Highly emissive and electrochemically stable thienylene vinylene oligomers and copolymers: An unusual effect of alkylsulfanyl substituents

Article abstract of DOI:10.1002/adfm.200902300

The synthesis, unexpected efficient photoluminescence, and reversible electrochemical p- and n-doping of new conjugated thienylene vinylene materials functionalized with alkylsulfanyl substituents poly(trithienylene vinylene) (PTTV) and poly(dithienylvinyl-co-benzothiadiazole) (PDTVB) along with dithienylvinylene-based oligomers is reported. The materials are studied by thermal and X-ray diffraction analysis, optical spectroscopy, cyclic voltammetry, and spectroelectrochemistry. Organic field-effect transistors (OFETs) are fabricated with PTTV and PDTVB. The polymers, prepared by Stille polycondensation, exhibit good thermal stability and a photoluminescent quantum yield in the range 34%-68%. Low bandgaps (1.5-1.8 eV), estimated by optical and electrochemical measurements along with high stability of both redox states, suggest that these structures are promising materials for photovoltaic applications. OFETs fabricated with PDTVB reveal a hole mobility of 7 × 10^-3 cm² V^-1 s^-1 with on/off ratio 10⁵, which are comparatively high values for completely amorphous polymer semiconductors.

Full text of DOI:10.1002/adfm.200902300

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Highly Emissive and Electrochemically Stable
Thienylene Vinylene Oligomers and Copolymers:
An Unusual Effect of Alkylsulfanyl Substituents
By Shehzad Jeeva, Olena Lukoyanova, Athan Karas, Afshin Dadvand,
Federico Rosei, and Dmitrii F. Perepichka*
devices by solution processing of conju-
gated polymers is particularly appealing for
large-area optoelectronic applications.[2]
Significant progress in the field of organic
electronics has already led to the commer-
cialization of highly efficient, bright, and
multicolored thin-film displays based on a
range of organic light-emitting diode
(OLED)devices.[3] Atthesametime,organic
field-effect transistors (OFETs)[4] and
organic photovoltaics (OPVs),[5] despite
major recent advancements, are still facing
issues of material performance and stability
that need to be addressed before wide
commercial applications are realized.
Polythiophenes are generally considered
as some of the most promising candidate
materials in large area optoelectronics.[6] In
particular, regioregular poly(3-hexylthio-
phene) (P3HT, Formula 1), which has a
bandgap of ꢀ1.9 eV, has shown superior
semiconducting performance with hole
The synthesis, unexpected efficient photoluminescence, and reversible
electrochemical p- and n-doping of new conjugated thienylene vinylene
materials functionalized with alkylsulfanyl substituents poly(trithienylene
vinylene) (PTTV) and poly(dithienylvinyl-co-benzothiadiazole) (PDTVB) along
with dithienylvinylene-based oligomers is reported. The materials are studied
by thermal and X-ray diffraction analysis, optical spectroscopy, cyclic
voltammetry, and spectroelectrochemistry. Organic field-effect transistors
(OFETs) are fabricated with PTTV and PDTVB. The polymers, prepared by
Stille polycondensation, exhibit good thermal stability and a
photoluminescent quantum yield in the range 34%–68%. Low bandgaps
(1.5–1.8 eV), estimated by optical and electrochemical measurements along
with high stability of both redox states, suggest that these structures are
promising materials for photovoltaic applications. OFETs fabricated with
PDTVB reveal a hole mobility of 7 T 10ꢁ3 cm2 Vꢁ1 sꢁ1 with on/off ratio 105,
which are comparatively high values for completely amorphous polymer
semiconductors.
mobility up to 0.1 cm2 Vꢁ1 sꢁ1 and is still the benchmark
polymeric material for OFETs and OPVs.[6] Nevertheless, there is
still a significant interest in tuning the electronic structure of
polythiophenes to further improve their charge carrier mobility,
enhance the luminescence quantum yield (which is rather low in
mostpolythiophenes),andreducethebandgaptobeabletocapture
a large portion of the solar spectrum in OPV applications. One of the
major approaches to such tuning is the reduction of the aromatic
stabilization (which hampers electron delocalization), for example, by
introducing donor–acceptor or aromatic-quinoidal structural distur-
bances.[7,8] Diluting the aromaticity by alternating benzene rings with
vinylenegroupshas been aparticularly fruitful approach leadingtothe
famous poly(phenylene vinylene) (PPV) and thousands of its
derivatives, which were the first and still are some of the most
extensively investigated polymers in OLED applications.[9]
The thiophene analog of PPV, poly(thienylene vinylene) (PTV),
however, has attracted relatively limited attention.[10–16] This is
somewhat surprising considering the substantial bandgap reduc-
tion in PTV (1.7 eV[10,14] compared to ꢀ2 eV for polythiophene[6]
and 2.5 eV for PPV[9c]), the relatively high hole mobility measured
in OFETs based on PTV[17] and its copolymers,[18,19] and the high
conductivity in the doped state.[20] One of the major drawbacks of
PTV materials is the dramatic loss of emissive properties
(quantum yield FPL ꢀ0.001% for parent PTV[21]). The lack of
1. Introduction
Conjugated polymers with aromatic and/or heteroaromatic
backboneshaveattractedwideattentionasplasticsemiconductors,
with a plethora of applications that span across the fields of light-
emitting diodes, photovoltaic cells, field-effect transistors, chemi-
cal and biosensors, and so on.[1] Much of the interest is due to the
fact that the properties of these materials can be readily tuned on a
molecular level to address specific applications. Moreover, the
prospect of manufacturing low-cost and/or flexible electronic
[*] Prof. D. F. Perepichka, Dr. S. Jeeva, Dr. A. Karas, Dr. O. Lukoyanova,
A. Dadvand
Department of Chemistry McGill University
Montreal, Quebec, H3A 2K6 (Canada)
E-mail: dmitrii.perepichka@mcgill.ca
Prof. F. Rosei, A. Dadvand
´
´
´ ´
Centre Energie, Materiaux et Telecommunications Institut
National de la Recherche Scientifique Varennes
Quebec J3X 1S2 (Canada)
Prof. D. F. Perepichka, Prof. F. Rosei
Centre for Self-Assembled Chemical Structures McGill
University Montreal, Quebec, H3A 2K6 (Canada)
DOI: 10.1002/adfm.200902300
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many thiophene-based polymers. Here, we describe the synthesis
and characterization of poly(trithienylene vinylene) (PTTV), its
donor–acceptor analog poly(dithienylvinylene-co-benzothiadiazole)
(PDTVB), and three model oligomers. We report tunable (deep-blue
to deep-red) and efficient PL, reversible electrochemical p- and n-
doping with correspondingin situ spectroscopic characterization, as
well as the OFET performance of the synthesized materials.
