Synthesis of liquid crystalline benzothiazole based derivatives: A study of their optical and electrical properties

Article abstract of DOI:10.1016/j.orgel.2009.09.012

Two new donor-acceptor type liquid crystalline semiconductors based on benzothiazole have been synthesized. Their structural, photophysical and electronic properties were investigated using X-ray diffraction, atomic force microscopy, cyclic voltammetry, UV-Vis, photoluminescence, and Raman spectroscopy. The liquid crystalline behaviour of the molecules was thoroughly examined by differential scanning calorimetry (DSC) and optical polarizing microscope. The DSC and thermogravimetric analysis (TGA) show that these materials posses excellent thermal stability and have decomposition temperatures in excess of 300 °C. Beyond 160 °C both molecules show a smectic A liquid crystalline phase that exists till about 240 °C. Field-effect transistors were fabricated by vacuum evaporating the semiconductor layer using standard bottom gate/top contact geometry. The devices exhibit p-channel behaviour with hole mobilities of 10^-2 cm²/Vs.

Full text of DOI:10.1016/j.orgel.2009.09.012

Organic Electronics 11 (2010) 1–9
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Synthesis of liquid crystalline benzothiazole based derivatives:
A study of their optical and electrical properties
a,
Gitish K. Dutta a, S. Guha b, , Satish Patil
*
*
a Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India
b Department of Physics and Astronomy, University of Missouri, Columbia, Missouri 65211, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 July 2009
Received in revised form 19 August 2009
Accepted 12 September 2009
Available online 18 September 2009
Two new donor–acceptor type liquid crystalline semiconductors based on benzothiazole
have been synthesized. Their structural, photophysical and electronic properties were
investigated using X-ray diffraction, atomic force microscopy, cyclic voltammetry, UV–
Vis, photoluminescence, and Raman spectroscopy. The liquid crystalline behaviour of the
molecules was thoroughly examined by differential scanning calorimetry (DSC) and optical
polarizing microscope. The DSC and thermogravimetric analysis (TGA) show that these
materials posses excellent thermal stability and have decomposition temperatures in
excess of 300 °C. Beyond 160 °C both molecules show a smectic A liquid crystalline phase
that exists till about 240 °C. Field-effect transistors were fabricated by vacuum evaporating
the semiconductor layer using standard bottom gate/top contact geometry. The devices
exhibit p-channel behaviour with hole mobilities of 10ꢀ2 cm2/Vs.
Keywords:
Benzothiazole
Donor–acceptor
Liquid crystal
Mobility
OFET
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
rials are also known [26–32]. The major challenge in device
fabrication is to achieve favourable alignment of molecules
p
-Conjugated organic materials have been the subject
between source and drain contacts enabling efficient
charge transport through the semiconducting medium
[33,34]. In this context, liquid crystalline materials are
promising candidates as active semiconductor layers in or-
ganic electronics [35–43]. Supramolecular organization of
the organic semiconductor in a thin film can be enhanced
through the utilization of the liquid crystalline material
[44–50]. Liquid crystalline phase can induce highly or-
dered, closely packed structures, leading to high mobilities
of recent research work due to their optical, electronic
properties as well as their applications in many fields such
as organic light-emitting diodes (OLED) [1–3], photovoltaic
cells [4–7], thin film organic field-effect transistors (OFETs)
[8–13]. The organic molecules used in electronic and opto-
electronic devices are generally divided into two groups:
small molecules and polymers. For field-effect transistors,
small organic molecules and oligomers are extensively
used rather than polymeric materials. The most notable
examples include pentacene [14–16], rubrene [17–20]
and oligothiophene derivatives [21–24]. In the best cases,
the room temperature mobility of crystalline organic
semiconductors can reach the value of around 0.1–
20 cm2 Vꢀ1 Sꢀ1 [25]. A majority of these organic molecules
are hole conductors although ambipolar and n-type mate-
within the domain boundaries [51,52]. Introduction of
p-
conjugated moieties into a liquid crystalline material gives
many advantages in charge transport properties arising
from their self-assembling nature [53–58]. They must con-
tain a rigid, conjugated core, which is responsible for the
semiconducting behaviour of the molecule. In the past
few years, considerable efforts have been made to design,
synthesize, and characterize such type of
p-conjugated
liquid crystalline materials [59–62]. In particular; smectic
liquid crystalline materials because of their spontaneous
alignment are good candidates for such applications. Their
* Corresponding authors. Tel.: +91 80 22932651; fax: +91 80 23601310.
