Bridgehead imine substituted cyclopentadithiophene derivatives: An effective strategy for band gap control in donor-acceptor polymers

Article abstract of DOI:10.1021/ma302569u

Bridgehead imine substituted cyclopentadithiophene structural units offer the opportunity to modify the electronic properties, in particular, the HOMO-LUMO energy levels of donor-acceptor polymers with unprecedented precision. Utilizing a common synthetic pathway, copolymers with high average molecular weights, a variety of functionality, and properties suitable for solar cell incorporation can be generated. The fabrication of organic photovoltaic devices with these new materials is demonstrated.

Full text of DOI:10.1021/ma302569u

Article
pubs.acs.org/Macromolecules
Bridgehead Imine Substituted Cyclopentadithiophene Derivatives:
An Effective Strategy for Band Gap Control in Donor−Acceptor
Polymers
Jason D. Azoulay,*,†,‡ Zachary A. Koretz,† Bryan M. Wong,‡ and Guillermo C. Bazan*,†
†Departments of Chemistry & Biochemistry and Materials, Center for Polymers and Organic Solids, University of California, Santa
Barbara, Santa Barbara, California 93106, United States
‡Materials Chemistry Department, Sandia National Laboratories, Livermore, California 94551, United States
S
* Supporting Information
ABSTRACT: Bridgehead imine substituted cyclopentadithiophene structural units offer the opportunity to modify the
electronic properties, in particular, the HOMO−LUMO energy levels of donor−acceptor polymers with unprecedented
precision. Utilizing a common synthetic pathway, copolymers with high average molecular weights, a variety of functionality, and
properties suitable for solar cell incorporation can be generated. The fabrication of organic photovoltaic devices with these new
materials is demonstrated.
iophene) (Egap = 1.9 eV), have afforded relatively high
efficiencies in combination with various fullerenes but only
harness a fraction of the incident solar irradiation.31−36
Derivatives based on 4H-cyclopenta[2,1-b:3,4-b′]dithiophene
(CPDT) lead to extended conjugation, lower HOMO−LUMO
energies, and stronger intermolecular interactions.37 An
alternative approach utilizes monomers of reduced aromaticity
whose decreased HOMO−LUMO separations are carried over
into their polymeric counterparts. As an example,
polycyclopenta[2,1-b:3,4-b′]dithiophen-4-one displays a Egap
of ≤1.2 eV as a result of the primary resonance contributor
(1a in Figure 1).38 Poly-4-dicyanomethylene-4H-cyclopenta-
[2,1-b:3,4-b′]dithiophene (1b) exhibits a reduced Egap of ∼0.8
eV as a result of the stronger electron-withdrawing substituent,
which enhances the participation of the primary resonance
contributor to the overall structure of the molecule.39 The
extent of intramolecular conjugation in these cases results from
the overlap of two interfering patterns (linear and cross)
leading to restricted conjugation and charge redistribution from
INTRODUCTION
■
Conjugated polymers offer the opportunity for the solution-
based fabrication of low-cost optoelectronic devices1−3 such as
light-emitting diodes (LEDs),4−9 field-effect transistors
(FETs),10−13 and solar cells.14−18 The most efficient organic
photovoltaic (OPV) devices employ a bulk-heterojunction
(BHJ) architecture, in which a polymeric electron donor and
fullerene acceptor form a network of nanoscale charge
separating heterojunctions throughout the bulk of the materi-
al.19,20 A persistent challenge in the development of polymer-
based donors has been fine-tuning of various structural and
electronic features in order to maintain solution processability,
suitable charge-carrier mobilities, wide-ranging optical absorp-
tion profiles, large absorption coefficients, the appropriate
energy band structure, and a favorable blend morphology.21−26
In order for molecular systems utilized in these devices to meet
the necessary performance requirements for broad commercial
utility, a more effective strategy to modulate their properties, in
particular the LUMO energy,27,28 and better understand
structure−function relationships must be established.14−18,27−30
The design of narrow band gap (Egap) conjugated polymers
has primarily relied on increasing the quinoid to aromatic
character and on variations in the occupancy of frontier orbitals.
