Tuning the band gap of low-band-gap polyselenophenes and polythiophenes: The effect of the heteroatom

Article abstract of DOI:10.1021/cm102395v

A series of new low-band-gap thieno-or selenolo-fused polyselenophenes (P5 and P6) and selenolo-fused polythiophene (P4) (as well as previously reported thieno-fused polythiophene, P3) was prepared systematically by electropolymerization (P4-P6) and by solid-state polymerization (P3, P5 and P6). The 2,5-dibrominated monomers (3Br₂, 5Br₂, and 6Br ₂) undergo solid-state polymerization under slight heating and produce insoluble P3, P5, and P6 as black conducting powders. The spectroelectrochemically measured optical band gaps of P4-P6 films are 0.96, 0.72, and 0.76 eV, respectively. DFT calculations performed on P3-P6 provide excellent estimations of the experimental band gaps of these polymers. The band gap of the polyselenophenes (P5 and P6) is 0.2 eV lower than that of the corresponding polythiophenes (P3 and P4). We introduced a new scheme for band gap control in conjugated polymers by replacing the sulfur atom with a selenium atom in the main and/or peripheral ring, which leads to significant and predictable changes in the band gap of the polymers. This is due to the lower aromaticity of a selenophene ring compared to a thiophene ring. Thus, we have achieved band gap control in very low band gap (～0.7-1.0 eV) polymers through the use of different combinations of selenium and sulfur atoms in the main and peripheral rings.

Full text of DOI:10.1021/cm102395v

896 Chem. Mater. 2011, 23, 896–906
DOI:10.1021/cm102395v
Tuning the Band Gap of Low-Band-Gap Polyselenophenes and
Polythiophenes: The Effect of the Heteroatom†
Asit Patra,‡ Yair H. Wijsboom,‡ Gregory Leitus,§ and Michael Bendikov*,‡
§
‡Department of Organic Chemistry and Chemical Research Support Unit,
Weizmann Institute of Science, Rehovot 76100, Israel
Received August 19, 2010. Revised Manuscript Received December 16, 2010
A series of new low-band-gap thieno- or selenolo-fused polyselenophenes (P5 and P6) and
selenolo-fused polythiophene (P4) (as well as previously reported thieno-fused polythiophene, P3)
was prepared systematically by electropolymerization (P4-P6) and by solid-state polymerization
(P3, P5 and P6). The 2,5-dibrominated monomers (3Br2, 5Br2, and 6Br2) undergo solid-state
polymerization under slight heating and produce insoluble P3, P5, and P6 as black conducting
powders. The spectroelectrochemically measured optical band gaps of P4-P6 films are 0.96, 0.72,
and 0.76 eV, respectively. DFT calculations performed on P3-P6 provide excellent estimations of
the experimental band gaps of these polymers. The band gap of the polyselenophenes (P5 and P6) is
0.2 eV lower than that of the corresponding polythiophenes (P3 and P4). We introduced a new
scheme for band gap control in conjugated polymers by replacing the sulfur atom with a selenium
atom in the main and/or peripheral ring, which leads to significant and predictable changes in the
band gap of the polymers. This is due to the lower aromaticity of a selenophene ring compared to a
thiophene ring. Thus, we have achieved band gap control in very low band gap (∼0.7-1.0 eV)
polymers through the use of different combinations of selenium and sulfur atoms in the main and
peripheral rings.
Introduction
The band gap of conjugated polymers (which are build
from aromatic units) depends (among other factors) on
the degree of a quinoid or aromatic character of the back-
bone. A powerful approach to designing low-band-gap
polymers is to use the donor-acceptor concept.2,5-7
Another very important strategy for designing low-
band-gap conducting polymers is to incorporate fused
aromatic ring(s) into the monomer skeleton to enhance
the quinoid character of the polymer backbone.8-10 The
first and classical example of the later strategy is the
synthesis of poly(isothianaphthene) (PITN).8,11 The aro-
maticity of the fused benzo or heterocyclic ring enforces a
quinoid structure on the backbones, which leads to a low
Conjugated polymers have attracted considerable in-
terest because of their tunable electronic properties and
wide range of possible applications.1 Special attention has
been devoted to low-band-gap conjugated polymers2
originally because of their unique electronic properties
and recently on account of their ability to efficiently
absorb solar light and the promise they show as materials
to improve photovoltaic cells.3,4
† Accepted as part of the “Special Issue on π-Functional Materials”.
*Corresponding author. E-mail: michael.bendikov@weizmann.ac.il.
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pubs.acs.org/cm
Published on Web 01/11/2011
2011 American Chemical Society

Article
Chem. Mater., Vol. 23, No. 3, 2011 897
band gap and a greater tendency to maintain planarity.
However, PITN is not planar12,13 because of repulsion
between the sulfur atom and hydrogen atom on the
phenyl ring and, consequently, its band gap is above
1.0 eV. Another example is thieno-fused thiophenes,14
such as poly(2-phenylthieno[3,4-b]thiophene) (P1)15 and
poly(2-decylthieno[3,4-b]thiophene) (P2),16 which are
expected to be planar. Using this strategy, Sotzing and
co-workers reported two unsubstituted thieno- and furan-
fused polythiophene-based polymers, poly(thieno[3,4-b]-
thiophene) (PT34bT; P3)17,18 and poly(thieno[3,4-b]furan)
(PT34bF)19 having low band gaps of 0.85 and 1.03 eV,
respectively. After thismanuscript was submitted, we came
across the article describing a new synthetic route for
monomers thieno[3,4-b]thiophene (3), its alkyl deivatives,
selenolo[3,4-b]thiophene(5) and thieno[3,4-b]furan in mul-
tistep synthesis,19c which is different from the one used in
our study. In the last year, the thieno[3,4-b]thiophene (3)
unit has attracted significant attention as a building block
for the construction of low-band-gap polymers for solar-
cell applications and the highest efficiency for an organic
solar cell, 6.77%, was achieved using a substituted thieno-
[3,4-b]thiophene building block.4c,20
PEDOT,21 and its derivatives) are well-studied and are
finding commercial applications. Recently, we reported22,23
poly(3,4-ethylenedioxyselenophene) (PEDOS), a selenium
analogue of PEDOT, which shows a low band gap, very
high stability in the oxidized state, and a well-defined
spectroelectrochemistry. PEDOS (Eg=1.4 eV) has a band
gap 0.2 eV lower than that of PEDOT (Eg = 1.6 eV).22
PEDOS derivatives also compare favorably with PEDOT
derivatives in some cases.24,25 Other polyselenophenes have
been reported recently.23,26,27 As a class, polyselenophenes
are expected to have advantages over polythiophenes. For
example, interchain charge transfer should be facilitated by
intermolecular Se Se contacts. Furthermore, both
3 3 3
experimental22,25 and theoretical studies28,29 indicate that
polyselenophenes should have a lower band gap, a more
quinoid character, and importantly, should be more diffi-
cult to twist25,29 than polythiophenes. Consequently, poly-
selenophenes are attractive candidates for the synthesis of
low-band-gap conjugated polymers. Selenophene also has a
lower aromaticity than thiophene,30 so a fused selenophene
ring should enforce a less quinoid structure on the polymer
backbone. We propose that these properties of polyseleno-
phenes and the fused selenophene ring can also be used for
band gap control purposes in conjugated polymers.