2. Results and Discussion
2.1. Synthesis
Formula 1. Structures of discussed conjugated polymers.
The synthesis of the key monomer (2, Scheme 1) involved the
conjugate addition-elimination of alkylthiol to 3-bromothiophene-
2-carbaldehyde to afford corresponding sulfide 1, followed by
McMurry coupling to give pure E-1,2-bis(3-(dodecylthio)thien-2-
yl)vinylene 2. Attempts to synthesize dibromo- and diiodo-
derivatives of 2 under various conditions failed, probably due to
the presence of an active vinylene unit. In fact, the intramolecular
cyclization of related 3-alkylthio-2-vinylthiophenes derivatives by
action of I2 was previously reported.[37] Successful functionaliza-
tion of 2 was, however, achieved by stannylation of the dilithium
derivative of 2 with tributyltin chloride (to give intermediate 3) as
well as oxidative dimerization of its monolithium salt with CuCl2
(to give oligomer 4).
The synthesis of PTTV 6 and PDTVB 7 was accomplished by
Stille polycondensation using equimolar amounts of 2,5-dibro-
mothiophene or 4,7-dibromobenzothiadiazole, respectively, and
monomer 3. The dark purple polymers were purified by
reprecipitation from dichloromethane (DCM) with methanol
followed by Soxlett extraction of short oligomers with hexane. The
molecular weight (MW) and the polydispersity index (PDI) were
assessed by gel-permeation chromatography (GPC) and matrix-
assisted laser-desorption ionization time-of-flight (MALDI-TOF)
analyses, which showed a moderate MW of 24 700 g mol–1 with
PDI of 1.45 for 6 and MW of 8000 g mol–1 with PDI of 2.0 for 7.
These relatively low MWs are typical for Stille coupling
polymerization and the wide distribution of polymer chain
lengths might be partially responsible for the amorphous nature
of 6 and 7. Another model oligomer 5 was synthesized using
similar Stille coupling with 2-bromothiophene.
photoluminescence (PL) in PTVand its derivatives has been noted
by several authors and is well-accepted in the literature.[21–25]
Although PL was reported for a few PTV copolymers with large
fluorophoric groups (fluorene,[26,27] biquinoline,[28] and carba-
zole[29]) their efficiency was relatively low; all PTV homopolymers
and copolymers with small fluorophoric groups (e.g., benzothia-
diazole,[30] thiophene-S,S-dioxide[24a]) are not emissive. To the
contrary, PTV acts as an efficient luminescence quencher.[13]
A
large number of thienylene vinylene oligomers have also been
studied,[31] yet no efficient emitters have been reported so far.
The reason for this loss of emissive properties is not clear. Early
studies[21] attributed the lack of PL to the effect of sulfur and
enhanced conjugation (which leads to fast exciton–exciton
recombination). While this is plausible, it does not explain why
the PL of PTVis much lower than that of polythiophene. Avery low
singlet-to-triplet gap in PTV oligomers, which could lead to fast
intersystem crossing and quench the singlet excited state, was
predicted by time-dependent density functional theory calcula-
tions (TD-DFT).[32] Also, the low efficiency of PL of the smallest
building block of PTV, dithienylvinylene (DTV) (which is still
higher than that of PTV: 1% in nonpolar solvents, 5% in
butyronitrile) was attributed to a symmetry-forbidden transition
from the lowest excited state.[33] It is also likely that synthesis-
related impurities and/or backbone defects provide an additional
quenching pathway for some PTV polymers. Indeed, the majority
of synthetic approaches to PTV and its derivatives were based on
either a thermal conversion route (heating a soluble precursor
[10,13–15,21]
>200 8C)
or oxidative[11,15] polymerization. While the
Thermogravimetric analysis (TGA) of the dithienyleneethene
monomer 2b and polymers 6 and 7 (in N2 atmosphere) reveals a
reasonably high thermal stability with decomposition onsets
>300 8C. Differential scanning calorimetry (DSC) shows no phase
transitions for both polymers in the temperature range ꢁ10–250 8C.
Also, X-ray diffraction (XRD) analysis shows no sign of crystallinity in
either of the polymers (see Supporting Information).
thermal conversion route does not appear to be a problem for PPV
materials, one could expect thiophene rings to be less stable in
these conditions. An important exception is due to alkyl-
substituted poly(thienylene cyanovinylene) (PTCV), which was
synthesized by more gentle Knoevenagel condensation and
showed near-IR (NIR; 950 nm) emission.[34,35] Most recently the
NIR PL (724 nm) was found in alkoxycarbonyl-substituted PTV,
although the PL quantum yield was not reported for either of these
polymers.[36]
2.2. Optical Properties
Revisiting PTV materials as possible precursors for ladder-type
thienoacene,[37,38] we have synthesized several alkylsulfanyl-
substituted thienylvinylene oligomers and surprisingly found these
to be highly luminescent materials. We thus decided to investigate
whether highly emissive properties can be generally engineered in
electron-rich PTV materials and if such properties can be combined
with a low bandgap and high charge mobility, which are typical for
The optical properties of new materials 2 and 4–7 were measured
in tetrahydrofuran (THF) solutions and in thin films (for 6 and 7).
