Effect of heterocycles on field-effect transistor performances of donor-acceptor-donor type small molecules

Article abstract of DOI:10.1016/j.cplett.2016.08.073

Two D–A–D small molecules comprising triphenylamine and diketopyrrolopyrrole were synthesized having either furan or thiophene connected to the fused lactam ring. In this design, furan/thiophene diketopyrrolopyrrole acts as an acceptor and triphenylamine acts as a donor. Propeller shaped triphenylamine has its effect on packing, processability and plays a vital role in determining the π-π molecular orbital stacking in such compounds and thus the mobility of charge carriers. With TDPPT and FDPPT, maximum hole carrier mobility obtained is 2.88?×?10^?3?cm²?V^?1?s^?1 and 1.60?×?10^?3?cm²?V^?1?s^?1, respectively using bottom gate bottom contact field-effect transistor.

Full text of DOI:10.1016/j.cplett.2016.08.073

Chemical Physics Letters 661 (2016) 107–113
Contents lists available at ScienceDirect
Chemical Physics Letters
journal homepage: www.elsevier.com/locate/cplett
Research paper
Effect of heterocycles on field-effect transistor performances
of donor-acceptor-donor type small molecules
⇑
Mrinmoy Kumar Chini , Rajashree Y. Mahale, Shyambo Chatterjee
Polymers and Advanced Materials Laboratory, CSIR-National Chemical Laboratory, CSIR-Network of Institutes for Solar Energy, Pune 411008, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 12 July 2016
In final form 27 August 2016
Available online 29 August 2016
Two D–A–D small molecules comprising triphenylamine and diketopyrrolopyrrole were synthesized
having either furan or thiophene connected to the fused lactam ring. In this design, furan/thiophene dike-
topyrrolopyrrole acts as an acceptor and triphenylamine acts as a donor. Propeller shaped triphenylamine
has its effect on packing, processability and plays a vital role in determining the
p-p molecular orbital
stacking in such compounds and thus the mobility of charge carriers. With TDPPT and FDPPT, maximum
Keywords:
ꢁ3
2
ꢁ1 ꢁ1
ꢁ3
2
ꢁ1 ꢁ1
hole carrier mobility obtained is 2.88 ꢀ 10 cm V
s
and 1.60 ꢀ 10 cm V
s , respectively using
Small molecules
Donor-acceptor-donor
Opto-electronic properties
Quadrupole
bottom gate bottom contact field-effect transistor.
Ó 2016 Elsevier B.V. All rights reserved.
Mobility
Field-effect transistor
1
. Introduction
small molecule is an attractive material as it incorporates the
advantages of both polymers and small molecules and show excel-
lent charge carrier properties despite having less ordered
microstructures.
Conjugated polymers (CPs) have been potentially used as an
active layer of organic electronic devices such as organic light
emitting diodes (OLEDs) [1], organic field-effect transistors (OFETs)
DPP is known to be a versatile building block [20,21] and also an
outstanding acceptor owing to its strong electron withdrawing nat-
ure and reasonably good photochemical stability [22–26]. We have
chosen triphenylamine (TPA) which is an excellent optoelectronic
organic material [27–29] owing to its good electron-donating and
hole-transporting abilities, as a donor to have D-A-D type structure
with DPP (acceptor). We observed that the solubility of TPA con-
taining molecules usually enhanced due to its propeller shaped
structure. Here, we have connected different heterocyclic mole-
cules (such as furan and thiophene) as a bridge in between TPA
and DPP functionality in the molecular backbone of the small mole-
cule. We are interested in studying the impact of the changes in the
bridging heterocyclic part of the molecular backbone on the OFET
performances of the small D-A-D type molecules. We report the
synthesis, electrochemical, optical and charge transport properties
of two D-A-D molecules FDPPT and TDPPT.
[
2] and organic photovoltaics [3]. The advantages of CPs are easy
solution processability, controllable band gap and excellent charge
transport properties [4–7]. The major drawbacks of the CPs have
been complex purification process, reproducibility [8], polydisper-
sity, and end group contaminations. Thus,
p-conjugated small
molecules have attracted a lot of attention owing to their synthetic
purity and reproducibility [9–11]. The main issue with the small
molecules is solution processability. To overcome this problem,
different methods have been reported [12]. These systems have
the limited chain to chain contacts that unfavourably impact the
device efficiency. In order to enhance the contact between chains,
small molecules containing
p-stacking functionalities have been
synthesized. Lactam containing diketopyrrolopyrrole (DPP) has
shown strong tendency to stack, driven by quadrupole-
quadrupole interaction [13].
Conjugated molecules having strong electron-donating sub-
stituent can sufficiently increase the charge carrier mobility
2
. Experimental section
[
14,15]. Comprising of electron deficient units with an electron rich
conjugated moiety is widely reported as the Donor–Acceptor–Do
nor (D–A–D) system [16–19]. Thus, a solution processable D–A–D
2.1. General methods and materials
Instruments and Materials: 1H and 13C NMR spectra were
⇑
Corresponding author.
E-mail address: mk.chini@ncl.res.in (M.K. Chini).
recorded on a Bruker arx 200 MHz and 100 MHz AVANS spectrom-
eter respectively using CDCl as the solvent unless otherwise
3
http://dx.doi.org/10.1016/j.cplett.2016.08.073
009-2614/Ó 2016 Elsevier B.V. All rights reserved.