Inspired by the high stability, low band gap, and pro-
mising properties of PEDOS22 and its derivatives,23-25
we decided to extend our study to the selenium analogues
of known P317 in order to achieve lowering of the band
gap and a more effective band gap control in conjugated
polymers. Here, we introduce a new method for band gap
control in very low band gap polymers. Our method takes
advantage of the different aromaticity of the selenophene
ring versus the thiophene ring to enable band gap tuning
in the range of 0.7-1.0 eV. We also present an efficient
synthetic method, characterization, and comparative
DFT calculations for new low-band-gap polymers, poly-
(selenolo[2,3-c]thiophene) (P4), poly(selenolo[3,4-b]-
thiophene) (P5), and poly(selenolo[3,4-b]selenophene)
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heterocyclic ring (e.g., poly(3,4-ethylenedioxythiophene),
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898 Chem. Mater., Vol. 23, No. 3, 2011
Patra et al.
(P6).31,32 We also report synthesis of P3, P5, and P6 via
solid-state polymerization (SSP) and show that polysele-
nophenes are attractive candidates for very low band gap
conjugated polymers.
at 100 mV/s and 5 was polymerized in TBAPF6/DCM at 50 mV/s
in cyclic voltammetry (CV) mode. Films were washed with
dichloromethane and stored under vacuum until needed. Spectro-
electrochemistry experiments were performed in 0.1 M TBAPC in
PC. Under these conditions, Fc/Fcþ=0.34 V vs SCE.
Synthesis of 2,3,4,5-Tetrabromoselenophene. Adapted from
the published procedure with modification.33 Bromine (94 g) in
CHCl3 (60 mL) was added dropwise to a stirred solution of
selenophene (15 g) in CHCl3 (60 mL) and AcOH (10 mL) at 0 °C
over the course of 1 h. The reaction mixture was warmed to
room temperature and stirred for 12 h, and then heated to 70 °C
for 5 h. Upon completion of the reaction, the mixture was
allowed to cool to room temperature and transferred to a large
beaker. Excess bromine was evaporated at room temperature
and the resulting mixture was diluted with CHCl3 (200 mL). The
organic phase was successively washed with water (80 mL),
dilute NaOH solution (50 mL), and brine (60 mL), and then
concentrated. The crude crystalline product was further purified
by column chromatography using hexane as an eluent to give a
white crystalline solid (46.5 g, 91% yield). mp. 98-99 °C; 13C
NMR (62.5 MHz, CDCl3) δ 117.9, 112.2.
Experimental Section
General Details. All reagents were purchased from Sigma-
Aldrich in reagent grade and were used without purification
unless noted. 1H NMR and 13C NMR spectra were recorded in
CDCl3 on a 250 MHz spectrometer with tetramethylsilane
(TMS) as the external standard. 77Se NMR spectra were recorded
on a 500 MHz NMR in CDCl3 with chemical shifts given with
respect to SeMe2 for which δ77Se = 0 ppm (selenophene was used
as an external standard for which δ77Se = 605 ppm). Differential
scanning calorimetry (DSC) measurements were performed on a
TA Q200 DSC instrument. High-resolution mass spectra were
measured on a Waters Micromass GCT Premier mass spectrom-
eter using field desorption (FD) ionization. Diethyl ether was
distilled from sodium/benzophenone under an atmosphere of dry
argon. In most of the column chromatographic separations,
hexane was used as eluents. Columns were prepared with silica
gel (60-230 mesh).
Electrochemistry. All electrochemical measurements were
performed using a PAR 263A potentiostat in a standard three-
electrode, one compartment configuration equipped with Ag/
AgCl wire, Pt wire, and Pt disk electrodes (diameter 1.6 mm) as
the pseudo reference electrode, counter electrode, and working
electrode, respectively. Most electrochemical experiments were
performed in anhydrous acetonitrile (ACN) or anhydrous di-
chloromethane (DCM) and propylene carbonate (PC) with 0.1 M
tetrabutylamonium perchlorate (TBAPC, Fluka) or tetrabutyla-
monium hexafluorophosphate (TBAPF6, Fluka) (dried under
vacuum) as the supporting electrolyte. All electrochemical solu-
tions were purged with dry N2 for 15 min. Ferrocene powder
(Fluka) was used to establish an electrochemical internal refer-
ence for all measurements as ferrocene/ferrocenium redox couple
(Fc/Fcþ=0.37 V vs saturated calomel electrode (SCE)). Mono-
mer concentration was ∼0.01 M.
Synthesis of 3,4-Dibromoselenophene. n-BuLi (1.6 M in hex-
anes, 29.5 mL, 47.0 mmol) was added dropwise to a stirred
solution of 2,3,4,5-tetrabromoselenophene (10 g, 22.4 mmol) in
anhydrous ether (80 mL) under a nitrogen atmosphere at -78 °C
over the course of 30 min, and the mixture was stirred for another
1.5 h at that temperature. Water (5 mL) was added slowly to
the reaction mixture, which was then allowed to warm to room
temperature. The mixture was then diluted with water (100 mL),
extracted with ether (3 ꢀ 75 mL), and the combined organic
phases werewashed with water (50mL), brine, driedover MgSO4,
and concentrated. The resulting colorless oil was purified by
column chromatography using hexane as an eluent to give 3,
1
4-dibromoselenophene (5.3 g, 82% yield) as a colorless oil. H
NMR (250 MHz, CDCl3) δ 7.93; 13C NMR(62.5 MHz, CDCl3) δ
127.4, 114.3. Selective debromination of 2,3,4,5-tetrabromosele-
nophene with 2 equiv. of n-BuLi to produce 3,4-dibromoseleno-
phene was mentioned inref 34, although synthetic details werenot
reported.
