Mixed selenium-sulfur fused ring systems as building blocks for novel polymers used in field effect transistors

Article abstract of DOI:10.1039/c0jm00602e

Eight novel polymer materials for transistor applications were prepared based on the fused ring building block selenolo[3,2-b]thiophene. The molecular structure of selenolo[3,2-b]thiophene was determined by single-crystal X-ray diffraction and was further modified for use as a monomer during the Stille synthesis of various copolymers containing substituted and unsubstituted thiophenes, thiazoles, dithienopyrroles (DTP) and benzothiadiazole subunits. The resultant polymers were analyzed for UV-Vis absorption and show a significant red shift when compared to regioregular poly(3-hexylthiophenes) (P3HT), attributed to the increased planarity and conjugation of the polymer backbone. Cyclic voltammetry experiments on the new polymers show that almost all of the new structures have a higher oxidation potential compared to P3HT, making them more stable toward oxidative doping under ambient atmospheres. The polymers were also evaluated for their electrical properties through their performance as the active layer in field effect transistors. The polymers showed relatively high mobility values, of which the highest observed value is 0.11 cm² V^-1 s^-1, with on/off ratios of about 10⁵.

Full text of DOI:10.1039/c0jm00602e

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Mixed selenium-sulfur fused ring systems as building blocks for novel
polymers used in field effect transistors†‡
Sarada P. Mishra, Anna E. Javier, Rui Zhang, Junying Liu, John A. Belot, Itaru Osaka
*
and Richard D. McCullough
Received 4th March 2010, Accepted 28th May 2010
DOI: 10.1039/c0jm00602e
Eight novel polymer materials for transistor applications were prepared based on the fused ring
building block selenolo[3,2-b]thiophene. The molecular structure of selenolo[3,2-b]thiophene was
determined by single-crystal X-ray diffraction and was further modified for use as a monomer during
the Stille synthesis of various copolymers containing substituted and unsubstituted thiophenes,
thiazoles, dithienopyrroles (DTP) and benzothiadiazole subunits. The resultant polymers were
analyzed for UV-Vis absorption and show a significant red shift when compared to regioregular
poly(3-hexylthiophenes) (P3HT), attributed to the increased planarity and conjugation of the polymer
backbone. Cyclic voltammetry experiments on the new polymers show that almost all of the new
structures have a higher oxidation potential compared to P3HT, making them more stable toward
oxidative doping under ambient atmospheres. The polymers were also evaluated for their electrical
properties through their performance as the active layer in field effect transistors. The polymers showed
relatively high mobility values, of which the highest observed value is 0.11 cm2 Vꢀ1 sꢀ1, with on/off
ratios of about 105.
Among these materials, polymers containing carbon-sulfur
1. Introduction
fused rings have been shown to some of the highest carrier
mobilities when used as the active layer in field effect
transistors.12 It is believed that the presence of fused rings with
a high degree of planarity and rigidity results in greater crystal-
linity, extended p overlap and result in overall improvements in
device performance. We also believe that the fused rings prevent
detrimental chain folding. Furthermore, the presence of fused
rings in the polymer backbone lowers the molecular reorgani-
zation energy and facilitates intermolecular charge hopping,
thereby increasing mobility.14 All of these factors have led many
groups to focus on the use of fused ring-containing organic
polymers to fabricate p-channel OFETs derived from thiophene-
based systems.11–13
Organic field effect transistors (OFETs) have recently become
more important because of their use in low cost devices, such as
smart cards, printable backplanes to drive circuits of large area
display devices, etc.1 The use of conjugated polymers in OFETs is
increasing exponentially even though their field effect mobilities
remain lower than corresponding inorganic thin film transistors.2
The increased interest is due to advantages of cheap
manufacturing and potential for printable electronics. Such
advantages have made them preferable for large-scale, low-cost
commercial applications.3 Another advantage of these materials
is that they allow for easy structural modifications through
molecular engineering, enabling the tuning of optical and elec-
tronic properties.4 One goal of this research is to produce OFETs
with high charge carrier mobilities as well as high on/off ratios
with better air stability.5 Solubility in common organic solvents
and reproducible mobilities are other requirements for easy
device fabrication by spin coating, stamping or inkjet printing.6
Many of the organic semiconductors used for p-channel OFETs
have been derived from thiophene based p-conjugated systems,7
oligo-thiophenes,8 acenes,9 porphyrins and phthalocyanines,10
polythienylenevinylenes11 and carbon-sulfur fused rings12 as well
as other fused-ring systems.13
Selenophenes are chalcogenophene homologues with chemical
and physical properties similar to those of thiophene, but have
not been well studied.15 Although various oligomeric fused ring
containing selenium atoms have been synthesized and the OTFT
properties studied, to the best of our knowledge polymers con-
taining fused selenophenes are unknown. This could be due to
low solubility16a and lack of suitable synthetic methods for
making such types of monomers and polymers.16 As a design
principle, it has been found that the replacement of the sulfur
atom by selenium can lead to electronic improvements, such as
the increase in room temperature conductivity in organic metals
such as tetrathiafulvalene (TTF) compared to tetraselenafulva-
lene (TSF) derivatives.17 Similarly, it has been shown in literature
that as compared to polythiophenes, polyselenophenes have
better conductivity and mobility due to the larger p-overlap in Se
derivatives due to the larger p-orbitals.18 Theoretical studies,19 as
well as experimental evidence,20 also indicate that poly-
selenophenes should have a lower band gap compared to poly-
thiophenes. Polyselenophenes are also expected to have lower
Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue,
Pittsburgh, PA, 15213, USA. E-mail: rm5g@andrew.cmu.edu
† This paper is part of a Journal of Materials Chemistry themed issue in
celebration of the 70th birthday of Professor Fred Wudl.
