Tuning the electronic properties of poly(thienothiophene vinylene)s via alkylsulfanyl and alkylsulfonyl substituents

Article abstract of DOI:10.1021/ma402018n

The use of alkylsulfanyl and alkylsulfonyl side chains are demonstrated to be a useful synthetic strategy for tuning the electronic properties of organic semiconductors, as shown in thienothiophene vinylene polymers. By changing the oxidation state of sulfanyl to sulfonyl, we lower the HOMO and LUMO energy levels of our substituted polymers, as well as enhance their fluorescence. Fine-tuning of the energy levels was achieved by combining sulfanyl and sulfonyl substituted thienothiophene monomers through random polymerization, yielding polymers with low-band gaps (1.5 eV) yet benefiting from a structurally uniform conjugated backbone. The effects of these functional side chains are presented through DFT calculations, UV-vis, fluorescence, and electrochemical measurements, as well as crystallographic analysis of a sulfanyl-substituted oligomer. The semiconducting properties of the new polymers are studied in OFET and OPV devices.

Full text of DOI:10.1021/ma402018n

Article
pubs.acs.org/Macromolecules
Tuning the Electronic Properties of Poly(thienothiophene vinylene)s
via Alkylsulfanyl and Alkylsulfonyl Substituents
Julia A. Schneider,† Afshin Dadvand,† Wen Wen,‡ and Dmitrii F. Perepichka†,*
†Department of Chemistry and the Center for Self-Assembled Chemical Structures, McGill University, 801 Sherbrooke Street
Montreal, QC, H3A 0B8 Canada
‡Department of Chemistry and Biochemistry, University of California at Santa Barbara, Santa Barbara, California, 93106, United
States
S
* Supporting Information
ABSTRACT: The use of alkylsulfanyl and alkylsulfonyl side chains are demonstrated to be a useful synthetic strategy for tuning
the electronic properties of organic semiconductors, as shown in thienothiophene vinylene polymers. By changing the oxidation
state of sulfanyl to sulfonyl, we lower the HOMO and LUMO energy levels of our substituted polymers, as well as enhance their
fluorescence. Fine-tuning of the energy levels was achieved by combining sulfanyl and sulfonyl substituted thienothiophene
monomers through random polymerization, yielding polymers with low-band gaps (1.5 eV) yet benefiting from a structurally
uniform conjugated backbone. The effects of these functional side chains are presented through DFT calculations, UV−vis,
fluorescence, and electrochemical measurements, as well as crystallographic analysis of a sulfanyl-substituted oligomer. The
semiconducting properties of the new polymers are studied in OFET and OPV devices.
backbone, a polymer’s energy levels can be directly tuned
through substituents, a method that is underutilized in
conjugated polymer chemistry.
INTRODUCTION
■
Thiophene-based conjugated polymers are important semi-
conducting materials with widespread applications in organic
light-emitting diodes (OLEDs), field-effect transistors
(OFETs), and photovoltaic solar cells (OPVs).1 Polythio-
phenes are now ubiquitous in the field of organic semi-
conductors since they are easily functionalized and show great
charge transport properties. Poly(3-hexylthiophene) (P3HT)
for example, has become the benchmark donor material for
bulk heterojunction solar cells, though its efficiency is limited
by a relatively large band gap (Eg = 1.9 eV) and high HOMO
level (−5.1 eV).2
Since P3HT, thiophene-based polymers have been tirelessly
modified to create new high performance materials. The
dominant approach is incorporating electron-deficient mono-
mer units with electron-rich thiophene to create donor−
acceptor systems that promote intrachain charge transfer,
lowering the band gap of the polymer.3 This, however, requires
the synthesis of two different monomer units, complicates the
structure-properties relationships and creates a polarization
along the conjugation backbone. Instead of altering the
For example, poly(3-alkoxythiophene) showed a smaller
band gap (1.60 eV) than P3HT due to increased rigidity
ascribed to S···O interactions, but the electron donating effect
of the alkoxy group raised the HOMO level (−4.47 eV).4
Poly(3-cyano-4-hexylthiophene), conversely, was found to have
a lower HOMO level (−6.1 eV), but a larger band gap (2.3
eV).5 Substituents such as fluorine,6 fluoroalkyl,7 thienyl,8
dicyanoethene,9 alkoxy groups,10 nitro groups,11 and esters12
have also been explored in polythiophenes.
Alkylsulfanyl substituents are of particular interest for several
reasons: the van der Waals radius of sulfur (∼0.18 nm) is less
than that of −CH2 (∼0.20 nm) which slightly reduces steric
strain compared to alkyl substituents;13 they impart solubility to
the polymer; and, in polythiophenes, they can act as mild
Received: October 2, 2013
Revised: November 11, 2013
Published: November 25, 2013
© 2013 American Chemical Society
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electron acceptors, lowering the HOMO levels.14,15 Further-
more, their electron withdrawing effect can be tuned by
changing the oxidation state of sulfur from sulfanyl (−SR) to
sulfinyl (−SOR) or sulfonyl (−SO2R). Sulfonyl groups have
been shown to increase fluorescence in polymers16 and
oligomers.17 They were recently employed as electron
withdrawing substituents in a donor−acceptor conjugated
polymer for OPVs,18 but there exists no detailed or
comparative studies of their electronic effects in semiconduct-
ing materials.
We were interested in a comparative analysis of the effect of
alkylsulfanyl and alkylsulfonyl substituents on the electronic
properties and behavior of conjugated polymers in semi-
conducting device applications. We have chosen poly(thieno-
[3,2-b]thiophene vinylene) as the conjugated backbone, taking
into consideration relative structural simplicity, planarity, and a
low expected band gap.