0

1
08
M.K. Chini et al. / Chemical Physics Letters 661 (2016) 107–113
noted. Chemical shifts are reported in parts per million (ppm),
2.2.3. 2,5-Dioctyl-3,6-di(thiophen-2-yl)-2,5-dihydropyrrolo[3,4-c]
pyrrole-1,4-dione (3)
In a three-necked, oven-dried 250 mL round-bottom flask, 3,6-
dithiophen-2-yl-2,5-dihydro pyrrolo[3,4-c]pyrrole-1,4-dione (1)
1
chemical shifts in H NMR were referenced to TMS (0.0 ppm) and
1
3
C NMR spectra were referenced to CDCl
TOF mass spectra were recorded on Voyager-De-STR MALDI-TOF
Applied Bio systems, Framingham, MA, USA) equipped with 337-
nm pulsed nitrogen laser. Sample solution (1 M) was mixed with
DHB (2,5 dihydroxy benzoic acid) matrix in CHCl and sonicated
3
(77.0 ppm). MALDI-
(
2 3
(3.00 g, 10.0 mmol) and anhydrous K CO (5.52 g, 30.0 mmol) were
l
dissolved in 100 mL of anhydrous N,N-dimethylformamide (DMF)
and heated to 120 °C under argon for 1 h. n-octylbromide (5.76 g,
30.0 mmol) was then added dropwise, and the reaction mixture
was further stirred and heated overnight at 130 °C. The reaction
mixture was allowed to cool down to room temperature; after that
it was poured into 400 mL of distilled water, and the resulting sus-
pension was stirred at room temperature for 1 h. The solid was col-
lected by vacuum filtration, washed with several portions of
distilled water, methanol, and then air-dried. The crude product
was purified by flash chromatography using dichloromethane-
hexane as eluent, and the solvent was removed in vacuo to obtain
a pure product, 2,5-dioctyl-3,6-di(thiophen-2-yl)-2,5-dihydropyr
3
before casting on 96-well stainless steel MALDI plate. Perkin Elmer
Lambda-35 spectrophotometer was used for UV/vis spectra. Perki-
nElmer STA 6000 thermogravimetric analyzer was used for Ther-
mogravimetric analysis (TGA). The X-ray diffraction data were
obtained on X’PertPro Panalytical Diffractometer at a wavelength
of 1.5406 Å. Atomic force microscopy studies were performed with
a Nanoscope IIIa microscope and carried out in tapping mode at
ambient temperature. All the electrochemical studies were done
using CH-Instruments. Agilent 4156 C semiconductor probe ana-
lyzer and semi probe station had been used for all the FET
measurements.
rolo[3,4-c]pyrrole-1,4-dione (3) as a purple brown shiny crystalline
1
Thiophene-2-carbonitrile, Furan-2-carbonitrile, Potassium tert-
butoxide, Diethyl succinate, Triphenylamine (TPA), Potassium
acetate, Bis(pinacolato)diboron (B2pin2), were purchased from
Sigma Aldrich and used without further purification. N-
Bromosuccinimide (NBS) were purchased from Sigma Aldrich and
powder (yield: 75.4%). H NMR (400 MHz, CDCl
3
) dppm: 8.94 (d,
2H), 7.64 (d, 2H), 7.29 (t, 2H), 4.08 (t, 4H), 1.75 (m, 4H), 1.27–
1
3
3
1.44 (m, 28H), 0.88 (t, 6H). C NMR (400 MHz, CDCl ) dppm:
161.04, 139.69, 134.92, 130.36, 129.44, 128.27, 107.35, 41.89,
31.43, 29.62, 28.87, 26.54, 22.30, 13.76.
0
used after recrystallization. 1,1 -Bis [(diphenylphosphino)ferro
cene] dichloropalladium (II) [Pd(dppf)Cl
tone)dipalladium (0) [Pd (dba) ], Tri (o-tolyl)phosphine [P(o-
tolyl) ] were purchased from Alfa Aesar chemicals. Sodium metal,
tert-amyl alcohol, Acetic acid, Chloroform (CHCl ), N, N-
dimethylformamide (DMF), Toluene, Methanol (MeOH), Potassium
carbonate (K CO ) were purchased from Loba Chemie. All the sol-
vents were dried by following reported procedures.
2
], Tris(dibenzylideneace
2
3
2.2.4. 3,6-Di(furan-2-yl)-2,5-dioctyl-2,5-dihydropyrrolo[3,4-c]
pyrrole-1,4-dione (4)
Compound 4 was synthesized following same procedure for
compound 3. By using compound O-DPP (2) (3 g, 11.18 mmol)
3
3
2
3
along with n-octylbromide (6.44 g, 33.55 mmol) and
K
2
CO
(6.17 g 44.73 mmol) (yield: 81.6%). H NMR (400 MHz, CDCl
dppm: 8.31 (d, 2H), 7.64 (d, 2H), 6.70 (t, 2H), 4.11 (t, 4H), 1.68
3
1
3
)
1
3
2
2
.2. Synthetic procedures
(m, 4H), 1.27–1.39 (m, 28H), 0.87 (t, 6H). C NMR (400 MHz,
CDCl ) dppm: 160.54, 144.83, 133.33, 120.18, 117.71, 113.60,
3
.2.1. 3,6-Di (thiophen-2-yl)-2,5-dihydropyrrolo [3,4-c] pyrrole-1,4-
106.10, 42.09, 31.47, 29.87, 28.85, 26.51, 22.30, 13.76.
dione (S-DPP) (1)
To argon filled oven-dried three-neck round-bottom flask
equipped with a magnetic stir bar, a dropping funnel and a reflux
condenser, potassium tert-butoxide (7.72 g, 68.9 mmol) and tert-
amyl alcohol (35 mL) were added. The mixture was heated to
2.2.5. 3,6-bis(5-Bromothiophen-2-yl)-2,5-dioctyl-2,5-dihydropyrrolo
[3,4-c]pyrrole-1,4-dione (5)
A 100 mL single-neck round-bottom flask was charged with a
stir bar, compound 3 (1 g, 1.72 mmol) was added to a solution in
chloroform (50 mL) under ambient conditions. Flask was wrapped
in aluminum foil to avoid the exposure of the reaction to light.
