The measured
PLQY is low (<1%) in both polar and non-polar solvents. However, a signal was detected in the
fluorimeter and the emission collected is in Figure 4.7.
The emission appears to red-shift in polar solvents compared to non-polar solvents, as the
maximum shifts from 503 nm in cyclohexane to 524 nm in tetrahydrofuran. This suggests that
the complex is capable of performing SBCT, as a red-shift in emission is expected with a
stabilization of the charge-transfer state. Further characterization of the excited state using a
400 500 600 700
0.0
0.2
0.4
0.6
0.8
1.0
t-bu-1-methoxyphenazine
Wavelength (nm)
Abs
Em
Figure 4.6 Absorption and emission of tbuMeOP.
75

technique such as transient absorption would afford insight into the nature of the excited state
and indicate whether a different state is formed dependent on solvent polarity. The low quantum
yield suggests that a non-radiative decay pathway quenches the excited state, which could be
attributed to bond breaking/forming upon excitation or other fluid-state dynamics at work. Study
of the material in the solid state or in a rigid matrix such as PMMA would be conducive to limit
the decay caused from the dynamics of the molecules in solution.

4.3.5.1 Comparison to Acridine Derivative
An acridine derivative of the ligand, with a single nitrogen substitution on the anthracene
ring at the 9- position, was synthesized by co-worker Narcisse Ukwitegetse (Figure 4.8, inset).
The photophysical characteristics of the ethoxy ligand are similar to that of the
methoxyphenazine ligand discussed above (Figure 4.8). The absorption profile displays a strong
400 500 600 700 800
0.0
0.2
0.4
0.6
0.8
1.0
ZntbuBOP
Wavelength (nm)
THF
THF em
Em Cychex
Cychex
Figure 4.7 Absorption and emission of ZntbuBOP in
polar (THF, blue traces) and non-polar (cyclohexane,
black trace) solvents.
76

allowed π-π
*
transition peak at approximately 350 nm, with a weaker n-π
*
transition shoulder at
lower energy.
The bis-acridine zinc complex was synthesized by Mr. Ukwitegetse following conditions
similar to those reported above used to synthesize the series of MBOP complexes (Figure 4.9,
inset). This compound exhibited low solubility in non-polar solvents, comparable to what was
observed with respect to the unsubstituted ZnBOAT complexes. However, it was possible to
characterize the solution state photophysical properties of the complex in polar solvents with
different dielectric constants.
Although the complex exhibited low emission intensity (PLQY <1%), a signal was
detected by the fluorimeter. The emission spectra of the zinc bis-o-acridine complex are shown
in Figure 4.9 in three different polar solvents. Tetrahydrofuran and dichloromethane have
comparable polarities, so it is not surprising that their emission profiles are relatively similar.
Figure 4.8 Absorption and emission of 1-ethoxyacridine in
dichloromethane. Inset: structure of 1-ethoxyacridine.
77

However, acetonitrile is more polar, and it is interesting to note that the emission profile is red-
shifted and therefore at a lower energy. This aligns with the expectation that this zinc complex
will perform symmetry-breaking charge transfer. It would be worthwhile to synthesize a
derivative designed to exhibit solubility in non-polar solvents, as was performed with the
MtbuBOAT complex. It would be interesting to observe whether the acridine derivative still
exhibits low emission intensity in very non-polar solvents, in which case the cause of the non-
radiative decay will need to be determined.
400 450 500 550 600
0.0
0.2
0.4
0.6
0.8
1.0

Normalized Intensity (a.u)
Wavelength (nm)
Emission of Zn(Acr)
2
THF
DCM
MeCN
Figure 4.9 Emission of zinc bis-o-acridine in different solvents.
Inset: structure of zinc-bis-o-acridine.
78

4.4 Conclusions
A series of MBOP complexes were synthesized and characterized. Changing the metal
affects the absorption of the molecule by altering the intensity of the LL’CT absorption peak,
with a small shift in the absorption maximum. Though the molecules were designed to perform
symmetry-breaking charge transfer, time resolved experiments are necessary to confirm whether
these materials are behaving as designed. Preliminary photophysical characterization of the
ZntbuBOP complex suggests that the complex may perform SBCT, but emission in a non-polar
solvent is too weak to draw definitive conclusions. Transient absorption studies would verify
whether the nature of the excited state is affected by solvent polarity. The broad absorption in
the visible spectrum and the energy levels of these complexes makes them candidates for use as
donor material in organic photovoltaics.