General Procedure for Synthesis of Compounds 7 and 8. This
procedure was adapted from the published procedure for com-
pound 7.35 3,4-Dibromothiophene or 3,4-dibromoselenophene
(8 mmol) was added to a stirred solution of trimethylsilylacety-
lene (813 mg. 1.17 mL, 8.30 mmol), PdCl2 (60 mg), and PPh3
(40 mg) in diethylamine (8 mL) under an inert atmosphere. To
this stirred mixture was added CuI (200 mg) and the mixture was
then warmed to 50 °C. After 5 h, the temperature of the reaction
mixture increased to 75 °C and stirring was continued for an
additional 2 h. After cooling to room temperature, 100 mL of
water was added. The aqueous layer was extracted with ether (3 ꢀ
50 mL), and the combined organic layers were washed with brine
andwater. The resulting ethersolutionwas driedover MgSO4 and
evaporated under a vacuum. The crude product was purified by
column chromatography using hexane as an eluent on silica gel to
obtain a pure product.
Spectroelectrochemistry. Spectra were taken in a UV-vis-
NIR quartz optical cell (100-QX, Hellma) with a JASCO V-570
UV-vis-NIR spectrophotometer. Monomers were polymer-
ized on an indium tin oxide (ITO) working electrode (5-15 Ω/0,
Delta Technologies, Stillwater). Films were electrodeposited as
follows: monomers 4 and 6 were polymerized in TBAPC/ACN
(31) After this work was completed, compounds 4-6 and their poly-
merization were disclosed in patent applications: (a) Zahn, S.;
Costello, C. A.; McLaws, M. US Pat. No. 18348, 2009. (b) Zahn,
S. US Pat. No. 14693, 2009. (c) Zahn, S.; Costello, C. A.; McLaws, M.
EP Pat. No. 2014664, 2009. The electrochemical polymerization of
monomer 4 and the optical band gap of P4 were reported in patent
applications: (d) Zahn, S. U.S. Pat. No. 140219, 2009. (e) Zahn, S. EP
Pat. No. 2014665, 2009. No other polymer characterization was
provided in these patent applications.
3-Bromo-4-(trimethylsilyl)ethynylthiophene (7) (refs 35 and
36). Colorless oil. 1H NMR (250 MHz, CDCl3) δ 7.47 (d, J =
3.2 Hz, 1H), 7.22 (d, J = 3.2 Hz, 1H), 0.26 (s, 9H). 13C NMR
(32) The formal names of these materials follow the rules of IUPAC
nomenclature and therefore relate first to the heaviest atom (Se)
regardless of if it is positioned on the backbone or a peripheral ring.
By contrast, from a materials chemistry perspective, the polymers
are defined by their backbones and, thus, we refer to P4 as a
polythiophene and P5 and P6 as polyselenophenes.
(33) Tung, D. T.; Villinger, A.; Langer, P. Adv. Synth. Catal. 2008, 350,
2109–2117.
(34) Ketcham, R.; Hoernfeldt, A.-B.; Gronowitz, S. J. Org. Chem. 1984,
49, 1117–1119.
(35) Brandsma, L.; Verkruijsse, H. D. Synth. Commun. 1990, 20, 2275–
2277.

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Chem. Mater., Vol. 23, No. 3, 2011 899
(62.5 MHz, CDCl3) δ 129.7, 124.6, 122.8, 113.9, 97.9, 97.8, -0.9.
HRMS for C9H11BrSSi [Mþ]: calcd, 257.9534 and 259.9513;
found, 257.9527 and 259.9507.
(hexanes) afforded 4,6-dibromo derivatives (3Br2-6Br2). Single
crystals of 3Br2 and 4Br2 were obtained by slow evaporation of
chloroform solution with a few drops of hexane (about 5%) in a
refrigerator (þ4 °C).
3-Bromo-4-(trimethylsilyl)ethynylselenophene (8) (ref 31).
1
Colorless oil. H NMR (250 MHz, CDCl3) δ 8.15 (d, J = 3.0
4,6-Dibromothieno[3,4-b]thiophene (3Br2). White crystalline
solid. 93% yield. M.p. 59-60 °C. 1H NMR (250 MHz, CDCl3) δ
7.35 (d, J = 5.6 Hz, 1H), 6.78 (d, J = 5.6 Hz, 1H). 13C NMR
(62.5 MHz, CDCl3) δ 147.1, 140.3, 133.9, 117.2, 97.1, 96.4.
4,6-Dibromoselenolo[2,3-c]thiophene (4Br2). White crystalline
solid. 90% yield. M.p. 52-53 °C. 1H NMR (250 MHz, CDCl3) δ
7.79 (d, J = 6 Hz, 1H), 7.07 (d, J = 6 Hz, 1H). 13C NMR (62.5
MHz, CDCl3) δ 148.9, 137.6, 133.8, 120.6, 100.3, 99.4. HRMS
for C6H2Br2SSe [Mþ]: calcd, 343.7409; found, 343.7387.
4,6-Dibromoselenolo[3,4-b]thiophene (5Br2). White crystalline
solid. 41% yield. M.p. 72-73 °C. 1H NMR (250 MHz, CDCl3) δ
7.43 (d, J = 5.6 Hz, 1H), 6.68 (d, J = 5.6 Hz, 1H). 13C NMR
(62.5 MHz, CDCl3) δ 149.5, 142.4, 133.8, 117.5, 101.6, 99.2.
HRMS for C6H2Br2SSe [Mþ]: calcd, 343.7409 and 345.7387;
found, 343.7408 and 345.7389.
Hz, 1H), 7.85 (d, J = 3.0 Hz, 1H), 0.26 (s, 9H). 13C NMR (62.5
MHz, CDCl3) δ 135.5, 127.1, 126.5, 114.2, 99.9, 96.9, -0.3.
General Procedure for Synthesis of Compounds 3-6. Adapted
from the published procedure for compound 3.17b,35 n-BuLi
(1.6 M in hexanes, 3.28 mmol) was added dropwise to a stirred
solution of compound 7 or 8 (3.28 mmol) in dry ether (40 mL)
at -78 °C (acetone/dry ice bath) under an inert atmosphere. The
resulting mixture was stirred at -78 °C for 1 h, after which sulfur
powder or elemental selenium (1.0 equiv.) was added to it. After
these additions, the solution was stirred for 1 h at -78 °C, and
the reaction mixture was brought to -10 °C over a period of 1 h
and further stirred for 30 min at -10 °C to fully dissolve the
sulfur or selenium. Then the resulting reaction mixture was
added to a separatory funnel containing a 100 mL ice and brine
mixture held at 0 °C. The extraction process was completed
within 1 min at 0 °C. The organic layer was washed again with
ice cold water (50 mL). The resulting aqueous part was slowly
heated to 70 °C for 1 h. After cooling, the aqueous layer was
extracted with ether (3 ꢀ 25 mL) and the combined organic
layers were washed with brine and water. The resulting ether
solution was dried over MgSO4 and evaporated under vacuum.
The crude product was purified by column chromatography
using hexane as an eluent on silica gel to obtain a pure product.
Thieno[3,4-b]thiophene (3) (ref 17). Colorless oil. 63% yield.