Thieno[3,2-b]thiophene’s (TT) fused unit fosters electron
delocalization by reducing the bond length alternation
(compared to simple thiophene).19 Owing to the 180° coupling
geometry of TT (cf. ≈ 150° for thiophene), its polymers
strongly prefer a linear rod-like structure and show increased
crystallinity and charge mobilities.20,21 A vinylene spacer
reduces the twist caused by repulsions between substituents
and “dilutes” the aromatic nature of the TT moieties. Both of
these effects promote electron delocalization and shrink the
band gap of the polymer.
In this paper we report the synthesis and spectroscopic,
structural, and device characterization of poly(3,6-
dialkylsulfanylthieno[3,2-b]thiophene vinylene), S-PTTV, its
oxidized derivative poly(3,6-dialkylsulfonylthieno[3,2-b]-
thiophene vinylene), SO2-PTTV, and the random copolymer
incorporating both sulfanyl and sulfonyl units, S/SO2-PTTV.
Comparing their properties to models of unsubstituted H-
PTTV and similarly substituted polymer without vinylene
linker (S-PTT) we explore the complex effect that substituents
play in these conjugated systems. Two dimers S-2TTV and
SO2-2TTV were also synthesized to understand the substituent
effects in a simplified system.
iophene)19) were not experimentally studied in this paper, but
are given for comparison.
Table 1. Calculated Energy Levels and Geometry of the
Polymers
a
polymer
P3MT
HOMO (eV) LUMO (eV) Eg (eV) dihedral (deg)
−4.31
−4.53
−5.45
−4.72
−4.67
−5.60
−5.15
−2.32
−2.74
−2.23
−2.79
−2.93
−3.76
−3.34
1.99
1.79
3.22
1.93
1.74
1.84
1.81
0
0
H-PTTV
S-PTT
55
0
b
S-PTTV
1
SO2-PTTV
S/SO2-PTTV
10
4
c
a
The dihedral angle between the thiophene ring and the next
b
conjugated unit. The dihedral angle is restrained to 0°, which
destabilizes the structure by 1.35 kcal/mol. PBC calculations of an
c
alternating polymer structure (AD)n. The block copolymer (AADD)n
showed a 0.06 eV lower band gap: HOMO = −5.12 eV; LUMO =
−3.37 eV.
As expected based on the behavior of sulfanyl-substituted
oligothiophenes,22 the alkylsulfanyl groups cause a large twist
(55°) in poly(thienothiophene) S-PTT, disrupting the
conjugation. Consequently, a very large band gap of 3.22 eV
is predicted for this polymer by DFT. This is higher than the
≈2.2 eV optical band gap reported13 for hexylsulfanyl S-PTT in
solution (in the solid state the band gap was further lowered to
1.74 eV). These observations suggest planarization of the
polymer chain (see below); indeed, calculations of a fully
planarized S-PTT polymer predict a band gap of 1.93 eV. The
steric repulsions of the substituents is removed by separating
the TT units with a vinylene spacer. Therefore, S-PTTV is
planar and is predicted to have a much smaller band gap of 1.74
eV. According to calculations, the sulfonyl substituents in SO2-
PTTV only cause a small twist (10°) in the polymer’s
backbone, increasing the band gap by 0.10 eV.
Both substituents induce an electron-withdrawing effect,
lowering both frontier orbitals by ≈0.15 eV for S-PTTV and
≈1.0 eV for SO2-PTTV, compared to unsubstituted H-PTTV
(Figure 1). Lower HOMO levels are beneficial for increased
stability in air and are expected to produce larger open circuit
voltages (VOC) in OPVs.23 At the same time, the substituents
do not significantly affect the π-conjugation along the chains.
The trends of MO energy vs reciprocal length of oligomers
(Figure 1) are shifted along the energy axis, but are otherwise
RESULTS AND DISCUSSION
■
Theoretical Calculations. Density functional theory
(DFT) calculations at the B3LYP/6-31G(d) level were
performed on a series of substituted TT oligomers (n = 1, 2,
..., 7) as well as for the infinite polymers, under periodic
boundary conditions (PBC). The side chains were modeled
with methyl groups. The HOMO, LUMO, and band gap values
of the corresponding polymers, along with optimized dihedral
angles are listed in Table 1. The model polymers S-PTT, H-
PTTV and P3MT (a model of regioregular poly(3-alkylth-
Figure 1. Calculated HOMO and LUMO energy levels of oligomers
and infinite chains modeled with PBC. (N = number of repeat units).
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nearly superimposable. Also, the topology of the HOMO and
LUMO of the S-PTTV, SO2-PTTV homopolymers and the
(alternating) copolymer S/SO2-PTTV are nearly superimpos-
able (Figure 2). Thus, the donor−acceptor motif should not
alter the electron delocalization along the backbone or the
intermolecular orbital coupling.
S···H interactions with the vinylene protons (the calculated S···
H distance is 2.80 Å).
Synthesis. Bisalkylsulfanyl-TT 1 was obtained from 3,6-
dibromothieno[3,2-b]thiophene25 by stepwise lithiation, fol-
lowed by quenching of the carbanion intermediate with
dialkyldisulfide (Scheme 1).26 Brominating 1 with NBS yielded
a
Scheme 1. Synthesis of Monomers 2 and 3
a
Key: (i) [1 equiv of nBuLi, 1 equiv of RS-SR] × 2, Et2O, −78 to +20
°C; (ii) NBS, CHCl3, −5 to +20 °C, Na2S2O3 work-up; (iii) m-CPBA,
CHCl3, 0 to 20 °C.
monomer 2. We note that partial oxidation of the sulfanyl
chains was occasionally observed, unless the excess of NBS is
quenched before an aqueous work-up. Full oxidation of the
alkylsulfanyl substituents into alkylsulfonyl was achieved with
m-chloroperoxybenzoic acid (m-CPBA) to give monomer 3.
Polymers were synthesized through the Stille polycondensa-
tion of monomers 2 and 3 with (E)-1,2-bis(tributylstannyl)-
ethene followed by end-capping with an excess of the
stannylated monomer, followed by iodobenzene (Scheme 2).