79

Chapter 4 References

1. Zou, Y.; Holmes, R. J., Correlation between the Open-Circuit Voltage and Charge
Transfer State Energy in Organic Photovoltaic Cells. ACS Applied Materials & Interfaces 2015,
7 (33), 18306-18311.
2. Schneider, M., Einfache Methode für die Auswertung von Messungen der
Transportphänomene. Berichte der Bunsengesellschaft für physikalische Chemie 1968, 72 (3),
399-400.
3. Grabowski, Z. R.; Rotkiewicz, K.; Rettig, W., Structural Changes Accompanying
Intramolecular Electron Transfer: Focus on Twisted Intramolecular Charge-Transfer States and
Structures. Chemical Reviews 2003, 103 (10), 3899-4032.
4. Lathrop, E. J. P.; Friesner, R. A., Simulation of optical spectra from the reaction center of
Rb. sphaeroides. Effects of an internal charge-separated state of the special pair. The Journal of
Physical Chemistry 1994, 98 (11), 3056-3066.
5. Wasielewski, M. R., Self-Assembly Strategies for Integrating Light Harvesting and
Charge Separation in Artificial Photosynthetic Systems. Accounts of Chemical Research 2009,
42 (12), 1910-1921.
6. Whited, M. T.; Patel, N. M.; Roberts, S. T.; Allen, K.; Djurovich, P. I.; Bradforth, S. E.;
Thompson, M. E., Symmetry-breaking intramolecular charge transfer in the excited state of
meso-linked BODIPY dyads. Chemical Communications 2012, 48 (2), 284-286.
7. Trinh, C.; Kirlikovali, K.; Das, S.; Ener, M. E.; Gray, H. B.; Djurovich, P.; Bradforth, S.
E.; Thompson, M. E., Symmetry-Breaking Charge Transfer of Visible Light Absorbing Systems:
Zinc Dipyrrins. The Journal of Physical Chemistry C 2014, 118 (38), 21834-21845.
8. Bartynski, A. N.; Gruber, M.; Das, S.; Rangan, S.; Mollinger, S.; Trinh, C.; Bradforth, S.
E.; Vandewal, K.; Salleo, A.; Bartynski, R. A.; Bruetting, W.; Thompson, M. E., Symmetry-
Breaking Charge Transfer in a Zinc Chlorodipyrrin Acceptor for High Open Circuit Voltage
Organic Photovoltaics. Journal of the American Chemical Society 2015, 137 (16), 5397-5405.
9. Ryazanova, O. A.; Voloshin, I. M.; Makitruk, V. L.; Zozulya, V. N.; Karachevtsev, V.
A., pH-Induced changes in electronic absorption and fluorescence spectra of phenazine
derivatives. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 2007, 66
(4–5), 849-859.
80

10. Mikami, N., Polarized absorption spectrum of phenazine crystal. Journal of Molecular
Spectroscopy 1971, 37 (1), 147-158.

81

Chapter 5. Metal bis-o-diazatetracene (MBOAT) Complexes Designed as Singlet Fission
Chromophores

5.1. Introduction
5.1.1 Singlet Fission in Thin Film
The motivation behind studying singlet fission dimers for OPV applications relates to the
theoretical maximum efficiency calculations. In order to harvest the most utility from a singlet
fission material in an OPV, there needs to be sufficient triplet yield, or singlet fission efficiency
in the thin film.
1
Materials that have been studied to have close to 200% triplet yield through
singlet fission in single-crystalline form will not necessarily yield the same efficiency in a thin
film. A thin film of diphenyltetracene (DPT) (Figure 5.1a) exhibited two triplet rise times; the
faster of the two is attributed to singlet fission occurring among appropriately-oriented
chromophores, and a slower component associated with diffusion of the singlet to a site
appropriate for singlet fission.
2
The overall triplet yield is 122%, and this system emphasizes the
importance of molecular orientation relating to singlet fission efficiency. It is desirable to
control the molecular orientation of singlet fission chromophores to ensure that each
chromophore is oriented in the ideal position with respect to a neighboring chromophore such
that every molecule is poised for singlet fission.