The DTV 2 isa pale-yellow solid with absorption maximain theUV
region of the spectrum (364 nm). Extending the conjugation
through endcapping with thiophene rings (5) or through
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Scheme 1. i) RSH, K2CO3, DMF, rt, 12 h; ii) TiCl4, Zn, THF, reflux, 2 h; iii) n-BuLi (2.5 M in hexanes), tributyltinchloride, THF, ꢁ78 8C to rt; iv) Pd(PPh3)4,
DMF, 130 8C, N2, 24 h; v) Pd(PPh3)4, toluene, reflux, N2, 24 h; vi) n-BuLi (2.5 M in hexanes), CuCl2, THF.
dimerization (4) leads to a predictable red shift (60–100 nm) of the
absorption (Table 1, Fig. 1). The much larger bathochromic shift
experienced by polymer 6 (>200 nm) suggests a rather long
effective conjugation length. The red-edge of the absorption band
gives the optical band gap of 1.83 eVfor 6. It can be further reduced
by employing a well-known donor–acceptor alternation strategy.[7]
Indeed, incorporating an electron-poor benzothiadiazole unit in 7
lowers the band gap to 1.65 eV. In the solid state the bandgap
absorption of both polymers undergoes a red shift of ꢀ50–100 nm
as a result of intermolecular p–p interactions (Fig. 2). The effect is
somewhat stronger for benzothiadiazole-containing 7, which
shows a very low optical bandgap of 1.48 eV in condensed phase
(compared to 1.75 eV for 6). No further bandgap decrease was
observed upon annealing the polymer film by exposing to CHCl3
vapors[39] or by heating at 120 8C for 48 h, in agreement with the
amorphous nature of the materials (see Supporting Information).
We note that the low bandgap and the very broad absorption of 7
that covers the entire visible spectrum make it of great potential
interest for photovoltaic applications.
Rather unexpectedly, all DTV derivatives, including 6 and 7,
showed bright PL (Table 1, Fig. 1). The emission color changes as
the bandgap is decreased with extending the conjugation and
Table 1. Optical characteristics of compounds 2b and 4–7.
THF solution
Thin films
Egopt [eV]
Egopt [eV]
PL
max
PL
abs:
max
abs:
Compound
FPL (%)
l
[nm]
l
[nm]
max
l
[nm]
l
[nm]
max
2b
4
364
426
32[d]
21[c]
49[c]
68[a]
34[b]
2.95
2.25
2.48
1.83
1.65
–
–
467
428
574
586
572
523
655
702
–
–
–
–
5
6
588
670
680
700
1.75
1.48
7
[a] Zinc phthalocyanine in 1% pyridine/toluene solution as a reference (l range 660–750 nm, FPL ¼ 0.30).[47] [b] Nile Blue in methanol as a reference
PL
max
(FPL ¼ 0.27).[47] [c] Fluorescein in 0.1 M aq. NaOH as a reference (¼0.79).[1] [d] Anthracene in ethanol as a reference (FPL ¼ 0.27).[47]
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Figure 3. DFT-optimized structure of model oligomer 2( showing a pre-
ferred out-of-plane orientation of alkylsulfanyl substituent enforced by
hydrogen bonding with vinyl protons.
reducing the HOMO energy from ꢁ5.20 eV in 2 (ꢁ5.19 eV in 2(()
to ꢁ5.37 eV for 2(. We note that the HOMO energy of 2( is in
excellent agreement with the value obtained for its longer alkyl
chainanalog2bbyelectrochemistry(seebelow).AloweredHOMO
reduces the risk of oxidation (doping) by oxygen, which is a
common problem in electron-rich polythiophenes and could
certainly contribute to the quenching of the PL in majority of PTV
derivatives. In fact, the only emissive PTVs reported are those
containing electron-withdrawing groups: cyano-substituted
PTCV[34,35] and recently reported alkoxycarbonyl-substituted
PTV.[36]
The minimal energy conformation of 2( (Fig. 3) shows a slight
twist (88) between the thiophene ring and the vinylene moiety,
which should not significantly affect the conjugation. The
methylsulfanyl substituents are out-of-plane, which allows the
sulfur atoms to engage its lone pair in a hydrogen-bonding
Figure 1. UV–Vis absorption (top) and PL (bottom) spectra of thienylene
vinylene derivatives 2 and 4–7 in THF solution.
PL
max
covers the full range from deep-blue (l 426 nm for 2b) to deep-
PL
max
red (l 702 nm for 7). The PL efficiency, FPL, of up to 68% (for 6,
Table 1) is not only much higher than that of all previously reported
thienylene vinylene polymers (most of which are not emissive) but
also higher than that of all thiophene homopolymers
(FPL ꢂ 25%).[22]
˚
interaction with vinylene protons (SꢃꢃꢃH distance is 2.63 A). This
We attribute this enhanced fluorescence to the effect of the
alkylsulfanyl group on the thiophene ring. Indeed, the analogs of
these materials with alkyl,[13,40] alkoxy,[23] phenyl,[14] or no
substituents[21] are not emissive. Such enhancement is rather
surprising since introducing sulfur atoms in a fluorophore usually
leads to enhanced intersystem crossing and dramatically
suppresses the PL quantum yield. In order to understand the
effect of the alkylsulfanyl group, we performed DFTcalculation of
the 2 and its derivatives with methylsulfanyl (2() and ethyl (2(()
substituents at the B3LYP/6-31G(d) level. The results clearly show
that alkylsulfanyl group acts as a moderate electron-acceptor,
conformation is preferred versus the fully planar structure by
1.2 kcal molꢁ1. The out-of-plane orientation of the alkyl chains
should result in ‘‘screening’’ of the conjugated polymer backbone
by substituents preventing close pꢃꢃꢃp interaction and further
reducing the risk of PL quenching through excimer formation.