The polymers were isolated by precipitation into MeOH and
washed in a Soxhlet apparatus with EtOH, acetone, hexane, and
finally extracted with chloroform and chlorobenzene.
Figure 2. DFT-optimized structures of tetramers showing the
topology of HOMO and LUMO.
The rigidity of the conjugated backbone is an important
factor defining the material properties of polythiophenes and
related structures.24 DFT predicts a very shallow rotation
barrier (≈1.5 kcal/mol) for both TT-TT and SO2TT-vinylene
connections (Figure 3). This means that nonplanar S-PTT
Scheme 2. Polymerization Reaction
S-PTTV was obtained as a dark, shiny material. Hexylsulfanyl
S-PTTVa precipitated during the reaction and displayed poor
solubility, signaling the need for longer side chains. Both
dodecyl-substituted S-PTTVb and branched 2-ethylhexyl
substituted S-PTTVc showed high MWs, but only S-PTTVc
was soluble enough for film spin-coating.
SO2-PTTV gave a lower percent yield of soluble material and
only low MWs could be achieved for both SO2-PTTVb and
SO2-PTTVc. Solid state 13C NMR confirmed the structure of
the insoluble fractions (see Supporting Information). This low
solubility is likely attributable to dipole−dipole interactions of
the sulfonyl substituents.
Figure 3. Energy barrier of rotation of thiophene/vinylene dihedral
angles for S-2TT (green), S-2TTV (blue), and SO2-2TTV (red).
could planarize due to solid state packing forces (as was in fact
observed13). Similarly, SO2-PTTV, predicted to be nearly
planar in the gas phase, could adopt a twisted conformation
under external factors (solvation, interchain interactions). In
fact, the energy penalty for twisting the SO2TT/vinylene
dihedral angle by 50° is smaller than RT (0.6 kcal/mol at room
temperature). In contrast, a much larger rotation barrier (7.9
kcal/mol per TT-vinylene connection) and thus high rigidity is
predicted for S-PTTV. This can be attributed to the smaller
size of the alkylsulfanyl group and perhaps to weak attractive
Random copolymers have only scarcely been applied in
OPVs, but they are easy to synthesize and can show increased
solubility and efficiencies.27 It is also possible to quickly realize
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Table 2. Molecular Weights, Polydispersities, and UV−Vis Absorption of Synthesized Polymers
c
MW (kg/mol)
PDI
yield (%)
λmax/onset solution (nm)
λmax/onset film (nm)
Egopt film (eV)
fwhm (cm‑1) soln/film
a
S-PTTVa
13.0
43.4
42.7
6.3
1.9
1.3
2.2
1.1
1.3
2.1
2.5
73
544/697
607/730
610/740
486/640
517/625
597/760
561/748
580/760
585/770
608/765
471/680
529/720
594/810
547/755
1.63
1.61
1.62
1.82
1.72
1.53
b
S-PTTVb
62
4450/5980
4540/6250
4680/5130
4810/5810
4770/6540
5340/6190
b
S-PTTVc
62
b
SO2-PTTVb
SO2-PTTVc
S/SO2-PTTV-1:1
S/SO2-PTTV-1:2
70
b
7.6
41
b
27.0
12.9
75
b
54
1.64
a
b
c
Total yield of the MeOH precipitated product. Yield of a polymer fraction(s) soluble in CHCl3 and PhCl. Egopt = 1240/λedge of film.
.
a series of random copolymers by simply varying the feed ratios
and thereby quantify the effect of a certain unit.
S/SO2-PTTV was synthesized by the random copolymeriza-
tion of a mixture of monomers 2 and 3 with (E)-1,2-
bis(tributylstannyl)ethene. The copolymer was soluble, with a
MW that averaged those of S-PTTV and SO2-PTTV (Table 2).
The relative fraction of the sulfanyl/sulfonyl substituted TT
units can be tuned by varying the ratio of the starting
1
monomers and easily quantified by H NMR. A 1:1 and 1:2
feed ratio of monomers 2:3 gave measured ratios of 1:0.88 and
1:1.6, respectively. Along with the lower MW of SO2-PTTV,
this indicates a reactivity preference for monomer 2 over 3,
likely due to the steric effect of the bulkier sulfonyl group and/
or coordination to palladium in the catalytic complex.28
To gain insight into the solid-state packing of the substituted
TTV systems, we synthesized two dimers by the Stille coupling
of monobrominated TTs 4 and 5 with (E)-1,2-bis-
(tributylstannyl)ethene to afford dimers S-2TTV and SO2-
2TTV, respectively. Monobromination of 1d was accomplished
by lithiation (nBuLi) followed by treatment with carbon
tetrabromide. 4 was then oxidized to 5 as described above
(Scheme 3). Single crystals of dimer S-2TTV were prepared as
orange needles from a mixture of ethanol, hexane, and toluene,
and were subjected to X-ray analysis (Figure 4).
Figure 4. Crystal structure of S-TTV showing (a) S···H distance with
vinylene hydrogen and the interplane distance and (b) the lack of π-
overlap between TT units.
a
Scheme 3. Synthesis of Model Dimers
Information). Similar out-of-plane geometry and even larger
planarization penalty was calculated for SO2−2TTV (Figure
S2). To accommodate the out-of-plane protruding substituents,
the molecules have to slip vs one another along the short
molecular axis, which significantly limits π···π overlap between
the TT units (Figure 4b).
Optical Properties. The UV−vis absorption spectra of the
prepared polymers were studied in chloroform solution and in
thin films (Figure 5, Table 2). All the polymers displayed
broader absorption spectra and red-shifts of the band edge in
the solid state compared to solution. The spin-coated films are
highly reflective, with characteristic metallic luster (Figure 5b).