1H NMR (250 MHz, CDCl3) δ 7.34 (d, J = 5.7 Hz, 1H), 7.35 (d,
J = 2.2 Hz, 1H), 7.24 (dd, J = 2.5 Hz, 0.7 Hz, 1H), 6.92 (dd, J =
5.5 Hz, 0.7 Hz, 1H).
4,6-Dibromoselenolo[3,4-b]selenophene (6Br2). White crystal-
line solid. 82% yield. M.p. 69-70 °C. 1H NMR (250 MHz,
CDCl3) δ 7.85 (d, J = 6 Hz, 1H), 6.99 (d, J = 6 Hz, 1H). 13
C
NMR (62.5 MHz, CDCl3) δ 151.2, 139.6, 133.2, 121.1, 104.1,
102.2.
Solid-State Polymerization (SSP). In a typical experimental
procedure, a dibromo compound (one of 3Br2-6Br2) was
placed in a 50 mL round-bottomed flask closed with a stopper.
The flask was heated to a temperature that was at least 5-10 °C
below the compound’s melting point for a few hours (see Table 1
for details), during which the original white color of the dibromo
compound turned black (except 4Br2). The appearance of
brown bromine vapor in the flask was indicative of the progress
of the SSP. After completion of the reaction, the resulting black
polymer was collected to afford the bromine-doped polymers.
Compounds 3Br2, 5Br2, and 6Br2 were polymerized using this
SSP method to obtain P3, P5, and P6, respectively. Compound
4Br2 does not undergo SSP when heated to below its melting
point (45 °C) for about 1 week. The conductivity of the as-
obtained bromine doped polymers was measured in a pressed
pellet by the two-probe method.
Selenolo[3,4-b]thiophene (4). Colorless oil; 61% yield; 1H
NMR (250 MHz, CDCl3) δ 7.76 (d, J = 6.0 Hz, 1H), 7.41 (d,
J=2.7 Hz, 1H), 7.26 (dd, J = 2.7 Hz, 0.7 Hz, 1H), 7.17 (dd, J=
6.0 Hz, 0.7 Hz, 1H). 13C NMR (62.5 MHz, CDCl3) δ 149.8,
135.7, 132.0, 120.3, 114.4, 114.2. 77Se NMR (CDCl3) δ= 434 ppm.
HRMS for C6H4SSe [Mþ]: calcd, 187.9199; found, 187.9194.
Selenolo[2,3-c]thiophene (5). Colorless oil; 51% yield 1H
NMR (250 MHz, CDCl3) δ 8.01 (d, J = 2.3 Hz, 1H), 7.89
(dd, J = 2.3 Hz, 0.8 Hz, 1H), 7.43 (d, J = 5.8 Hz, 1H), 6.79 (dd,
J = 5.9 Hz, 0.8 Hz, 1H). 13C NMR (62.5 MHz, CDCl3) δ 150.2,
141.2, 132.6, 117.4, 117.2, 114.8. 77Se NMR (CDCl3) δ=737
ppm. HRMS for C6H4SSe [Mþ]: calcd, 187.9199; found,
187.9205.
Details of Computations. All calculations were carried out
using the Gaussian 03 program.37 All molecules and polymers
were fully optimized without symmetry constraints using a
hybrid density functional method,38 Becke’s three-parameter
exchange functional combined with the LYP correlation func-
tional (B3LYP)39 and with the 6-31G(d) basis set (B3LYP/
6-31G(d)). The calculations for polymers were performed using
Periodic Boundary Conditions (PBC) as implemented in Gauss-
ian 03.40 The polymer unit cell consists of two monomer units
oriented anti to each other. It has been shown that band gaps
predicted for conducting polymers using the hybrid B3LYP
functional coupled with PBC are in excellent agreement with the
experimental values.28b,41
Selenolo[3,4-b]selenophene (6). Low melting point white solid;
57% yield; 1H NMR (250 MHz, CDCl3) δ 8.08 (d, J = 2.25 Hz,
1H), 7.90 (dd, J = 0.7 Hz, 2.25 Hz, 1H), 7.84 (d, J = 6 Hz, 1H),
7.04 (dd, J = 6 Hz, 0.7 Hz, 1H). 13C NMR (125 MHz, CDCl3) δ
152.3, 137.0, 131.5, 120.8, 119.9, 118.0. 77Se NMR (CDCl3) δ =
736, 434 ppm.
General Procedure for Bromination of 3-6 to Obtain Dibro-
mo-Derivatives 3Br2-6Br2. NBS (4 mmol; recrystallized from
hot water) was added to a stirred solution of compounds 3-6
(2 mmol) in CHCl3 at 0 °C. The resulting reaction mixture was con-
tinuously stirred at 0 °C for 20 min. The reaction was followed
by TLC and, upon completion of the reaction, the mixture was
diluted with water (100 mL). The resulting aqueous layer was
extracted with CHCl3 (3ꢀ40 mL). The combined organic phases
were washed with water, brine, and concentrated. Purification
of the crude residue by column chromatography on silica gel
(37) All calculations were performed using the Gaussian 03 program.
Frisch, M. J.et al. Gaussian 03, Revision C.02; Gaussian, Inc.:
Wallingford, CT, 2004. See the Supporting Information for complete
citation.
(38) (a) Parr, R. G.; Yang, W. Density-Functional Theory of Atoms and
Molecules; Oxford University Press: New York, 1989. (b) Koch, W.;
Holthausen, M. C. A Chemist’s Guide to Density Functional Theory;
Wiley-VCH: New York, 2000.
(39) (a) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (b)
Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(36) Arnanz, A.; Marcos, M.-L.; Delgado, S.; Gonzalez-Velasco, J.;
Moreno, C. J. Organomet. Chem. 2008, 693, 3457–3470.
(40) (a) Kudin, K. N.; Scuseria, G. E. Chem. Phys. Lett. 1998, 289, 611.
(b) Kudin, K. N.; Scuseria, G. E. Phys. Rev. B 2000, 61, 16440.

900 Chem. Mater., Vol. 23, No. 3, 2011
Table 1. Conditions for Solid-State Polymerizations (SSP) and Conductivities for As-Prepared P3, P5, and P6
Patra et al.
Scheme 1. Synthesis of Compounds 3-6 and 3Br2-6Br2
Results and Discussion
of NBS in CHCl3 at 0 °C led to the formation of 5Br2 with
a relatively low yield (41%). Bromination of 5 was per-
formed also at -28 °C; however, the product was again
obtained with a relatively low yield. The low yield from
the bromination of 5 may be due to partial polymeriza-
tion of 5Br2 during the reaction, and indeed 5Br2 poly-
merizes easily under SSP conditions (see below). Unlike 5,
bromination of 6 with 2 equiv. of NBS gave compound
6Br2 with a good yield (82%).
