Dithienylbenzothiadiazole-based donor-acceptor organic semiconductors and effect of end capping groups on organic field effect transistor performance

Article abstract of DOI:10.1071/CH12421

Donor-Acceptor-Donor (D-A-D) based conjugated molecules 4,7-bis(5-(4-butoxyphenyl)thiophen-2-yl)benzo[c][1,2,5]thiadiazole (BOP-TBT) and 4,7-bis(5-(4-trifluoromethyl)phenyl)thiophen-2-yl)benzo[c][1,2,5]thiadiazole (TFP-TBT) using thiophene-benzothiadiazole-thiophene central core with trifluoromethyl phenyl and butoxyphenyl end capping groups were designed and synthesised via Suzuki coupling. Optical, electrochemical, thermal, and organic field effect transistor (OFET) device properties of BOP-TBT and TFP-TBT were investigated. Both small molecules possess two absorption bands. Optical band gaps were calculated from the absorption cut off to be in the range of 2.06-2.25eV. Cyclic voltammetry indicated reversible oxidation and reduction processes and the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels were calculated to be in the range of 5.15-5.40eV and 3.25-3.62eV, respectively. Upon testing both materials for OFET, trifluoromethylphenyl end capped material (TFP-TBT) shows n-channel behaviour whereas butoxyphenyl end capped material (BOP-TBT) shows p-channel behaviour. Density functional theory calculations correlated with shifting of HOMO-LUMO energy levels with respect to end capping groups. Vacuum processed OFET of these materials have shown highest hole carrier mobility of 0.02cm ²/Vs and electron carrier mobility of 0.004cm²/Vs, respectively using Si/SiO₂ substrate. By keeping the central D-A-D segment and just by tuning end capping groups gives both p- and n-channel organic semiconductors which can be prepared in a single step using straightforward synthesis.

Full text of DOI:10.1071/CH12421

CSIRO PUBLISHING
Aust. J. Chem. 2013, 66, 370–380
Full Paper
http://dx.doi.org/10.1071/CH12421
Dithienylbenzothiadiazole-Based Donor-Acceptor
Organic Semiconductors and Effect of End Capping Groups
on Organic Field Effect Transistor Performance
A D
,
A C
,
A
Prashant Sonar,
Samarendra P. Singh,
Ting Ting Lin,
A B D
, ,
and Ananth Dodabalapur
A
Institute of Materials Research and Engineering, 3 Research Link Singapore 117602,
Republic of Singapore.
B
Permanent address: Microelectronics Research Center, The University of Texas
at Austin, Austin, TX, 78758, USA.
C
Current address: Shiv Nadar University, Greater Noida, 203207, India.
D
Corresponding authors. Email: sonarp@imre.a-star.edu.sg;
ananth.dodabalapur@engr.utexas.edu
Donor-Acceptor-Donor (D-A-D) based conjugated molecules 4,7-bis(5-(4-butoxyphenyl)thiophen-2-yl)benzo[c][1,2,5]
thiadiazole (BOP-TBT) and 4,7-bis(5-(4-trifluoromethyl)phenyl)thiophen-2-yl)benzo[c][1,2,5]thiadiazole (TFP-TBT)
using thiophene-benzothiadiazole-thiophene central core with trifluoromethyl phenyl and butoxyphenyl end capping
groups were designed and synthesised via Suzuki coupling. Optical, electrochemical, thermal, and organic field effect
transistor (OFET) device properties of BOP-TBT and TFP-TBT were investigated. Both small molecules possess two
absorption bands. Optical band gaps were calculated from the absorption cut off to be in the range of 2.06–2.25 eV. Cyclic
voltammetry indicated reversible oxidation and reduction processes and the highest occupied molecular orbital (HOMO)
and the lowest unoccupied molecular orbital (LUMO) energy levels were calculated to be in the range of 5.15–5.40 eV and
3.25–3.62 eV, respectively. Upon testing both materials for OFET, trifluoromethylphenyl end capped material (TFP-TBT)
shows n-channel behaviour whereas butoxyphenyl end capped material (BOP-TBT) shows p-channel behaviour. Density
functional theory calculations correlated with shifting of HOMO-LUMO energy levels with respect to end capping groups.
Vacuum processed OFET of these materials have shown highest hole carrier mobility of 0.02 cm2/Vs and electron carrier
mobility of 0.004 cm2/Vs, respectively using Si/SiO2 substrate. By keeping the central D-A-D segment and just by tuning
end capping groups gives both p- and n-channel organic semiconductors which can be prepared in a single step using
straightforward synthesis.
Manuscript received: 23 September 2012.
Manuscript accepted: 23 November 2012.
Published online: 9 January 2013.
Introduction
highest occupied molecular orbital (HOMO) and the lowest
unoccupied molecular orbital (LUMO) energy levels can be
prepared.[5–20] Among reported hole transporting and electron
transporting materials, p-conjugated small molecules are an
important category of organic semiconductors due to their well
defined, monodisperse, discrete structure with synthetic repro-
ducibility.[21–26] Small molecules are easier to purify and
therefore result in less batch-to-batch variation. Small mole-
cules have been studied extensively for OFET applications and
among the most important materials classes are oligoacene and
oligothiophene derivatives. Among all these materials, recently
donor-acceptor-donor (D-A-D) based small molecules are
becoming attractive candidates for OFET applications due to
their strong intramolecular interactions.[27] Additionally, due to
donor-acceptor charge transfer and low band gap, these
Organic field effect transistors (OFET) using small molecule
organic semiconductors have received great attention due to
their potential applications in low cost organic electronics.[1–4]
OFET using organic semiconductors can potentially be utilised
in applications such as drivers for large-area flexible displays or
low end data storage smart cards. Organic materials possess the
advantages of easy tuning of chemical structures, simpler pro-
cessing and proven compatibility with plastic substrates.