After the reaction mixture was stirred in an ice bath at 0 °C for
20 min, N-bromosuccinimide (NBS) (0.76 g, 4.30 mmol) was added
in three portions to it, the solution stirred at room temperature for
48 h. Resulting crude product was extracted with chloroform,
1
00–110 °C for 1.5 h. To this mixture 2-thiophenenitrile (5.0 g,
4
5.8 mmol) was injected in one portion and the stirring continued
at 105 °C for 30 min. A mixture of diethyl succinate (4.00 g,
2.9 mmol) in tert-amyl alcohol (10 mL) was added drop wise over
a period of 1 h with rapid stirring. The mixture was then stirred at
00–110 °C for a further 4 h, and then cooled to 50 °C. Then the
2
1
mixture was diluted with of methanol (30 mL) and neutralized
with acetic acid (5 ml). The reaction mixture was then heated to
reflux for 45 min before cooling to room temperature. The suspen-
sion was filtered over a Buchner funnel and the solid was washed
with hot methanol and water several times and dried under vac-
uum at 80 °C for 16 h to give the product, 3,6-di (thiophen-2-yl)-
2 4
washed with water, and dried over anhydrous Na SO . Solvent
was removed under reduced pressure and the product was purified
using silica gel chromatography eluting with a mixture of hexane
1
and DCM to get dark red purple solid. Yield: (1.2 g, 72%). H NMR
(CDCl
3
, 400 MHz) dppm: 8.69 (d, 2H), 7.25 (d, 2H), 3.99 (t, 4H),
1
3
2
,5-dihydropyrrolo [3,4-c] pyrrole-1,4-dione (S-DPP) (1). Yield:
1.72 (m, 4H), 1.42–1.28 (m, 20H), 0.89 (t, 6H). C NMR (CDCl ,
3
3
.6 g (26%) as a red solid. This compound was used without further
400 MHz) dppm: 160.77,138.74,135.09, 131.38, 130.86, 118.88,
107.55, 42.03, 31.50, 29.71, 28.88, 26.56, 22.37, 13.82.
1
purification. H NMR (DMSO-d
7
6
, 400 MHz) dppm: 7.30 (dd, 2H,),
13
.95 (d, 2H), 8.22 (d, 2H), 11.21 (s, 2H); C NMR (DMSO-d
6
,
4
00 MHz) dppm: 108.53, 128.65, 130.76, 131.23, 132.58, 136.11,
and 161.58.
.2.2. 3,6-Di (furan-2-yl)-2,5-dihydropyrrolo [3,4-c] pyrrole-1,4-dione
2.2.6. 3,6-bis(5-Bromofuran-2-yl)-2,5-dioctyl-2,5-dihydropyrrolo[3,4-
c]pyrrole-1,4-dione (6)
2
Compound 6 was synthesized by following the same procedure
used for the synthesis of compound 5. By using compound 4 (1.5 g,
(
O-DPP) (2)
Compound 2 was synthesized following same procedure for
3.05 mmol) and NBS (1.35 g, 7.61 mmol). Yield: (1.35 g, 78%). 1
H
compound 1. Yield: 3.1 g (21%). 1H NMR (DMSO-d
, 400 MHz)
NMR (CDCl
3
4H), 1.70 (m, 4H), 1.41–1.28 (m, 20H), 0.88 (t, 6H). C NMR (CDCl ,
6
3
, 400 MHz) dppm: 8.26 (d, 2H), 6.64 (d, 2H), 4.06 (t,
1
3
13
dppm: 6.83 (dd, 2H), 7.65 (d, 2H), 8.04 (d, 2H), 11.17 (s, 2H).
NMR (DMSO-d , 400 MHz) dppm: 107.57, 113.71, 116.79, 131.27,
43.75, 146.91, and 161.71.
C
6
400 MHz) dppm: 160.65, 143.54, 132.63, 122.03, 115.26, 110.25,
105.97, 45.94, 39.83, 30.23, 28.45, 22.93, 22.89, 13.81.
1

M.K. Chini et al. / Chemical Physics Letters 661 (2016) 107–113
109
2
.2.7. N,N-diphenyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)
aniline (8)
A 3-neck 250 mL round bottom flask was charged with a stir
reacted with NBS to obtain compound 5 and 6 according to
reported procedure [30]. Then borylation of compound 7 was car-
ried out by Miyaura borylation reaction using Pd(dppf)Cl catalyst
2
bar, 4-bromotriphenylamine (4.00 g, 12.3 mmol) (7), bis(pinaco-
and anhydrous dioxane as solvent to get compound 8. The final
compounds 9 and 10 were obtained with the Suzuki-Miyaura cou-
pling using the following reagents, bis(dibenzylideneacetone)palla
lato)diboron (B2pin2) (3.26 g, 12.8 mmol), anhydrous potassium
0
acetate (KOAc) (3.28 g, 33.41 mmol), dichloro[1,1 -bis(diphenylpho
sphino)-ferrocene]palladium(II) dichloromethane adduct (Pd
dium (0) (Pd
drous K CO in anhydrous toluene solvent. The UV–vis absorption
spectra were recorded in chloroform (CHCl ) solution. It can be
2 3 3
(dba) ), tri-o-tolylphosphine (P(o-tol) ) and anhy-
(
dppf)Cl ꢂCH
2 2
Cl
2
) (0.243 g, 0.334 mmol) and degassed dioxane
2
3
(
120 mL). After the reaction mixture was heated at 85 °C for 16 h,
3
it was extracted with diethyl ether and washed with distilled
water. The organic extract was dried over MgSO , and solvent
was removal under reduced pressure yielded brown viscous oil.
noticed that both the FDPPT and TDPPT molecules exhibits well-
4
defined two peaks both in solution and in thin film. The lower
⁄
wavelength absorption bands ascribed to the
p–p
transitions
Purification with flash chromatography (25% hexanes in CH
yielded 4.30 g of compound 8 as tacky off-white solid (95%).