‡ Electronic supplementary information (ESI) available: Experimental
details and microstructure characterization. CCDC reference numbers
763008. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c0jm00602e
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J. Mater. Chem., 2011, 21, 1551–1561 | 1551

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oxidation and reduction potentials, easier polarizability and to
be more suitable to inter-chain charge transfer due to the inter-
molecular Se-Se interactions,21 all of which are attractive prop-
erties for organic electronics.
exposed to air while being delivered to the probe station for
measurement. While measuring the current–voltage curves and
transfer curves, VG was scanned from ꢀ80 V to +40 V. The field
effect mobilities were obtained from the transfer curves in the
saturation regime at VDS ¼ ꢀ80 V.
Considering the superior properties of selenophenes over
thiophenes, it was expected that the replacement of sulfur by
selenium could be an interesting approach toward different
materials for organic thin film transistors. In this paper, we
report on the synthesis, characterization and performance in field
effect transistors of a series of polymers containing seleno-
pheno[3,2-b]thiophene used in conjunction with other electron
rich and electron poor components to modify the polymer
optoelectronic properties.
2.3 Monomer synthesis
The dibromo compounds of B,22 C,23 G24 and H25 and the dis-
tannyl compound of D26 and E27 were synthesized as reported in
literature.
Ethyl 2-(thiophen-3-ylselanyl)acetate (1). A stirred solution
of 3-bromothiophene (30.0 g, 0.18 mol) in Et2O (300 mL)
ꢁ
was cooled to ꢀ78 C and n-BuLi (2.5 M solution in hexane,
2. Experimental
73.6 mL) was added dropwise over 30 min. After stirring for 1h,
selenium powder (16.6 g, 0.210 mol) was added in portions. The
2.1 General
ꢁ
solution stirred for another 1h at ꢀ78 C and then warmed to
Gel permeation chromatography (GPC) of all polymers was
performed on a Water 2690 separations module and a Water
2487 dual l absorbance detector, with chloroform as the eluent
and a series of three Styragel columns (104, 500 and 100 Aꢁ;
Polymer Standard Service). To determine the molecular weight,
toluene was taken as an internal standard and polystyrene was
used for calibration. 1H NMR was recorded on a Bruker
Advance 300 MHz spectrometer. UV-vis-NIR spectra of all the
polymers were recorded on a Varian Cary 5000 spectrometer in
chloroform solution and polymer thin films cast onto 22 mm
square cover glass slide.
ꢁ
ꢀ30 C for 4h. Ethyl 2-bromoacetate (35.1 g, 0.210 mol) was
added dropwise to the reaction and it warmed gradually to
room temperature overnight. The reaction was then quenched
with H2O (100 mL) and the organic layer separated. The
aqueous layer was back extracted with Et2O (2 ꢂ 50 mL) and
the combined organics dried over MgSO4 and evaporated in
vacuo. The residue was distilled under reduced pressure to yield
1
the required compound as a light yellow liquid. Yield 78%; H
NMR (CDCl3, 300 MHz) d 7.42 (dd, J1 ¼ 3.0 Hz, J2 ¼ 1.2 Hz,
1H), 7.29 (dd, J1 ¼ 5.1 Hz, J2 ¼ 3.0 Hz, 1H), 7.14 (dd, J1 ¼
5.1 Hz, J2 ¼ 1.2 Hz, 1H), 4.11 (q, J ¼ 7.2 Hz, 2H), 3.39 (s, 2H),
1.19 (t, J ¼ 7.2 Hz, 3H); 13C NMR (CDCl3, 75 MHz) d 170.76,
132.35, 128.90, 126.46, 121.85, 61.27, 28.15, 14.06; GC MS:
250 [M]+, 163, 97.
Cyclic voltammetry was carried out on an Autolab PGSTAT
100 using a three-electrode cell. A platinum stick electrode coated
with a polymer thin film was used as the working electrode and
a platinum wire was the counter electrode. An Ag/Ag+ electrode
(Ag in a 0.01 mol/L of AgNO3) was used as the reference
electrode. At the beginning of the experiment, the instrument
was calibrated against the Fc/Fc+ couple (0.01 V vs. Ag/Ag+).
Ethyl 2-(2-formylthiophen-3-ylselanyl)acetate (2). To a solu-
tion of ethyl 2-(thiophen-3-ylselanyl)acetate (1) (30 g, 0.12 mol)
and DMF (17.5 g, 0.238 mol) in 1,2-dichloroethane (200 mL) at
An anhydrous 0.1
M solution of tetrabutylammonium-
0
ꢁC was added POCl3 (27.5 g, 0.179 mol) drop wise. After
tetrafluoroborate (Bu4N+BF4ꢀ) in acetonitrile, saturated with
addition, the cooling bath was removed and the reaction heated
at reflux for 4h. It was subsequently cooled to rt and added to an
ice-cold aqueous solution of sodium acetate. The organic layer
was separated and the aqueous layer was extracted with chlo-
roform. The combined organics were washed several times with
H2O, dried over MgSO4 and evaporated under reduced pressure.