Additionally, their opto-electronic properties can be tuned
easily by selecting appropriate conjugated building blocks in the
backbone. Using such blocks, a library of both electron donor
(hole transporting) and electron acceptor (electron transporting)
materials with high thermal and chemical stability, facile
processability, high charge carrier mobility, and controllable
Journal compilation Ó CSIRO 2013
www.publish.csiro.au/journals/ajc

Low Band Gap Organic Semiconductors
371
S
S
materials also have been used successfully as a donor compo-
nent in organic photovoltaics (OPV).[28,29] The widely studied
D-A-D based materials are mostly related with thiophene (D)
and benzothiadiazole (A) building blocks due to their: (i) easy
availability, (ii) possibility for structural modification,
(iii) straightforward synthesis, and (iv) promising performance
in organic electronic devices. The thiophene-benzothiadiazole
conjugated system also has a benefit of forming self-assembled
molecular assemblies in the solid state due to sulfur-nitrogen
interactions.[27] Such intramolecular and intermolecular inter-
actions through p-p stacking are expected to provide a good
charge transportation pathway. Yamashita has reviewed several
such classes of functional materials for p- and n-channel OFET
applications.[27] Our group has also been working on D-A-D
based materials over the past few years, trying to understand the
structure-property relationship of these materials through fine
tuning in the structural design and the effect on various
optoelectronic properties (absorption, emission, energy level
tuning, morphology, and charge transport behaviour).[30–32]
These materials have been successfully used as active layers in
OFET and organic photovoltaic devices. In this work, we report
the design and synthesis of two D-A-D based materials 4,7-bis
(5-(4-butoxyphenyl)thiophen-2-yl)benzo[c][1,2,5]thiadiazole
(BOP-TBT) and 4,7-bis(5-(4-trifluoromethyl)phenyl)thiophen-
2-yl)benzo[c][1,2,5]thiadiazole (TFP-TBT).[33,34] Although the
synthesis of both compounds have been reported previously,
there is no systematic comparison of both materials in terms of
various properties (optical, electrochemical, thermal, and elec-
trical) and their comparative performance in OFET. Both
D-A-D materials have a similar conjugated backbone except for
the difference in end capping groups. The rationale behind using
alkoxy and trifluoromethyl substituted phenylene end capping
groups for synthesising these D-A-D materials is to check the
effect of electron donating and electron accepting end groups on
the optical, electrochemical, and charge transport properties.
These materials were characterised by various tools such as
matrix-assisted laser desorption ionisation time-of-flight
(MALDI-TOF), absorption and emission spectroscopy, cyclic
voltammetry (CV), thermogravimetric analysis (TGA), differ-
ential scanning calorimetry (DSC), and by theoretical model-
ling. OFET made from BOP-TBT using vacuum deposition on
octyltrichlorosilane (OTS) treated Si/SiO2 substrate exhibited a
hole mobility of 2 ꢀ 10ꢁ2 cm2/Vs, whereas TFP-TBT deposited
devices exhibited an electron mobility of 4 ꢀ 10ꢁ3 cm2/Vs with
gold source and drain electrodes. We demonstrated that by using
different end capping groups with similar D-A-D core, we can
create both p-channel and n-channel organic semiconductors
with minimal synthetic effort. Due to high hole and electron
mobility and low band gap, these materials are also potential
candidates for bilayer OPV devices. The detailed synthesis,
characterisation, and effect of end capping group on device
performance are discussed below.
C4H9O
OC4H9
N
N
S
BOP-TBT
B(OH)2
C4H9O
Pd(0)
Ethanol
Toluene/2MK2CO3
S
S
Br
Br
N
N
S
Pd(0)
Toluene/2MK2CO3
F3C
B(OH)2
Ethanol
S
S
F3C
CF3
N
N
S
TFP-TBT
Scheme 1. Synthesis of D-A-D organic semiconductors BOP-TBT and
TFP-TBT for organic field effect transistors.
Synthesis
4,7-Bis(5-(4-butoxyphenyl)thiophen-2-yl)benzo[c]
[1,2,5]thiadiazole (BOP-TBT)
To a Schlenk flask 4-(butoxyphenyl)boronic acid (0.425 g,
2.19 mmol) and 4, 7-bis(5-bromothiophene-2-yl) benzothiadia-
zole (0.400 g, 0.87 mmol), potassium carbonate (K2CO3) (2 M
aqueous solution, 7 mL), and 3 mL of ethanol were dissolved in
toluene (15 mL). The solution was purged with argon for 30 min,
and then tetrakis(triphenylphosphine) palladium (20 mg,
0.017 mmol) was added instantly under argon flow. The reaction
mixture was degassed by three freeze-pump thaw cycles and
then stirred at 808C for 48 h under argon. After completion of the
reaction, toluene was removed and the product was precipitated
in methanol; the resulting solid was filtered off and washed with
copious amount of methanol. The crude product was purified
using vacuum train sublimation at high temperature (250 to
3008C) yielding the pure product as dark red crystalline needles
(77 % yield). Due to low solubility of the material we could not
measure the NMR spectra. We confirmed the purity of the
compound using elemental analysis and MALDI-TOF
techniques.
Found: C 68.4, H 5.2, N 4.6, S 16.0. C34H32N2O2S3 requires
C 68.4, H 5.4, N 4.7, S 16.1 %. m/z (MALDI-TOF) 596.26; [M]þ
requires 596.16.