NMR (400 MHz, CDCl
2
Cl
2
)
H
and the strongest absorption peak results from the intramolecular
charge transfer (ICT) [31]. Absorption spectra of compound FDPPT
in chloroform shows two peaks at kmax 587.2 and 635.1 nm and
1
3
) dppm: 7.69 (d, 2 H), 7.27 (t, 4 H), 7.23 (d,
13
4
1
2
H), 7.13–7.02 (m, 4 H), 1.34 (s, 12 H). C (400 MHz, CDCl
50.65, 147.54, 136.01, 129.45, 125.26, 123.87, 121.79, 83.76,
5.26.
3
) dppm:
TDPPT in chloroform shows two peaks at kmax 593.2 and
633.9 nm (Fig. 1a). Both the molecules have shown similar charac-
teristic in their UV/vis absorption spectra possibly due to their sim-
ilar molecular structures. The maximum absorption wavelength
(
1
k
max) of both the TDPPT and FDPPT are red-shifted as much as
2
.2.8. 3,6-bis(5-(4-(Diphenylamino)phenyl)thiophen-2-yl)-2,5-
5 to 45 nm compared to their absorption in solution. This can
dioctyl-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (9)
attributed to the better packing (p–p stacking) of the small mole-
A 50 mL Schlenk tube was charged with materials, 5 (0.800 g,
cules in solid thin film state. The absorption spectrum of TDPPT
becomes remarkably broad compared to the FDPPT spectrum in
solid state which indicates that the different heterocycle has its
influence on molecular packing and
cal band gaps are estimated from the absorption edges of thin films
using the following formula E
FDPPT and TDPPT are 1.82 eV and 1.79 eV, respectively. The Eg
of FDPPT is noticeably larger than that of TDPPT.
The frontier orbital energy levels (HOMO-LUMO) of these mate-
rials were determined by cyclic voltammetry (CV) techniques. Plat-
inum wire and platinum foil were used as working and counter
1
.17 mmol), 8 (0.958 g, 2.58 mmol), bis(dibenzylideneacetone)
palladium (0) (Pd (dba)3) (0.021 g, 0.023 mmol), tri-o-
tolylphosphine (P(o-tol) (0.028 g, 0.094 mmol), anhydrous
CO (1.54 g, 11.13 mmol), 2 drops of aliquat 336, freeze-pump-
thawed toluene (20 mL) and freeze-pump-thawed distilled water
6.0 mL). The reaction mixture was heated at 90 °C for 16 h before
being precipitated into 250 mL of MeOH. The precipitates were fil-
tered through a 20 m nylon membrane. Purification by flash chro-
matography (10% hexanes in chloroform) yielded compound 9 as a
2
3
)
p–p stacking in film. The opti-
K
2
3
opt
g
= 1240/konset. The band gaps of
(
opt
l
1
metallic purple solid (56%). H NMR (400 MHz, CDCl
3
) dppm: 8.96
(
(
s, 2H), 7.52–7.06 (m, 30H), 4.09 (s, 4 H), 1.76 (m, 4H), 1.55–1.24
+
m, 20H), 0.82 (t, 6H). 1 C NMR (400 MHz, CDCl
3
electrode, respectively. Ag/Ag (0.1 M of AgNO3 in acetonitrile)
3
) dppm: 161.02,
52.75, 149.57, 148.24, 146.76, 141.78, 138.90, 136.58, 129.12,
27.48, 124.67, 123.36, 107.41, 41.95, 31.48, 29.69, 28.90, 26.59,
electrode was used as reference electrode. The cyclic voltammetry
were performed using tetra butyl ammonium perchlorate (0.01 M
TBAP in acetonitrile) solution as supporting electrolyte at room
temperature with a scan rate of 50 mV/s and ferrocenium/fer-
1
1
2
1
2.30, 13.76 ESI–HRMS calcd for C66
011.87.
66 4 2 2
H N O S 1011.40, found
+
rocene (Fc /Fc) redox couple as external reference. The CVs of
TDPPT and FDPPT are shown in Fig. 1b. HOMO values were calcu-
lated using the formula, HOMO = ꢁ[Eox (onset) + 4.8] eV assuming
that the absolute energy level of Fc is ꢁ4.8 eV [32]. The LUMO level
was estimated from the optical band gap values. The HOMO and
the LUMO level of TDPPT (ꢁ5.2 eV and ꢁ3.4 eV) is both deeper
than FDPPT (ꢁ5.0 eV and ꢁ3.2 eV), while both the small molecules
are identical because they have same electron donating (TPA) and
accepting unit (DPP) except two different heterocyclic units (furan/
thiophene). The sulphur atom in thiophene unit is responsible for
lower HOMO/LUMO level for TDPPT. It is quite often seen in oxy-
gen vs sulphur systems. The optical band gap is weakly affected
by this. The energy level diagram of the TDPPT and FDPPT is shown
in Fig. 1d.
2
.2.9. 3,6-bis(5-(4-(Diphenylamino)phenyl)furan-2-yl)-2,5-dioctyl-
2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (10)
Compound 10 was synthesized by following the procedure used
for the synthesis of compound 9. Herein, 6 (0.800 g, 1.23 mmol), 8
(
(
(
1.01 g, 2.71 mmol), bis(dibenzylideneacetone)palladium (0)
Pd
P(o-tol)
2
(dba)
3
)
(0.023 mg,
(0.030 g, 0.098 mmol), anhydrous
0.0245 mmol),
tri-o-tolylphosphine
CO (1.60 g,
3
)
K
2
3
1
1.68 mmol), 2 drops of aliquat 336 were used, (Yield = 62.5%).