Soluble polythiophenes, such as regioregular poly(3-hexylth-
Received: December 14, 2012
Revised: January 21, 2013
Published: February 7, 2013
© 2013 American Chemical Society
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constituent copolymer segments through inductive effects due
to substitutions on the Ar ring. As a result of competing
electronic effects, the HOMO−LUMO energies can be
modulated with unprecedented precision, which affords a new
strategy in the design of materials for light harvesting
applications.
RESULTS AND DISCUSSION
■
In order to probe if imine substitution would lead to
unanticipated electronic perturbations as compared to 1c, we
chose to compare the long oligomer systems (n = 4) of 1c and
1d in order to ascertain an accurate description of the
electronic and optical properties.41,42 Optimized ground-state
(S0) geometric structures as determined at the B3LYP/6-
31G** level of theory with periodic boundary conditions43 are
illustrated pictorially along with representations of the single-
electron HOMO and LUMO wave functions (Figure 2).
Analysis of the HOMO in each case reveals a well-delocalized
wave function. This implies that the positively charged polaron
of the fully charge-separated state could be delocalized to a
similar extent across the conjugated backbones in both
polymers. The LUMO is mainly localized on the BT units
but shows a varying degree of delocalization based on the
component parts and in 1d extends onto the imine
functionality. The spectra of 1c and 1d exhibit one dominant
low-energy transition with a large oscillator strength (f)
followed by a second, high-energy transition (or series of
transitions) with smaller oscillator strengths (see Supporting
Information). The oscillator strength of 1c (f = 3.09) is
modestly reduced in comparison to 1b (f = 3.34) as a result of
the electron-withdrawing imine substituent and reduced
transition dipole in the system. As illustrated in Figure 2,
there are not large noticeable differences with regard to the
ground-state geometric shapes of the backbones. Importantly,
the imine substituent stabilizes the HOMO further, while also
maintaining an energetically stabilized LUMO.
Figure 1. Molecular structures considered in this study: 1a = poly-4H-
cyclopenta[2,1-b:3,4-b′]dithiophen-4-one; 1b = poly-4-dicyanomethy-
lene-4H-cyclopenta[2,1-b:3,4-b′]dithiophene; 1c = poly[2,6-(4,4-bis-
(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7-(2,1,3-
benzothiadiazole)] (PCPDTBT), and 1d.
the electron-rich to the electron-deficient portions of the
molecule.
The incorporation of alternating electron-rich (donor) and
-poor (acceptor) moieties along the linearly conjugated
backbone gives rise to broad intramolecular charge-transfer
excitations that can be more appropriately tuned to match the
solar spectrum. A well-appreciated class of polymers incorpo-
rates CPDT and 2,1,3-benzothiadiazole (BT) (structure 1c in
Figure 1) (Egap ∼ 1.4 eV).40 Polymeric materials based on a
donor−acceptor (DA) architecture have been used extensively
for the preparation of narrow Egap conjugated polymers and
represent the dominant class of high-performance materials to
date.
Recognizing these considerations, we identified a framework
in which the electronic structure can be more finely tuned. In
this contribution, we examine DA copolymers incorporating an
aryl (Ar) imine substituent at the bridgehead position of
CPDT. We find that the imine substituent can be used to
systematically influence the aromatic stabilization of the
The utility of derivatives based on imine substitution stems
from the potential for the introduction of a variety of backbone
Figure 2. Optimized ground-state (S0) geometric structures as determined at the B3LYP/6-31G** level of theory with periodic boundary conditions
of 1c (top) and 1d (bottom) and pictorial representations of the single-electron HOMO and LUMO wave functions projected over a single repeat
unit for clarity.