2.3. Electrochemistry
The electrochemical properties of oligomers 2b, 4, and 5 were
investigated by cyclic voltammetry (CV, Fig. 4) and the obtained
redox potentials were used to determine the HOMO/LUMO
energies (Table 2). The shortest oligomer 2b shows an irreversible
oxidation wave with current peak potential of Epa ¼ 0.57 V versus
Fc/Fcþ, indicating a chemical instability of a radical cation on an
isolated thiophene ring. The following partially reversible redox
waves at 0.88 and 1.25 V versus Fc/Fcþ must be due to the
product(s) of the first oxidation. However, no polymer film
formation was observed on the electrode surface upon multiple
potential cycling (for concentrations up to 20 mM). No reduction
process was detected by CV within the potential window,
consistent with the large optical bandgap of 2b. The bithiophene
moiety in the structure of 5 stabilizes the ion radical species and
two reversible one-electron oxidation waves as well as a partially
reversible reduction wave were observed in its CV.
Figure 2. UV–Vis absorption of 6 and 7 in thin films.
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intramolecular reaction, rather than electro-
oxidative polymerization, contributes to the
electrochemical irreversibility.
Both 6 and 7 show a further reduction of the
HOMO–LUMO gap from the oligomers 4 and
5. Both polymers also exhibit reversible p- and
n-doping behavior, as seen from their CVs in
Figure 5. The bandgaps, 1.98 eV for 6 and
1.67 eV for 7, were determined from differ-
ential pulse voltammetry (DPV, see Supporting
Information). These values were somewhat
higher, yet in reasonable agreement with the
optical bandgaps (Table 1). Replacing the
thiophene moiety with electron-deficient ben-
zothiadiazole in 7 significantly facilitates the n-
doping, while the p-doping is only slightly
disfavored as compared to 6. The electroche-
mical bandgap of 7 is also consistent with the
bandgaps reported for polymers with the same
backbone structures, but different substitution
pattern (alkoxy groups).[30] Interestingly, the
alkylsulfanyl substituents seem to significantly
increase the electrochemical n-doping stability
of 7. Two discernible reduction processes were
observed for 7 in propylene carbonate and
acetonitrile (see Supporting Information) since
it was sufficiently stable at more negative
potentials.Multiplepotentialcycling(100cycles
in propylene carbonate) of the first reduction
Figure 4. CVs of 1.1 mM oligo(thienylene vinylene)s in 0.1 M TBAPF6/CH2Cl2; scan rate:
0.1 V sꢁ1
.
process,withintheꢁ1.8toꢁ0.1 VversusFc/Fcþ
The first oxidation peak of 5 is 250 mV less positive than that of
DTV 2, indicating a raised HOMO level due to the extended
conjugation. A very similar effect was observed for the DTV
potential range, resulted in less than 9% loss of cathodic peak
current intensity (see Supporting Information). The second
reduction is slightly less reversible and an additional 60 cycles,
including first and secondreduction processes (i.e., up toꢁ2.2 V vs.
Fc/Fcþ), resulted in a ꢀ40% loss of electrochemical signal.
Spectroelectrochemical experiments further confirmed excel-
lent stability of both polymers toward p-doping, even without any
protection from the atmospheric oxygen and moisture. Polymer 6
exhibits distinct color switching from purple to black upon
electrochemical oxidation. The absorption spectrum of the neutral
polymerfilm began to change at 0.6 V(vs. Ag wire), consistent with
the onset p-doping potential under these experimental conditions.
The bandgap absorption band at lmax ¼ 650 nm decreases while
two new broad polaronic absorption bands (at lmax ꢀ1000 nm and
>1600 nm) start to grow (Fig. 6, solid lines). An isosbestic point at
.
þ
dimer 4. However, while the first oxidation giving 4 was
(quasi)reversible, removing the second electron (presumably from
the terminal monothiophene moieties) is irreversible. The second
oxidationisfollowedbyfourconsecutiveoxidationwaves(twicethe
number observed for corresponding ‘‘monomer’’ 2b at 0.72, 0. 95,
1.07, and 1.25 V versus Fc/Fcþ. These redox waves were not
observed for the shorter oligomer 5, highlighting the reactivity of
the monothiophene-centered radical-cation in these structures.
Similar oligomers, without alkylsulfanyl groups, were reported to
undergo only two reversible electrochemical oxidations under
similar conditions.[41] This fact, along with the lack of concentra-
tion dependence and of film growth on the electrode suggest that
Table 2. Redox potentials[a] and HOMO–LUMO levels of 2b, 4, and 5 in solution and 6 and 7 in thin films.
pa
red
pa
/Epc
ox1 ox1
Erpecd/E
E
1=2
red
1=2
E
1ox
Compnd
HOMO[b]
LUMO[b]
Egap[b]
E
2b
4
–
–
0.57/–
–
ꢁ5.37
ꢁ5.1
>2.9
2.36
2.58
1.98
1.67
ꢁ2.02/ꢁ2.16
ꢁ2.08/ꢁ2.40
2.09
2.24
0.38/0.22
0.38/0.29
0.30
0.34
ꢁ2.71
ꢁ2.56
ꢁ2.89
ꢁ3.25
5
ꢁ5.14
ꢁ4.96
ꢁ5.06
6
ꢁ1.91[c]
ꢁ1.55[c]
0.16[d]
0.26[d]
7
[a] All potentials are reported in volts versus Fc/Fcþ redox couple. [b] HOMO and LUMO energies and bandgaps are determined from CV for solution (0.1 M
TBAPF6/CH2Cl2) and DPV for films (0.1 M TBAPF6/propylene carbonate as an electrolyte) according to EMO ¼ ꢁ4.8 – Ered/ox versus Fc/Fcþ and reported in
onset
red
onset
ox
eV. [c] E
. [d] E
.
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Figure 5. CVs of 6 and 7 films in 0.1 M TBAPF6/propylene carbonate; scan
rate: 0.1 V sꢁ1
.