The crude product was purified by column chromatography on
silica gel (3 : 1 hexane and ethylacetate mixture) to yield the pure
compound 2 as a yellow oil. Yield 81%; 1H NMR (CDCl3,
300 MHz) d 9.98 (s, 1H), 7.73 (dd, J1 ¼ 5.1 Hz, J2 ¼ 0.6 Hz, 1H),
7.33 (d, J ¼ 5.1 Hz, 1H), 4.12 (q, J ¼ 7.2 Hz, 2H), 3.54 (s, 2H),
1.20 (t, J ¼ 7.2 Hz, 3H); 13C NMR (CDCl3, 75 MHz) d 182.75,
170.15, 138.32, 135.18, 134.74, 132.05, 61.60, 27.10, 14.01; GC
MS: 278 [M]+, 191.
nitrogen, was used as the supporting electrolyte.
2.2 Fabrication and characterization of FET devices
FET devices were fabricated in a bottom gate, bottom contact
configuration on n-type silicon substrates as the gate and ther-
mally grown 250 nm SiO2 as the dielectric layer. Source and drain
electrodes were patterned using standard photolithography and
were deposited on SiO2 by sputter deposition of 5 nm of titanium
(for adhesion) and 50 nm of gold. The devices were cleaned using
a UV cleaner (Novascan PSD-UVT) for 40 min by exposure to
UV light in air at a temperature of 120 ꢁC. The devices were
surface treated by immersing in a 30 mM solution of octyltri-
chlorosilane (OTS-8, Acros) in anhydrous hexadecane at room
temperature for 2.5–3 h. The devices were then washed several
times with HPLC grade toluene, dried with N2 flow followed by
vacuum for at least 1h. A 5 mL solution of 1 mg/mL polymer in
chloroform was deposited on the devices and allowed to evapo-
rate slowly in a glass Petri dish saturated with chloroform. After
film formation, the OFETs were kept under vacuum overnight
prior to being tested. All measurements were carried out under
a slow continuous flow of argon. However the devices were
Ethyl selenopheno[3,2-b]thiophene-5-carboxylate (3). In
a 500 mL round bottom flask, ethyl 2-(2-formylthiophen-3-
ylselanyl)acetate (2) (25.0 g, 89.9 mmol) was taken with DMF
(125 mL) and K2CO3 (37.2 g, 0.269 mol) was added. The mixture
was stirred at room temperature for 24h under N2. It was
quenched by adding H2O (100 mL) and the compound extracted
with ethylacetate. The organic phase was washed several times
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with H2O, dried over MgSO4 and evaporated in vacuo. Purifi-
cation was accomplished by column chromatography using
silica gel and hexane-ethylacetate mixture (9 : 1) to obtain
the yellow oil. Yield 89%; 1H NMR (CDCl3, 300 MHz)
d 8.25 (d, J ¼ 0.6 Hz, 1H), 7.57 (d, J ¼ 5.1 Hz, 1H), 7.35 (dd,
J1 ¼ 5.4 Hz, J2 ¼ 0.9 Hz, 1H), 4.39 (q, J ¼ 7.2 Hz, 2H), 1.41
(t, J ¼ 7.2 Hz, 3H); 13C NMR (CDCl3, 75 MHz) d 163.65, 143.70,
140.11, 138.46, 131.0, 128.25, 122.99, 61.45, 14.93; GC MS: 260
[M]+, 215, 188.
The mixture was heated at reflux with vigorous stirring for 48 h
under N2. The excess solvent was distilled and the compound
extracted with chloroform. The organic phase was washed with
water, dried over MgSO4 and evaporated. The residue was
purified by column chromatography using hexane and chloro-
form (9 : 1) to obtain a yellow solid. Yield 53%, 1H NMR
(CDCl3, 300 MHz) d 7.38 (d, J ¼ 0.3 Hz, 1H), 7.26 (d, J ¼ 0.6 Hz,
1H), 7.19 (dd, J1 ¼ 5.4 Hz, J2 ¼ 3.6 Hz, 2H), 6.94
(dd, J1 ¼ 5.1 Hz, J2 ¼ 1.2 Hz, 2H), 2.74–2.80 (m, 4H), 1.64 (quin,
J ¼ 7.2 Hz, 4H), 1.25–1.40 (m, 36H), 0.87 (t, J ¼ 6.9 Hz, 6H); 13
C
Selenopheno[3,2-b]thiophene-5-carboxylic acid (4). Ethyl sele-
nopheno[3,2-b]thiophene-5-carboxylate (3) (22.0 g, 84.6 mmol)
was dissolved in a mixture of THF and methanol (1 : 1, 200 mL)
and 10% aqueous solution of NaOH (70 mL) was added. The
reaction was stirred at rt for 3h. The excess solvents were distilled
off and the white solid dissolved in H2O. To the aqueous solu-
tion, conc. HCl was slowly added to maintain a pH between 2–3.
The white precipitate was filtered, washed several times with H2O
until the filtrate became neutral and vacuum dried to afford the
NMR (CDCl3, 75 MHz) d 140.81, 140.54, 140.18, 139.85, 139.03,
137.41, 133.02, 130.67, 130.13, 130.09, 124.26, 121.18, 120.27,
31.99, 30.87, 29.73, 29.68, 29.63, 29.55, 29.43, 29.35, 29.32, 22.76,
14.18.
2,5-bis(5-bromo-3-dodecylthiophen-2-yl)selenopheno[3,2-b]thio-
phene (8). A THF (50 mL) solution of 2,5-bis(3-dodecylthiophen-
2-yl)selenopheno[3,2-b]thiophene (7) (2.0 g, 2.9 mmol) was
ꢁ
cooled to 0 C and NBS (1.08 g, 6.09 mmol) was slowly added.