Experimental
Materials
4,7-Bis(5-(4-trifluoromethyl)phenyl)thiophen-2-yl)
benzo[c][1,2,5]thiadiazole (TFP-TBT)
All commercially available materials [(4-butoxyphenyl)
boronic acid, 4-(trifluoromethyl)phenyl)boronic acid, tetrakis
(triphenylphosphine)] were used as received unless otherwise
noted. 4,7-Bis(5-bromothiophene-2-yl) benzothiadiazole was
synthesised according to a published procedure.[35] All reactions
were carried out using Schlenk techniques under an argon or
nitrogen atmosphere in anhydrous solvents.
4-(Trifluoromethyl)phenylboronic acid (0.500 g, 2.50 mmol),
(0.500 g,
4,7-bis(5-bromothiophene-2-yl)benzothiadiazole
1.09 mmol), and sodium hydroxide (2 M aqueous solution,
5 mL) were added to a Schlenk flask and then dissolved in
toluene (30 mL). The solution was purged with argon for 30 min,

372
P. Sonar et al.
Comment 1
Comment 2
(a)
S
S
4000
3000
2000
1000
0
C4H9O
OC4H9
N
N
S
Chemical formula: C34H32N2O2S3
Exact mass: 596.16
Molecular weight: 596.83
500
1000
1500
2000
2500
m/z
(b)
D:\data\sonar\(cf3)2-ph-t-btz-t-p-(cf3)2\0_P23\1
Comment 1
Comment 2
Without matrix
S
S
5000
4000
3000
2000
F3C
CF3
N
N
S
Chemical formula: C28H14F6N2S3
Exact mass: 588.02
Molecular weight: 588.61
1000
0
500
1000
1500
2000
2500
m/z
Fig. 1. Matrix assisted laser desorption/ionisation time-of-flight spectra of BOP-TBT and TFP-TBT organic
semiconductors.
and then tetrakis(triphenylphosphine)palladium (150 mg,
0.12 mmol) was added quickly under argon flow. The reaction
mixture was degassed by three freeze-pump thaw cycles and
then stirred at 808C for 48 h under argon. After completion of the
reaction, toluene was removed and the product was precipitated
in methanol; the resulting solid was filtered off and washed with
copious amount of methanol. The crude product was purified
using vacuum train sublimation at high temperature (250 to
3008C) yielding the pure product as a dark red crystalline solid
(61 % yield). Again due to limited solubility of this material we
could not perform NMR spectroscopy. Purity of the compound
was confirmed by elemental analysis and MALDI-TOF
techniques.
a Shimadzu model 2501-PC. DSC was carried out under nitro-
gen on a TA Instrument DSC Q100 with a scanning rate of
108C minꢁ1. TGA was carried out using a TA Instrument TGA
Q500 and a heating rate of 108C minꢁ1. Cyclic voltammetry
experiments were performed using an Autolab potentiostat
(model PGSTAT30) by Echochimie. Measurements were
recorded in dichloromethane solutions of BOP-TBT and TFP-
TBT under argon at room temperature with a conventional
three-electrode arrangement consisting of a platinum wire
working electrode, a gold counter electrode, and an Ag/AgCl in
3 M KCl reference electrode, respectively. HOMO values were
calculated from the corresponding oxidation onsets which were
converted to saturated calomel electrode (SCE), based on an
ꢁ4.4 eV SCE energy level relative to vacuum using ionisation
potential, IP ¼ ꢁ(Eox-onset þ 4.4) eV. Likewise, LUMO levels
were obtained from the reduction onsets via electron affinity,
EA ¼ ꢁ(Ered-onset þ 4.4) eV.
Found: C 57.0, H 2.2, N 4.6, S 15.7. C28H14F6N2S3 requires
C 57.1, H 2.4, N 4.8, S 16.3 %. m/z (MALDI-TOF) 588.10; [M]þ
requires 588.02.
Characterisation and Measurements
OFET Fabrication and Characterisation
MALDI-TOF mass spectra were obtained on a Bruker Autoflex
TOF/TOF instrument using dithranol as the matrix. Elemental
analysis was conducted on a CHNSO Elemental Analyzer for
quantitative measurements using FLASH EA-112 series by
Thermo Electron Corporation. UV-Vis spectra were recorded on
Top contact/bottom gate OFET devices were fabricated using
nþ-Si/SiO2 substrates where nþ-Si and SiO2 work as gate
electrode and gate dielectric, respectively. Substrates were
cleaned using ultrasonication in acetone, methanol, and

Low Band Gap Organic Semiconductors
373
(a)
1.2
de-ionized water consecutively. The cleaned substrates were
dried under a nitrogen flow and heated at 1008C for 5 min. The
substrates were then treated in UV-ozone for 20 min. The sub-
strate was then kept in a desiccator with a few drops of OTS. The
desiccator was evacuated for 3 min and placed in an oven at
1108C for 3 h. The substrate was removed from the desiccator,
thoroughly rinsed with isopropanol, and dried under a nitrogen
flow. For all sets of OFET, thin films of BOP-TBT and TFP-
TBT were vacuum deposited on OTS treated Si/SiO2 substrates.
Patterned gold layers of thickness ,100 nm were deposited for
use as source and drain electrodes through a shadow mask. For
typical OFET devices reported here, the source-drain channel
length and channel width were 100 mm and 1 mm, respectively.
The device characteristics of the OFET were measured at room
temperature under nitrogen with a Keithley 4200 parameter
analyser.