The variation of the solution absorption band with different
alkyl side-chains (S-PTTVa/b/c) are in line with the changes
in molecular weights of these polymers. Interestingly, very little
difference was observed in the solid state. The absorption of
solid S-PTTVc, λedge = 765 nm (Eg = 1.62 eV) is red-shifted
from analogous alkyl-substituted poly(3,6-dihexyl)thieno[3,2-
b]thiophene vinylene C6-PTTV which showed a band edge at
700 nm (1.77 eV)29 and from poly(3,6-dioctylsulfanylthieno-
[3,2-b]thiophene), SC8-PTT, whose band edge appears at 713
nm (1.74 eV).13 Evidently, both the sulfanyl substituents and
the vinylene spacer play a part in shrinking the band gap.
Replacing sulfanyl sides chains with sulfonyl (SO2-PTTV)
results in a ∼0.1 eV increase of the optical band gap (720 nm,
1.72 eV), as predicted by DFT (Table 1). This increase should
a
Key: (i) 1 equiv nBuli, 1 equiv CBr4, Et2O, −78 to +20 °C; (ii) m-
CPBA, CHCl3, 0 to 20 °C; (iii) 0.5 equiv of Bu3SnCHCHSnBu3,
Pd(PPh3)4, 100 °C
In accordance with the DFT calculations, the conjugated
backbone of S-2TTV is nearly planar (TT-vinylene dihedral
angle = 5.2°). The crystals formed a slipped-stacked structure
with an interplane distance of 3.50 Å (Figure 4a). Importantly,
the “inner” alkylsulfanyl substituents bend out of the plane at
∼80°, thus protruding above and below the aromatic plane.
This confirms the DFT prediction that placing this substituent
in plane destabilizes the structure by ≈4 kcal/mol, compared to
the optimal 83° twisted geometry (Figure S2, Supporting
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Figure 5. Absorption spectra of polymers in (a) chloroform solutions and (b) thin films (the inset shows a flask coated in a film of S-PTTVc).
in part be attributed to the low MW36 of SO2-PTTV (Table 2)
although its lower rigidity (Figure 3) should also play a role and
must be responsible for the unexpectedly large band gap in
solution (1.98 eV). SO2-PTTV displays the smallest increase in
the fwhm between solution and solid state, which suggests a
weakened intermolecular exciton coupling caused by the
sulfonyl substituents.
Copolymer S/SO2-PTTV-1:1 retains the broad absorption
(fwhm = 6540 cm−1) of S-PTTV and shows a further lowered
band gap of 1.53 eV, that can be attributed to the donor−
acceptor interactions between sulfanyl and sulfonyl-substituted
units. The balance between the latter appears important as the
increased amount of −SO2 substituted TT units in S/SO2-
PTTV-1:2 increases the band gap (1.64 eV), possibly due to
disrupted solid-state packing (cf. small Δfwhmfilm‑sol = 850
cm−1, Table 2), but also due to a lower MW.
Comparing the absorption of the dimers S-2TTV and SO2-
2TTV, unperturbed by the uncertainty of polymer lengths,
supports the above observations. Their 0−0 transitions (450
and 444 nm, determined as a crossing of the absorption and
emission curves) are 2.76 and 2.79 eV for S-2TTV and SO2-
2TTV, respectively (Figure 6). The sulfanyl-substituted S-
2TTV showed a more pronounced vibronic splitting of the
absorption band, as expected based on its predicted rigidity
(Figure 3).
dramatic enhancement of fluorescence in thienylene−vinylene
oligomers and polymers upon the introduction of sulfanyl side
chains, which was unexpected due to the additional sulfur
atoms.31 As a further surprise, we now observed that S-PTTV
containing fused thienothiophene rings, is again nonemissive,
although the model dimer S-2TTV exhibits a moderate
emissivity (λmax = 485 nm, PLQY = 15%, Figure 4). We
could not find a good explanation for these variations, but we
note that both alkyl-substituted PTTV (C6-PTTV29) and
alkylsulfanyl-substituted PTT (SC8-PTT, PLQY = 26%13) are
reported to show strong photoluminescence.
In line with previous reports,16 oxidizing the sulfanyl group
into sulfonyl enhances the photoluminescence. The dimer SO2-
2TTV shows quantum yield three times larger (λmax = 483 nm,
PLQY = 45%) than its sulfanyl analogue S-2TTV (Figure 6).
Moderately strong luminescence was also observed for SO2-
PTTV at λmax = 585 nm (PLQY = 17%), but introducing
sulfanyl-substituted units in the copolymer again quenches the
luminescence, and only very week emission at 690 nm was
observed for S/SO2-PTTV-1:1 (see Supporting Information).
Electrochemical Properties. The redox behavior of the
synthesized polymers was studied by cyclic voltammetry (CV)
for thin films drop-cast on Pt electrodes (Figure 7, Table 4).
Generally, oligo- and polythiophenes are weak-to-moderate
emitters, a fact attributed to the heavy atom effect of sulfur
enhancing the intersystem crossing.30 A majority of thienyle-
nevinylene oligomers and polymers and their derivatives are
almost completely nonemissive. We have earlier reported a
Figure 7. Cyclic voltammograms of synthesized polymers (0.1 M
Bu4NPF6 /propylene carbonate as electrolyte; scan rate 0.1 V/s).
All three polymers show reversible reduction (n-doping) and
oxidation (p-doping) waves, except for the most electron-
deficient SO2-PTTV, which revealed a quasi-reversible
oxidation (ie. corresponding cathodic peaks at much more
negative potentials), indicating trapping of the positive charges.
Compared to the alkyl-substituted C6-PTTV29 the sulfanyl
substituents show a moderate electron withdrawing effect,
Figure 6. Absorption () and emission (− −) spectra of S-2TTV and
SO2-2TTV dimers.