1
required compound. Yield 78%, H NMR (CD3OD, 300 MHz)
The reaction was stirred overnight at rt. The THF was evapo-
rated and the compound dissolved with EtOAc. The organic
phase was washed several times with H2O, dried over MgSO4 and
evaporated. The crude product was purified by column chro-
matography (silica gel, hexane) to obtain a yellow solid. Yield
d 8.21 (s, 1H), 7.68 (d, J ¼ 5.1 Hz, 1H), 7.39 (d, J ¼ 5.1 Hz, 1H);
13C NMR (CDCl3, 75 MHz) d 165.47, 144.02, 140.10, 138.68,
131.01, 128.25, 122.83.
1
Selenopheno[3,2-b]thiophene (5). To a solution of seleno-
pheno[3,2-b]thiophene-5-carboxylic acid (4) (15.0 g, 64.9 mmol)
in quinoline (75 mL) was added 2 CuO.Cr2O3 (0.760 g,
2.59 mmol) and this was heated to 200 ꢁC. When the evolution of
CO2 stopped (3h), the reaction cooled to rt and was added slowly
to a 50% solution of HCl (300 mL). It was extracted twice with
chloroform and the organic layer washed several times with H2O
until the aqueous layer became neutral. The chloroform layer
was dried over MgSO4 and evaporated. Purification was done by
column chromatography using silica gel and hexane-chloroform
mixture (95 : 5) to obtain an off white solid. Yield 83%, 1H NMR
(CDCl3, 300 MHz) d 7.98 (dd, J1 ¼ 5.7 Hz, J2 ¼ 1.5 Hz, 1H),
7.53 (dd, J1 ¼ 5.7 Hz, J2 ¼ 0.6 Hz, 1H), 7.40 (dd, J1 ¼ 5.4 Hz,
J2 ¼ 1.5 Hz, 1H), 7.34 (dd, J1 ¼ 5.1 Hz, J2 ¼ 0.6 Hz, 1H); 13C
NMR (CDCl3, 75 MHz) d 141.0, 138.7, 130.54, 126.97, 122.52,
122.05; GC MS: 188 [M]+.
73%, H NMR (CDCl3, 300 MHz) d 7.32 (d, J ¼ 0.6 Hz, 1H),
7.20 (d, J ¼ 0.6 Hz, 1H), 6.90 (d, J ¼ 0.9 Hz, 2H), 2.66–2.72
(m, 4H), 1.60 (quin, J ¼ 7.8 Hz, 4H), 1.24–1.40 (m, 36H), 0.86
(t, J ¼ 6.9 Hz, 6H); 13C NMR (CDCl3, 75 MHz) d 140.90, 140.66,
140.56, 139.60, 139.27, 136.30, 134.28, 132.77, 132.73, 131.94,
121.48, 120.62, 111.19, 111.15, 31.95, 30.65, 29.68, 29.59, 29.45,
29.39, 29.22, 22.72, 14.14.
2,5-bis(3-dodecyl-5-(trimethylstannyl)thiophen-2-yl)selenopheno-
[3,2-b]thiophene (9). To a clear solution of 2,5-bis(5-bromo-3-
dodecylthiophen-2-yl)selenopheno[3,2-b]thiophene (8) (1.50 g,
1.77 mmol) in THF (40 mL), n-BuLi (1.80 mL, 2.5 M solution in
hexane) was added dropwise at ꢀ78 ꢁC. The solution was stirred
for 1 h and a THF solution of trimethyltinchloride (5.30 mL, 1M
solution) was added. The solution was then stirred at rt for an
additional 4 h and quenched with H2O. The compound was
extracted with Et2O, washed several times with H2O, dried over
MgSO4 and evaporated. The dark, sticky liquid obtained was
washed with methanol and dried to obtain the required
compound in pure form. Yield 73%, 1H NMR (CDCl3, 300 MHz)
d 7.35 (s, 1H), 7.24 (s, 1H), 7.00(s, 2H), 2.74–2.81 (m, 4H), 1.63
(quin, J ¼ 7.5Hz, 4H), 1.19, 1.31 (m, 36H), 0.88 (t, J ¼ 6.6 Hz,
6H), 0.36 (s, 18H); 13C NMR (CDCl3, 75 MHz) d 141.20, 140.91,
140.54, 138.94, 138.79, 138.40, 137.54, 137.18, 137.12, 136.42,
130.86, 128.82, 120.65, 119.69, 31.96, 30.96, 29.70, 29.51, 29.51,
29.39, 29.27, 29.22,22.73, 14.15, ꢀ8.19.
2,5-dibromoselenopheno[3,2-b]thiophene (6). To an ice-cooled
stirred solution of selenopheno[3,2-b]thiophene (5) (8.00g,
42.7 mmol) in DMF (100 mL), NBS (15.2 g, 85.5 mmol) was
added in portion and the reaction mixture was allowed to stir
overnight at room temperature. The mixture was poured into
H2O and the compound filtered using a Buckner funnel. The
filtrate was washed several times with H2O and recrystallized in
MeOH to obtain a light yellow solid. Yield 87%, 1H NMR
(CDCl3, 300 MHz) d 7.36 (s, 1H), 7.17 (s, 1H); 13C NMR (CDCl3,
75 MHz) d 139.53, 138.28, 124.81, 124.64, 115.15, 113.37; GC
MS: 346 [M]+, 265, 186, 106.