Absorption Spectroscopy. The electronic spectra of
monomers 3-6 were obtained and compared with the
results of DFT calculations. The UV-vis spectra of 3-6
(Figure 1) consist of three absorption peaks. Peaks at
around 234-249 nm correspond to the absorption of the
main thiophene and selenophene rings, with the seleno-
phene ring absorbing at a longer wavelength than the
thiophene ring. We note that parent thiophene absorbs at
Synthesis of the Monomers. The known 3-bromo-
4-(trimethylsilyl)ethynylthiophene (7) and 3 were synthe-
sized according to the adapted literature procedure17b,35
from commercially available 3,4-dibromothiophene. The
selenium analogue of 7, 3-bromo-4-(trimethylsilyl)ethy-
nylselenophene (8), was obtained by the palladium cata-
lyzed Sonogashira coupling reaction of 3,4-dibromosele-
nophene with trimethylsilylacetylene in the presence of
CuI and diethylamine in toluene at 50 °C. The peri-
pheral42 heterocyclic ring was constructed by the lithia-
tion of 7 or 8 with 1 equivalent of n-BuLi in ether followed
by the addition of elemental sulfur or selenium to produce
the thiolate or selenolate anion that, after cyclization,
gives 3-6 (Scheme 1).
Bromination of 3 with 2 equiv. of NBS in CHCl3 at 0 °C
gives 4,6-dibromothieno[3,4-b]thiophene (3Br2) as the
sole product with a 93% yield. Two doublets at δ 7.35
and 6.78 with a coupling constant J = 5.6 Hz correspond-
ing to the 2,3-hydrogens in the peripheral ring supports
the interpretation that bromination occurs selectively on
the main42 ring. The structure of 3Br2 was also confirmed
by X-ray analysis (see below). Bromination of 4 with 2
equivalents of NBS furnished 4Br2 with a 90% yield
(Scheme 1). Interestingly, bromination of 5 with 2 equiv.
λ
max=232 nm and parent selenophene absorbs at λmax=
249 nm. Compounds 3-6 have two additional weaker
absorption peaks at ∼266-277 nm and at ∼293-314 nm,
which apparently originate from the interaction between
the two heterocyclic rings. Again, these peaks are at
longer wavelengths for compounds 5 and 6, which have
a selenium atom in the main ring, than for compounds 3
and 4, which have a sulfur atom in the main ring. The
presence of three absorption peaks in the region of
220-320 nm for compounds 3-6 is also nicely repro-
duced by TD-DFT calculations. The calculated (TD-
B3LYP/6-311þG(2df,p)//B3LYP/6-31G(d)) absorption
peaks for these compounds (see the Supporting Informa-
tion; Table S1) are in excellent agreement with the experi-
mental values.
(41) (a) Pino, R.; Scuseria, G. E. J. Chem. Phys. 2004, 121, 8113. (b)
Cao, H.; Ma, J.; Zhang, G.; Jiang, Y. Macromolecules 2005, 38,
1123. (c) Kertesz, M.; Choi, C. H.; Yang, S. Chem. Rev. 2005, 105,
3448.
(42) Throughout the paper, we refer to the ring that undergoes poly-
merization (bottom ring in Scheme 1) as the main ring and the ring
that does not undergo polymerization (top ring in Scheme 1) as the
peripheral or annulated ring.

Article
Electropolymerization. Monomers 4, 5, and 6 were
Chem. Mater., Vol. 23, No. 3, 2011 901
that the polymers are electroactive and stable during first
few cycles at different scan rates (insets in Figure 2).
Spectroelectrochemistry. Spectroelectrochemical stud-
ies of the electrochemically produced polymers, P4-P6,
were performed to evaluate their electronic properties
and band gaps and, consequently, to elucidate the effect
of the aromaticity of the annulated heterocyclic ring on
the electronic properties of the conjugated polymers. The
spectroelectrochemistry of P4-P6 films obtained on an
ITO electrode are shown in Figure 3 (in neutral and
p-doped states). For P4 and P6, the measurements (electr-
opolymerization and spectroelectrochemistry) were per-
formed in two different solvents (acetonitrile and PC) and
the spectroelectrochemical results are very similar in both
solvents (Figure 3a,b,d,e). For P5, the measurements
were performed in dichloromethane (Figure 3c) because
it does not electrodeposit sufficiently in either acetonitrile
or PC. The spectroelectrochemically measured optical
band gap (extracted from the onset on the π-π*
transition) of P4, which has a polythiophene backbone
and a selenium atom in the peripheral ring, is 0.96 eV
(∼1290 nm) (Figure 3a,b), which is comparable to the
reported optical band gap (0.9 eV).31 For comparison,
the optical band gap of P3, in which the heteroatom in the
peripheral ring is sulfur, is 0.85 eV.17a P4 is deep-blue in
the neutral state and transmissive gray in the p-doped
state. Polyselenophene-based P5, which has a sulfur
heteroatom in the peripheral ring, is predicted to have a
very low band gap of about ∼0.6 eV (after empirical
correction of computational results, see computational
section below) and the electrodeposited film has an
polymerized electrochemically by repetitive cycles over
the anodic redox active range of the compounds (Figure 2
for compounds 4-6 and Figure S1 in the Supporting
Information for the known17 electropolymerization of
compound 3). All compounds show an irreversible anodic
peak corresponding to the oxidation of the monomers.
Electropolymerization of thiophene 4 produces films that
are quite stable electrochemically; however, electropoly-
merization of selenophenes 5 and 6 leads to less electro-
chemically stable films (electrochemical polymerization
of monomer 4 was reported to provide P4).31 For com-
parison, polyselenophenes of the PEDOS type usually
produce films as electrochemically stable as films of
PEDOT type polymers.22,24 The CVs of P4, P5, and P6
recorded in a monomer-free electrolytic medium show
Figure 1. Normalized UV-vis absorption spectra of monomers 3-6 in
acetonitrile.
Figure 2. Multisweep electropolymerization of (a) monomer 4 on Pt electrode in TBAPC/acetonitrile; (b) monomer 5 on Pt electrode in TBAPF6/
dichloromethane;(c) monomer6onPtelectrode in TBAPC/acetonitrile. The insetsshowthe CVfor thepolymersinmonomer-freesolutionatdifferentscan
rates between 25 to 150 mV/s.

902 Chem. Mater., Vol. 23, No. 3, 2011
Patra et al.