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a
b
Table 4. Redox Potentials, HOMO−LUMO Levels, and
c
Band Gaps of Polymers Determined by CV of Thin Films
Ered
(V)
Eox
(V)
LUMO
(eV)
HOMO
(eV)
EgCV
(eV)
C6−PTTV29
S-PTTV
−1.83
−1.63
−1.20
−1.37
−1.14
0.24
0.32
0.91
0.50
0.74
−2.97
−3.17
−3.60
−3.43
−3.66
−5.04
−5.12
−5.71
−5.30
−5.54
2.07
1.95
2.11
1.87
1.88
SO2−PTTV
S/SO2−PTTV-1:1
S/SO2−PTTV-1:2
a
All potentials reported versus Fc/Fc+ in 0.1 M Bu4NPF6/propylene
b
carbonate. Calculated from the onset of the oxidation and reduction
potentials, assuming HOMO of Fc at −4.8 eV.32 EgCV = HOMO −
c
LUMO.
Figure 9. Current−voltage characteristics of bulk-heterojunction cells
of S/SO2-PTTV-1:1 and S-PTTV in 1:1 blends with PC70BM.
shifting the oxidation and reduction of S-PTTV by 0.08 and
0.20 eV, respectively (Table 4). A much larger shift is observed
for the sulfonyl-substituted polymer, SO2-PTTV (0.67 and 0.63
V shifts for oxidation and reduction, respectively). The random
copolymers S/SO2-PTTV-1:1 and S/SO2-PTTV-1:2 show the
electronic effects of both monomer units with reduction and
oxidation onsets between those of S-PTTV and SO2-PTTV,
giving them the smallest electrochemical gaps at 1.87 and 1.88
eV, respectively.
Device Studies. S-PTTV and copolymer S/SO2-PTTV-1:1
were tested in OFETs and OPVs, but unfortunately SO2-PTTV
was not soluble enough to be spin-cast into working devices.
Thin-film transistors were prepared from toluene solutions of
the polymers spin-cast on Si/SiO2 followed by the deposition
of patterned Au electrodes. The output and transfer character-
istics of the S-PTTVc transistor are shown in Figure 8. A rather
Table 5. Photovoltaic Properties of ITO/PEDOT:PSS/
Polymer:PC70BM (1:1)/Ca/Al Devices
HOMOCV
(eV)
Isc (mA/
Voc
PCE
(%)
polymer
C6-PTTV29
cm2)
(V)
FF
−5.0
−5.12
−5.30
1.27
3.16
1.64
4.46
0.60
0.68
0.82
0.70
0.37
0.37
0.29
0.27
0.28
0.79
0.39
0.84
S-PTTV
S/SO2-PTTV
S/SO2-PTTV and 3%
CN
S/SO2-PTTV and
0.5% DIO
4.07
0.66
0.29
0.78
The performances of both the S-PTTV and S/SO2-PTTV-
1:1 devices are limited by low short-circuit currents (Isc) and fill
factors (FF). This can be related to the low charge mobility
seen in the transistor devices, although C6-PTTV, that showed
higher hole mobility in OFETs, produced even lower Isc in
analogous OPV devices. Poor mobilities mean more chances
for recombination at the interface before the hole and electron
can diffuse away and increase the serial resistance which leads
to the low FFs observed.33 Similar results were obtained with
Ca vs Al electrodes, so poor charge extraction at the electrode
interface is unlikely to be the issue.
Preliminary optimizations attempts for S/SO2-PTTV-1:1
using chloronaphthalene (CN) and 1,8-diiodooctane (DIO)
additives led to improvements in the current densities, but not
to FFs. The best Isc (4.5 mA/cm2) was achieved for devices
with 3% CN additive, which is still half the Isc seen in analogous
P3HT devices without additives. The fact that the FFs did not
increase suggests that while the additives may reduce the phase
segregation, thereby improving the exciton dissociation, the
larger interface area also increases charge recombination,
leaving the low mobility of the polymers as the limiting factor.
Figure 8. Output (left) and transfer (right) characteristics of
transistors made from S-PTTVc (μh ∼ 1 × 10−5 cm2/V s).
low hole mobility of ∼1 × 10−5 cm2/V·s was measured for S-
PTTVc, and no further improvement was achieved upon
annealing. The copolymer-based devices showed no transistor
activity (μ+ <10−7 cm2/V·s) and no n-channel activity (electron
conductance) was observed for either polymers. This compares
unfavorably with the reported OFETs based on C6-PTTV with
hole mobilities up to 1.9 × 10−2 cm2/V·s (for annealed films, a
40× increase from unannealed). The out-of-plane protrusion of
the side chains and the resulting poor overlap between TT
units, as observed in the S-2TTV crystals, is likely the reason
for poor transistor characteristics of these materials.
Bulk-heterojunction solar cell devices from polymer:PC70BM
blends were tested. Figure 9 shows the I−V curves of two of the
polymers, and the photovoltaic characteristics obtained are
listed in Table 5.
S-PTTV displayed slightly higher open-circuit voltages
(VOC) than reported for C6-PTTV, as is expected due to its
lower HOMO level. The trend continues for S/SO2-PTTV-
1:1, which has a high VOC of 0.8 V.
CONCLUSIONS
■
We have reported new thienothiophene vinylene polymers
whose electronic properties can be tuned by changing the
oxidation state of the sulfur-containing side-chains. The sulfanyl
and sulfonyl substituents show moderate and strong electron-
withdrawing effects and their combination in random
copolymers is a means to fine-tune the energy levels and
achieve a low band gap (1.5 eV) without affecting the
uniformity of the conjugated backbone. Bulk-heterojunction
photovoltaics based on the S/SO2-PTTV-1:1 random copoly-
mer (with PC70BM) displayed high VOC of 0.8 V, but suffered
from low ISC due to poor charge mobility. The latter was
attributed to poor of π−π overlap between TT units caused by
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allowed to warm to room temperature overnight. The next day the
reaction mixture was cooled back to −75 °C and a second dose of n-
BuLi added. After an hour, dihexyldisulfide was added and the reaction
was allowed to warm to room temperature and stirred overnight a
second time. The reaction was then quenched with H2O (20 mL), the
organics were washed with NH4Cl solution (20 mL), H2O (2 × 20
mL), then dried over Na2SO4, and concentrated in vacuo. The crude
oil was then distilled in a Kugelrohr apparatus (170 °C, 0.16 bar) to
out-of-plane substituents, as corroborated by X-ray analysis.