5.1.2 Intramolecular Singlet Fission
If a molecule could be synthesized such that two chromophores are locked in the
preferred orientation for efficient singlet fission, then theoretically these molecules would be
capable of 100% efficient intramolecular singlet fission. A thin film of these chromophores
82

would be expected to yield 200% triplet generation for every singlet exciton, and therefore meet
one of the necessary requirements for achieving OPV efficiency greater than the Shockley-
Queissar limit through carrier multiplication.
1a, 3

5.1.2.1 Tetracene Dimers
On a fundamental level, it is interesting to study singlet fission in molecular dimers of
chromophores in order to analyze the efficiency in a two-body system. The first tetracene dimers
were analyzed by Bardeen et al. with very low triplet yield; though the orientation of these
chromophores was such that any communication between the two tetracene chromophores was
through-bond rather than through-space.
4
A tetracene dimer in which the two chromophores
have overlapping π systems for through-space interactions was synthesized to observe the
orientation of the chromophores with respect to each other and the singlet fission efficiency in
solution (Figure 5.1b).
5
This system was observed to form the multi-exciton (ME) correlated
triplet pair in solution, but without a third site for the triplet to diffuse to, the ME state gradually
relaxed back to the ground state.

Figure 5.1 a.) Diphenyltetracene b.) bis-alkynyltetracene c.) metal-bis-o-diazatetracene
(MBOAT), M = Zn, Cu, Ni.
83

5.1.2.2 Pentacene Dimers
Other groups studying pentacene dimers, where the formation of two triplets is an
exothermic process, have observed an appreciable triplet yield and singlet fission efficiency in
solution.
6
This suggests that the exothermic driving force is sufficient to separate the ME state
into free triplets within the system. It would be interesting to study the singlet fission efficiency
of these systems in a thin film to determine whether 100% efficiency (200% triplet yield) is
reached. The dimers could be incorporated into OPVs to enhance the photocurrent within the
active layer to achieve high EQE without optical modifications.

5.1.3 Proposed System
A series of metal-bis-diazatetracene complexes were proposed to determine whether
these metal complexes would act as singlet fission dimers (Figure 5.1c). It is worth noting that
in the present literature, there are no singlet fission dimers where the chromophores are joined
through a metal center. According to DFT calculations performed on the zinc metal complex,
the expected singlet energy (1.7 eV) is greater than twice that of the triplet (0.86 eV), suggesting
that the energy levels of the proposed material system are appropriate to perform singlet fission.
Calculations also suggest that the singlet and triplet energies of the free ligand are sufficient for
singlet fission to occur within the ligand. Singlet fission in the diazatetracene molecule is more
energetically favorable than in tetracene, so it would be interesting to characterize efficiency of
this phenomenon within the proposed system.

84

5.2. Experimental Section
Materials were purchased from Sigma Aldrich. Synthesis of 1-methoxy-2,9-diazatetracene was
carried out according to the literature procedure.

5.2.1 1-hydroxy-2,9-diazatetracene
In a clean, dry round bottom flask, 1-methoxy-2,9-diazatetracene (100 mg, 0.38 mmol) was
dissolved in 5 mL dry dichloromethane and cooled to -78 °C with a dry ice/acetone bath. Four
equivalents of boron tribromide in hexanes (1 M, 1.5 mL, 1.5 mmol) was slowly added via
syringe. The ice bath was removed and the reaction stirred for 4 hrs at room temperature. The
excess BBr
3
was carefully quenched with water until evolution of HBr ceased. The reaction was
poured over a saturated aqueous solution of NaHCO
3
, which was extracted with dichloromethane
(3 x 50 mL). The combined organic layers were washed with water, brine, dried over anhydrous
magnesium sulfate and filtered, then concentrated via rotary evaporation and loaded onto silica
gel. The crude reaction mixture was purified via column chromatography.