Figure 3. Spectroelectrochemistry of films of (a) P4 in acetonitrile, (b) P4 in PC, (c) P5 in DCM, (d) P6 in acetonitrile, and (e) P6 in PC.
optical band gap is 0.72 eV (Figure 3c). Although optical
band gap of P5 is slightly higher than predicted, P5 is a
very low band gap polymer. The spectroelectrochemistry
of polyselenophene-based P6, in which the heteroatom in
the peripheral ring is selenium, is represented in panels d
and e in Figure 3 in acetonitrile and PC, respectively. The
absorption peak is estimated to onset at 1620 nm, which
corresponds to an optical band gap of 0.76 eV in excellent
agreement with calculated values (see below) and makes
P6 a very low band gap polymer. P5 and P6 in the
electrochemically p-doped state do not have strong ab-
sorption in the near IR range, in contrast to their poly-
thiophene analogues, P317 and P4, and to their poly-
selenophene analogue PEDOS22 and its derivatives.24
Using the optically measured band gap of P3-P6 we
have shown that systematic changes in the band gap can
be achieved by replacing the sulfur atom with a selenium
atom in the main and peripheral rings,42 which enables
band gap control in this family of low-band-gap conduct-
ing polymers. We can summarize that replacing the sulfur
atom with a selenium atom in the main heterocyclic ring
(i.e., switching from a polythiophene to a polyseleno-
phene backbone) lowers the band gap; however, replacing
the sulfur atom with a selenium atom in the peripheral
heterocyclic ring increases the band gap because of the
reduced aromatic character of selenophene compared to
thiophene (e.g., P4 and P6 have higher band gaps than P3
and P5, respectively). The combination of these two trends
allows the band gap to be tuned. Importantly, the experi-
mentally obtained band gaps of P4 and P6 (measured from
the onset of the UV-vis-NIR absorption spectra of the
polymers in the neutral state) are in excellent agreement (to
within 0.1 eV) with the calculated values (see below).
N-Type Doping Behavior. Polythiophenes are known to
be more stable under p-type doping than under n-type
doping and most polythiophene-based conducting poly-
mers do not show reversible n-type behavior.43 Spectro-
electrochemistry of P4 under n-type doping is shown in
(43) Holze, R. In Handbook of Advanced Electronic and Photonic
Materials and Devices; Nalwa, H. S, Ed.; Academic Press: San Diego,
CA, 2001; Chapter 6, pp 209-301.

Article
Chem. Mater., Vol. 23, No. 3, 2011 903
Figure 4. (a) Spectroelectrochemistry of P4 film on ITO showing n-type doping. CV of p- and n-type behavior of (b) P4, (c) P5, and (d) P6.
Figure 4a. P4 can be fully oxidized at 0.8 V (Figure 3a and
3b) and then reduced stepwise to -0.8 V to the neutral
state. Further reduction to -1.6 V resulted in n-type
doping that resembled the p-type in terms of absorption
with relatively strong absorption in the NIR range. To
the best of our knowledge, this is the first spectroelec-
trochemical study of n-type doping behavior of such type
of polymers. Selenophene-based P5 and P6 show irrever-
sible n-type behavior and are not as stable under repeated
CV cycles in the n-type region.
Chemical Solid-State Polymerization. Thiophene-based
low-band-gap conductive polymers are well studied and
their electropolymerization was frequently reported.2,9
However, limited number of reports on chemical polymer-
izations leading to low-band-gap conductive polymers
have appeared. Pomerantz et al., reported16 the chemical
polymerization of P2 with 1 equiv. of FeCl3 to produce a
soluble polymer, whereas polymerization with 4 equiv. of
FeCl3 produces an insoluble polymer with a relatively low
conductivity of 7.2 ꢀ 10-2 S cm-1 and band gap of 0.92-
0.98 eV. More recently, it was reported that chemical
polymerization of 3 with FeCl3 produces conductive P3
(conductivity of 2 S cm-1) in the oxidized state.18b Oxida-
tive chemical polymerizations of monomers 3-6 are
expected to produce defects in the polymer chains because
of the possibility for branching through the peripheral ring,
which may lead to two-dimensional propagation of the
polymer chains and decrease the conjugation. To obtain
highly conjugated and well-ordered conductive polymers
and avoid cross-linkages, we used a recently developed
solid-state polymerization (SSP)44,45 method that requires
mild polymerization conditions and is especially useful for
the preparation22 of polyselenophenes because of their
lower polymerization temperature (Scheme 2).
Compound 3Br2 was polymerized via SSP under slight
heating (50 °C) for 2 days (Table 1). The resulting
bromine-doped black polythiophene P3 is completely
insoluble in common organic solvents and as-obtained
P3 has a relatively high conductivity of ∼6 S cm-1 (mea-
sured in a pressed pellet by the two probe method).46,47
The conductivity value is similar to that of similarly
prepared PEDOT and PEDOS measured by the same
method and it is even slightly higher than the previously
reported conductivity of chemically polymerized P3 (2 S
cm-1).18b Our attempt to use SSP for the preparation
of P4 failed because of the low melting point of 4Br2
(52 °C). Selenophene-based dibromo-compounds 5Br2
and 6Br2 undergo SSP more easily than thiophene-based
dibromo-compound 3Br2. Compounds 5Br2 and 6Br2
were successfully polymerized using the solid-state meth-
od by slight heating at 45 °C for 4-8 h to obtain black
polymers P5 and P6, respectively (Table 1). The resulting
bromine-doped polymers are also completely insoluble in
common organic solvents.48 However, P5 and P6 have
(44) (a) Meng, H.; Perepichka, D. F.; Wudl, F. Angew. Chem., Int. Ed.
2003, 42, 658–661. (b) Meng, H.; Perepichka, D. F.; Bendikov, M.;
Wudl, F.; Pan, G. Z.; Yu, W.; Dong, W.; Brown, S. J. Am. Chem.
Soc. 2003, 125, 15151–15162. (c) Spencer, H. J.; Berridge, R.;
Crouch, D. J.; Wright, S. P.; Giles, M.; McCulloch, I.; Coles,
S. J.; Hursthouse, M. B.; Skabara, P. J. J. Mater. Chem. 2003, 13,
2075–2077.
ꢀ
ꢀ
(45) Lepeltier, M.; Hiltz, J.; Lockwood, T.; Belanger-Gariepy, F.;
Perepichka, D. F. J. Mater. Chem. 2009, 19, 5167–5174.
(46) Wudl, F.; Bryce, M. R. J. Chem. Educ. 1990, 67, 717–718.
(47) We note that actual conductivity might be significantly higher
because of relatively large contact resistance for materials with high
conductivity in the two-probe method. See also ref 45.

904 Chem. Mater., Vol. 23, No. 3, 2011
Patra et al.