1
H
NMR (400 MHz, CDCl
3
) dppm: 8.37 (s, 2 H), 7.55–7.57
(
d, 4 H), 7.29–7.24 (t, 8 H), 7.12–7.05 (m, 16 H), 4.17 (t, 4 H),
1
3
1
.76 (m, 4H), 1.57–1.18 (m, 20H), 0.78 (t, 6H).
C NMR
(
400 MHz, CDCl ) dppm: 160.81, 157.04, 148.55, 147.02, 143.50,
3
1
4
32.18, 129.44, 125.44, 125.09, 123.76, 122.69, 122.50, 107.97,
2.56, 31.72, 30.35, 29.43, 29.22, 27.13, 22.56, 14.04. ESI–HRMS
DFT studies were done using B3LYP functional and polarized 6-
⁄
3
1g basis set. The surface plots of both the molecules show that
66 4 4
calcd for C66H N O 979.28, found 979.35.
the wave function is well delocalized within the
p-backbone for
HOMO since both TPA and furan/thiophene DPP is contributing.
However, the wave function is delocalized preferably over the
DPP part in case of LUMO for both the compounds. This may be
attributed to its electron accepting nature. The surface energy plots
are shown in Fig. S18 (ESI). The optimized ground state geometry
of FDPPT and TDPPT were taken for determining the dihedral angle
at the junction of the furan/thiophene part of the DPP and phenyl
part of the TPA connected to furan/thiophene. The dihedral angle
was 175.49° for FDPPT whereas the angle was found to be
165.41° for TDPPT. Optimized ground state structures of FDPPT
and TDPPT and dihedral angle are shown in Fig. S19. The dihedral
3
. Results and discussion
The synthetic steps for the synthesis of the DPP molecules are
shown in Scheme 1. The structure of all the key intermediates
and target D-A-D molecules has been confirmed by 1H NMR,
NMR and ESI–HRMS.
13
C
The O-DPP and S-DPP were synthesized by Stobbe condensation
between diethyl succinate and 2-furonitrile or 2-
thiophenecarbonitrile, respectively. The O-DPP and S-DPP were
subjected to alkylation using 1-bromo octane, which was then

1
10
M.K. Chini et al. / Chemical Physics Letters 661 (2016) 107–113
OH
R
N
R
N
H
K+ButO-,
100-110 °C , 4-6 hrs
N
O
X
O
NBS,
CHCl3
O
Br
Br-C H
X
X
CN
X = S/O
8
17
X
O
X
X
RT, 48
hrs
X
Br
K2CO3, DMF,120 °C
O
O
O
N
H
N
R
N
R
O
O
O
1
2
S-DPP
O-DPP
3 S-DPPR
4 O-DPPR
5 S-DPPR-Br
6 O-DPPR-Br
O
O
O
B
B
5
6
S-DPPR-Br
O-DPPR-Br
O
R
N
Pd(dppf)Cl .CH Cl
Pd (dba) , P(o-tolyl)
2 3 3
2
2
2
O
O
O
X
KOAc
K2CO3, aliquat 336
N
Br
N
B
N
N
H O/Toluene
X
Dioxane, 85 °C, 16h
2
O
90 °C, 16h
N
R
7
Br-TPA
8
TPA-BE
9 S-DPPT (TDPPT)
1
0 O-DPPT (FDPPT)
R= n-Octyl
Scheme 1. Synthesis scheme of FDPPT and TDPPT.
Fig. 1. (a) UV/vis spectra in solution and in thin film state, (b) cyclic voltammograms showing oxidation, (c) the schematic diagram of FET device and (d) energy levels of
FDPPT and TDPPT.
angle between the planes indicates that the DPP and specified phe-
nyl part of the TPA are in plane with a deviation of ꢃ5° for FDPPT
which is higher for TDPPT (ꢃ15°).
presence of propeller structured TPA moiety. The thin film mor-
phologies of deposited FDPPT and TDPPT were investigated by
atomic force microscopy (AFM) before and after annealing under
similar conditions, used for FET device fabrication. The morphology
of FDPPT was found to be a non uniform film with appearance of
aggregation of small rods at random places. They become more
pronounced upon annealing (Fig. 2b). On the other hand, TDPPT
also showed a film with nano rods (Fig. 2c). The thin films of FDPPT
and TDPPT deposited at room temperature showed a root mean
square (rms) roughness value (Rq) of 18.5 nm and 14.9 nm after
The thermal stability of the molecules was examined using
thermogravimetric analysis. Significant weight loss was not
observed until 230 °C in the thermogram, indicating good thermal
stability of the molecules. The X-ray diffraction (XRD) measure-
ment on the thin films deposited at 80 °C was also carried out
and the diffraction pattern was given in Fig. 4b. This suggests that
the thin film molecules are amorphous in nature due to the

M.K. Chini et al. / Chemical Physics Letters 661 (2016) 107–113
111
Fig. 2. (a and b) Represents AFM amplitude images of FDPPT, (c and d) represents AFM amplitude images of TDPPT before and after annealing of the thin films, respectively.
annealing showed the Rq values of 23.8 nm and 18.1 nm respec-
tively. The roughness had increased after annealing. The interac-
tion between alkyl chains of the modified substrates and the
small molecules is majorly responsible for the change in the sur-
face roughness [33]. This phenomenon is known for conjugated
small molecules.