Absorbance
Emission
615 nm
352 nm
1.0
0.8
0.6
0.4
0.2
0
504 nm
300
400
500
600
700
800
900
Wavelength [nm]
(b)
Absorbance
Emission
574 nm
341 nm
1.0
0.5
0
Results and Discussion
475 nm
Syntheses and Characterisation
The synthesis of dithienylbenzothiadiazole based D-A-D
materials with different end capping groups is outlined in
Scheme 1. First, dithienylbenzothiadiazole (93 % yield) was
synthesised using commercially available 4,7-dibromobenzo[c]
[1,2,5]thiadiazole and 2-tributylstannylthiophene in dry N,N-
dimethylformamide in presence of tetrakis(triphenyl phopshine)
palladium catalyst at 808C for 24 h by Stille coupling. Bro-
mination of dithienylbenzothiadiazole was carried out using
N-bromosuccinimide (NBS) in DMF at room temperature for
24h producing 4,7-bis(5-bromothiophene-2-yl)benzothiadiazole
(62 % yield). This dibromo derivative was used as a common
block for synthesising the other two final compounds. Molecular
300
400
500
600
700
800
900
Wavelength [nm]
Fig. 2. Absorbance and emission spectrum of (a) BOP-TBT and
(b) TFP-TBT D-A-D organic semiconductors.
(a)
4.0µ
0
12.0µ
8.0µ
ꢀ4.0µ
5.20 eV
4.0µ
3.25 eV
ꢀ8.0µ
0
ꢀ12.0µ
ꢀ4.0µ
0
0.4
0.8
1.2
1.6
ꢀ1.6
ꢀ1.2
ꢀ0.8
ꢀ0.4
0
E [V] vs Ag/Agꢁ
E [V] vs Ag/Agꢁ
80.0µ
60.0µ
40.0µ
20.0µ
0
(b)
0
ꢀ8.0µ
3.62 eV
5.50 eV
ꢀ16.0µ
ꢀ24.0µ
ꢀ32.0µ
ꢀ20.0µ
ꢀ1.6
ꢀ1.2
ꢀ0.8
ꢀ0.4
0
0
0.4
0.8
1.2
1.6
E [V] vs Ag/Agꢁ
E [V] vs Ag/Agꢁ
Fig. 3. Cyclic voltammogram analysis of (a) BOP-TBT and (b) TFP-TBT D-A-D organic semiconductors.

374
P. Sonar et al.
120
100
80
1.2
1.0
0.8
0.6
0.4
0.2
(a)
S
S
C4H9O
OC4H9
N
N
S
Chemical formula: C34H32N2O2S3
Exact mass: 596.16
Molecular weight: 596.83
60
40
20
0
0
ꢀ0.2
1000
0
200
400
600
800
Universal V4.4A TA instruments
Temperature [ꢂC]
120
100
80
1.2
(b)
S
S
F3C
CF3
1.0
0.8
0.6
0.4
N
N
S
Chemical formula: C28H14F6N2S3
Exact mass: 588.02
Molecular weight: 588.61
60
40
0.2
0
20
0
ꢀ0.2
0
200
400
600
800
1000
Universal V4.4A TA instruments
Temperature [ꢂC]
Fig. 4. Thermogravimetric analysis of (a) BOP-TBT and (b) TFP-TBT D-A-D organic semiconductors.
organic semiconductors 4,7-bis(5-(4-butoxyphenyl)thiophen-2-
yl)benzo[c][1,2,5]thiadiazole (BOP-TBT) and 4,7-bis(5-(4-
trifluoromethyl)phenyl)thiophen-2-yl)benzo[c][1,2,5]thiadiazole
(TFP-TBT) were synthesised using 4-(butoxyphenyl)boronic
acid and 4-(trifluoromethyl)phenyl)boronic acid with 4,7-bis(5-
bromothiophene-2-yl)benzothiadiazole, as described in the
Synthesis section, above. The respective MALDI-TOF of
compounds BOP-TBT and TFP-TBT are shown in Fig. 1. Due to
the low solubility of these organic semiconductors in organic
solvents, we were unable to obtain 1H or 13C NMR spectra.
transfer associated with thiophene-benzothiadiazole moieties
(higher wavelength region) in the D-A-D backbone. In the lower
wavelength region BOP-TBT and TFP-TBT exhibited absorp-
tion maxima at 352 and 341 nm respectively. In the longer
wavelength region, BOP-TBT exhibits an absorption maximum
at 504 nm whereas that for TFP-TBT is at 475 nm. Although the
conjugated backbone of both materials have similar conjugated
blocks, the 29 and 11 nm blue shift in longer and shorter
wavelength region observed in TFP-TBT compared with BOP-
TBT, respectively, is attributed to the stabilisation of the LUMO
by the electron-withdrawing trifluoromethyl group. Similar
observations have been reported recently for pyridine-
benzothiadiazole based co-oligomers.[36] The optical band gaps
calculated from the absorption cut off values are 2.06 and
2.25 eV for BOP-TBT and TFP-TBT respectively. The fluo-
rescence spectra of BOP-TBT and TFP-TBT exhibit red
photoluminescence maxima at 615 and 574 nm respectively.
Optical Properties
The absorbance and photoluminescence spectra of BOP-TBT
and TFP-TBT were measured in chloroform using 10ꢁ5 M
solutions and are shown in Fig. 2a and 2b respectively. Both
materials exhibit two absorption peaks, attributed to the thio-
phene constituent (lower wavelength region) and the charge

Low Band Gap Organic Semiconductors
375
15
10
5
(a)
S
S
C4H9O
OC4H9
N
N
S
Chemical formula: C34H32N2O2S3
Exact mass: 596.16
Molecular weight: 596.83
0
ꢀ5
0
50
100
150
200
250
300
Exo up
Universal V4.4A TA instruments
Temperature [ꢂC]
(b)
6
S
S
F3C
CF3
N
N
S
4
2
Chemical formula: C28H14F6N2S3
Exact mass: 588.02
Molecular weight: 588.61
0
ꢀ2
0
50
100
150
200
250
300
Exo up
Universal V4.4A TA instruments
Temperature [ꢂC]
Fig. 5. Differential scanning calorimetry thermograms of (a) BOP-TBT and (b) TFP-TBT D-A-D organic
semiconductors.