E-mail addresses: guhas@missouri.edu (S. Guha), satish@sscu.iisc.
ernet.in (S. Patil).
1566-1199/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.orgel.2009.09.012

2
G.K. Dutta et al. / Organic Electronics 11 (2010) 1–9
2. Results and discussion
N
S
2.1. Synthesis and characterization
C10H21
S
S
The synthetic route to compounds (a) and (b) are sum-
marized in the Scheme 1. Benzothiazole core were synthe-
sized according to method described in literature [63] and
bithiophene derivatives were synthesized by reported pro-
cedure [64,65]. The final derivatives were synthesized by
Wittig reaction and Stille cross coupling and purified by
column chromatography [66,67].
a
b
N
S
C10H21
S
S
Fig. 1. Chemical structures of compounds a and b.
2.2. Optical and electrochemical properties of the compounds
self-assembling property produces multidomain films with
no physical grain boundaries between the domains thus
facilitating hopping of charge carriers more easily.
The optical and electrochemical properties of the com-
pounds are summarized in the Table 1. The photophysical
properties of these two derivatives were investigated
using UV–Vis and photoluminescence spectroscopy. The
absorption and emission spectra of these two compounds
in solution (CHCl3) and solid state are depicted in Fig. 2.
Compound (a) shows two strong absorption peaks at
318 nm and 395 nm in solution and photoluminescence
peak at around 504 nm (in solution) and 587 nm (in solid
In our search for organic semiconductors for OFET
application we focused on benzothiazole based organic
molecules (Fig. 1) because of their liquid crystalline
properties. We have designed and synthesized two do-
nor–acceptor (D–A) type benzothiazole based molecules
showing interesting liquid crystalline and semiconducting
properties. In both molecules, the core benzothiazole ring
acts as an electron acceptor unit because of the presence
of sulphur and nitrogen atoms and alkyl substituted thio-
phene ring as a donor because of its high electron donating
ability and charge carrier mobilities. These molecules show
a smectic A liquid crystalline phase above the melting tem-
perature, which results in a good alignment and packing of
the molecule in the solid state. Also, the materials exhibit
p-type FET behaviour with hole mobility of 10ꢀ2 cm2/Vs
and on/off current ratio of 1 ꢁ 103. With additional surface
modification of the gate dielectric layer, it is conceivable
that the transport properties of OFETs may further im-
prove. The AFM studies of the annealed samples show a
dewetting of the surface with smooth morphology.
state). The absorption peak at 318 nm belongs to the p–p*
transition and the absorption peak at 395 nm corresponds
to a charge transfer band of the donor–acceptor system
[68]. Compound (b), on the other hand predominantly
shows only one strong absorption peak at 390 nm in
solution which belongs to the charge transfer band; the
photoluminescence is observed at ꢂ462 nm in solution
and 550 nm in solid state. In case of compound (a) the do-
nor and acceptor units are linked through a vinyl bond
which inhibits a charge transfer process from the donor
thiophene ring to acceptor benzothiazole ring. As a conse-
*
quence both
p–p transition and charge transfer bands
are observed in the absorption spectrum. In compound
S
N
CHCl3/ t-BuO-K+
+
CHO
C10H21
S
S
Ph3Br-
1
2
N
S
C10H21
S
S
a
b
S
Pd(0)
S
+
Br
S
SnBu3
C10H21
S
S
N
3
4
N
C10H21
S
Scheme 1. Synthesis routes to a and b.