Scheme 2. Solid-State Polymerization of 3Br2, 5Br2, and 6Br2
similar distances in 3Br2. For comparison, the shortest
intermolecular Br Br distance in 2,5-dibromo-3,4-ethyl-
3 3 3
enedioxythiophene (DBEDOT) is 3.45 A.44b It could be
suggested that the short intermolecular Br Br distances
3 3 3
are responsible for the observed solid-state polymerization
of 3Br2; however, 4Br2 has even shorter intermolecular
Br Br distances and does not undergo SSP (although the
3 3 3
melting point of 4Br2 is 7 °C lower than that of 3Br2, which
prevents direct comparison). Indeed, recent work on SSP
concluded that “the intermolecular Hal/Hal contacts,
which were suggested earlier to be important, do not
appear to be critical for the solid-state polymerization”,45
which is in line with our observations. The interplane
distance between π-π-stacked molecules in 3Br2 is
3.49 A, which is practically the same as in DBEDOT44b
(3.50 A), whereas the interplane distance between π-π-
stacked molecules in 4Br2 is slightly larger (3.57 A) because
of the larger size of the selenium atom (the interplane
distance between π-π-stacked molecules in 2,5-dibromo-
3,4-ethylenedioxyselenophene (DBEDOS)22 is 3.64 A).
Differential Scanning Calorimetric (DSC) Studies of
Solid-State Polymerization (SSP). We used DSC to fol-
low the SSP process of 3Br2-6Br2. Heating the dibromo
compounds 3Br2-6Br2 at a scan rate of 10 °C/min results
first in endothermic melting followed by exothermic
polymerizations of the melted samples (Figure 7). The
melting of compounds 3Br2-6Br2 requires 4.0, 3.7,
4.0, and 5.0 kcal/mol, respectively. Selenophene-based
dibromo compounds 5Br2 and 6Br2 show sharp exother-
mic polymerization peaks at significantly lower tempera-
tures than their thiophene-based analogues 3Br2 and 4Br2
(Figure 7).
The kinetics of the SSP of 5Br2 and 6Br2 was studied by
isothermal DSC measurements at 40-60 °C (below their
melting points) (Figure 8). Extrapolation of the Arrhe-
nius plot50 to 20 °C predicts the half lifetime of compound
5Br2 to be ∼2.5 days, which is in good agreement with our
observation of SSP of 5Br2 at room temperature. By
comparison, similar extrapolations to 20 °C predict half
lifetimes of about 20 days for DBEDOS22 and of about
70 days for DBEDOT.44b A comparison of the SSP data
from the DSC measurements indicates that compound
5Br2 polymerizes at a temperature that is ∼20 °C lower
than the polymerization temperature of DBEDOS and
∼40 °C lower than that of DBEDOT. The DSC-measured
activation energies for the SSP of 5Br2 (30.5 kcal/mol)
and 6Br2 (34.7 kcal/mol)51 are higher than for DBEDOS
(27.5 kcal/mol)22 and DBEDOT44b (27.1 kcal/mol). How-
ever, the half lifetimes of 5Br2 and 6Br2 are shorter than
those of DBEDOT and DBEDOS because of their higher
activation entropies.
relatively low conductivities of ∼0.01 and 0.22 S cm-1
,
respectively, in the doped state (measured in a pressed
pellet by the two-probe method)46 in contrast to P3,
which has a relatively high conductivity (Table 1).
X-ray Analysis. Only a few crystal structures of thieno
fused thiophene derivatives have been reported in the
literature,49 and no crystal structure of thieno[3,4-b]thio-
phene (3) or of any selenolo fused thiophene derivatives
are known. Single crystals of thiophene-based 3Br2 and
4Br2 were grown from a mixture of chloroform and
hexane (∼5%) by slow evaporation at low temperature
(∼4 °C). The unit cell of 3Br2 crystals comprises two
independent molecules. The peripheral42 ring of one of
the molecules has a disordered structure (Figure 5). For
discussion, we use the geometry of the other independent
molecule, which is not disordered. The unit cell of 4Br2
crystals also comprises two independent molecules, both
having disordered structures (Figure 6). The average
bond lengths in two independent molecules are used for
the discussion of 4Br2. Both 3Br2 and 4Br2 are planar and
their crystals exhibit two types of packing: π-π stacking
and a herringbone pattern (see Figures S2 and S3 in the
Supporting Information). All C-C bond lengths in 3Br2
are very similar (within 0.01 A) to those in 4Br2, which is
in agreement with the calculated (B3LYP/6-31G(d)) geom-
etries of 3Br2 and 4Br2 (see Table S3 in the Supporting
Information). In 3Br2, the bond lengths and angles in the
main thiophene ring42 are similar to those of many other
previously reported thiophene derivatives, whereas the
annulated42 thiophene ring has somewhat different bond
lengths, consistent with the presence of two exocyclic
double bonds. X-ray analysis of 3Br2 reveals its shortest
intermolecular Br Br, Br S, and S S distances to be
3 3 3
3 3 3
3 3 3
3.60, 3.56, and 3.58 A, respectively (see Figure S2 in the
Supporting Information). X-ray analysis of 4Br2 reveals its
shortest intermolecular Br Br, Br S, Br Se, S
3 3 3
3 3 3
3 3 3
3 3 3
S, and Se Se distances to be 3.49, 3.41, 5.24, 3.49, and
3 3 3
3.62 A, respectively (see Figure S3 in the Supporting
Information), which are within the van der Waals radii of
Br (1.85 A) and S (1.80 A) and somewhat shorter than
(48) The possibility of significant direct bromination by released mole-
cular bromine of the polymer produced by SSP can be excluded
based on IR spectrum of the produced polymer (see Figure S4 in the
Supporting Information). The calculated (B3LYP/6-31G(d))
C-Br vibration stretch (strong peak) in brominated compound 5
is predicted at 868 or 916 cm-1 (depending on isomer; frequencies
are unscaled). However, there is no peak around 900 cm-1 in the IR
spectrum of bromine-doped P5.
(49) (a) Amaresh, R. R.; Lakshmikantham, M. V.; Baldwin, J. W.;
Cava, M. P.; Metzger, R. M.; Rogers, R. D. J. Org. Chem. 2002, 67,
2453–248. (b) Matsumura, N.; Tanaka, H.; Yagyu, Y.; Mizuno, K.;
Inoue, H.; Takada, K.; Yasui, M.; Iwasaki, F. J. Org. Chem. 1998,
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(50) For the Arrhenius plots, we used the maximum reaction rate (peak
maximum at DSC curves) as a substitute for t1/2, following ref 44b.
We note that while there is no solid theoretical support for the
validity of such Arrhenius plots it was successfully applied
earlier22,44b and gives very reasonable estimation of half lifetimes.
In each experiment, the same amount of compound from the same
batch was used to eliminate the effect of impurities, crystallite size,
etc.
(51) We note that Arrhenius plot (Figure 8b) of SSP of 6Br2 is deviating
from linearity.

Article
Chem. Mater., Vol. 23, No. 3, 2011 905
Figure 5. Crystal structure of 3Br2 (presents two independent molecules in the unit cell).