A blue shift of 41 nm is observed for TFP-TBT compared with
BOP-TBT which is again related to stabilisation of the LUMO
due to the electron withdrawing trifluoromethyl end capping
group. Stokes shifts of 111 and 99 nm were observed for BOP-
TBT and TFP-TBT respectively. This result clearly indicates
that although possessing similar conjugated blocks in the
backbone, the different alkyl end capping groups play a signif-
icant role in modulating photophysical properties.
electrode, a gold counter electrode, and an Ag/AgCl reference
electrode under argon using tetrabutylammonium hexa-
fluorophosphate as an electrolyte at a scan rate of 50 mV sꢁ1
.
Both compounds exhibited reversible anodic and cathodic redox
waves. HOMO-LUMO energy levels and the corresponding
electrochemical band gaps of BOP-TBT and TFP-TBT were
calculated from oxidation-reduction onset potentials. These
onset potentials were converted to SCE and the corresponding IP
and EA values were derived from the onset redox potentials,
based on ꢁ4.4 eV as the SCE energy level relative to vacuum
(EA ¼ Ered-onset þ 4.4 eV, IP ¼ Eox-onset þ 4.4 eV). The CV
curves for BOP-TBT and TFP-TBT are shown in Fig. 3. BOP-
TBT clearly exhibits two dominant oxidation peaks arising from
two electron donating thiophene blocks whereas only one
reduction peak is observed which is due to the single ben-
zothiadiazole unit incorporated in the D-A-D backbone. TFP-
TBT also shows one strong and one weak oxidation peak
Electrochemical Properties
The electrochemical properties of BOP-TBT and TFP-TBT
were investigated by CV in order to estimate the electrochemical
band gaps and HOMO/LUMO energy levels as a function of the
end capping groups. All CV measurements were recorded at
room temperature in dichloromethane (compounds were spar-
ingly soluble indichloromethane) using a conventional three
electrode configuration consisting of a platinum wire working

376
P. Sonar et al.
LUMO
(a)
Optimized geometry
HOMO
BOP-TBT
ꢀ4.602
ꢀ3.288
DFP-TBT
ꢀ5.336
ꢀ3.796
(b)
Fig. 6. The optimised geometries (a) and electron density isocontours of HOMO and LUMO of BOP-TBT and TFP-TBT and (b) the
torsion angles between central D-A-D segment and alkyl substituted phenylene end capped group for the optimised structures obtained
at GGA-PW91-dspp/dnp level and quality ‘fine’ using DMol3 code.
whereas there are two reduction peaks due to benzothiadiazole
(strong peak) and the trifluoromethyl group (weak peak). The
oxidation onsets for BOP-TBT and TFP-TBT were observed at
0.8 and 1.10 V respectively. The HOMO values calculated for
BOP-TBT and TFP-TBT are 5.20 and 5.50 eV while the LUMO
values were determined to be 3.25 and 3.62 eV, respectively,
calculated from the reduction onsets. The 0.30 and 0.37 eV
lower values of HOMO and LUMO for TFP-TBT is due to the
electron withdrawing effect of the trifluoromethyl group. From
these values it can be clearly seen that HOMO-LUMO levels can
be altered by using appropriate end capping groups, despite
having the same conjugated backbone. Electrochemical band
gaps calculated for BOP-TBT and TFP-TBT from the HOMO-
LUMO differences are 1.95 and 1.88 eV respectively. These
values are found to be in good agreement with the optical band
gaps calculated from the UV-vis absorption onset.
compounds show a 5 % weight loss (high decomposition tem-
perature) at 4008C and 3308C, respectively, as shown in Fig. 4.
This clearly indicates excellent thermal stability of the materi-
als. Such a high stability is prerequisite for the purification of
insoluble organic semiconductors using a vacuum sublimation
method at high temperature. The high thermal stability of these
compounds is due to the rigid backbone. Such thermally stable
compounds allow fabrication of devices via vacuum sublima-
tion without any decomposition. BOP-TBT shows better ther-
mal stability than TFP-TBT due to the longer alkyl chain
(butoxy) substituted on the phenyl ring compared to the shorter
alkyl group (trifluoromethyl). This has also been confirmed by
derivative weight loss curves for both compounds where the
weight loss is most apparent. The DSC scans of BOP-TBT and
TFP-TBT are depicted in Fig. 5 and this curve exhibits melting
points at 2228C and 2368C respectively. BOP-TBT exhibits
slightly lower melting temperature than TFP-TBT which is due
to the longer alkyl chain (butoxy) substituted on the phenyl.