while holding the V
respectively (determined from the output characteristics). The
charge carrier mobility ( ) in the saturation regime was calculated
T
using the following relationship, I ) ]. W and
L are the semiconductor channel width and length, respectively. Ci
(Ci = 14.9 nF) is the capacitance per unit area of the gate dielectric
D
at ꢁ40 V and ꢁ60 V for FDPPT and TDPPT,
l
2
D
= (l
CW/2L) [(V
G
ꢁ V
Finally after all the molecular characterization, we have done
the transistor measurements using prefabricated field effect tran-
layer. V
T
is the threshold voltage. The device parameters are sum-
marized in Table 1. The hole mobility of FDPPT and TDPPT based
ꢁ4
2
ꢁ1 ꢁ1
ꢁ4
2
ꢁ1 ꢁ1
sistors having bottom gate (SiO
uration on top of the silicon wafer substrates. The thickness of the
gate dielectric, SiO was 210 nm. The substrates were cleaned
2
) and bottom contact (Au) config-
transistorswas3.47 ꢀ 10 cm V
s
and6.29 ꢀ 10 cm V
s
measured for unannealed thin films, respectively. Upon annealing
2
at 80 °C, the mobility for FDPPT and TDPPT was increased to
ꢁ3
2
ꢁ1 ꢁ1
ꢁ4
2
ꢁ1 ꢁ1
using ultrasonication in chloroform, acetone and isopropanol
respectively prior to use. The substrates were modified by dipping
them into 5% (V/V) hexamethylenedisilazane (HMDS) solution of
1.59 ꢀ 10 cm V
s
and 7.79 ꢀ 10 cm V
s , respectively.
This is attributed to the better packing upon annealing. Further-
more, on annealing at 100 °C the hole mobility was reduced to
ꢁ
4
2
ꢁ1 ꢁ1
ꢁ5
2
ꢁ1 ꢁ1
CHCl
sphere since devices based on unmodified substrates showed no
measurable current (I ) upon sweeping the drain voltage (V ) at
various gate voltages (V ) due to the possible poor interaction
between the SiO and the studied molecules [34]. The FDPPT and
TDPPT molecules were then spin coated from chloroform solution
10 mg/mL) on top of the HMDS modified substrates and the device
3
followed by annealing at 100 °C for 10 min under N
2
atmo-
5 ꢀ 10 cm V
s
and 7 ꢀ 10 cm V
s , respectively. The
maximum mobility obtained for the annealed FDPPT and TDPPT
ꢁ3
2
ꢁ1 ꢁ1
ꢁ3
2
ꢁ1 ꢁ1
D
D
thin film was 2.88 ꢀ 10 cm V
s
and 1.60 ꢀ 10 cm V
s
G
respectively. The hole carrier mobility is comparable for both
FDPPT and TDPPT. This is likely due to the TPA donor units nullify-
ing the possible packing facilitated furan and thiophene. In such
cases, morphology can impact the charge carrier mobility [35].
However, both FDPPT and TDPPT showed similar morphology,
hence the charge carrier mobility is not significantly different.
2
(
characterization was carried out under inert conditions. The
annealing temperature could enhance the device performances.
But beyond certain annealing temperature mobility decreased
rapidly. Thus, beyond that particular annealing temperature, we
had not annealed the thin film devices. To check the change in
FET performances for the FDPPT and TDPPT, the prepared devices
were further annealed at temperatures such as 80 °C and 100 °C
We observed the contact resistance (R
temperature. The R was calculated using the relationship, R
+ RCh. Where, R and RCh are the total and the channel resistance,
respectively. The R for the FDPPT unannealed thin film device is
2 ꢀ 10 O at RT whereas upon annealing R was reduced little bit
for the TDPPT as such spin
was
C
) varied with annealing
C
T
=
R
C
T
C
9
C
9
for 10 min each inside glove box. The V
D
was swept between 0
to 1 ꢀ 10 O at 80 °C (Fig. 4c). The R
C
9
and ꢁ100 V while holding constant negative V
G
. The output char-
coated thin film device is 5 ꢀ 10 O whereas on annealing R
C
8
acteristics of the devices showed obvious linear and saturation
regimes for FDPPT (Fig. 3a) and TDPPT (Fig. 3c) and indicated that
both of the molecules are hole transporting (p type) in nature.
reduced by order to 1 ꢀ 10 O at 80 °C (Fig. 4d). This can be corre-
lated to the increase in mobility values. On further annealing at
higher temperature, the R
C
values increase which in turn correlates
The transfer characteristics were obtained by sweeping the V
G
to the lower charge carrier mobility obtained.

1
12
M.K. Chini et al. / Chemical Physics Letters 661 (2016) 107–113
Fig. 3. Output (a) and transfer characteristics curve (b) of FDPPT. Output (c) and transfer characteristics (d) of TDPPT after annealing, respectively.
Table 1
OFET parameters at various temperatures.
ꢁ1
ꢁ1
h,avg (cm2
ꢁ1 ꢁ1
Molecules
FDPPT
T (°C)
l
h,max (cm2
V
s
)
l
V
s
)
V
T
(V)
I
on/off
ꢁ
ꢁ
ꢁ
4
ꢁ4
2
1
1
As-cast
8.56 ꢀ 10
2.88 ꢀ 10
4.15 ꢀ 10
1.53 ꢀ 10
1.60 ꢀ 10
1.04 ꢀ 10
3.47 ꢀ 10
1.59 ꢀ 10
5.04 ꢀ 10
6.29 ꢀ 10
7.79 ꢀ 10
6.94 ꢀ 10
ꢁ46
ꢁ62
ꢁ58
ꢁ22
ꢁ18
ꢁ15
5.5 ꢀ 10
6.5 ꢀ 10
4.5 ꢀ 10
4.0 ꢀ 10
1.0 ꢀ 10
5.3 ꢀ 10
3
4
ꢁ3
ꢁ4
8
1
0
00
ꢁ3
ꢁ3
ꢁ4
ꢁ4
ꢁ4
ꢁ5
2
3
2
TDPPT
As-cast
8
1
0
00
C G
Fig. 4. (a) The TGA curve, (b) thin film XRD of both FDPPT and TDPPT annealed at 80 °C shows amorphous nature, (c and d) represents the R vs V plot for FDPPT and TDPPT
respectively.

