Synthesis and characterization of thieno[3,4-b]thiophene-based copolymers bearing 4-substituted phenyl ester pendants: Facile fine-tuning of homo energy levels

Article abstract of DOI:10.1021/ma201501g

A study was conducted to design and synthesize PTB-based polymers bearing 4-substituted phenyl esters in place of the alkyl esters as the pendants of the thieno[3,4-b]thiophene (TT) unit. It was found that their highest occupied molecular orbital (HOMO) energy levels were readily fine-tuned by changing the substituents on the phenyl groups. Four novel TT monomers bearing various phenyl ester groups (TT1-TT4) were easily prepared in one pot from commercially available phenol derivatives and the key precursor compound, 4,6-dibromothieno[3,4-b]thiophene-2-carboxylic acid, which was synthesized by modified literature procedures. The molecular weights of the obtained polymers were determined by size-exclusion chromatography (SEC) in tetrahydrofuran (THF). The thermal stability of these polymers was investigated by thermal gravimetric analysis (TGA) under a nitrogen atmosphere. The results show that all polymers have good thermal stability with 5% weight loss temperatures (T_d) above 300 °C, which is adequate for application in PSC devices.

Full text of DOI:10.1021/ma201501g

COMMUNICATION TO THE EDITOR
pubs.acs.org/Macromolecules
Synthesis and Characterization of Thieno[3,4-b]thiophene-Based
Copolymers Bearing 4-Substituted Phenyl Ester Pendants: Facile
Fine-Tuning of HOMO Energy Levels
Tomoyuki Yamamoto,† Tomoyuki Ikai,*,† Mitsuhiro Kuzuba,† Takayuki Kuwabara,†,‡ Katsuhiro Maeda,*,†,‡
Kohshin Takahashi,†,‡ and Shigeyoshi Kanoh†
†Graduate School of Natural Science and Technology and ‡Research Center for Sustainable Energy and Technology,
College of Science and Engineering, Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan
S Supporting Information
b
olymer solar cells (PSCs) have been attracting much interest
because of their advantages; they are low cost, lightweight,
In the present study, we designed and synthesized PTB-based
polymers bearing 4-substituted phenyl esters instead of the
previously reported alkyl esters as the pendants of the TT unit
and found that their HOMO energy levels could be readily fine-
tuned by changing the substituents on the phenyl groups.
The synthetic routes to the monomers and polymers are shown in
Scheme 1. Four novel TT monomers bearing various phenyl ester
groups (TT1ꢀTT4) were easily prepared in one pot from commer-
cially available phenol derivatives and the key precursor compound,
4,6-dibromothieno[3,4-b]thiophene-2-carboxylic acid (1), which
was synthesized by modified literature procedures.6b,10 These TT
monomers were obtained in good yields (>70%). Initially, copoly-
merization of TT1 and bis(trimethylstannyl) BDT comonomer
BDTa with 2-ethylhexyloxy substituents, by Stille cross-coupling,
was carried out using Pd(PPh3)4 as a catalyst in a toluene/N,N-dime-
thylformamide (DMF) mixture (4/1, v/v) to yield PTT1BDTa.6
However, the resulting PTT1BDTa exhibited very low solubility in
common organic solvents such as tetrahydrofuran (THF), chloro-
form, and chlorobenzene, even at high temperatures. To improve the
solubility of the polymer, a novel bis(trimethylstannyl) BDT bearing
longer branched alkyl side chains, 2-octyldodecyloxy groups (BDTb),
was synthesized and used as the comonomer instead of BDTa.
The results of the copolymerizations of TT1ꢀTT4 with BDTb
are summarized in Table 1 (runs 1ꢀ4). The resulting polymers were
purified by successive Soxhlet extractions with methanol (MeOH)
and hexane to remove byproducts and oligomers and subsequent
filtration through Celite to remove the metal catalyst. The four poly-
mers, PTT1BDTb, PTT2BDTb, PTT3BDTb, and PTT4BDTb,
were obtained in relatively good yields (60ꢀ85%). The introduction
of 2-octyldodecyloxy substituents on the BDT unit enhanced the
solubility of the resulting polymers, and the obtained polymers exhi-
bited good solubility in THF, chloroform, and chlorinated benzene.
For comparison, we also prepared the corresponding alkyl ester type
polymer PTT5BDTb (run 5 in Table 1) and the reported PTB-based
polymer PTT5BDTa (run 6 in Table 1).6b The molecular weights
(Mn) of the obtained polymers were determined by size-exclusion
chromatography (SEC) in THF. All the polymers have a similar mole-
cular weight, so the structural modification of the ester groups in the
TT units seems to have little effect on the polymerizability of these
P
and flexible and have the potential for the production of large-area
devices.1 Bulk heterojunction (BHJ) PSCs have become the most
successful device configurations. In such devices, conjugated poly-
mers, as an electron donor, and fullerene derivatives, as an electron
acceptor, form an interpenetrating network on the nanometer scale.2
For BHJ PSCs, regioregular poly(3-hexylthiophene) (P3HT) has
been extensively used as the electron-donor material because of its
solubility, stability, and compatibility with the typical electron
acceptor [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM).1,3
However, P3HT can only harvest less than ca. 23% of the available
solar photons because its absorption band is located at wavelengths
of less than 650 nm.4 Extensive research efforts have therefore been
devoted to developing novel conjugated polymers with low bandgaps
to better harvest the solar energy and improve the power-conversion
efficiencies (PCEs) of PSCs.4,5
Recently, Yu et al. reported π-conjugated polymers consisting
of alternating thieno[3,4-b]thiophene (TT) and benzo[1,2-b:4,5-
b0]dithiophene (BDT) units (PTB-based polymers), promising
donor materials for PSCs, whose absorption region reached ca.
800 nm as a result of the stabilization of the quinoidal resonance
structure in the polymer main chain by the thieno[3,4-b]thiophene
unit.6 The extended absorption of sunlight directly contributed to
increasing the short-circuit current density (Jsc), and these PSCs
exhibited high PCEs of over 5%, while the open-circuit voltages (Voc)
of the PSCs were relatively low, at∼0.6 V. It is known that the Voc of
a PSC is closely related to the difference between the highest
occupied molecular orbital (HOMO) of the donor component
and the lowest unoccupied molecular orbital (LUMO) of the
acceptor, and deepening the HOMO energy level of the donor
polymer is a potential way of developing highly efficient PSCs with a
high Voc.7 The following two strategies have been used to deepen the
HOMO energy level of PTB-based polymers: the introduction of a
fluorine atom, which is the atom with the highest electronegativity, in
the TT ring6b and the replacement of the alkyl ester pendants of the
TT unit with alkyl ketone pendants.8 These modifications reduced
the HOMO energy levels of the polymers from ꢀ5.01 to ꢀ5.12 eV,
and the PCEs were effectively improved from 5.15 to 6.10 and 6.58,
respectively. An excellent PCE of 7.73% was achieved by combining
the two strategies.9 However, the synthesis of PTB-based polymers
bearing a fluorine atom on the polymer main-chains is very time-
consuming, and further structural modifications are quite limited.
Received: July 1, 2011
Revised:
August 4, 2011
Published: August 16, 2011
r
2011 American Chemical Society
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dx.doi.org/10.1021/ma201501g Macromolecules 2011, 44, 6659–6662
|