Figure 7. Output (left) ꢁa5nd transfer (right) characteristics of OFET based
V
ꢁ1sꢁ1; on/off ratio 103) and 7 (bottom,
ꢀ750 nm is maintained during these spectral changes, up to
þ0.95 V potential, suggesting a clean transition between the
neutral and polaronic state. Different spectral characteristics (a
band at ꢀ800 nm and a very broad absorption >1100 nm) start to
develop above þ0.95 V, which is consistent with formation of
bipolaronic states (Fig. 6, dashed lines). Upon reduction back to
0 V, the absorption spectrum resumed its original (undoped) shape.
Very similar spectroelectrochemical behavior was observed for
7 (see Supporting Information), although due to limited spectral
changes in the visible region (400–700 nm), the polymer retained
its dark-blue appearance throughout electrochemical oxidation in
the 0 to 1.3 V (vs. Ag wire) range with spectral recovery upon re-
reduction to the neutral form.
on 6 (top, mh ¼ 4 ꢄ 10 cm2
VT ¼ ꢁ10 V. 7: mh ¼ 7 ꢄ 10ꢁ3 cm2 Vꢁ1
s
ꢁ1; on/off ratio 105; VT ¼ ꢁ25 V).
of 6 and 7, while having the advantage of simplified device
manufacturing (no need of annealing and uniform performance
unaffected by the degree of crystallinity), clearly limits the charge
mobilityinthesematerials.Nosignsofcrystallization(basedeither
on XRD or optical absorption data) was observed during annealing
of the polymer films by heating at temperatures up to 120 8C or by
exposure to chloroform vapor. Accordingly, these treatments did
not improve device performance. While mobilities approaching
10ꢁ2 cm2 Vꢁ1 sꢁ1 are far from the champion values for organic
semiconductors in crystalline form, they are quite close to the best
mobilities observed in amorphous organic semiconductors.[19,42]
We also note that the on/off ratio of 105 achieved by 7 is quite high
as for solution-processed device.
2.4. FET Studies
Thin-film transistors were prepared in bottom-contact configura-
tionbyspin-coating achlorobenzenesolution of6 and7 onSi/SiO2
substrates prepatterned with Au electrodes. The transistor output
and transfer characteristics are shown in Figure 7. OFETs based on
3. Conclusions
6 showed low hole mobilities (up to 4 ꢄ 10ꢁ5 cm2 Vꢁ1
s
ꢁ1).
However, much higher hole mobilities (up to 7 ꢄ 10ꢁ3 cm2 Vꢁ1 sꢁ1
)
were found for thiadiazole-containing 7. The amorphous natures
In summary, we have demonstrated the synthesis and character-
ization of highly emissive, low-bandgap conjugated polymers 6
and 7, along with corresponding model oligomers 2, 4, and 5. We
found that the alkylsulfanyl group, aside from a predictable
improvement of solubility, causes unexpected electronic effects
dramatically increasing the emissivity of these thienylene vinylene
derivatives and improving their electrochemical stability. Indeed,
while most of thienylene vinylene derivatives show no or marginal
fluorescence, a PL quantum yield of 68% and 34% was measured
for 6 and 7, respectively. DFT calculations reveal the electron-
withdrawingeffectofthealkylsulfanylgroupandthepreferredout-
of-plane orientation of the alkyl chain that could minimize the
quenching of the PL by oxygen-doped states and formation of
excimers, respectively. Further synthetic exploration of the
alkylsulfanyl substituents in conjugated materials thus seems
justified.[43]
Excellent stability of both p- and n-doped states of the
synthesized polymers was demonstrated by repeated CV as well
as spectroelectrochemical studies. The bandgap energies (ꢀ1.7 eV
Figure 6. Spectroelectrochemistry of 6 film in 0.1 M TBAPF6/propylene
carbonate; potential is versus Ag wire pseudo-reference electrode, spectra
collected in 25 mV intervals.
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The charge-carrier mobilities of OFETs were calculated in the saturation
regime on the reverse scan, from a plot of the square root of the drain
current versus gate voltage using
for 6, ꢀ1.5 eV for 7) make these polymers potential candidates for
OPV. The OFETdevice fabricated with 7 showed an un-optimized
hole mobility of up to 7 ꢄ 10ꢁ3 cm2 Vꢁ1 sꢁ1, which is a reasonably
high value for completely amorphous solution-processed materi-
als. Further work is under progress to fabricate a photovoltaic
device from 7.
2
IDS ¼ CimðW=2LÞðVG ꢁ VTÞ
(1)
where IDS is the drain-source current, Ci the capacitance per unit area of
the gate dielectric (18 nF cmꢁ2), L the channel length, W the channel
width, and VT and VG are the threshold voltage and gate voltage,
respectively. Electrical measurements were carried out at room tempera-
ture under vacuum (10ꢁ6 mbar) using a LakeShore probe station and
Keithley 4200 semiconductor parameter analyzer.
4. Experimental
Materials and Methods: All reactions were carried out under dry
nitrogen atmosphere. The reagents n-butyllithium (1.6 and 2.5 M in
hexanes), tributyltin chloride, palladium (II) chloride (Alfa Aesar),
triphenylphosphine, N-bromosuccinimide, and thiophene were obtained
commercially (from Aldrich, except where noted) and used without further
purification. IR spectra were obtained on a Nicolet 6700 FTIR spectrometer
in an ATR mode. 1H NMR and 13C NMR spectra were obtained on a Varian
Mercury 300 or 400 MHz NMR spectrometer. Mass spectra were run on a
Kratos 7525 RFA (EI, 70 eV) or ThermoFisher Orbitrap (ESI and APCI)
mass spectrometer. UV–Vis spectra were measured with a Varian Cary
5000 spectrometer in 1 cm cuvettes or in films spin-coated on
microscope glass slide. GPC analysis was performed on a Viskotek
(TDA model 301) instrument using polystyrene standards, conventional
calibration (1.0 mL minꢁ1 in THF). Fluorescence spectra were measured
with a Varian Eclipse spectrofluorometer in 1 cm cuvettes (in solution) or in
films spin-coated on microscope glass slide. FPL of 2b and 4–7 was
measured in THF solution related to appropriate standards (Table 1) as
described before [44].