The oxidation state of the side chains controls the emissive
properties of these materials; sulfonyl groups demonstrated
dramatically increased fluorescence in dimers (PLQY of 45% vs
15% for sulfanyl) and polymers (PLQY of 17% vs ∼0% for
sulfanyl).
Introducing a sulfur atom in various oxidation states
(sulfanyl/sulfonyl) between the alkyl chain and the conjugated
backbone represents a simple and predictable method for
tuning the electronic and optical properties of organic
semiconductors. This method can be a welcome alternative
to designing more complex donor−acceptor motifs for new
polymeric semiconductors.
1
yield pure 1a as a colorless oil (0.319 g, 67%). HNMR (acetone-d6,
400 MHz): δ 7.54 (s, 2H), 2.99 (t, J = 7.2 Hz, 4H), 1.63 (m, 4H), 1.44
(m, 4H), 1.27 (m, 8H), 0.87 (t, J = 6.8 Hz, 6H). 13C NMR (CDCl3,
125 MHz): δ 141.2, 127.0, 125.0, 34.6, 31.3, 29.6, 28.2, 22.5, 14.0. HR-
MS (ESI): m/z = 373.1150 [M + 1] (calcd for C18H29S4, m/z =
373.1147).
3,6-Bis(dodecylsulfanyl)thieno[3,2-b]thiophene (1b). The
compound was synthesized in the same manner as described for 1a.
Kugelrohr distillation was not effective for separation, so the product
was recrystallized from pentane in a −18 °C in freezer to give tan
EXPERIMENTAL SECTION
■
All lithiation and polymerization reactions were performed under inert
atmosphere, with flame-dried glassware and anhydrous solvents. 3,6-
dibromothieno[3,2-b]thiophene was synthesized in four steps
following literature procedures.34
1
crystals in a 30% yield. H NMR (acetone-d6, 400 MHz): δ 7.54 (s,
2H), 3.00 (t, J = 7.2 Hz, 4H), 1.63 (m, 4H), 1.44 (m, 4H), 1.27 (m,
1
16H), 0.88 (t, J = 6.8 Hz, 6H). H NMR (CDCl3, 400 MHz): δ 7.24
(s, 2H, overlaps with CHCl3), 2.90 (t, J = 7.6 Hz, 4H), 1.61 (m, 4H),
1.40 (m, 4H), 1.24 (m, 16H), 0.88 (t, J = 7.2 Hz, 6H). 13C NMR
(CDCl3, 125 MHz): δ 141.1, 126.9, 125.0, 34.6, 31.9, 29.6, 29.6, 29.5,
29.3, 29.1, 28.5, 22.7, 14.1.
Molecular weight measurements of the polymers were performed
on a GPC PL 50 in THF at 30 °C. UV/vis absorption and
photoluminescence spectra were measured in CHCl3 with a JACSO
V670 UV−vis−NIR spectrometer and a Varian Eclipse Fluorometer,
respectively. The fluorescence quantum yields of the polymers were
determined versus cresyl violet in MeOH (PLQY = 0.54), while the
quantum yields of the dimers were determined versus fluorescein in
0.1 M NaOH (PLQY = 0.79). Cyclic voltammetry was performed on a
CH670 potentiostat from CH-Instruments in a three-electrode cell
using a propylene carbonate solution of 0.1 M (TBA)PF6 as an
electrolyte, at scan rates of 100 mV s−1. Pt disk and Pt wire were used
as the working and counter electrodes, respectively, and a Ag/AgCl or
Ag/AgNO3 electrode was used as the reference. All potentials were
adjusted vs ferrocene (internal standard).
OPV devices were prepared on ITO-coated glass substrates that had
been cleaned with detergent and rinsed by sonicating in deionized
water, acetone, and finally isopropanol for 20 min each. PEDOT:PSS
(Clevios P Al 4083) was spin-cast onto the ITO-coated glass at 4,000
rpm for 30 seconds and then baked at 140 °C for 10 minutes.
Solutions of PC71BM and polymer in 1:1 ratios and 1 mg/mL
concentrations in chlorobenzene were prepared by stirring overnight
at 70 °C under inert atmosphere. The photoactive layer was spin-cast
at various rates for 60 s each. Next, thermal evaporation of 5 nm of Ca
and/or 100 nm of Al was performed to form the top contacts.
Standard solar cell characterizations were carried out in a nitrogen
environment under simulated 100 mW/cm2 AM1.5G irradiation from
a 300 W Xe arc lamp with an AM 1.5 global filter. All measurements
were repeated on at least 4 devices.
3,6-Bis(2-ethylhexylsulfanyl)thieno[3,2-b]thiophene (1c).
The compound was synthesized and purified as described for 1a to
1
give a clear oil in 78% yield. H NMR (CDCl3, 400 MHz, 25 °C): δ
7.24 (s, 2H), 2.92 (d, J = 6.0 Hz, 4H), 1.40 (m, 9H), 1.25 (m, 9H),
0.87 (m, 12H). HR-MS (ESI): m/z = 429.1782 [M + 1] (calcd for
C22H37S4, m/z = 429.1773).