Representative example for the polymerization by stille cross-
coupling reaction. Tris(dibenzylideneacetone)dipalladium(0)
(4.32 mg, 2.00 mol %) and tri(o-tolyl)phosphine (5.74 mg,
8.00 mole %) were dissolved in chlorobenzene and allowed to stir
for 10 min. A solution of 2,5-bis(5-bromo-3-dodecylthiophen-2-
yl)selenopheno[3,2-b]thiophene (8) (200 mg, 0.236 mmol)
and 2,5-bis(3-dodecyl-5-(trimethylstannyl)thiophen-2-yl)seleno-
pheno[3,2-b]thiophene (9) (239.6 mg, 0.236 mmol) was added.
2,5-bis(3-dodecylthiophen-2-yl)selenopheno[3,2-b]thiophene (7).
To a stirred THF (50 mL) mixture of 2-(3-dodecylthiophen-2-yl)-
4,4,5,5-tetramethyl-1,3,2-dioxaborolane (6.56 g, 17.3 mmol),
2,5–dibromoselenopheno[3,2-b]thiophene (6) (2.00 g. 5.78
mmol), and tetrakis(triphenylphosphine)palladium (0.26 g, 0.23
mmol) was added 2 M aqueous potassium carbonate (50 mL).
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The resulting solution was heated to reflux under N2 for 72h.
After cooling to room temperature, the reaction mixture was
added to 100 mL of MeOH containing conc. HCl (5 mL) and
stirred for 2h. The polymers were filtered, then Soxhlet extracted
with methanol and hexane to remove the inorganic impurities
and oligomers. Finally, the soluble fraction was collected by
extracting with chloroform and evaporated to obtain the poly-
mer PMA.
rigidity. This effect should manifest itself as a red shift in the
UV-vis peaks between the solution and solid-state and is sup-
ported by the observed data (vide infra). Fig. 1B shows
hydrogen bonding between the hydrogen on C2 and a neigh-
boring Se/S atom appears to strongly influence the crystal
00
ꢀ
packing; the 3.90 A C2/Se/S separation produces a short H/
ꢀ
Se/S contact distance of 3.04 A. The two ring centers of gravity
ꢀ
for a single molecule of 5 are separated by 2.12 A and the
shortest intermolecular ring center of gravity separation is
ꢀ
4.59 A. The normals to the least-squares mean planes for the
two molecules of 5 shown in Fig. 1b make an angle of 93.5ꢁ with
each other. Although one could speculate that the chalcogenide
atoms would also seek to interact with other H-atoms in the
polymer, we do not believe this is the case since the low band
gaps and UV-vis data for the derived polymers completely
support a cofacial arrangement of polymer units, to be dis-
cussed later.
3. Results and discussion
3.1. Monomer synthesis and polymerization
The selenopheno[3,2-b]thiophene (5) was synthesized from 3-
bromothiophene by a multi-step synthetic route (Scheme 1). First,
3-bromothiophene was lithiated at ꢀ78 ꢁC in dry ether; then
selenium powder was added and the reaction warmed to ꢀ30 ꢁC to
facilitate insertion of selenium metal into the thiophene ring.
Finally subsequent addition of ethyl 2-bromoacetate gave
compound 1 in a 78% yield. Compound 1 was then converted to
the corresponding 2-subtituted carbonyl compound (2) by
a Vilsmeier formylation reaction. Compound 2 was then treated
with K2CO3 in DMF to yield the ring closed product seleno-
pheno[3,2-b]thiophene-2-ethylester (3) as a yellow oil in 89%
yield. Compound 3 was hydrolyzed by aqueous NaOH in a THF/
methanol mixture followed by acidification to obtain the cor-
responding acid 4. The hydrolysis reaction was found to proceed
only in aqueous THF, using a mixture of methanol and THF
increased the reaction rate. Acid 4 was then decarboxylated with
copper chromite in quinoline (as a solvent) to afford seleno-
pheno[3,2-b]thiophene (5) as off-white crystals in very good
yield.
Selenopheno[3,2-b]thiophene (5) was then di-brominated
using NBS in DMF at low temperature to yield compound 6. It
was observed that the presence of trace mono-bromo product
resulted in product decomposition. However the di-bromo
compound 6, without the mono-bromo derivative, was suffi-
ciently stable under ambient conditions. Di-bromoseleno-
pheno[3,2-b]thiophene (6) was used for the coupling reaction
with the 2-boronic ester of 3-dodecyl thiophene to yield
compound 7 as a yellow solid. Compound 7 was di-brominated
using NBS in THF to yield monomer 8 that was used as
a precursor for the polymers. The di-stannyl monomer 9 was
easily synthesized from precursor 8, by dilithiation at ꢀ78 ꢁC,
followed by the addition of trimethyltinchloride. The di-stannyl
compound 9 was found to be unstable when attempts were made
to purify by either silica or basic alumina column chromatog-
raphy, and thus, was used without further purification. However,
after washing the crude compound with methanol to remove the
unreacted tin(IV)chloride and some dark-colored material,
compound 9 was found to be reasonably pure for polymerization
reactions. For synthesizing the polymers, either monomer 8 or
monomer 9 was used with the other di-stannyl or di-bromo
precursor respectively, keeping in mind that the di-stannyl
compound should be the more electron rich aromatic precursor
during Stille coupling.