M.K. Chini et al. / Chemical Physics Letters 661 (2016) 107–113
113
[
12] A. Arulkashmir, R.Y. Mahale, S.S. Dharmapurikar, M.K. Jangid, K.
Krishnamoorthy, Supramolecular interaction facilitated small molecule films
for organic field effect transistors, Polym. Chem. 3 (2012) 1641.
4
. Conclusion
In summary, we have synthesized and characterized DPP and
[13] A.L. Kanibolotsky, F. Vilela, J.C. Forgie, S.E.T. Elmasly, P.J. Skabara, K. Zhang, B.
Tieke, J. McGurk, C.R. Belton, P.N. Stavrinou, D.D.C. Bradley, Well-defined and
monodisperse linear and star-shaped Quaterfl uorene-DPP molecules: the
significance of conjugation and dimensionality, Adv. Mater. 23 (2011) 2093.
[
[
[
TPA based small molecules with octyl side chains. The synthesized
molecules are solution processable and form good films on HMDS
modified substrates. Before annealing, the HMDS modified devices
have shown proper FET characteristics and the obtained charge
carrier mobility values for both FDDPT and TDPPT were low but
comparable. On annealing the modified thin films at 80 °C, the hole
carrier mobility values for both D-A-D molecules have increased by
approx. one order to give relatively high values due to better pack-
ing and improved quadrupole-quadrupole interaction arising from
the DPP units enhanced the interchain contacts. The maximum
14] Z. Bao, J.A. Rogers, H.E. Katz, Printable organic and polymeric semiconducting
materials and devices, J. Mater. Chem. 9 (1999) 1895.
15] C.J. Brabec, N.S. Sariciftci, J.C. Hummelen, Plastic solar cells, Adv. Funct. Mater.
11 (2001) 15.
16] P. Sonar, S.P. Singh, Y. Li, M.S. Soh, A. Dodabalapur,
A
low-bandgap
diketopyrrolopyrrole-benzothiadiazole-based copolymer for high-mobility
ambipolar organic thin-film transistors, Adv. Mater. 22 (2010) 5409.
17] M. Zhang, H.N. Tsao, W. Pisula, C. Yang, A.K. Mishra, K. Müllen, Field-effect
transistors based on a benzothiadiazole–cyclopentadithiophene copolymer, J.
Am. Chem. Soc. 129 (2007) 3472.
[
ꢁ
3
[18] Z. Cai, Y. Guo, S. Yang, Q. Peng, H. Luo, Z. Liu, G. Zhang, Y. Liu, D. Zhang, New
donor–acceptor–donor molecules with Pechmann dye as the core moiety for
solution-processed good-performance organic field-effect transistors, Chem.
Mater. 25 (2013) 471.
mobility obtained for FDPPT and TDPPT thin film was 2.88 ꢀ 10
2
ꢁ1 ꢁ1
ꢁ3
2
ꢁ1 ꢁ1
cm V
s
and 1.60 ꢀ 10 cm V
s
while annealed at 80 °C,
respectively. Overall, we can conclude that heterocyclic moieties
have little impact on charge carrier mobility of these small mole-
cules while connected to TPA moiety.
[
19] L.E. Polander, L. Pandey, S. Barlow, S.P. Tiwari, C. Risko, B. Kippelen, J.-L. Bredas,
S.R. Marder, Benzothiadiazole-dithienopyrrole donor–acceptor–donor and
acceptor–donor–acceptor triads: synthesis and optical, electrochemical, and
charge-transport properties, J. Phys. Chem. C 115 (2011) 23149.
Acknowledgements
[20] E.D. Glowacki, H. Coskun, M.A. Blood-Forsythe, U. Monkowius, L. Leonat, M.
Grzybowski, D. Gryko, M.S. White, A.A. Guzik, N.S. Sariciftci, Hydrogen-bonded
diketopyrrolopyrrole (DPP) pigments as organic semiconductors, Org.
Electron. 15 (2014) 3521.
[21] Y. Zhang, C. Kim, J. Lin, T.Q. Nguyen, Solution-processed ambipolar field-effect
transistor based on diketopyrrolopyrrole functionalized with benzothiadiazole
group, Adv. Funct. Mater. 22 (2012) 97.
MKC and RYM thank CSIR for fellowship. We are grateful to Dr.
K. Krishnamoorthy for his support. We also thank CSIR for financial
support through NWP 54 (TAPSUN). We thank Mrs. Pooja Mud-
dellu for AFM imaging.
[
22] S.S. Zade, M. Bendikov, Study of hopping transport in long oligothiophenes and
oligoselenophenes: dependence of reorganization energy on chain length,
Chem. Eur. J. 14 (2008) 6734.
Appendix A. Supplementary material
[
23] Y. Qiao, Y. Guo, C. Yu, F. Zhang, W. Xu, Y. Liu, D. Zhu, Diketopyrrolopyrrole-
containing quinoidal small molecules for high-performance, air-stable, and
solution-processable n-channel organic field-effect transistors, J. Am. Chem.
Soc. 134 (2012) 4084.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.cplett.2016.08.
[
24] H. Zhong, J. Smith, S. Rossbauer, A.J.P. White, T.D. Anthopoulos, M. Heeney, Air-
stable and high-mobility n-channel organic transistors based on small-
molecule/polymer semiconducting blends, Adv. Mater. 24 (2012) 3205.