G.K. Dutta et al. / Organic Electronics 11 (2010) 1–9
3
Table 1
Optical and electrochemical properties of the compounds.
Compounds
Film
Solution
CV
onset
ox
kabs (nm)
kem (nm)
kabs (nm)
kem (nm)
Eg (eV)
EHOMO (eV)
LUMO (eV)
E
(V)
a
b
318
325
587
550
395
390
504
462
2.6
2.8
0.35
0.37
ꢀ5.17
ꢀ5.19
ꢀ2.57
ꢀ2.39
1.4
1.2
1.0
0.8
0.6
0.4
0.2
absorption of solution
Pl of solution
absorption of film
absorption of solution
Pl of solution
Compound a
Compound b
1.2
0.8
0.4
0.0
absorption of film
Pl of film
Pl of film
300
400
500
600
700
200
300
400
500
600
700
Wavelength (nm)
Wavelength (nm)
Fig. 2. UV–Vis absorption and photoluminescence spectra of compounds a and b.
(b) the donor and acceptor units are directly linked
allowing a strong charge transfer process, and as a result
only one peak is observed in the absorption spectrum. For
both the molecules there is no sharp absorption peak in
the solid state due to aggregation. The emission maxima
in the solid state were observed at longer wavelengths
than those in solution (80–90 nm red shift) indicating
the presence of a strong interaction between the mole-
cules (packing effect) [69] in the solid state, which is
essential for good device performance. The optical band
gaps, Eg, were approximated from the onset of the low en-
ergy side of the absorption spectra (konset, solution) to the
baseline. Further, the HOMO–LUMO energy levels were
calculated by cyclic voltammetry measurements. The oxi-
dation onset for the compound (a) and (b) were 0.35 and
0.37 eV, respectively, and their corresponding HOMO
energy levels are ꢀ5.17 and ꢀ5.19 eV (Table 1). HOMO
with d-spacings of 15.2 Å and 10.5 Å corresponds to inter-
molecular distances. Similarly, compound (b) shows two
sharp diffraction peaks at 2h = 6.0° and 8.7° with d-spac-
ings of 14.6 Å and 10.0 Å corresponds to intermolecular
distances. For both the compounds, several reflections in
the wide-angle regions (between 7.6 Å and 3.6 Å), occur
because of the organization within the 3D lattice and the
reflections at around 3.6 Å may be due to
mesogens [40].
p–p stacking of
Raman scattering is particularly informative on chemi-
cal composition, segmental orientation, conformational
distribution and phase identification in organic semicon-
ductors. Fig. 4 shows the Raman spectra of as-is powder
samples for both compounds. They were measured in a
perfect backscattering geometry with less than 2 mW inci-
dent laser power on a 50 l
m2 area of the sample. The
1620 cmꢀ1 Raman peak in compound (a) originates from
the C@C stretch motion of the vinyl group [70], which is
absent in compound (b). The Raman peaks in the
1600 cmꢀ1 region in both compounds are from the intra-
ring C–C stretch motion (ring breathing) of the phenyl
group [71]. The 1564/1560 cmꢀ1 peak arises from a C@C
stretch motion of the phenyl ring. The Raman peaks in
the 1470–1490 cmꢀ1 region in both compounds are attrib-
uted to the benzothiazole unit [72]. These peaks arise from
C–H bending and C–N stretch motions. The C–N stretch
mode is usually an infrared active mode that may become
Raman active due a lowering of the symmetry of the mol-
ecule. In compound (a) the 1487 cmꢀ1 peak is seen only as
a shoulder. The 1447 cmꢀ1 Raman frequency in both com-
pounds are from the C@C ring stretch motion of the thio-
phene unit. In thiophene-based polymers the 1447 cmꢀ1
appears as an intense peak in the Raman spectrum [73].
Future Raman scattering studies from biased OFET struc-
energy levels have been calculated by using equation
onset
ox
HOMO = ꢀ(E
+ 4.8) (eV). The LUMO levels were esti-
mated from HOMO values and values of optical band gaps
according to the equation: LUMO = HOMO + Eogpt (eV).