3,6-Bis(butylsulfanyl)thieno[3,2-b]thiophene (1d). The com-
pound was synthesized and purified as described for 1a to give a clear
1
oil in 43% yield. H NMR (acetone-d6, 400 MHz, 25 °C): δ 7.53 (s,
2H), 2.99 (t, J = 8.0 Hz, 4H), 1.61 (m, 4H), 1.46 (m, 4H), 0.89 (t, J =
7.2 Hz, 6H). HR-MS (ESI): m/z = 317.0524 [M + 1] (calcd for
C14H21S4, m/z = 317.0521).
2,5-Dibromo-3,6-bis(dodecylsulfanyl)thieno[3,2-b]-
thiophene (2b). Thiophene 1b (82.4 mg, 0.152 mmol) was dissolved
in CHCl3 (2 mL) and acetic acid (2 mL) and cooled to −40 °C. NBS
(82.6 mg, 0.464 mmol) was added and the reaction warmed slowly to
room temperature protected from light with foil. After being stirred for
24 h, the reaction was treated with saturated Na2S2O3 solution (5 mL).
The reaction mixture was then diluted with H2O and extracted with
CHCl3 (2 × 20 mL). The solvent was removed in vacuo and the
resulting product was passed through a silica plug with hexane to yield
pure bromide 2b as a white solid (77.5 mg, 73%). 1H NMR (CD2Cl2,
400 MHz): δ 2.89 (t, J = 7.2 Hz, 4H), 1.54 (m, 4H), 1.40 (m, 4H),
1.25 (m, 16H), 0.89 (t, J = 6.8 Hz, 6H).
DFT calculations were performed using the Gaussian 09W program
at the B3LYP level with a 6-31G(d) basis set.35 All geometry
optimization calculations were started with the substituents in the anti-
orientation, at 90° angles to the TT plane. The dihedral angles
reported were averaged from internal TT−vinylene dihedrals in
geometry-optimized oligomers. The PBC calculation for polymer were
performed at the same level, with various unit cell size to enable
different dihedral angles (one monomer per unit cell necessarily
affords a planar polymer; two monomers allow the optimization of the
dihedral angle, with alternating clock-wise/anticlockwise rotation; six
monomers per units cell allows helical twisting with 60° pitch, close to
the optimized 55° for S-PTT). The energy of rotation of a TT unit vs
the vinylene spacer was performed on optimized dimers while freezing
the dihedral angle to the second TT unit. The energy diagrams of full,
360° rotations of substituents were calculated from optimized dimers
with both TT−vinylene dihedral angles frozen.
2,5-Dibromo-3,6-bis(hexylsulfanyl)thieno[3,2-b]thiophene
(2a). The compound was synthesized as described for 2b to give an
orange oil in 33% yield. 1HNMR (acetone-d6, 400 MHz): δ 2.97 (t, J =
7.2 Hz, 4H), 1.55 (p, J = 7.6 Hz, 4H), 1.44 (p, J = 7.6 Hz, 4H), 1.26
(m, 8H), 0.86 (t, J = 7.2 Hz, 6H). 13CNMR (CDCl3, 75 MHz): δ
139.3, 125.5, 119.1, 34.6, 31.3, 30.0, 28.2, 22.5, 14.1. HR-MS (ESI):
m/z = 528.9348 [M + 1] (calcd for C18H27Br2S4, m/z = 528.9357).
2,5-Dibromo-3,6-bis((2-ethylhexyl)sulfanyl)thieno[3,2-b]-
thiophene (2c). Compound was synthesized as described above to
1
give an orange oil in 42% yield. HNMR (CDCl3, 300 MHz): δ 2.87
(d, J = 3.0 Hz), 1.42 (m, 9H), 1.25 (m, 9H), 0.87 (t, J = 7.2 Hz, 12H).
HR-MS (APCI): m/z = 584.9954 [M + 1] (calcd for C22H34Br2S4, m/
z = 584.9983).
2,5-Dibromo-3,6-bis(dodecylsulfonyl)thieno[3,2-b]-
thiophene (3b). Sulfide 2b (0.1195 g, 0.1710 mmol) was dissolved in
CHCl3 (5 mL) and cooled to 0 °C under nitrogen gas. MPCBA,
purified from commercial reagent by washing with pH 7.5 phosphate
buffer, (0.1520 g, 0.8808 mmol) was added, and the reaction mixture
was allowed to warm to room temperature and stirred for 20 h. The
reaction was then quenched with 0.1 M NaOH solution (25 mL),
extracted with CHCl3 (2 × 20 mL), and dried over Na2SO4. The
3,6-Bis(hexylsulfanyl)thieno[3,2-b]thiophene (1a). n-BuLi
(0.60 mL, 1.5 mmol) was added at −75 °C to a stirring solution of
3,6-dibromothieno[3,2-b]thiophene (0.379 g, 0.127 mmol) in diethyl
ether (20 mL) and the reaction mixture stirred for 1 h.
Dihexyldisulfide (0.42 g, 1.4 mmol) was added and the reaction was
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solvent was removed in vacuo and the product was passed through a
silica plug to afford pure sulfone 3b as a white solid (0.122 g, 93%). 1H
NMR (CDCl3, 300 MHz, 25 °C): δ 3.34 (t, J = 7.5 Hz, 4H), 1.75 (p, J
= 7.8 Hz, 4H), 1.40 (m, 4H), 1.24 (m, 16H), 0.87 (t, J = 6.6 Hz, 6H).
13C NMR (CDCl3, 75 MHz, 25 °C): δ 135.9, 131.0, 122.1, 55.4, 31.9
29.5, 29.5, 29.4, 29.3, 29.2, 28.9, 28.1, 22.7, 22.2, 14.1. HR-MS (ESI)
m/z = 761.1021 [M + 1] (calcd for C30H51Br2O4S4, m/z = 761.1031).