The diffraction-derived molecular structurex of colorless
crystals (from EtOH) of compound 5 is shown in Fig. 1A. Each
individual molecule consists of a selenophene ring fused to
a thiophene ring. The similarity of size and shape for these fused
rings produces a pseudo-centrosymmetric molecule that can
pack in a neatly ordered fashion but with two possible orien-
tations for the Se/S vector. The two packing orientations for
5 would have nearly identical positions for the carbon and
hydrogen atoms but the Se and S positions would be inter-
changed. It is therefore not surprising that the nearly planar
Copolymerizations using either compound 8 or 9 (mentioned
as M in Scheme 2) with different thiophene derivatives (A-H)
were performed by reacting equimolar amounts of the di-bromo
and di-distannyl derivatives and a catalytic amount of tris-
(dibenzylideneacetone)dipalladium (0) and tri(o-tolyl)phosphine
in chlorobenzene at 150 ꢁC for 72 h to obtain the polymers
PMA-PMH. After the polymerization reaction was complete,
the polymers were obtained by precipitation with acidic meth-
anol and purified by successive Soxhlet extractions, using
methanol, hexane and finally chloroform to give polymer yields
of 76–89%. However, only a very small fraction of polymer PME
was found to be soluble in chloroform and thus the yield was
found to be comparatively low. In a similar case, polymer PMF
was found to be insoluble in most solvents. Hence it could not be
characterized and will not be discussed further here.
ꢀ
(to within 0.005 A) molecules of 5 utilize a crystallographic
inversion center in the crystal. This inversion center lies at the
midpoint of the common C–C bond for the two fused rings. The
Ci-related sites specified as Se/S and SeS0 in Fig. 1a are therefore
each occupied by a Se atom 50% of the time and a S atom 50%
of the time. This planar conformation is highly desirable
since its incorporation in the polymer structure could decrease
the polymer p-stacking distance as well as increase polymer
x CCDC 763008 contains the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Crystal data for Selenolo[3,2-b]thiophene: C6H4SSe, FW 187.11,
ꢀ
ꢀ
orthorhombic, space group Pbca, a ¼ 9.9543(7) A, b ¼ 5.9678(4) A,
ꢁ
ꢁ
ꢁ
3
ꢀ
ꢀ
c ¼ 10.1404(7) A, a ¼ 90 , b ¼ 90 , g ¼ 90 , V ¼ 602.39 A , Z ¼ 4,
rcalcd ¼ 2.063 g/cm3, m ¼ 6.450 mmꢀ1, T ¼ 100 K, 6449 reflections
collected, 916 unique (Rint ¼ 0.054), R1 ¼ 0.0201, wR2 ¼ 0.05585 [I >
2s(I)]. Supporting information contains additional experimental
procedures, 1H-NMR and X-ray data.
Before performing any material study, all polymers were kept
under vacuum overnight. Molecular weights were determined by
GPC analysis using chloroform as eluent and polystyrene as
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Scheme 1 Synthesis of selenopheno[3,2-b]thiophene containing monomer.
Fig. 1 Diffraction derived molecular structure of 5 (thermal ellipsoids at 50%) and the H-bond.
standard. The number average molecular weight of all the
polymers were obtained ranging from 10,000 to 33,000 as shown
in Table 1 with the exception of PME in which Mn was found to
be 5,600.
regioregular poly(3-alkylthiophene)s, a distinct red shift was
observed for all the polymers in both solution and thin film. This
red shift is attributed to the presence of the rigid selenothiophene
unit, which leads to the slightly more planar structure of the
polymer chain even in solution, thus increasing its effective
conjugation. In dilute chloroform solutions, the polymers
exhibited blue shifted spectra as compared to the thin films. This
blue shift for the solutions’ spectra is because of the more
disturbed interchain association in the liquid state as compared
to the solid state.
3.2 Optical properties
The solution UV-vis spectra of all polymers were studied using
chloroform solutions, and thin films cast from chloroform
solutions before and after annealing. The spectra of the cast films
are shown in Fig. 2 and the absorption maxima for all
measurements are summarized in Table 2. When compared to
For thin films, polymers PMB, PMC, PME and PMH showed
absorption maxima between 520 nm to 533 nm whereas PMA
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Scheme 2 Polymerization of selenopheno[3,2-b]thiophene containing monomer with various other monomers.
of the polymer chains leading to an increase in effective conju-
gation length. However for polymers PMB and PMC, a signifi-
cant red shift was not identified for the lmax but a shoulder
toward longer wavelength was observed, which indicates better
ordered structures.28
Table 1 Molecular weight and the yield of the synthesized polymers
a
a
Polymer
Mn
Mw/Mn
Yieldb
PMA
PMB
PMC
PMD
PME
PMF
PMG
PMH
10,000
15,000
21,000
21,000
5,600
insoluble
20,000
33,000
1.8
1.6
1.6
1.3
1.2
-na-
1.8
1.9
81%
76%
89%
89%
27%
-na-
87%
83%
Optical band gaps were calculated from the onset of UV-vis
absorption spectra and are shown in Table 2.29 The band gaps of
polymers PMA-PMF and PMH were found to be 1.72–1.93 eV,
which are slightly smaller or closer to the band gap of rr-P3HT
(1.9 eV). However polymer PMG showed a band gap of 1.57 eV,
which is much smaller than that of rr-P3HT, mainly due to the
strong donor–acceptor interaction between the electron rich
selenopheno[3,2-b]thiophene and the electron poor benzothia-
zole. Another factor to consider is that the large number of fused
rings in PMG may have led to the increased planarity in the
structure, which can lead to extended conjugation in the polymer
chain and a lower bandgap. Because of its decreased bandgap
and absorption in a broad area of the visible region, polymer
PMG may be a viable material for organic photovoltaic
applications.
a
Calculated from GPC (using chloroform as eluent w.r.t polystyrene
standards). Yields are calculated based on the chloroform soluble
fraction.
b
showed slightly red-shifted spectra compared to these. This could
be due to the presence of a larger number of selenolo[3,2-b]-
thiophene in the polymer, which makes the backbone more
planar and thus more conjugated. Similarly, polymer PMD
showed an increased red shift than polymer PMA, probably
because of the increased planarity of the polymer backbone due
to the presence of the larger DTP fused ring structure.28
Surprisingly, polymer PMG showed very broad absorption
spectra having two absorption maxima at 472 nm and 623 nm.