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substituents, solubilizing and pendant groups. To better
illustrate this concept, calculations aimed at exploring the
effect of a variety of substituents capable of inductively
withdrawing electron density from the donor were carried
out and are compared in Table 1. Introduction of an electron-
bromosuccinimide (NBS) in anhydrous DMF at 25 °C to
afford 2,6-dibromo-4H-cyclopenta[2,1-b:3,4-b′]dithiophen-4-
one (1) in 94% yield. Imine formation from the respective
aniline does not proceed using typical condensation reaction
procedures. A method for the reaction of anilines with 1 was
therefore developed which utilizes TiCl4 and triethylamine
(NEt3) in dichloromethane and provides the desired monomers
in greater than 90% conversion. In this reaction, TiCl4 is added
to a solution of aniline and excess NEt3 at −35 °C. A solution
of 1 is subsequently added, and the reaction mixture is allowed
to warm to room temperature and stirred overnight.45
Conversion is nearly quantitative when greater than 1 equiv
of aniline is used. Introduction of a solubilizing ethylhexyl
substituent (in the case of P3 and P4) was carried out by the
reaction of the commercially available starting materials 2-(4-
aminophenyl)-1,1,1,3,3,3-hexafluoropropan-2-ol and 4-amino-
2,3,5,6-tetrafluorobenzoic acid with 2-ethylhexyl bromide in
anhydrous DMF in the presence of K2CO3 at 65−80 °C
(Scheme 1).
Synthetic entry into the bis-stannylated monomers was
achieved using ∼3.5 equiv of hexamethylditin (Me3SnSnMe3)
and 10 mol % Pd(PPh3)4 as the catalyst in toluene at 80 °C. As
a result of the statistical nature of the reaction, yields are higher
when a larger excess of Me3SnSnMe3 is utilized; however,
minimal differences in product distribution and yield were
noted above 3.5 equiv. Purification of the monomers is
accomplished using reverse phase chromatography with ethanol
(1% NEt3) as the eluent. The overall process is illustrated in
Scheme 1 with yields being substrate dependent, but on the
order of 50−70%. The X-ray crystal structure of the bis-
stannylated monomer 3 is shown in the Supporting
Information and confirms the identity of the molecule.46 The
imine functionality is in a planar arrangement with the
bithiophene backbone, a unique environment compared to
CPDT as the bridging atom is no longer tetrahedral. This
structural feature should afford better interchain ordering. The
Table 1. Calculated Data for the Octamer (n = 4) at the
B3LYP/6-31G** Level of Theory
S0 → S1
electronic
d
a
a
b
c
HOMO LUMO
Eg
Evert
f
configuration
1c
−4.47
−4.72
−4.78
−4.99
−2.91
−3.11
−3.16
−3.35
1.56
1.33
1.35
1.35
1.36
3.34 HOMO →
LUMO (95%)
1d (Ph)
1e (p-F)
1.61
1.62
1.64
3.09 HOMO →
LUMO (93%)
3.07 HOMO →
LUMO (94%)
1f
(C6F5)
2.84 HOMO →
LUMO (92%)
a
Frontier molecular orbital energies as determined at the B3LYP/6-
b
31G** level of theory. S0 to S1 vertical transition energy (Evert).
c
d
Oscillator strength. Electronic configurations were determined with
TDDFT at the B3LYP/6-31G** level of theory. All energies are in eV.
withdrawing fluorine substituent at the para-position (Ar = p-F-
C6H4) (1e) allows for further reduction of the HOMO−
LUMO energies while maintaining the Egap of the material. A
more strongly withdrawing pentafluorophenyl (Ar = C6F5)
derivative (1f) results in further stabilization of the LUMO. In
addition, there is a systematic decrease in the oscillator strength
going across the series. There is however, little perturbation in
the electronic configuration and S0 to S1 vertical transition
energies.