2,5-Dibromo-3,6-bis(2-ethylhexylsulfonyl)thieno[3,2-b]-
thiophene (3c). The compound was synthesized in the same manner
as described above to give a yellow solid in 82% yield. 1HNMR
(acetone-d6, 300 MHz, 25 °C): δ 3.45 (d, J = 6.0 Hz, 4H), 1.44 (m,
10H), 1.24 (m, 8H), 0.86 (t, J = 7.2 Hz, 12H). 13CNMR (CDCl3, 125
MHz): δ 135.8, 131.8, 122.0, 58.8, 34.6, 32.5, 28.2, 25.9, 22.6, 14.0,
10.3. HR-MS (ESI): m/z = 648.9771 [M+1] (calcd for
C22H35Br2O4S4, m/z = 648.9779).
[M + 1] (calcd for C30H41S8, m/z = 657.0968). Single crystals were
grown by dissolving S-2TTV in a hot toluene/hexane/ethanol mixture
and cooling overnight.
(E)-1,2-Bis(3,6-bis(butylsulfonyl)thieno[3,2-b]thiophen-2-yl)-
ethene (SO2-2TTV). The same procedure as above was used with 5 as
starting material. The product was flashed through silica with DCM,
concentrated, then recrystallized from propanol and centrifuged to
remove the propanol supernatant to afford 30.2 mg (45% yield) of
pure SO2-2TTV as an orange powder. 1HNMR (CDCl3, 300 MHz, 25
°C): δ 8.20 (s, 2H), 8.01 (s, 2H), 3.24 (t, J = 8.1 Hz, 8H), 1.79 (m,
8H), 1.46 (m, 8H), 0.937 (t, J = 7.2 Hz, 6H), 0.913 (t, J = 7.2 Hz, 6H).
13CNMR (dimethyl sulfoxide-d6, 500 MHz, 83 °C): δ 148.5, 139.9,
139.2, 134.0, 132.6, 129.7, 123.9, 56.8, 55.9, 24.7, 24.7, 21.08, 21.05,
13.56, 13.55. HR-MS (ESI): m/z = 785.0569 [M + 1] (calcd for
C30H41O8S8, m/z = 785.0562). Mp >300 °C (dec.)
2-Bromo-3,6-bis(butylsulfanyl)thieno[3,2-b]thiophene (4).
3,6-Bis(butylsulfanyl)thieno[3,2-b]thiophene (0.1667 g, 0.5266
mmol) was cooled to −75 °C in dry Et2O. nBuLi was added dropwise
and the reaction was stirred for 40 min at −75 °C before being
warmed to −10 °C for 1 h. The reaction mixture was then cooled back
to −75 °C, and carbon tetrabromide (0.1665 g, 0.5021 mmol),
dissolved in 1 mL of dry Et2O, was added dropwise. After 2 h, the
reaction mixture was quenched with H2O (10 mL), diluted in Et2O
(25 mL), washed with H2O and NaHCO3 solution, and then dried
over Na2SO4. The product was purified by silica-gel column
chromatography with hexane and CHCl3 as eluent (1:0 to 9:1
gradient) to afford a yellow oil (0.126 g, 61% yield). 1HNMR
(acetone-d6, 400 MHz, 25 °C): δ 7.65 (s, 1H), 2.985 (t, J = 7.2 Hz,
2H), 2.978 (t, J = 7.2 Hz, 2H) 1.61 (m, 4H), 1.46 (m, 4H), 0.895 (t, J
= 7.2 Hz, 3H), 0.870 (t, J = 7.2 Hz, 3H).
2-Bromo-3,6-bis(butylsulfonyl)thieno[3,2-b]thiophene (5).
Compound was synthesized in the same manner as described for
compound 3 to give a yellow solid in 86% yield. 1HNMR (acetone-d6,
300 MHz, 25 °C): δ 8.59 (s, 1H), 3.50 (t, J = 7.5 Hz, 2H), 3.39 (t, J =
7.8 Hz, 2H), 1.72 (m, 4H), 1.45 (m, 4H), 0.89 (t, J = 6.9 Hz, 6H).
General Procedure for Stille Polycondensation Polymer-
ization. The TT and (E)-1,2-bis(tributylstannyl)ethene monomers
were weighed out and transferred to a pear-shaped flask and placed
under a stream of nitrogen gas. Pd0(PPh3)4 (10 mol %) was weighed
out into a dry 25 mL Schlenk tube in a glovebox. The monomer
mixture was transferred to the reaction flask, rinsing with dry toluene
(2 mL). The mixture was degassed by freeze−pump−thaw (2 × 15
min at −75 °C). The reaction was then heated to reflux (110 °C) and
sealed under nitrogen gas. The polymerization was monitored by UV
over 48 h, but no further shift in the UV absorption onset was
observed after 15 h. The reaction product was then precipitated by
dropwise addition into MeOH (100 mL) and collected by filtration on
PTFE membrane. The polymers are purified by Soxhlet extraction by
sequential washing with ethanol, acetone, and hexane and finally
collected in chloroform or chlorobenzene.
ASSOCIATED CONTENT
■
S
* Supporting Information
1H and 13C NMR spectra of the synthesized polymers (solution
and solid state) and monomer precursors, emission spectra of
SO2-PTTV and S/SO2-PTTV-1:1. CV of S/SO2-PTTV-1:2,
additional device performance data. DFT optimized geometries
of calculated polymers, AFM images of polymer blend
morphology, and an X-ray crystallographic data (.cif) file.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: (D.P.) dmitrii.perepichka@mcgill.ca.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was funded by NSERC-Discovery. JAS thanks
NSERC for Vanier fellowship and McGill University for
funding her visit to Prof. Bazan’s lab (UCSB). W.W.’s work is
funded by the NSF (DMR-SOLAR 1035480). We thank
Francine Belanger-Gariepy (University of Montreal) for X-ray
crystallographic analysis.
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1
(46% yield) of S-2TTV as an orange powder. H NMR (CDCl3, 300
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2.822 (t, J = 7.2 Hz, 4H), 1.50 (m, 16H), 0.863 (t, J = 7.2 Hz, 6H),
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