The probable explanation for this broadness is the presence of
a donor–acceptor group in the highly electron rich selenothio-
phene and the electron poor benzothiazole in the same backbone,
resulting in a lower band gap polymer.
3.3 Electrochemical properties
The redox properties of all the polymers, as well as rr-P3HT,
were determined by cyclic voltammetry. All polymers displayed
a partial reversible oxidation process as seen in Fig. 3. The onset
of oxidation (Eox) was calculated for each polymer, and was
found to be between 0.08V to 0.35V vs. Ag/Ag+, most of which
is higher than the Eox of rr-P3HT. Because of their higher
oxidation potential, these polymers are expected to be more
stable in their neutral state than rr-P3HT. Table 2 summarizes
the HOMO levels of all the polymers, which were calculated
from the onset of the oxidation peak (Eox), assuming that the
To study the effect of annealing, all polymer films were
ꢁ
annealed at a temperature of 150 C for 30 min inside a glove
box. An obvious red shift in the absorption spectra was observed
for most of the polymers, which may be due to increased ordering
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Fig. 2 UV-vis absorption spectra of the polymer thin films.
Table 2 Optical and electronic properties of the synthesized polymers
Polymer
lmax (film)
l
max (film after annealing)a
lmax (solution)
Eg (eV)b
Eox (V vs. Ag/Ag+)/Homo (eV)c
LUMO (eV)d
PMA
PMB
PMC
PMD
PME
PMG
PMH
rr-P3HT
538
520
520
554
543
520
520
556
524
467, 627
533
465
453
459
514
1.8
1.8
1.9
1.7
1.9
1.6
1.9
1.9
0.30/ꢀ5.03
0.50/ꢀ5.23
0.48/ꢀ5.21
0.58/ꢀ5.31
0.30/ꢀ5.03
0.50/ꢀ5.23
0.40/ꢀ5.13
0.35/ꢀ5.08
ꢀ3.2
ꢀ3.4
ꢀ3.3
ꢀ3.6
ꢀ3.1
ꢀ3.7
ꢀ3.2
ꢀ3.2
521
478
472, 623
533
504
420, 539
470
442
a
Films were annealed at a temperature of 150 ꢁC for 30 min in nitrogen atmosphere. b Optical band gap evaluated from the onset of absorption spectra
c
of the polymer film. The potentials are reported vs. Fc/Fc+ based on the assumption that the redox couple of Fc/Fc+ is 4.8 eV relative to vacuum.
Calculated from the optical band gap and homo from CV.
d
energy level of ferrocene/ferrocenium (Fc/Fc+) is at 4.8 eV from
vacuum.30 The calculated HOMO energy levels of all the poly-
mers were found to be between ꢀ5.0 to ꢀ5.3 eV, which are all
deeper when compared to that of rr-P3HT (ꢀ4.8 eV). LUMO
levels were then calculated from the HOMO level and the
optical band gap.
devices. The calculated field effect mobilities were found to be
consistent and reproducible from batch to batch.
Polymers PMA and PMH showed comparatively high
mobility compared to the other polymers. All the components in
these polymers comprise of highly electron-rich groups, which
could enhance charge injection, which in turn, could be a factor
in having a relatively high hole mobility.31 It should also be noted
that the solubilizing alkyl groups present in these polymers are
well separated and symmetrical, which may enable the polymers
to better self-assemble into lamellar p-stacking microstructures
that could provide efficient pathways for charge transport.32 We
also feel that the homogeneity of the p-system appears to be
3.4 Field effect transistor
Table 3 reports the average field-effect mobilities, on/off ratios
and threshold voltages for all the polymers. All measurements
were performed on at least five different days, using different
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Fig. 3 Cyclic voltammetry of the polymers at a scan rate of 100 mV sꢀ1
.
Table 3 Field effect mobility, on/off ratio and threshold voltage (VT) of the polymer devices before and after annealing
a b
,
Mobility (cm2 Vꢀ1 sꢀ1
)
On/off ratioa c
,
Polymer
Ae
Bf
Ag
Bh
VT (V)d
PMA
PMB
PMC
PMD
PME
PMG
PMH
0.07–0.08
0.1–0.11
0.007–0.008
0.03
5 ꢂ 104 to 4 ꢂ 105
4 ꢂ 103 to 6 ꢂ 103
4 ꢂ 103 to 1 ꢂ 104
5 ꢂ 101 to 1 ꢂ 102
5 ꢂ 101 to 2 ꢂ 102
6 ꢂ 102 to 3 ꢂ 103
3 ꢂ 104 to 9 ꢂ 104
2 ꢂ 105 to 4 ꢂ 105
7 ꢂ 103 to 8 ꢂ 103
4 ꢂ 104 to 7 ꢂ 104
113–146
2 ꢂ 102 to 4 ꢂ 102
3 ꢂ 104 to 7 ꢂ 104
3 ꢂ 105 ꢀ4 ꢂ 106
ꢀ10
ꢀ29
ꢀ11
ꢀ14
ꢀ23
ꢀ9
0.006–0.008
0.003–0.03
0.052–0.054
10ꢀ4-10ꢀ5
0.032–0.034
0.08–0.11
0.03
10ꢀ4-10ꢀ5
0.029–0.039
0.10
+4
a
Devices have L ¼ 10 mm. b Evaluated from the saturated regime at VSD ¼ ꢀ80 V. c Calculated from ISD ¼ ꢀ80 V (on) and 40 V (off). d Evaluated from
the onset of the transfer curve showing highest mobility. e mobility before annealing. f mobility after annealing at 150 ꢁC for 30 min. g on/off ratio before
h
annealing. on/off ratio after annealing at 150 ꢁC for 30 min.