3-Bromothiophene-2-carbaldehyde [47]: To a solution of diisopropyla-
mine (25.0 mL, 184 mmol) in anhydrous THF (300 mL) in a three-neck 1 L
round bottom flask equipped with a thermometer, magnetic stirrer bar, and
septum seal, was added n-BuLi (73.5 mL, 2.5 M in hexane, 184 mmol) and
the mixture was stirred at ꢀ0–8 8C for 20 min. To the prepared solution of
lithium diisopropylamide (LDA), 3-bromothiophene (30.0 g, 184 mmol,
17.3 mL) was added dropwise via syringe keeping the temperature <5 8C.
The resultant mixture was stirred for 30 min with continuous cooling in ice
water, followed by dropwise addition of N-methylformanilide (22.7 mL,
184 mmol) over ca. 20 min. Following a period of 3 h with stirring, 1.0 M HCl
(200 mL) was added and the heterogeneous mixture was stirred for 30 min.
Et2O (100 mL) was added and the organic phase was separated and
washed with 1.0 M HCl (200 mL) and brine (200 mL) and dried over
anhydrous MgSO4. After filtration through a plug of silica and evaporation
in vacuo, the crude material was dried under high vacuum, giving the
yellow oil of the desired product (31.3 g, 89%), which was of sufficient
purity for further reaction. 1H NMR (300 MHz, CDCl3, d): 9.98 (s, 1H;
CHO), 7.72 (d, J ¼ 5.1 Hz, 1H; Ar-H), 7.16 (d, J ¼ 5.4 Hz, 1H; Ar-H); IR:
Electrochemical Measurements: All electrochemical measurements were
performed at room temperature in a three-electrode cell using a CHI-770
Electrochemical Workstation. Electrochemical investigations of oligomers
2b, 4, and 5 were conducted in anhydrous CH2Cl2 with a Pt disk
(d ¼ 1.6 mm) as the working electrode, Pt wire as the auxiliary electrode,
and Ag/Agþ as the non-aqueous reference electrode. Solutions of 6 and 7
n ¼ 1649 cmꢁ1
.
4,7-Dibromobenzothiadiazole: This was prepared according to the
published procedure [46] by bromination of benzothiadiazol, which results
in a white solid (1.1 g, 50% yield), m.p. 187–188 8C [46]. The monomer was
used without further characterization.
in chloroform were drop-cast on
a Pt disk and electrochemical
measurements were performed in propylene carbonate with Pt wire and
Ag/AgCl electrodes. All potentials were referenced against Fc/Fcþ redox
couple, which in our conditions show 0.38 V versus Ag/Agþ in CH2Cl2 and
0.41 V versus Ag/AgCl in propylene carbonate. Bu4NPF6 (0.1 M) was used
as a supporting electrolyte. The electrolyte solution was purged with Ar gas
before and between electrochemical measurements.
Spectroelectrochemistry: For spectroelectrochemical measurements,
solutions of 6 and 7 in chloroform were drop-cast on freshly cleaned
(sonication in acetone, than O2-plasma) indium tin oxide (ITO)-coated
glass and the substrate was dried in vacuum. This polymer-coated working
electrode was immersed in a 1-cm quartz cuvette containing 0.1 M
Bu4NPF6/PC, and Pt gauze and Ag wire were placed in the same cuvette as
the counter and pseudo-reference electrodes, respectively. Prior to spectral
characterization, the polymer films were electrochemically cycled within 0
to 1 V until a reproducible electrochemical response was obtained. The
progressive doping potentials were then set to the ITO electrode and after
the doping was complete (evaluated by dropping of the current, ca. 3 min)
the spectral absorption was recorded.
OFET Device Fabrication and Testing: OFETs were fabricated using
single-channel Au source/drain electrodes (bottom-contact configuration)
patterned on 190-nm-thick SiO2 thermally grown on heavily n-doped
Si (rꢀ 0.01–0.02 Ohm cm). The thickness of the electrodes was 25 nm. Au
patterning was achieved by lift off using a 5-nm-thick Cr adhesion layer [45].
Prior to deposition, substrates were cleaned by sonication in acetone and
isopropyl alcohol followed by plasma cleaning. Then, Si/SiO2 surfaces were
modified with hexamethyldisilazane (HMDS). The substrates were placed
in HMDS/toluene (10%) solution and held at 90 8C for 1 h, then washed
with acetone to remove excess of HMDS and dried under a N2 gas stream.
The single-channel devices had channel length of 6 mm and widths of
1880 mm. The semiconductor films were deposited from a concentrated
solution of both compounds by spin-coating with velocity ranging from
500 to 1200 rpm.
3-Dodecylsulfanylthiophene-2-carbaldehyde (1b): K2CO3 (27.0 g, 195 mmol),
3-bromothiophene-2-carbaldehyde [47] (15.0 g, 78.1 mmol), and dodecane-
1-thiol (19.0 g, 22.5 mL, 93.7 mmol) were added to DMF (250 mL). The
resultant amber brown mixture was stirred at room temperature for 2 days
during which time it changes to brick brown colored. The mixture was
poured into brine (400 mL) and extracted with EtOAc (500 mL). The
organic phase was separated and dried over MgSO4. Evaporating the
solvent gives a pale yellow oil (30.7 g) that was purified via column
chromatography (1:20 EtOAc/Hex) followed by recrystallization in EtOH to
give 1b as yellow crystals (17.0 g, 71%), m.p. 24–25 8C. 1H NMR (300 MHz,
CDCl3, d): 10.08 (s, 1H; CHO), 7.71 (d, J ¼ 4.8 Hz, 1H; Ar-H), 7.19 (d,
J ¼ 5.4, 1H; Ar-H,), 2.98 (t, J ¼ 7.6 Hz, 2H, SCH2), 1.65 (m; CH2), 2.4–1.2
(m; CH2), 0.87 (t, J ¼ 6.8 Hz, 3H; CH3); 13C NMR (75 MHz, CDCl3, d):
182.1 (C¼O), 145.0, 138.5, 134.4, 129.4, 34.6, 31.9, 29.6, 29.5, 29.4, 29.3,
29.1, 28.6, 22.6, 14.1; IR: n ¼ 1649 cmꢁ1; HRMS (EI, m/z): [Mþ] calcd for
C12H28OS2, 312.15816; found, 312.15719.