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COMMUNICATION TO THE EDITOR
Scheme 1. Synthesis and Structures of Monomers and Polymers
Table 1. Polymerization Results and Thermal and Optical Properties of the Polymers
run
polymer
yield (%)a
Mn (104)b
PDIb
Td (°C)c
λmax (nm)
λonset (nm)
Eg (eV)d
HOMO (eV)e
LUMO (eV)f
1
2
3
4
5
6
PTT1BDTb
PTT2BDTb
PTT3BDTb
PTT4BDTb
PTT5BDTb
PTT5BDTa
62
60
85
82
77
71
5.9
6.1
7.1
6.7
6.1
4.7
3.0
3.6
3.3
2.3
2.4
2.7
313
316
315
313
316
326
692
686
682
684
671
675
761
755
748
751
736
758
1.63
1.64
1.66
1.65
1.68
1.63
ꢀ5.60
ꢀ5.39
ꢀ5.23
ꢀ5.12
ꢀ5.13
ꢀ5.01
ꢀ3.97
ꢀ3.75
ꢀ3.57
ꢀ3.47
ꢀ3.45
ꢀ3.38
a MeOH and hexane-insoluble part. b Determined by SEC (eluent: THF, PSt standard). c The 5% weight-loss temperatures with a heating rate of 10 °C/
min in N2. d Calculated from Eg = 1240/λonset. e Measured by photoelectron spectroscopy in air. f Calculated from LUMO = Eg + HOMO.
Figure 1. Absorption spectra of the polymers in chlorobenzene (a) and in the film state (b).
TT-based monomers. The thermal stability of these polymers was
investigated by thermal gravimetric analysis (TGA) under a nitrogen
atmosphere (Supporting Information, Figure S1). All polymers
showed good thermal stability with 5% weight loss temperatures (Td)
above 300 °C, which is adequate for application in PSC devices.
Figure 1 shows the absorption spectra of the polymers in
chlorobenzene and in the film state. The absorption spectra of the
polymers in solution are very similar to those in the solid state,
suggesting that the polymers possess a similar rigid-rod conformation
in both states. The absorption bands of the polymers with phenyl
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Macromolecules
COMMUNICATION TO THE EDITOR
2.0 ꢁ 10ꢀ5, and 2.7 ꢁ 10ꢀ5 cm2 Vꢀ1 sꢀ1, respectively, which are
comparable to that of PTT5BDTb bearing alkyl ester groups (3.9 ꢁ
10ꢀ5 cm2 Vꢀ1 ꢀ1). Replacement of the alkyl esters with phenyl
s
esters therefore seems to bring about only slight changes in the
hole mobility for this type of polymer. However, the hole mobilities
of these polymers were relatively low compared with that of
PTT5BDTa with 2-ethylhexyloxy side chains on the BDT unit
(8.4 ꢁ 10ꢀ5 cm2 Vꢀ1 sꢀ1) measured with our hole-only device.14
The bulkier branched side chains, 2-octyldodecyloxy groups, on the
BDT unit may increase the steric hindrance and make it difficult to
form efficient intermolecular πꢀπ-stacking, which is very important
in transporting the holes inside the polymer films. It is therefore
necessary to optimize the side-chain structures in consideration of
both the solubility and the charge transport properties.
In conclusion, a series of PTB-based polymers consisting of
alternating thieno[3,4-b]thiophene units bearing 4-substituted phen-
yl ester pendants and benzo[1,2-b:4,5-b0]dithiophene units were
synthesized, and their thermal and optical properties were investi-
gated. We found that the HOMO energy levels of the polymers could
be finely tuned in the range ꢀ5.12 to ꢀ5.60 eV by changing the
substituents on the phenyl ring, and there was a relatively good linear
correlation between the HOMO energy levels and the Hammett
substituent constants. Since PTT1BDTb and PTT2BDTb with
electron-withdrawing substituents possess deeper HOMO energy
levels, at ꢀ5.60 and ꢀ5.39 eV, respectively, compared with those
of previously reported PTB-based polymers,6b,8,9,15 the PSCs based
on these polymers are expected to show higher Voc values. Further
improvements in hole mobility by optimizing the side-chain struc-
tures on the benzodithiophene units should therefore yield promis-
ing results for realizing high-efficiency PSCs. Detailed investigations