These results show that the introduction of vinyl linkage
between benzothiazole and bithiophene unit inhibits do-
nor–acceptor charge transfer and increases the effective
conjugation length, thus lowering the band gap by about
0.2 eV.
2.3. Structure and morphology
To study the crystallinity of the materials, X-ray diffrac-
tion studies were performed from drop cast films that were
annealed at 120 °C (Fig. 3). From the diffraction pattern it
seems that both the materials are crystalline. Compound
(a) exhibits distinct diffraction peaks at 2h = 5.7° and 8.3°

4
G.K. Dutta et al. / Organic Electronics 11 (2010) 1–9
2500
2000
1500
1000
500
2000
Compund a
Compound b
1500
1000
500
0
0
10
20
2θ
30
40
10
20
30
40
2θ
Fig. 3. Room temperature powder XRD of thin films of compounds a and b.
1474
Compound a
Compound b
100
1487
1600
80
60
40
20
0
1447
1447
1564
1620
compound a
compound b
1607
1560
0
100
200
300
400
500
600
1200
1300
1400
1500
1600
TemperatureºC
Raman Shift (cm-1)
Fig. 5. TGA of compounds a and b.
Fig. 4. Raman spectra of compounds a and b.
250 °C as shown in DSC (Fig. 6). The optical polarizing
images of smectic A phase and crystalline phase were gi-
ven in Fig. 7. All the phase transition temperatures includ-
ing their change of enthalpy are given in the Table 2.
tures of both compounds will yield information on the nat-
ure of electron–phonon coupling associated with charge
transport mechanism in such devices.
2.4. Thermal and liquid crystalline properties
2.5. AFM analysis
The thermal stability of the compounds (a) and (b) were
investigated using thermal gravimetric analysis (TGA) and
differential scanning calorimetry (DSC). Both the com-
pounds have an excellent thermal stability, with decompo-
sition temperatures of 320 °C (compound a) and 300 °C
(compound b) as shown in Fig. 5. At temperature around
158 °C compound (a) starts melting and a liquid crystalline
phase appears. This liquid crystalline phase exist up to
240 °C and a new nematic phase appears between 240 °C
and 253 °C before it turns into a completely isotropic phase
at around 260 °C. The existence of the nematic phase for
compound (a) has been confirmed from a clear transition
at 240 °C in DSC (Fig. 6). The optical polarizing images of
the nematic and smectic A phase are shown in Fig. 7. Com-
pound (b), on the other hand, starts to melt at around
177 °C and forms a smectic A phase existing up to 239 °C,
beyond which it becomes a completely isotropic phase at
A thin film of compound (a) was evaporated on cleaned
Si wafers under vacuum evaporation (10ꢀ6 mbar). The
thickness of the films ranged from 60 to 80 nm. Two films
were utilized for the AFM studies: one as-is and the other
thermally annealed. The latter was placed in an oven (un-
der nitrogen atmosphere) and slowly heated to 120 °C and
kept at this temperature for a few hours. It was then
heated to 160 °C, left at this temperature for a few hours
and then slowly brought back to room temperature. These
temperatures were chosen to match the changes in the
crystalline phase with temperature. We simultaneously re-
corded AFM topography and phase images. Fig. 8 shows
the topography and phase images of the as-grown [(a)
and (b)] and annealed sample [(c) and (d)]. The annealed
samples clearly show a dewetting of the surface but with
a very smooth morphology. The rms roughness of the
two samples for
1
l
m ꢁ 1
lm area decreased from

G.K. Dutta et al. / Organic Electronics 11 (2010) 1–9
5
10
5
15
compound a
Compound b
10
5
Heating
Heating
0
0
Cooling
150
-5
-10
Cooling
-5
50
100
200
250
80
160
240
TemperatureºC
TemperatureºC
Fig. 6. DSC of compounds a and b (scan rate is 10 °C/min under N2).
Fig. 7. Optical polarizing microscopic images of compounds a and b.
Table 2
Phase transition temperatures and their corresponding Enthalpy changes.