Scheme 5.1 Synthesis of ligand and metal complex. M = Zn, Ni, Cu.
85

5.2.2 Metal-bis-o-diazatetracene (MBOAT) General Procedure
To a clean round bottom flask, 1-hydroxy-2,9-diazatetracene (50 mg, 0.20 mmol) was added.
This was dissolved in 5 mL ethanol and stirred vigorously. The appropriate metal acetate
hydrate (0.5 eq, 0.10 mmol) was dissolved in 5 mL ethanol, then added to the ligand and stirred
overnight for 12 hrs. An immediate color change was noted. The product was collected via
vacuum filtration, then washed with ethanol and hexanes until solvent ran clear.

5.3. Results and Discussion
5.3.1 Synthesis of the Ligand and Metal Complexes
The 1-methoxy-2,9-diazatetracene (MeOAT) ligand was readily synthesized according to
the literature procedure. Synthetic conditions that were discussed previously to make 1-
hydroxyphenazine were applied to the synthesis of 1-hydroxy-2,9-diazatetracene (HOAT). The
use of four equivalents of BBr
3
was carried through, and no optimization was performed so
unreacted starting material was recovered as well as the desired product. The synthesis of the
metal complexes was achieved in a similar manner to the MBOP complexes discussed in Chapter
4, by combining 0.5 equivalent of a M
2+
acetate with one equivalent of the ligand.
Recrystallization of these materials was not achieved due to very low solubility in a multitude of
solvents.

5.3.2 Characterization of the Free Ligand
The absorption and emission of the MeOAT ligand are displayed in Figure 5.2. In
solution, the lower-energy absorption band is similar to that of tetracene in terms of the vibronic
progression of the molecule. The band edge at 550 nm correlates to a singlet energy of 2.25 eV,
86

in agreement with DFT calculations performed. The triplet energy of the complex was
calculated at 0.90 eV, meeting the 2T
1
≤ S
1
requirement for singlet fission to occur. The
emission profile of the material is structureless, and the material has a PLQY = 40%.

A thin film of this material was spin-cast onto a quartz substrate. The absorption
spectrum is displayed in Figure 5.3. Although the absorption bands broaden, the vibronic
progression can still be identified in the lower energy absorption band. The absorption intensity
of the first band is weakened relative to the lower energy vibronic bands. Low PLQY (<1%) was
observed from the thin film, and no emission was detectable in the fluorimeter. This suggests
that the ligand performs singlet fission in the thin film, though transient absorption experiments
to characterize the absorption characteristics of this new state are necessary to confirm this
hypothesis.
Figure 5.2 Absorption (solid line) and emission (dashed line) of
MeOAT ligand.
87

5.3.3 Characterization of Metal Complexes
The metal-bis-o-diazatetracene (MBOAT) complexes were synthesized in fair yield
according to the procedures outlined above, adding 0.5 equivalents of the appropriate metal

acetate to the ligand in ethanol. All three metal complexes precipitated out of ethanol and
exhibited minimal solubility in a variety of solvents. The metal center affected the appearance of
the final complex: ZnBOAT was a dark purple/black color, CuBOAT a blue solid, and NiBOAT
a green solid. Photophysical characterization was performed in tetrahydrofuran, as the
complexes were found to be soluble only in tetrahydrofuran, and is displayed in Figure 5.4.
As with the MBOP complexes discussed in Chapter 4, a new low-energy absorption peak
is observed upon metalation of the ligand. The metal center affects the intensity and peak
maximum of this band. ZnBOAT exhibits the highest energy peak at 594 nm and moderate
absorption intensity. NiBOAT has an intermediate absorption peak at 619 nm with the strongest
absorption. CuBOAT has the weakest absorption with the lowest energy maximum at 642 nm.
400 500 600 700
0.0
0.2
0.4
0.6
0.8
Absorbance (au)
Wavelength (nm)
Thin Film
Figure 5.3 Absorption of a thin film of MeOAT.
88