Our synthetic approach is shown in Scheme 1 and begins
with the preparation of 4H-cyclopenta[2,1-b:3,4-b′]dithiophen-
4-one.44 Bromination at the 2,6-positions proceeds using N-
Scheme 1. (a) Synthesis of Aniline Derivatives; (b) Synthetic Route toward Bis-Stannylated Monomers; (c) Microwave-
Mediated Copolymerization of Bis-Stannylated Monomers with 4,7-Dibromo-5,6-bis(alkyloxy)benzo-2,1,3-thiadiazoles: 3.5 mol
% Pd(PPh3)4, Xylenes, 120 °C (5 min), 140 °C (5 min), 170 °C (60 min)
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Table 2. Optical and Electrochemical Properties of P1−P5
a
a
b
c
d
e
f
Mn
PDI
λmax [nm]
λonset [nm]
Egopt [eV]
EHOMO [eV]
ELUMO [eV]
Egelec [eV]
EgDFT [eV]
P1
P2
P3
P4
P5
29
33
19
24
28
1.9
2.5
1.8
2.4
2.0
720
733
737
710
715
850
870
860
880
885
1.46
1.43
1.44
1.41
1.40
−5.38
−5.44
−5.46
−5.48
−5.61
−3.73
−3.99
−4.00
−4.08
−4.12
1.65
1.45
1.46
1.40
1.49
1.53
1.54
1.57
1.59
1.57
a
Mn [kg mol−1] and PDI (Mw/Mn) were determined by gel-permeation chromatography (GPC) at 150 °C in 1,2,4-trichlorobenzene relative to
polystyrene standards. In o-DCB at 25 °C. Films spin-cast from a CHCl3 solution. Determined from the onset of the film. Highest occupied
molecular orbital energy level (EHOMO) calculated from the onset of oxidation and lowest unoccupied molecular orbital energy level (ELUMO
b
c
d
e
)
f
calculated from the onset of reduction. Energy band gap calculated from the difference between EHOMO and ELUMO. Band gaps obtained with
periodic boundary conditions at the B3LYP/6-31G(d,p) level of theory.
Figure 3. (a) UV−vis absorption spectra of P1−P5 in o-DCB at 25 °C and normalized AM1.5G solar photon flux. (b) Comparison of the
electrochemical band gaps and LUMO offsets relative to PC71BM.
aryl ring is arranged roughly perpendicular to the CPDT
backbone and oriented toward one side of the CPDT core. In
accordance with solution (NMR) characterization, both
thiophene protons are inequivalent as noted by large differential
syn−anti shifts.
the maximum photon flux at ∼700 nm.50 The optical band gap
(Egopt) of P1 as estimated from the absorption onset of the film
at 850 nm is 1.46 eV. Cyclic voltammetry (CV) shows that the
HOMO is located at −5.38 eV and the LUMO at −3.73 eV, as
determined by the oxidation and reduction onset, respectively.
This gives an electrochemical bandgap (Egelec) of 1.65 eV.
Substituents capable of withdrawing electron density were
explored next. Addition of a para-fluorine substituent (P2)
leads to a narrowing of the band gap and moderate decrease in
Copolymerization of the bis-stannylated donors with the
appropriate solubilizing acceptors (1, 2, 4, and 5 with 4,7-
dibromo-5,6-bis(dodecyloxy)benzo-2,1,3-thiadiazole and 3 with
4,7-dibromo-5,6-bis(hexyloxy)benzo-2,1,3-thiadiazole)47 was
carried out via microwave irradiation using Pd(PPh3)4 as the
catalyst in xylenes. This methodology afforded the desired
copolymers in >75% yields following purification via Soxhlet
extraction and allowed us to carefully probe the effects of
various substitution patterns.48,49 The structure of the
copolymers was confirmed by 1H NMR and 19F NMR
spectroscopies in C6D4Cl2 at 120 °C (necessary to suppress
polymer aggregation) and FT-IR (see Supporting Information).
Gel-permeation chromatography (GPC) at 150 °C in 1,2,4-
trichlorobenzene relative to polystyrene standards was used to
determine the number average molecular weight (Mn) of the
products. In general, polymers with Mn ≥ 19 kg mol−1 could be
generated with minimal changes to the reaction procedure.
These data are summarized in Table 2.