Compounds
Tt (Cr–SA)
DHðCr—SAÞ (J/gm)
Tt (SA–N)
DHðSA—NÞ (J/gm)
Tt (N–I)
DH(N–I) (J/gm)
a
b
158 °C
177 °C
18.40
30.35
239
–
1.85
–
250
234
1.08
15.91
Transition temperatures and enthalpies determined by DSC at scan rate = 10 °C/min. (Cr = crystalline phase, SA = smectic A phase, N = nematic phase and
I = isotropic phase, Tt is transition temperature and H = enthalpy change.)
D
ꢂ3.2 nm in the as-grown sample to ꢂ0.7 nm in the an-
of the annealed sample may be avoided by treating the Si
surface differently. We are currently exploring different
nealed sample. At this point it is not clear if the dewetting

6
G.K. Dutta et al. / Organic Electronics 11 (2010) 1–9
Fig. 8. AFM images of as-grown and annealed samples of compound a.
10-5
10-6
10-7
10-8
chemical treatments of Si and Si/SiO2 interfaces for thin
film growth of such liquid crystalline molecules since the
small rms roughness of annealed films is extremely bene-
ficial from the device aspect if the film coverage were bet-
ter. Due to the issues with the annealed films all devices
were fabricated with as-is evaporated films of compounds
(a).
VGS= 0V
VGS= -10V
VGS= -20V
0.2
0.0
VGS= -15V
-0.2
-0.4
-20
-15
-10
-5
0
VDS(V)
2.6. Field-effect transistor performance
Thin film transistors were fabricated by vacuum deposi-
tion of compound (a). The devices were fabricated using a
top-contact bottom gate structure. In this work no surface
modification of the Si/SiO2 layers were done and all organic
films were grown at room temperature. The devices exhi-
bit p-channel performance. Although the output transistor
characteristics from FETs fabricated with compound (b)
were similar to compound (a), the former exhibited large
leakage currents. It is not clear whether the leakage cur-
rent in compound (b) mainly originates from the morphol-
ogy of the film owing to grain boundaries and whether this
can be circumvented using different gate dielectric layers.
Systematic studies for improving FET characteristics using
different surface modification and gate dielectrics from
both compounds are in progress.
Forward Bias
Reverse Bias
-80
-60
-40
-20
VGS (V)
0
20
40
Fig. 9. The transfer characteristics for a forward and reverse sweep and
output characteristics (inset) from an OFET fabricated using compound a.
drain-source voltages were swept from 0 to ꢀ20 V. The
highest hole mobility, calculated from the saturation
regime (VDS = ꢀ60 V), was estimated as 0.01 cm2/Vs. The
on/off ratio was obtained as 103 from IDS at VGS = 0 V and
VGS = ꢀ60 V; the threshold voltages were relatively low at
Fig. 9 shows the transfer characteristics for a forward
and reverse sweep from an OFET using compound (a);
the inset shows the output characteristics where the

G.K. Dutta et al. / Organic Electronics 11 (2010) 1–9
7
ꢂ7 V. We point out that there is a lot of room for improve-
4.1.2. Synthesis of 2-(4-(5-(5-decylthiophen-2-yl)thiophen-
2-yl)phenyl)benzo[d]thiazole (b)
ment for device performance. It is well established that
surface modification by octyltrichlorosilane (OTS) or hex-
amethyldisilazane (HMDS) self-assembled monolayers
can improve charge carrier mobilities and on/off ratios by
more than an order of magnitude [67]. The results pre-
sented here are a proof of concept that the liquid crystal-
line benzothiazole compounds are excellent candidates
for application in organic electronics.
Tributyl(5-(5-decylthiophen-2-yl)thiophen-2-yl)stann-
ane (3) (0.515 gm, 0.86 mmol) and 2-(4-bromophe-
nyl)benzo[d]thiazole (4) (0.250 g, 0.86 mmol) were
dissolved in 25 ml of dry DMF. Pd(PPh3)4 (50 mg,
0.04 mmol) was added and the mixture was stirred for
16 h at 95 °C. After cooling to room temperature the sol-
vent was removed in rotavapor and the yellow color solid
compound was purified by column chromatography (SiO2/
chloroform:Hexane (v:v 1:1)) to give b as a yellow solid.