of the photovoltaic properties of the PSCs produced using these
PTB-based polymers are currently in progress and will be reported
in due course.
Figure 2. Plots of HOMO energy levels of the polymers against the
Hammett constants σ of the substituents.
ester groups and those of PTT5BDTb were located at around 500ꢀ
800 nm; this was consistent with the spectra of previously reported
PTB-based polymers with alkyl ester groups, such as PTT5BDTa.6b
The optical bandgaps (Egopt = 1240/λonset) were estimated based on
the absorption onset wavelength (λonset) of the polymer films and are
summarized in Table 1. Compared with PTT5BDTb bearing an
alkyl ester group, the polymers bearing phenyl ester groups exhibited
somewhat narrower Egopt values, depending on the type of sub-
stituent. As the electron-withdrawing ability of the substituent
increases, the optical bandgaps tend to be slightly narrower. These
results mean that the phenyl ester groups can contribute to elonga-
tion of the π-conjugation length of the polymer main chains.
The HOMO energy levels of the polymers were estimated
with a photoelectron spectrometer by measuring the ionic potential
of the polymer films in air (Supporting Information, Figure S2), and
the LUMO energy levels were calculated from the values of the
HOMO energy level and the Egopt (LUMO = Egopt + HOMO); the
values are listed in Table 1. When electron-withdrawing groups such
as trifluoromethyl and fluoro groups were introduced on the phenyl
groups, the HOMO energy levels became deeper compared with
those for the nonsubstituted PTT3BDTb (ꢀ5.23 eV). In contrast,
electron-donating substituents such as a methoxy group raised the
HOMO energy level to ꢀ5.12 eV. To visualize the substituent effect,
the HOMO energy levels of the polymers are plotted against the
Hammett constants (σ)11 of the substituents in Figure 2. A relatively
good linear relationship is observed between the HOMO energy
levels and the σ values. Computational studies using density func-
tional theory (DFT) at the B3LYP/6-31G(d,p) level12 were addi-
tionally performed for the corresponding model compounds to eva-
luate the influence of these substituents on the HOMO energy levels
of the polymers (Supporting Information, Figure S15). The calcu-
lated HOMO energy levels for the model compounds also showed a
similar tendency to the experimental values for the polymers, al-
though the calculated values were rather higher than the experimental
ones (Supporting Information, Figure S16). These results suggest
that the substituents on the phenyl groups can perturb the electron
density of the main chain through an ester linkage, and one can finely
tune the HOMO energy levels of the polymers by selecting the
substituents on the phenyl groups.
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental details, NMR
b
spectra of all TT monomers and polymers, TGA curves, photo-
electron spectra, DFT-calculated frontier molecular orbitals, plots of
calculated HOMO energy levels against the Hammett constants,
and IꢀV curves for the hole-only devices. This material is available
free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: ikai@t.kanazawa-u.ac.jp (T.I.), maeda@t.kanazawa-u.ac.jp
(K.M.).
’ ACKNOWLEDGMENT
We thank Prof. T. Yamagishi (Kanazawa University) for permis-
sion to use the TGA instrument. This work was supported by
the Environment Research and Technology Development Fund
(B-0807) of the Ministry of the Environment, Japan.
The hole mobilities of these polymers were measured by a
method based on the space-charge-limited current (SCLC) model
using the corresponding hole-only devices.13 The hole mobilities of
PTT1BDTb, PTT2BDTb, PTT3BDTb, and PTT4BDTb bearing
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