Thermal Analysis
In order to determine the thermal stability and thermal transi-
tions of these compounds, TGA and DSC were conducted under
a nitrogen flow. TGA measures the amount and rate of change in
the weight of a material as a function of temperature or time in
a controlled atmosphere. Both BOP-TBT and TFP-TBT
Density Functional Theory Electronic Structure Calculation
In order to study the planarity of the materials and the effects of
end capping groups on electronic properties, density functional
theory (DFT) calculations of their electronic state were

Low Band Gap Organic Semiconductors
377
(a)
S
O
HOMO (ꢀ4.917 eV)
LUMO (ꢀ1.718 eV)
Isovalue ꢃ 0.03
Isovalue ꢃ 0.01
(b)
S
F3C
HOMO (ꢀ5.759 eV)
LUMO (ꢀ2.583 eV)
Isovalue ꢃ 0.03
Isovalue ꢃ 0.01
Fig. 7. The optimised geometries and electron density isocontours of HOMO and LUMO (measured at isovalue ¼ 0.03 and 0.01) of end capping
groups (a) butoxyphenyl and (b) trifluoromethylphenyl used for synthesising BOP-TBT and TFP-TBT respectively. These values are obtained at
GGA-PW91-dspp/dnp level and quality ‘fine’ using DMol3 code.
performed at the level of GGA-PW91-dspp/dnp using the
DMol3 module in the Materials Studio Modelling software
package (Accelrys Inc.). The optimised geometries and electron
density isocontours of HOMO and LUMO of BOP-TBT and
TFP-TBT with their torsion angles (between central D-A-D
segment and alkyl substituted phenylene end capped groups) are
shown in Fig. 6. All the isocontour values are 0.02 atomic unit
(a.u.). In all materials, the HOMO and LUMO are localised
along the conjugated backbone. Localisation of the HOMO/
LUMO orbital on such conjugated blocks has been commonly
observed in previous theoretical studies.[31] The theoretical
calculated HOMO-LUMO values and the band gaps of BOP-
TBT and TFP-TBT are comparable with the experimental
results and exhibit a similar trend (CV and optical data). The
attachment of trifluoromethyl (an electron withdrawing group)
to phenylene causes lowering of HOMO and LUMO levels of
TFP-TBT compared with BOP-TBT in spite of having similar
conjugated backbone. Such trends are expected due to higher
electronegativity/electron affinity of the trifluoromethyl group
compared with the butoxy group. The theoretically predicted
HOMO and LUMO energy levels are ,0.51 to 0.73 eV higher
than experimentally calculated values. The observed discrep-
ancy of experimental and theoretical values are related with
various effects such as effect of electrolyte, effect of solvent, and
conformational order in the bulk state which is not taken into
account in theoretical calculations. In order to confirm the effect

378
P. Sonar et al.
2.4 ꢄ 10ꢀ3
of end capping groups on HOMO-LUMO shifting of BOP-TBT
and TFP-TBT, we also studied molecular modelling on indi-
vidual trifluoromethylphenylene and butoxyphenylene end
groups attached to thiophene. The optimised geometries and
electron density isocontours at isovalue 0.01 and 0.03 of HOMO
and LUMO of trifluoromethylphenylene and butoxyphenylene
moieties attached to thiophene end groups are shown in Fig. 7.
The HOMO and LUMO values for butoxyphenylene thiophene
end group are calculated as ꢁ4.917 eV and ꢁ1.718 eV, whereas
for trifluoromethylphenylene thiophene these values are
ꢁ5.759 eV and ꢁ2.583 eV respectively. Just by changing the
electron donating butoxy group with an electron withdrawing
trifluoromethyl group, the LUMO value has been shifted by
0.87 eV. This theoretical data is significant to check the exper-
imental results and we believe that theoretical modelling is a
useful tool for designing and synthesising novel electron
transporting organic semiconductors.
10ꢀ6
10ꢀ7
10ꢀ8
10ꢀ9
10ꢀ10
1.6 ꢄ 10ꢀ3
8.0 ꢄ 10ꢀ4
0
0
ꢀ15
ꢀ30
ꢀ45
ꢀ60
ꢀ75
VGS [V]
OFET Characteristics
The electrical properties of BOP-TBT and TFP-TBT were
investigated by fabricating and characterising OFET devices
using thin films of BOP-TBT and TFP-TBT. A bottom gate top-
contact OFET device configuration was used to characterise the
charge transport properties of BOP-TBT and TFP-TBT. The
devices were fabricated on either pþ-Si (or nþ-Si) nþ-doped
silicon wafers with a thermally grown silicon oxide layer
(,200 nm) having a capacitance of ,17 nF cmꢁ2. The þ-Si or
nþ-Si served as the gate electrode while the silicon oxide layer
acted as the gate dielectric. The BOP-TBT and TFP-TBT were
deposited on Si/SiO2 substrate by vacuum deposition method.
Gold was used as source and drain electrodes, and was thermally
evaporated on top of the organic semiconductor thin film using a
shadow mask. OFET devices have a channel length of 100 mm
and a channel width of 1 mm. The evaluation of transistor per-
formance was carried out in a glove box under nitrogen using a
Keithley 4200 SCS semiconductor characterisation system. The
output and transfer characteristics of OFET using BOP-TBT and
TFP-TBT channel semiconductor are shown in Fig. 8 and Fig. 9
respectively. The carrier mobility in the saturated regime, msat, is
calculated according to Eqn (1):
ꢀ4
ꢀ3
ꢀ2
ꢀ1
0
ꢀ70V
ꢀ60V
ꢀ50V
ꢀ40V
ꢀ30V
0
ꢀ15
ꢀ30
ꢀ45
ꢀ60
ꢀ75
VDS [V]
Fig. 8. Transfer (VGS versus IDS) and output (VDS versus IDS) character-
istics of BOP-TBT based organic field effect transistors (L ¼ 100 mm;
W ¼ 1 mm) on OTS treated pþ-Si/SiO2 substrate devices.