Yield 65%.
3. Conclusions
1H NMR (400 MHz, CDCl3): d 8.11–8.045 (m, 3H), 7.93–
7.91 (d, J = 8 Hz, 1H), 7.72–7.70 (d, J = 8.3 Hz, 2H), 7.52–
7.49 (dt, 1H), 7.41–7.38 (dt, 1H), 7.33–7.32 (d, J = 3.7,
1H), 7.11–7.10 (d, J = 3.7 Hz, 1H), 7.05–7.04 (d, J = 3.5 Hz,
1H), 6.71–6.70 (d, J = 3.4 Hz, 1H), 2.82–2.79 (t, J = 8 Hz.,
2H), 1.71–1.66 (m, 4H), 1.36–1.19 (m, 12H), 0.89–0.86
(t, J = 6.9 Hz, 3H).
Two new liquid crystalline donor–acceptor type benzo-
thiazole derivatives have been synthesized by simple syn-
thetic methods and characterized by optical absorption
and photoluminescence spectroscopies, Raman scattering,
differential scanning calorimetry, and thermogravimetric
analysis. The field-effect transistor performance of one of
the benzothiazole compounds shows that these materials
have a great potential for future application in organic
devices.
13C NMR (400 MHz, CDCl3): d 146.32, 140.94, 138.79,
136.37, 134.22, 128.15, 126.45, 125.75, 125.26, 124.76,
124.00, 123.68, 123.12, 121.61, 31.90, 31.59, 30.21, 29.55,
29.34, 29.08, 22.67, 14.09.
ESIMS [Found: m/z 516.2 [M+H]+, Calcd for
4. Experimental details
C31H33NS3[M+H]+: 515.17.
Anal, Calcd. for C31H33NS3:C, 72.19; H, 6.45; N, 2.72; S,
18.65. Found C, 72.26; H, 6.29; N, 3.33; S, 18.21.
4.1. Synthesis
All chemicals were purchased from S.D. Fine Chemicals
Ltd.; Mumbai, India and Spectrochem Pvt. Ltd.; Mumbai,
India, except tributyltinchloride, Pd(PPh3)4 and potassium
tert-butoxide which were bought from Sigma Aldrich.
1H NMR and 13C NMR spectra were recorded using Bru-
ker 400 MHz. Chemical shifts were given in parts per mil-
lion and coupling constants (J) in Hertz.
4.2. Instrumental analysis
Thermal properties were studied under nitrogen atmo-
sphere on Mettler DSC-1 instrument. Thermal gravimetric
analysis (TGA) was conducted on Mettler Toledo TGA/SDTA
851e (temperature rate 10° C/min under N2). The electro-
chemical properties of both the molecules were examined
by using cyclic voltammetry (CH instrument). The electro-
lyte solution employed were 0.1 M Bu4NPF6/THF solution
using platinum as working electrode and Ag/Ag+ as the ref-
erence electrode at a scan rate of 100 mVsꢀ1 under Argon
atmosphere.
Absorption spectra were recorded on Perkin Elmer
(Lambda 35) UV–Vis Spectrometer and fluorescence emis-
sion spectra were recorded by using Perkin Elmer Spectro-
fluorophotometer (LS 50B) for both solution as well as
solid state. Films of the two molecules were fabricated on
quartz substrate from orthodichlorobenzene solution by
spin casting. The absorption and fluorescence spectra of
solution state were taken in chloroform (conc.
1 ꢁ 10ꢀ5 mol/L). Optical polarizing microscopic images of
liquid crystalline phase were taken by Olympus- BX-51
polarizing microscope.