Figure 6. Crystal structure of 4Br2 (presents two independent molecules in the unit cell).
Computational Study. Structures of P3-P6 were calcu-
lated at the PBC/B3LYP/6-31G(d) level of theory. Im-
portantly, the optimized geometries of P3-P6 are planar,
whereas their analogue PITN12 is calculated to be twisted
because of steric repulsions, including between sulfur and
hydrogen in the benzene ring. Because deviation from
planarity should lead to a significant increase in band
gap,52 the band gap of P3-P6 is expected to be lower than
the band gap of PITN, which is in line with the experi-
mental results presented above. Consequently, compounds
based on P3-P6 should have obvious advantages over
PITN for the construction of low-band-gap polymers.
The calculated band gaps for P3-P6 are in excellent
agreement (note that calculations at the B3LYP/6-31G(d)
level of theory consistently overestimate band gaps by
∼0.1 eV except of P5 were calculated and experimental
values are practically the same) with the values from
spectroelectrochemistry. The calculated band gaps of poly-
selenophenes P5 (Eg = 0.69 eV) and P6 (Eg = 0.83 eV)
are about 0.27 and 0.24 eV lower than those of P3 (Eg=
0.96 eV) and P4 (Eg=1.07 eV), respectively (Table 2 and
Figure 9). This is in line with the band gap differences of
about 0.2 eV that have previously been observed (both
experimentally and computationally) between polyseleno-
phenes and polythiophenes.22,24,28,29
The thiophene ring is more aromatic than the seleno-
phene ring, which leads to quinoid resonance structures
playing a greater role in polyselenophenes than in poly-
thiophenes.29,30 The aromaticity of the peripheral42 ring
stabilizes the quinoid form of the corresponding polymers
and results in a decrease in the band gap. Consequently,
the aromaticity of the peripheral ring should significantly
influence the band gap of the corresponding polymers.
The different aromaticities of selenophene and thiophene
rings can be used as a tool to govern the band gap of low-
band-gap polymers, in general, and of the P3-P6 series,
Figure 7. DSC scans of the dibromo compounds 3Br2-6Br2 at a heating
scan rate of 10 °C/min.
in particular. Indeed, according to our calculations, the
band gap of P3 is 0.11 eV lower than that of P4 and the
band gap of P5 is 0.14 eV lower than that of P6. The dif-
ferences in the aromaticity of selenophene and thiophene
and the greater role of the quinoid structure in polymers
that have a polyselenophene backbone is exemplified by
the differences in the calculated backbone bond lengths
for P3-P6. The inter-ring C-C bond distances (Table 2)
of polyselenophenes P5 and P6 (1.412 and 1.415 A, re-
spectively) are ∼0.011 A shorter than these bond lengths
in the corresponding polythiophenes, P3 and P4 (1.424 A
and 1.426 A, respectively). This bond length difference is
similar to the calculated differences in inter-ring C-C
bond lengths found between other polyselenophenes and
polythiophenes.22,29
We can summarize that, in P3-P6, lower aromaticity
in the main ring42 decreases the band gap because of the
more important contribution of the quinoid structure of
the main chain, while lower aromaticity in the peripheral
ring42 leads to greater aromaticity in the main ring, which
increases the band gap because of a decrease in the
quinoid character of the main chain. Thus, using different
combinations of selenium and sulfur atoms in the main
and peripheral rings42 leads, for the first time, to the
(52) Zade, S. S.; Bendikov, M. Chem.;Eur. J. 2007, 13, 3688–3700.

906 Chem. Mater., Vol. 23, No. 3, 2011
Patra et al.
Figure 8. Isothermal DSC curves and (insets) Arrhenius plots for the solid-state polymerization at different temperatures of compounds (a) 5Br2
and (b) 6Br2.
Table 2. Calculated Bond Lengths, HOCO (Highest Occupied Crystal Orbital) and LUCO (Lowest Unoccupied Crystal Orbital) Energies (at B3LYP/
6-31G(d)), and Experimental Optical Band Gaps for P3-P6
band gap (eV)
polymers
fused (A)a
inter-ring (A)b
HOCO (eV)
LUCO (eV)
calcdc
exptd
P3
P4
P5
P6
1.424
1.428
1.427
1.429
1.424
1.426
1.412
1.415
-4.02
-4.07
-3.92
-4.00
-3.06
-3.00
-3.23
-3.17
0.96
1.07
0.69
0.83
0.8517b
0.96
0.72
0.76
a Fused C-C bond length. b Inter-ring C-C bond length. c Calculated band gap. d Experimental optical band gap obtained from spectroelec-
trochemical data.
the spectroelectrochemically measured band gaps of these
polymers are 0.96, 0.72, and 0.76 eV, respectively. DFT
calculations performed on P3-P6 provide excellent esti-
mations of the band gaps of the corresponding polymers.
We introduced a new scheme for band gap control in
conjugated polymers by replacing the sulfur atom with a
selenium atom in the main and/or peripheral ring,42
which leads to significant and predictable changes in the
band gap of the polymers. This is due to the different
aromaticities of thiophene and selenophene rings, with
the selenophene ring being less aromatic than the thio-
phene ring. The use of different combinations of selenium
and sulfur atoms in the main and peripheral rings42 leads,
for the first time, to the achievement of band gap control
in very low band gap (<1.0 eV) conjugated polymers.
Figure 9. Comparison of the calculated energy levels of P3-P6 at
B3LYP/6-31G(d).
achievement of band gap control in very low band gap
(<1.0 eV) conjugated polymers.
Conclusions
Acknowledgment. We thank the Israel Science Founda-
tion, the Estate of George Talis Foundation, and the Helen
and Martin Kimmel Center for Molecular Design for finan-
cial support. M.B. is the incumbent of the Recanati career
development chair, a member ad personam of the Lise
Meitner-Minerva Center for Computational Quantum Chem-
istry, and acknowledges DuPont for a Young Professor
Award.
The paper reports new strategy for the band gap con-
trol in very low band gap (<1.0 eV) polymers. A series of
new low-band-gap conjugated polyselenophenes and
polythiophenes was systematically prepared by electro-
polymerization (P4, P5, and P6) and by solid-state poly-
merization (P3, P5, and P6). We have also presented the
synthesis of monomers 4, 5, and 6, followed by their
bromination. The first X-ray structures of thieno[3,4-b]-
thiophene and selenolo[2,3-c]thiophene derivatives, i.e.,
of 3Br2 and 4Br2, are also presented. Electropolymeriza-
tion of 4, 5, and 6 affords P4, P5, and P6, respectively, and
Supporting Information Available: CIF files; electrochemical
data for compound 3, details of calculations, and complete
citation of reference 37 (PDF). This material is available free
of charge via the Internet at http://pubs.acs.org.