25] A. Babel, S.A. Jenekhe, Alkyl chain length dependence of the field-effect carrier
mobility in regioregular poly (3-alkylthiophene)s, Synt. Met. 148 (2005) 169.
26] M.H. Yoon, S.A. DiBenedetto, A. Facchetti, T.J. Marks, Organic thin-film
transistors based on carbonyl-functionalized quaterthiophenes: high
mobility N-channel semiconductors and ambipolar transport, J. Am. Chem.
Soc. 127 (2005) 1348.
0
73.
[
[
References
[
1] J. Pei, W.L. Yu, W. Huang, A.J. Heeger, A novel series of efficient thiophene-
based light-emitting conjugated polymers and application in polymer light-
emitting diodes, Macromolecules 33 (2000) 2462.
[
2] Y. Huang, X. Guo, F. Liu, L. Huo, Y. Chen, T.P. Russell, C.C. Han, Y. Li, J. Hou,
[
27] S. Ando, R. Murakami, J. Nishida, H. Tada, Y. Inoue, S. Tokito, Y. Yamashita, N-
type organic field-effect transistors with very high electron mobility based on
thiazole oligomers with trifluoromethylphenyl groups, J. Am. Chem. Soc. 127
Improving the ordering and photovoltaic properties by extending
conjugated area of electron-donating units in polymers with D-A structure,
Adv. Mater. 24 (2012) 3383.
p–
(
2005) 14996.
[
3] F. Liu, C. Wang, J. Baral, W. Zhao, L. Zhang, J.J. Watkins, A.L. Briseno, T.P. Russell,
Relating chemical structure to device performance via morphology control in
diketopyrrolopyrrole-based low band gap polymers, J. Am. Chem. Soc. 35
[
28] M. Facchetti, M.H. Mushrush, G.R. Yoon, M.A. Hutchison, T.J. Ratner, Marks,
building blocks for n-type molecular and polymeric electronics.
Perfluoroalkyl-versus alkyl-functionalized oligothiophenes (nT; n = 2–6).
systematics of thin film microstructure, semiconductor performance, and
modeling of majority charge injection in field-effect transistors, J. Am. Chem.
Soc. 126 (2004) 13859.
(
2013) 19248.
[
[
[
4] A. Facchetti, P-Conjugated polymers for organic electronics and photovoltaic
cell applications, Chem. Mater. 23 (2011) 733.
5] X. Zhao, X. Zhan, Electron transporting semiconducting polymers in organic
electronics, Chem. Soc. Rev. 40 (2011) 3728.
6] L. Biniek, B.C. Schroeder, C.B. Nielsen, I. McCulloch, Recent advances in high
mobility donor–acceptor semiconducting polymers, J. Mater. Chem. 22 (2012)
[
29] Y. Shirota, H. Kageyama, Charge carrier transporting molecular materials and
their applications in devices, Chem. Rev. 107 (2007) 953.
[
30] R. Rabindranath, Y. Zhu, I. Heim, B. Tieke, Red emitting N-functionalized poly
(
1,4-diketo-3,6-diphenylpyrrolo[3,4-c]pyrrole) (poly-DPP): a deeply colored
polymer with unusually large stokes shift, Macromolecules 39 (2006) 8250.
31] E. Ripaud, Y. Olivier, P. Lariche, J. Cornil, J. Roncali, low-bandgap
1
4803.
7] F. Huang, K.S. Chen, H.L. Yip, S.K. Hau, O. Acton, Y. Zhang, J.D. Luo, A.K.Y. Jen,
Development of new conjugated polymers with donor- -bridge-acceptor side
chains for high performance solar cells, J. Am. Chem. Soc. 131 (2009) 13886.
8] W.R. Saaneck, R.H. Friend, J.L. Bredas, Electronic structure of conjugated
polymers: consequences of electron–lattice coupling, Phys. Rep. 319 (1999)
[
[
[
[
A
p
diketopyrrolopyrrole-benzothiadiazole-based copolymer for high-mobility
ambipolar organic thin-film transistors, J. Phys. Chem. B 115 (2011) 9379.
[
32] C. Popere, A.M. Della Pelle, S. Thayumanavan, BODIPY-based donor-acceptor
conjugated alternating copolymers, Macromolecules 44 (2011) 4767.
p-
2
31.
[
33] C. Bock, D.V. Pham, U. Kunze, D. Kafer, G. Witte, C. Woll, Improved morphology
and charge carrier injection in pentacene field-effect transistors with thiol-
treated electrodes, J. Appl. Phys. 100 (2006) 114517.
9] B. Walker, C. Kim, T.Q. Nguyen, Small molecule solution-processed bulk
heterojunction solar cells, Chem. Mater. 23 (2011) 470.
[
10] R. Fitzner, E. Mena-Osteritz, A. Mishra, G. Schulz, E. Reinold, M. Weil, C. Körner,
H. Ziehlke, C. Elschner, K. Leo, M. Riede, M. Pfeiffer, C. Uhrich, P. Bäuerle,
[
[
34] M.K. Chini, C. Das, S. Chatterjee, Chem. Phys. Lett. 657 (2016) 26.
35] S.S. Dharmapurikar, A. Arulkashmir, C. Das, P. Muddellu, K. Krishnamoorthy,
Enhanced hole carrier transport due to increased intermolecular contacts in
small molecule based field effect transistors, ACS Appl. Mater. Interface 5
Correlation of
p-conjugated oligomer structure with film morphology and
organic solar cell performance, J. Am. Chem. Soc. 134 (2012) 11064.
11] Y. Lin, Y. Li, X. Zhan, Small molecule semiconductors for high-efficiency
organic photovoltaics, Chem. Soc. Rev. 41 (2012) 4245.
[
(
2013) 7086.