4.1.1. Synthesis of 2-(4-((E)-2-(5-(5-decylthiophen-2-
yl)thiophen-2-yl)vinyl)phenyl)benzo[d] thiazole (a)
To a solution of 5-(5-decylthiophen-2-yl)thiophene-2-
carbaldehyde (1) (0.617 g, 1.08 mmol) and Wittig salt (2)
(0.347 g, 1.03 mmol) in dry CHCl3 was added t-BuOꢀK+
(0.139 g, 1.2 mmol) in ethanol at room temperature. After
allowing the mixture refluxing for 4 h, water was added
and extracted with CHCl3. The organic phase was dried
over sodium sulphate. After removal of the solvent, the res-
idue was purified by silica gel column chromatography
with 30% chloroform and hexane as the eluent. Yield 67%.
1H NMR (400 MHz, CDCl3): d 8.02–8.00 (m, 3H), 7.85–
7.83 (d, J = 8 Hz, 1H), 7.51–7.49 (d, J = 8 Hz, 2H), 7.45–
7.41 (dt, 1H), 7.34–7.30 (dt, 1H), 7.23–7.19 (d, J = 16 Hz,
1H), 6.95–6.39 (m, 3H), 6.84–6.80 (d, J = 16 Hz, 1H), 6.62–
6.60 (d, J = 8 Hz, 1H), 2.75–2.71 (t, J = 8 Hz, 2H), 1.63–1.53
(m, 4H), 1.30–1.18 (m, 12H), 0.83–0.80 (t, J = 8 Hz, 3H).
13C NMR (400 MHz, CDCl3): d 146.00, 140.71, 137.63,
129.53, 128.10, 127.95, 127.72, 126.40, 125.20, 124.90,
123.47, 123.44, 123.41, 121.61, 31.88, 31.58, 30.19, 29.55,
29.32, 29.06, 22.67, 14.10.
The AFM measurement was performed with a Nano-
scope IIIa (Veeco Instruments, Inc.) operating in the tap-
ping mode. Commercial ultra-sharp, rectangular silicon
cantilevers made by Micromasch (250 ꢁ 35 ꢁ 1.7
l
m3)
were used having a nominal spring constant of ꢂ0.35
ESIMS [Found: m/z 542.1 [M+H]+, Calcd. for C33H35NS3
N/m and a resonance frequency of ꢂ33 kHz.
[M+H]+: 541.19.
The Raman spectra were collected using an Invia Reni-
shaw spectrometer attached to a confocal microscope with
a ꢁ 50 long working distance objective and a 785 nm line
Anal. Calcd. for C33H35NS3: C, 73.15; H, 6.51; N, 2.59; S,
17.75. Found C, 71.84; H, 6.19; N, 3.64; S, 16.10.

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G.K. Dutta et al. / Organic Electronics 11 (2010) 1–9
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of a diode laser as the excitation wavelength. Typical laser
power was a few mW on the sample.
OFET Fabrication: The devices were fabricated using a
top-contact bottom gate structure with gold source and
drain electrodes. A heavily p-doped silicon (1 0 0) wafer
was used as the gate electrode with 200 nm bare SiO2 layer
as the gate dielectric insulator. Thin films (100 nm) of com-
pound (a) were deposited on SiO2 by vacuum sublimation
at a rate of 0.2 Å/s under a pressure of 10ꢀ6 mbar. The sub-
strate was kept at room temperature. Patterned Au layers
of thickness 60 nm were deposited for the source (S)–drain
(D) contacts. Typical S–D channel length (L) and width (W)
for OFETs were 500 lm and 5 mm, respectively. The device
characteristics were measured at room temperature with
two source meters, Keithley 2400 and Keithley 236, config-
ured together using a program written in LabView. The
source–drain current and the source-gate leakage currents
were measured with Keithley 236, which has a resolution
of 10 fA.
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Acknowledgement
We thank department of science and technology (DST)
for financial support and G.K. Dutta thanks UGC for Junior
Research Fellowship. SG acknowledges the IREE award
from the National Science Foundation (NSF) ECCS-
0523656 and NSF ECCS-0823563 for partial support of this
work.
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