The first absorption peak at 409 nm is identical for all three complexes. The emission efficiency
of these metal complexes was low (PLQY<1%) and no signal was detected in the fluorimeter for
any of the metal complexes. The band edge of ZnBOAT is at 700 nm which correlates to a

singlet energy of 1.77, in close agreement to the calculated energy. This suggests that the system
is energetically able to perform singlet fission.
It is possible that the low PLQY is due to intramolecular singlet fission or, at the very
least, formation of the triplet correlated ME state. To verify this, further time-correlated
experiments are required. Another possible explanation for low PLQY in the polar solvent
tetrahydrofuran could be SBCT. If the rate of the formation of the CT state outcompetes the rate
of singlet fission, the MBOAT complexes would form that state first. The CT state, energetically
lower than the S
1
state, would trap the excited state and cause singlet fission to be inaccessible.
Figure 5.4 Absorption spectra of ZnBOAT (black trace),
NiBOAT (red trace) and CuBOAT (blue trace).
89

Synthesis of a more soluble MBOAT derivative would give insight into whether there is solvent
polarity dependence on the emission of the MBOAT complexes. If so, then SBCT would
negatively affect the ability of the chromophore to exhibit efficient singlet fission in solution or
in the solid state.

5.4 Conclusions and Future Outlook
The diazatetracene ligand appears to undergo singlet fission in a thin film, as the
emission that is observed in solution is quenched in the solid state which indicates the presence
of an interchromophoric excited state. The MBOAT complexes are non-emissive in solution,
suggesting that the complexes could perform intramolecular singlet fission within the metal
complexes. This is backed by singlet energy calculations that are in agreement with the band
edge absorption of ZnBOAT, though time-dependent studies would be helpful to definitively
determine the nature of the excited state. The broad absorption in the visible region of the solar
spectrum suggests that these materials would perform well in OPVs, and future work would
focus on determining whether these materials are capable of performing singlet fission in a dimer
or in thin film.

90

Chapter 5 References

1. (a) Hanna, M. C.; Nozik, A. J., Solar conversion efficiency of photovoltaic and
photoelectrolysis cells with carrier multiplication absorbers. Journal of Applied Physics 2006,
100 (7), 074510; (b) Greyson, E. C.; Stepp, B. R.; Chen, X.; Schwerin, A. F.; Paci, I.; Smith, M.
B.; Akdag, A.; Johnson, J. C.; Nozik, A. J.; Michl, J.; Ratner, M. A., Singlet Exciton Fission for
Solar Cell Applications: Energy Aspects of Interchromophore Coupling†. The Journal of
Physical Chemistry B 2010, 114 (45), 14223-14232.
2. Roberts, S. T.; McAnally, R. E.; Mastron, J. N.; Webber, D. H.; Whited, M. T.; Brutchey,
R. L.; Thompson, M. E.; Bradforth, S. E., Efficient Singlet Fission Discovered in a Disordered
Acene Film. Journal of the American Chemical Society 2012, 134 (14), 6388-6400.
3. (a) Smith, M. B.; Michl, J., Singlet Fission. Chemical Reviews 2010, 110 (11), 6891-
6936; (b) Lee, J.; Jadhav, P.; Reusswig, P. D.; Yost, S. R.; Thompson, N. J.; Congreve, D. N.;
Hontz, E.; Voorhis, T.; Baldo, M. A., Singlet Exciton Fission Photovoltaics. Accounts of
Chemical Research 2013, 46 (6), 1300-1311; (c) Havlas, Z.; Wen, J.; Michl, J., Singlet fission:
Towards efficient solar cells. AIP Conference Proceedings 2015, 1702, 090017.
4. (a) Müller, A. M.; Avlasevich, Y. S.; Müllen, K.; Bardeen, C. J., Evidence for exciton
fission and fusion in a covalently linked tetracene dimer. Chemical Physics Letters 2006, 421 (4–
6), 518-522; (b) Müller, A. M.; Avlasevich, Y. S.; Schoeller, W. W.; Müllen, K.; Bardeen, C. J.,
Exciton Fission and Fusion in Bis(tetracene) Molecules with Different Covalent Linker
Structures. Journal of the American Chemical Society 2007, 129 (46), 14240-14250.
5. Korovina, N. V.; Das, S.; Nett, Z.; Feng, X.; Joy, J.; Haiges, R.; Krylov, A. I.; Bradforth,
S. E.; Thompson, M. E., Singlet Fission in a Covalently Linked Cofacial Alkynyltetracene
Dimer. Journal of the American Chemical Society 2016, 138 (2), 617-627.
6. (a) Zirzlmeier, J.; Lehnherr, D.; Coto, P. B.; Chernick, E. T.; Casillas, R.; Basel, B. S.;
Thoss, M.; Tykwinski, R. R.; Guldi, D. M., Singlet fission in pentacene dimers. Proceedings of
the National Academy of Sciences 2015, 112 (17), 5325-5330; (b) Zirzlmeier, J.; Casillas, R.;
Reddy, S. R.; Coto, P. B.; Lehnherr, D.; Chernick, E. T.; Papadopoulos, I.; Thoss, M.;
Tykwinski, R. R.; Guldi, D. M., Solution-based intramolecular singlet fission in cross-conjugated
pentacene dimers. Nanoscale 2016, 8 (19), 10113-10123.