the HOMO−LUMO energies as determined by CV (EHOMO
=
−5.44; ELUMO = −3.99 eV). Introduction of a more strongly
electron withdrawing C6F5 substituent (P5) leads to subse-
quent decrease in the HOMO−LUMO energies to −5.61 and
−4.12 eV. While P2 had attractive energy levels, the low
solubility of this material prompted the investigation of
copolymers with an additional solubilizing unit on the donor
(i.e., P3 and P4). Introduction of a p-C(CF3)2OC8H17
substituent, which we anticipated to have similar electron
withdrawing characteristics to P2, leads to a polymer with
comparable energy levels (EHOMO = −5.46 eV; ELUMO = −4.00
eV; Egelec of 1.46 eV). Due to the presence of the bulky p-
C(CF3)2OC8H17 substituent, only C6H13-alkoxy subtituents on
the BT acceptor are required for sufficient solubility of the
polymer product. The absorption spectrum of P3 is indicative
of a stronger tendency toward aggregation when compared to
the other materials as a result of a reduction in side chain
density. Heating to 120 °C (see Supporting Information) leads
The UV−vis absorption spectra of P1−P5 are compared in
Figure 3 and illustrate broad absorption profiles with
absorption maxima (λmax) in solution (25 °C in o-DCB)
occurring between 710 and 740 nm, which coincides well with
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Figure 4. Solar cell characteristics of bulk heterojunction devices prepared from P1 or P3 and PC71BM. (a) Current−voltage plots of solar cells
composed of P1:PC71BM (dashed line) and P3:PC71BM (solid line) under air mass 1.5 global (AM 1.5G) irradiation at 100 mW cm−2. (b) The
external quantum efficiency (EQE) spectra of the same devices.
to the disappearance of the shoulder and structure in the
absorption band together with a blue shift (∼50 nm) in the λmax
of the polymer, suggesting the break-up of interchain
aggregates.
In the case of P4, additional electron withdrawing
substituents lead to a further decrease in the HOMO−
LUMO energies (EHOMO = −5.48 eV; ELUMO = −4.08 eV;
Egelec of 1.40 eV) and a broad red-shifted absorption profile
(Egopt of 1.41 eV at 880 nm). A large increase in the solubility of
P5 as compared to P6 is apparent. It is worth noting that P1,
P3, and P5 are all highly soluble (>10 mg/mL) in common
organic solvents including THF and exhibit ideal band
structures and absorption profiles for light harvesting
applications in combination with PC71BM.51,52
To demonstrate the ultimate utility of copolymers based on
bridgehead imine substitution, the photovoltaic properties of
P1 and P3 were briefly examined utilizing a standard device
architecture of ITO/PEDOT(40 nm)/P:PC71BM (80−90
nm)/LiF(1 nm)/Al(100 nm). Solar cells were fabricated
using previously published procedures, and full details can be
found in the Supporting Information. We found that P1 and P3
performed comparably to 1c using a 1:1 ratio of polymer to
PC71BM and a solvent mixture of 3% diiodooctane in
chlorobenzene. Figure 4 illustrates the current−voltage (J−V)
characteristics of solar cells composed of P1 and P3 under AM
1.5G irradiation at 100 MW cm−2 and the external quantum
efficiency (EQE) spectra of the same devices. The J−V
characteristics of the device for P1 show that the short circuit
current density (Jsc) is −11.2 mA cm−2 with an open circuit
voltage (Voc) of 0.56 V and a fill factor (FF) of 0.49. The EQE
illustrates that current generation extends to almost 900 nm,
which leads to a power conversion efficiency (ηe) of 2.88%. A
similar device made with P3 exhibits: Jsc = −9.8 mA cm−2, Voc =
0.70 V, FF = 0.44, and ηe = 3.02%. In accord with
electrochemical measurements there is an increase in the Voc
of the device, which correlates with the difference in the
HOMO of P1 and P3. We note here that a strong correlation
between Mn, Jsc, and FF has been demonstrated for 1c.49
harvesting applications with unprecedented precision. More-
over, we demonstrate a method to tune both the HOMO and
LUMO energy levels of these materials. Due to the wide variety
of commercially available and synthetically accessible anilines, it
is reasonable to assume that numerous electronic and structural
modifications can be explored without the need for the lengthy
requisite chemical transformations typically required for the
synthesis of a new polymer derivative. We anticipate that
further optimization of device fabrication conditions and
materials properties will enable improvements in performance
and insights into the features governing photovoltaic perform-
ance in BHJ solar cells.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental details, the synthesis of monomers and polymers,
device fabrication, thin-film UV−vis spectra, and cyclic
voltammetry. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: jdazoul@sandia.gov (J.D.A.); bazan@chem.ucsb.edu
(G.C.B.).
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by Sandia National Laboratories, a
multiprogram laboratory operated by Sandia Corporation, a
Lockheed Martin Company, for the United States Department
of Energy under Contract DE-AC04-94AL85000.
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In short, we demonstrate a synthetic strategy to systematically
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