2
Is:d: ¼ Ci msat ðW=2 LÞðVG ꢁ VTÞ
ð1Þ
molecules T4B1[30] might be due to the weak D-A interaction
induced by the phenylene moiety (weak donor) than thiophene
(strong donor). When the TFP-TBT molecule was used as a
channel semiconductor in OFET devices, n-channel perfor-
mance was observed with the best electron mobility
0.004 cm2/Vs with threshold voltage 45.0 Volts calculated from
the saturation regime. Both small molecules have similar
conjugated backbone with the only difference being the alkyl
group substituent. Due to the electron withdrawing trifluoro-
methyl group substitution on phenylene ring in TFP-TBT the
LUMO is lowered resulting in n-channel performance, whereas
BOP-TBT having butoxy group on phenylene shows p-channel
performance. There are several reports of discrete p-channel and
n-channel organic semiconductors but there are very few reports
of making both p and n-channel organic semiconductors using a
straightforward synthetic approach, using the same conjugated
backbone, and just by tuning alkyl chain substitution. From
these observations it is clear that different end capping group
substitution is an effective synthetic method for the generation
where Is.d. is the drain current, W and L are, respectively, the
semiconductor channel width and length, Ci is the capacitance
per unit area of the gate dielectric, and VG and VT are,
respectively, the gate voltage and threshold voltage. VT of the
device was determined from linear extrapolation of the linear fit
of the (Is.d.
)
1/2 versus VG curve in the saturation regime at ID ¼ 0.
The BOP-TBT based devices show p-channel performance with
the best calculated hole mobility of 0.02 cm2/Vs and threshold
voltage of ꢁ33.0 Volts. The OFET devices fabricated on OTS
treated Si/SiO2 substrates have shown two orders of magnitude
higher hole mobility as compared with the transistors fabricated
on bare Si/SiO2 substrates (mh ¼ 6.2 ꢀ 10ꢁ4 cm2/Vs). Higher
hole mobility exhibited in OFET devices fabricated on OTS
treated substrates can be attributed to better packing of BOP-
TBT thin films on self-assembled monolayer treated substrates.
These mobility values are within the range of previously
reported thiophene-benzothiadiazole based D-A-D small mole-
cules.[27,30,31] The lower mobility of BOP-TBT compared with
our earlier reported thiophene-benzothiadiazole based small

Low Band Gap Organic Semiconductors
379
8.0 ꢄ 10ꢀ4
6.0 ꢄ 10ꢀ4
4.0 ꢄ 10ꢀ4
2.0 ꢄ 10ꢀ4
0
10ꢀ6
Acknowledgements
The authors acknowledge the Visiting Investigatorship Program (VIP) of the
Agency for Science, Technology and Research (A*STAR), Republic of
Singapore for financial support.
10ꢀ7
References
10ꢀ8
[1] Y. Shirota, H. Kageyama, Chem. Rev. 2007, 107, 953. doi:10.1021/
CR050143þ
[2] M. T. Lloyd, J. E. Anthony, G. G. Malliaras, Mater. Today 2007, 10,
34. doi:10.1016/S1369-7021(07)70277-8
10ꢀ9
[3] F. Garnier, Acc. Chem. Res. 1999, 32, 209. doi:10.1021/AR9800340
[4] M. Mas-Torrent, C. Rovira, Chem. Soc. Rev. 2008, 37, 827.
doi:10.1039/B614393H
[5] S. Allard, M. Forster, B. Souharce, H. Thiem, U. Scherf, Angew. Chem.
Int. Ed. 2008, 47, 4070. doi:10.1002/ANIE.200701920
[6] H. Klauk, Organic Electronics: Materials, Manufacturing and Appli-
cations 2006 (Wiley-VCH: Weinheim).
10ꢀ10
0
15
30
45
60
75
VGS [V]
[7] K. Mu¨llen, G. Wegner, Electronic Materials: The Oligomer Approach
1998 (Wiley-VCH: Weinheim).
200
150
100
50
[8] H. E. Katz, A. J. Lovinger, C. Klock, T. Siegrist, W. Li, Y. Y. Lin,
A. Dodabalapur, Nature 2000, 404, 478. doi:10.1038/35006603
[9] B. Crone, A. Dodabalapur, Y. Y. Lin, R. W. Filas, A. LaDuca,
R. Sarpeshkar, H. E. Katz, W. Li, Nature 2000, 403, 521.
doi:10.1038/35000530
[10] B. Yoo, B. A. Jones, D. Basu, D. Fine, T. Jung, S. Mohapatra,
A. Facchetti, K. Dimmler, M. R. Wasielewski, T. J. Marks,
A. Dodabalapur, Adv. Mater. 2007, 19, 4028. doi:10.1002/ADMA.
200700064
[11] M. H. Yoon, S. A. DiBenedetto, A. Facchetti, T. J. Marks, J. Am.
Chem. Soc. 2005, 127, 1348. doi:10.1021/JA045124G
[12] A. Facchetti, Y. Deng, A. C. Wang, Y. Koide, H. Sirringhaus, T. J.
Marks, R. H. Friend, Angew. Chem. Int. Ed. 2000, 39, 4547.
doi:10.1002/1521-3773(20001215)39:24,4547::AID-ANIE4547.3.0.
CO;2-J
[13] J. A. Letizia, A. Facchetti, C. L. Stern, M. A. Ratner, T. J. Marks, J. Am.
Chem. Soc. 2005, 127, 13476. doi:10.1021/JA054276O
[14] S. Ando, R. Murakami, J. Nishida, H. Tada, Y. Inoue, S. Tokito,
Y. Yamashita, J. Am. Chem. Soc. 2005, 127, 14996. doi:10.1021/
JA055686F
50V
40V
30V
20V
60
0
0
15
30
45
75
VDS [V]
Fig. 9. Transfer (VGS versus IDS) and output (VDS versus IDS) character-
istics of TFP-TBT based organic field effect transistors (L ¼ 100 mm;
W ¼ 1 mm) on OTS treated pþ-Si/SiO2 substrate devices.
[15] K. Ito, T. Suzuki, Y. Sakamoto, D. Kubota, Y. Inoue, F. Sato, S. Tokito,
Angew. Chem. Int. Ed. 2003, 42, 10.