3-Octylsulfanylthiophene-2-carbaldehyde (1a): This was prepared exactly
as 1b using octanethiol. The product was obtained as a yellow oil (2.98 g,
89%). 1H NMR (300 MHz, CDCl3, d): 10.08 (s, 1H; CHO), 7.71 (d,
J ¼ 5.2 Hz, 1H; Ar-H), 7.11 (d, J ¼ 5.2, 1H; Ar-H), 2.99 (t, J ¼ 7.6 Hz, 2H;
SCH2), 1.66 (m, 2H; CH2), 1.44 (m, 2H; CH2), 1.26 (m, 8H; CH2), 0.87 (t,
J ¼ 6.8 Hz, 3H; CH3); IR: n ¼ 1649 cmꢁ1
.
E-1,2-Bis(3-(dodecylsulfanylthienyl-2)ethene (2b): To an ice-cold stirred
suspension of Zn dust (0.68 g) in anhydrous THF (100 mL) was added TiCl4
(3.6 g, 2 mL) dropwise. The resultant mixture was refluxed for 90 min,
during which time dissolution of yellow-green precipitate was attained to
afford a black solution. On cooling at 45 8C the aldehyde 1b (2.0 g,
6.4 mmol) dissolved in THF (15 mL) was added in one portion via septum
seal and the reaction mixture was stirred overnight at room temperature. A
saturated solution of NaHCO3 (200 mL) and CHCl3 (60 mL) was added and
the mixture was stirred for 2 h after which it was filtered, separated into
organic and aqueous phases, and the aqueous phase extracted with CHCl3
Adv. Funct. Mater. 2010, 20, 1661–1669
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(30 mL). The combined organic phases were dried over MgSO4, filtered,
and evaporated in vacuo to give a crude brownish-yellow oil, which was
purified by column chromatography (EtOAc/petroleum ether 1:30) to
afford the yellow product 2b (1.77 g, 93% yield), m.p. 64–66 8C (from
CHCl3-methanol). 1H NMR (300 MHz, CDCl3, d): 7.36 (s, 2H; ¼CH), 7.17
(d, 2H, J ¼ 5.2 Hz; Ar-H), 7.01 (d, J ¼ 5.6 Hz, 2H; Ar-H), 2.78 (t, J ¼ 7.0 Hz,
4H; SCH2), 1.56 (m, 4H; CH2), 1.24 (m, 36H; CH2), 0.87 (t, J ¼ 6.6 Hz, 6H;
CH3); 13C NMR (75 MHz, CDCl3, d): 141.9, 132.0, 130.7, 123.5, 120.9, 36.5,
32.0, 29.7, 29.6, 29.5, 29.3, 29.3, 28.7, 22.8, 14.2; IR: n ¼ 1462 cmꢁ1; HRMS
(EI, m/z): [Mþ] calcd for C34H56S4, 592.3265; found, 592.3259; TGA:
Tdec ¼ 293 8C (onset).
PTTV 6: To a mixture of monomer 3 (597 mg, 0.51 mmol) and 2,5-
dibromothiophene (122 mg, 0.51 mmol) in thoroughly degassed DMF
(10 mL) was added Pd(Ph3P)4 (10 mol%) and the mixture was stirred at
90 8C under N2 for 48 h. The reaction mixture was cooled to room
temperature and then passed through a plug of silica and evaporated in
vacuo. The dark purple residue was purified by repeated precipitation (ꢄ3)
from CHCl3 solution with MeOH, then by extraction with hexane and dried
under vacuum to give 6 (50%). 1H NMR (400 MHz, CDCl3, d): 7.19–6.89
(broad), 2.84 (broad, s), 1.67–1.23 (broad, m), 0.86 (broad, m); IR:
n ¼ 1452 cmꢁ1
;
TGA: Tdec ¼ 347.3 8C (onset); UV/Vis (THF):
lred-edge ¼ 667 nm; GPC (THF): MW ¼ 24 700 g mol–1
.
PDI ¼ 1.45;
E-1,2-Bis(3-octylsulfanylthien-2-yl)ethene (2a): Prepared exactly as 2b as
yellow powder (730 mg, 52% yield); m.p. 46–48 8C (from CHCl3-methanol).
1H NMR (300 MHz, CDCl3, d): 7.37 (s, 2H; ¼CH), 7.17 (d, 2H, J ¼ 4.8 Hz;
Ar-H), 7.01 (d, J ¼ 5.2 Hz, 2H; Ar-H), 2.79 (t, J ¼ 7.5, 4H; SCH2), 1.56 (m,
4H; CH2), 1.40 (m, 4H; CH2), 1.24 (m, 16H; CH2), 0.87 (t, J ¼ 6.9 Hz, 6H;
CH3); 13C NMR (75 MHz, CDCl3, d): 142.1, 132.2, 130.9, 123.6, 121.0, 36.7,
32.0, 29.9, 29.44, 29.38, 28.8, 22.9, 14.3; TGA: Tdec ¼ 300 8C (onset).
MALDI-TOF: 42 000 (maximum intensity). Further purification can be
achieved by prolonged (12 h) Soxhlet extraction with hexane. The hexane
insoluble fraction of 6 (25%) shows s slightly shifted absorbance
(lred-edge ¼ 680 nm) but no improvement in hole mobility measured in
OFET devices. GPC (THF): MW ¼ 30 700 g mol–1. PDI ¼ 1.33.