important in these polymers.This could also explain why PMB
and PMD, although composed of electron rich units, showed
relatively lower mobility. The higher side-chain density of PMB
could be interfering with its packing, while the unsymmetrical
DTP in PMD, which promotes poorly-formed nanofibrillar
morphologies, could also be a factor in the decreased transport
properties. PMB and PMC have similar structures, so
though they both have similar mobility values. However, the
electron-poor thiazole group in PMC increases its ionization
potential, making it more stable to oxidative doping and having
a higher on/off ratio. Both PMC and PMG include electron-rich
and electron-poor units, and both of them have good on/off
ratios, which can be attributed to their higher oxidation poten-
tial. The on/off ratio of polymer PMD was found to be much
smaller because of the presence of the highly electron rich DTP
ring (D) in the polymer backbone, which is prone to oxidation
(doping) that can increase the off current. On the other hand,
PME showed a very low mobility, which could be due to its
relatively low molecular weight. Thermal annealing led to an
increase in on/off ratios and could be attributed to the removal of
charge traps within the polymer films.12 On the other hand,
mobilities showed only a very slight increase after thermal
annealing.
(chloroform) onto OTS treated devices. TMAFM characteriza-
tions of thin film surface morphologies were then performed
directly inside the channel of OFET devices. In order to facilitate
the equilibration of nanostructures, polymer deposition was
carried out under saturated solvent vapor by placing the
substrate in a covered Petri dish partially filled with the solvent.
Sample thin films were subjected to additional thermal annealing
at 150 ꢁC for 30 min followed by slowly cooling to room
temperature in a glove box under an inert atmosphere. Polymer
thin films were examined before and after annealing to study the
evolution of surface morphologies and microstructures.
Surprisingly, initial results from tapping mode atomic force
microscopy analysis revealed that surface morphologies
exhibited no discernable variations resulting from thermal
annealing for all of the polymer thin films. Consequently, only
the surface morphologies after thermal treatment are subjected
to consideration and discussion. Fig. 4 provides sample
TMAMF images of the surface morphologies of representative
polymer thin films inside the transistor channel. The left figure
for each polymer shows topographic images where the right
figures shows phase contrast images. Close examination of
phase contrast TMAFM images revealed that the morphologies
of the selenolo[3,2-b]thiophene containing polymers seemed to
highly depend on the molecular nature of the copolymer units.
Most of the polymer thin films exhibited a relatively featureless
and smooth surface with the exception of polymers PMD
and PMH. The morphology of the PMD thin film exhibited
fibrillar features, most likely corresponding to PMD micro-
crystalline structures (Fig. 4 A and B). The needle-shaped
3.5 Microstructure characterization
The surface morphologies of thin films were analyzed by tapping
mode atomic force microscopy (TMAFM). Polymer sample thin
films were prepared by drop-casting from dilute solutions
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Fig. 4 Tapping mode AFM images of the polymer films PMD (A, B), PMH (C, D) and PMA (E, F) drop cast on OTS treated surfaces. (Left column
represents the topographic images and the right column shows phase images).
features self-assembled into bundles with a width of approxi-
mately 20 nm. The formation of needle-shaped features in PMD
thin films is most likely due to the presence of the large, highly
planar DTP rings in the polymer chain, which forces the
polymer chain to maintain extended conjugation and improve
inter-planar interactions resembling the mechanism of nano-
fibril formation in rr-P3HTs. In comparison, TMAFM images
of the PMH thin film, presented in Fig. 4 C and D, shows
undulating nanofibrillar morphology. The relatively larger PDI
values of the PMH may have led to larger phase separation of
the polymer, thus leading to the non-uniformity of the fibrils.
The polymer could actually be capable of forming more
compact structures, but didn’t due to polymer’s relatively high
polydispersity. Azimuthally averaged radial profiles of the 2D
Fourier transforms of the phase contrast image exhibited
a sharp peak centered at q ¼ 0.1097 nmꢀ1 revealing a domain
size of around 57 nm.
The rest of the other polymer films exhibited smooth surfaces
and a general lack of features, a representative example of which
is polymer PMA (Fig. 4 E and F). Such morphology may be
beneficial in achieving charge carrier transport in semiconducting
polymer thin films, as in the case of PMA. It has been said that
the formation of nanofibrillar morphologies or large crystalline
domains is believed to give rise to high mobility,30 however, we
believe that well-defined fibril boundaries or crystalline domain
boundaries create rate-limiting boundaries for charge transport.
The quality of having smooth surfaces with indiscernible grain
boundaries is highly desirable to assisting the development of
inter-domain/interchain connectivity (presumably facilitated by
sparse placement of alkyl side chains), thus becoming more
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We surveyed a series of novel and solution processable semi-
conducting polymers based on selenolo[3.2-b]thiophene. Poly-
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voltammetry, and show excellent optoelectronic properties.
Polymer analysis of device performance in field-effect transistors
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