[16] C. D. Dimitrakopoulos, P. R. L. Malenfant, Adv. Mater. 2002, 14, 99.
doi:10.1002/1521-4095(20020116)14:2,99::AID-ADMA99.3.0.
CO;2-9
[17] D. J. Gundlach, Y.-Y. Lin, T. N. Jackson, S. F. Nelson, D. G. Schlom,
IEEE Electron Device Lett. 1997, 18, 87. doi:10.1109/55.556089
[18] H. E. Katz, Z. Bao, S. L. Gilat, Acc. Chem. Res. 2001, 34, 359.
doi:10.1021/AR990114J
of potential p- and n-channel semiconductors for high mobility
OFET devices.
Conclusions
[19] F. Garnier, A. Yassar, R. Hajlaoui, G. Horowitz, F. Deloffre, B. Servet,
S. Ries, P. Alnot, J. Am. Chem. Soc. 1993, 115, 8716. doi:10.1021/
JA00072A026
D-A-D based conjugated molecules BOP-TBT and TFP-TBT
using a thiophene-benzothiadiazole-thiophene central core with
butoxyphenyl and trifluoromethyl phenyl end capping groups
have been designed, synthesised, and characterised. Both small
molecules have shown optical band gaps ranging from 2.06 to
2.25 eV. HOMO and LUMO energy levels are calculated to be in
the range of 5.15–5.4 eV to 3.25–3.62 eV from CV measure-
ments. Upon testing both materials for OFET, tri-
floromethylphenyl end capped material show n-channel
behaviour, whereas the butoxyphenyl end capped material
shows p-channel behaviour. Vacuum processed OFET of BOP-
TBT and TFP-TBT have shown maximum hole mobility of
0.02 cm2/Vs and electron mobility of 4 ꢀ 10ꢁ3 cm2/Vs, respec-
tively. Keeping the same central D-A-D conjugated segment and
just tuning the end capping groups gives both p- and n-channel
organic semiconductors in a single step using straightforward
synthesis.
[20] D. J. Gundlach, Y.-Y. Lin, T. N. Jackson, D. G. Schlom, Appl. Phys.
Lett. 1997, 71, 3853. doi:10.1063/1.120524
[21] Z. Bao, A. J. Lovinger, J. Brown, J. Am. Chem. Soc. 1998, 120, 207.
doi:10.1021/JA9727629
[22] M. M. Shi, H. Z. Chen, J. Z. Sun, J. Ye, M. Wang, Chem. Commun.
2003, 14, 1710. doi:10.1039/B304141G
[23] Y. Sakamoto, T. Suzuki, M. Kobayashi, Y. Gao, Y. Fukai, Y. Inoue,
F. Sato, S. Tokito, J. Am. Chem. Soc. 2004, 126, 8138. doi:10.1021/
JA0476258
[24] C. Di, J. Li, G. Yu, Y. Xiao, Y. Guo, Y. Liu, X. Qian, D. Zhu, Org. Lett.
2008, 10, 3025. doi:10.1021/OL8008667
[25] M. Mas-Torrent, C. Rovira, Chem. Soc. Rev. 2008, 37, 827.
doi:10.1039/B614393H
[26] C. R. Newman, C. D. Frisbie, D. A. da. S. Filho, J. L. Bredas,
P. C. Ewbank, K. R. Mann, Chem. Mater. 2004, 16, 4436.
doi:10.1021/CM049391X

380
P. Sonar et al.
[27] Y. Yamashita, Sci. Technol. Adv. Mater. 2009, 10, 024313.
doi:10.1088/1468-6996/10/2/024313
[32] P. Sonar, S. G. Santamaria, T. T. Lin, A. Sellinger, H. Bolink, Aust. J.
Chem. 2012, 65, 1244. doi:10.1071/CH12171
[28] B. Walker, A. B. Tamayo, X.-D. Dang, P. Zalar, J. H. Seo, A. Garcia,
M. Tantiwiwat, T.-Q. Nguyen, Adv. Funct. Mater. 2009, 19, 3063.
doi:10.1002/ADFM.200900832
[29] J. A. Kong, E. Lim, K. K. Lee, S. Lee, S. H. Kim, Sol. Energy Mater.
Sol. Cells 2010, 94, 2057. doi:10.1016/J.SOLMAT.2010.06.016
[30] P. Sonar, S. P. Singh, S. Sudhakar, A. Dodabalapur, A. Sellinger,
Chem. Mater. 2008, 20, 3184. doi:10.1021/CM800139Q
[31] P. Sonar, S. P. Singh, P. Lecle`re, M. Surin, R. Lazzaroni, T. T. Lin,
A. Dodabalapur, A. Sellinger, J. Mater. Chem. 2009, 19, 3228.
doi:10.1039/B820528K
[33] T. Kono, D. Kumaki, J. i. Nishida, T. Sakanoue, M. Kakita, H. Tada,
S. Tokito, Y. Yamashita, Chem. Mater. 2007, 19, 1218. doi:10.1021/
CM062889þ
[34] G. Myhre, A. Sayyad, S. Mataka, S. Pau, Appl. Phys. Lett. 2011, 99,
091108. doi:10.1063/1.3631670
[35] J. Huang, Y. Xu, Q. Hou, W. Yang, M. Yuan, Y. Cao, Macromol.
Rapid Commun. 2002, 23, 709. doi:10.1002/1521-3927(20020801)
23:12,709::AID-MARC709.3.0.CO;2-E
[36] M. Akhtaruzzaman, M. Tomura, J. Nishida, Y. Yamashita, J. Org.
Chem. 2004, 69, 2953. doi:10.1021/JO035800H