PDTVB 7: To a mixture of monomer 3 (501 mg. 0.42 mmol) and 4,7-
dibromobenzothiadiazole (125 mg, 0.42 mmol) in thoroughly degassed
DMF (10 mL) was added Pd(Ph3P)4 (10 mol %) and the mixture was stirred
at 90 8C under the atmosphere of N2 for 48 h. The reaction mixture was
cooled to room temperature and passed through a plug of silica and
evaporated. The dark purple residue was purified by repeated precipitation
process using CHCl3 as solvent to solubilize and cold MeOH to precipitate.
After the repeated precipitation process (ꢄ3) from CHCl3 solution with
MeOH and the further purification by Soxlett using hexane the dark purple
precipitate was dried under high vacuum to give 7 (75%). 1H NMR
(400 MHz, CDCl3, d): 8.2–8.0 (broad) 7.85–7.0 (broad)–6.89 (broad), 2.91
(broad, m), 1.9–0.6 (broad, m); IR: n ¼ 1482 cmꢁ1; TGA: Tdec ¼ 346.7 8C
E-1,2-Bis(3-dodecylsulfanyl-5-(tributylstannyl)thienyl-2)ethene (3): To
a
solution of 2b (200 mg, 0.33 mmol) in ahnydrous THF (80 mL) was added
n-BuLi (0.34 mL, 0.84 mmol, 2.5 M in Hex) dropwise at ꢁ78 8C and stirred
at room temperature for 2 h. The reaction mixture was again cooled to
ꢁ78 8C and to the above intermediate, tributyltin chloride (0.2 mL,
0.74 mmol) was added via syringe and the reaction mixture was stirred for
36 h at room temperature. A saturated solution of Na2CO3 (150 mL) was
added, followed by the addition of EtOAc (100 mL). The organic layer was
dried (MgSO4), filtered, and evaporated under vacuum to give yellowish
brown oil. The yellow oil was passed through a plug of silica column
(hexane) to give tin derivative 3 (361 mg, 95%), which was used without
further purification. 1H NMR (300 MHz, CDCl3, d): 7.35 (s, 2H), 7.01 (s,
2H), 2.78 (t, J ¼ 6.8 Hz, 4H; SCH2), 1.61–1.07 (m, 76H; CH2), 0.87 (m,
24H; CH3); IR: n ¼ 1463 cmꢁ1; MS (APCl) 1171.53 [M–Hþ].
(onset); UV/Vis (THF): lred-edge ¼ 750nm; GPC (THF): MW ¼ 8000g mol–1
,
PDI ¼ 2.0.
4,4(-Bis(octylsulfanyl)-5,5(-bis[2-(3-octylsulfanylthien-2-yl)vinyl-1]-
[2,2(]bithiophene (4): 2a (1.00 g, 2.07 mmol) was charged into a flame-
dried three-neck round-bottomed flask equipped with a thermometer.
Under an argon atmosphere, Et2O (20 mL) was added and the resultant
green/yellow solution was cooled to ꢁ78 8C. To the above suspension, n-
BuLi (2.5 M in hexane; 0.9 mL, 2.25 mmol) was added dropwise and
warmed to 0 8C for 3 h. CuCl2 was added under positive pressure and a red
color formed instantly. The resultant reaction mixture was left stirring at
room temperature overnight. The resultant blood-red mixture was diluted
with Et2O (20 mL) and washed with deionized water (20 ꢄ 3 mL) and brine
(20 mL). The organic layer was separated and dried over anhydrous
MgSO4, filtered, and evaporated in vacuum to afford a viscous red oil.
Purification by column chromatography (hexane/EtOAc; 20:1) gave red
Acknowledgements
We acknowledge support from NSERC of Canada through Discovery
Grants and a CRD grant supported by Plasmionique Inc. We are grateful to
the CFI for infrastructure support. F.R. acknowledges the Canada Research
Chairs program for partial salary support. We thank Nadim Saade (McGill
University) and Dr. Patrick Pribil (MDS Analytical Technologies) for mass-
spectrometry analyses and Petr Fiurasek (CSACS, McGill University) for
GPC. Supporting Information includes detailed electrochemical and
spectroelectrochemical analyses, XRD and MALDI-TOF data for the
polymers, and details of DFT calculations. Supporting Information is
available online from Wiley InterScience or from the authors.
1
solid product (520 mg, 52%), m.p. 56–58 8C. H NMR (400 MHz; CDCl3,
d): 7.34 (s, 4H), 7.18 (d, J ¼ 5.2, 2H), 7.10 (s, 2H), 7.02 (d, J ¼ 5.2 Hz, 2H),
2.83 (m, 8H), 1.60 (m, 8H), 1.42 (m, 8H), 1.26 (m, 32H), 0.86 (m, 12H);
13C (75 MHz; CDCl3, d): 141.5, 137.0, 134.4, 132.18, 132.15, 128.5, 124.0,
121.2, 120.5, 36.71, 36.68, 32.1, 29.790, 29.85, 29.5, 29.4, 28.88, 28.85,
22.9, 14.4; IR: n ¼ 1466 cmꢁ1; UV/Vis (THF): lmax (e) ¼ 467 nm; PL (THF):
lmax (e) ¼ 572 nm; HRMS (APCI, m/z): [MþHþ] calcd for C52H79S8,
959.3942; found, 959.3927.
Received: December 7, 2009
Revised: January 22, 2010
Published online: April 7, 2010
E-1,2-Bis(3-(dodecylsulfanyl)-5-(thien-2-yl)thien-2-yl)ethene (5): To an
oven dried round-bottomed flask, 2-bromothiophene (0.17 mg,
1.06 mmol), 3 (250 mg, 0.48 mmol), and Pd(PPh3)4 (4 mol %) were added
and dissolved in toluene (10 mL). The system was degassed and then
heated to 60 8C under argon atmosphere overnight. The reaction mixture
was cooled to room temperature. The organic layer was washed with
deionized water (50 ꢄ 3 mL) and brine (50 mL). The organic layer was
separated and dried over anhydrous MgSO4, filtered, and evaporated in
vacuum to afford a crude red solid that was purified by recrystallization
from DCM/EtOH mixture to give red needles of 5b (86 mg, 24%), m.p. 84–
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