Abstract (if available)

Abstract Problems associated with consumption of the limited resource of fossil fuels, inclusive of climate change, pollution and acidification of the ocean due to increased atmospheric levels of carbon dioxide, have driven scientists to seek out alternative sources of energy. Solar energy in the form of photovoltaics presents as an attractive option because of the abundance of energy supplied to the earth by the sun. An efficient method of capturing the energy for use in applications that are currently fueled by non-renewable resources is of interest. Organic photovoltaics (OPVs) embody the third generation of solar cell technology that has made strides in efficiency and stability toward commercial applications. However, as a recent technology, there are improvements to be made in the field before the technology is on par with silicon photovoltaics. This work seeks to utilize the excitonic excited state in OPVs to achieve carrier multiplication through singlet fission and to decrease the energetic cost of separating the exciton into free carriers through symmetry-breaking charge transfer. Chapter 2 introduces novel materials that can act as singlet fission acceptors within the active layer to replace the commonly-used fullerenes. Chapter 3 examines one of the candidates in terms of singlet fission efficiency as well as device performance. The concept of symmetry-breaking charge transfer is introduced in Chapter 4, along with the synthesis and characterization of materials that were proposed for this purpose. Chapter 5 examines the utility of using a metal center to hold two singlet fission chromophores in communication, ideally to achieve an increase in singlet fission yield compared to the free chromophores.

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University of Southern California Dissertations and Theses

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Asset Metadata

Creator Femia, Denise (author)

Core Title Improving the field of organic photovoltaics through the development of new active layer materials with unique photophysical properties

School College of Letters, Arts and Sciences

Degree Doctor of Philosophy

Degree Program Chemistry

Publication Date 05/09/2017

Defense Date 07/28/2016

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

Tag OAI-PMH Harvest,organic photovoltaics,photophysical chemistry,singlet fission

Format application/pdf (imt)

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Thompson, Mark E. (committee chair)

Creator Email drfemia@gmail.com,femia@usc.edu

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c40-320010

Unique identifier UC11214556

Identifier etd-FemiaDenis-4905.pdf (filename),usctheses-c40-320010 (legacy record id)

Legacy Identifier etd-FemiaDenis-4905.pdf

Dmrecord 320010

Document Type Dissertation

Format application/pdf (imt)

Rights Femia, Denise

Type texts

Source University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Access Conditions The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the a...

Repository Name University of Southern California Digital Library

Repository Location USC Digital Library, University of Southern California, University Park Campus MC 2810, 3434 South Grand Avenue, 2nd Floor, Los Angeles, California 90089-2810, USA