Synthesis and characterization of quinoxaline derivative as organic semiconductors for organic thin-film transistors

Article abstract of DOI:10.1166/jnn.2017.13841

Three new quinoxaline-based derivatives, end-functionalized with 2,3,5,8-tetraphenyl (3), 2,3-diphenyl-5,8-di(thiophen-2-yl) (4), and 2,3-diphenyl-5,8-bis(5-phenylthiophen-2-yl) (5) were synthesized, characterized, and incorporated as organic semiconducto

Full text of DOI:10.1166/jnn.2017.13841

Article
Journal of
Nanoscience and Nanotechnology
Vol. 17, 5530–5538, 2017
Copyright © 2017 American Scientific Publishers
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Synthesis and Characterization of Quinoxaline
Derivative as Organic Semiconductors for
Organic Thin-Film Transistors
1
ꢀ†
2ꢀ†
3
Hyekyoung Kim , M. Rajeshkumar Reddy , Seong-Soo Hong ,
1ꢀ∗
2ꢀ∗
Choongik Kim , and SungYong Seo
1
Department of Chemical and Biomolecular Engineering, Sogang University, Seoul 04107, Korea
2
Department of Chemistry, Pukyong National University, Busan 48513, Korea
3
Department of Chemical Engineering, Pukyong National University, Busan, 48513, Korea
Three new quinoxaline-based derivatives, end-functionalized with 2,3,5,8-tetraphenyl (3), 2,3-
diphenyl-5,8-di(thiophen-2-yl) (4), and 2,3-diphenyl-5,8-bis(5-phenylthiophen-2-yl) (5) were syn-
thesized, characterized, and incorporated as organic semiconductors in top-contact/bottom-gate
organic thin-film transistors (OTFTs). Thermal, optical, and electrochemical properties of the newly
developed compounds were fully investigated. For the fabrication of thin films of all three com-
pounds, solution-shearing (SS) and vacuum deposition method were employed. Thin films of com-
−
5
2
pound 5 sh Do we el i vd epr e- c dh ab ny n Ienl gc eh na tr aa ctt oe :r i Ss t ti ca st ew Ui t hn i hv eo lres i mt yo ob fi l iNt i ee sw a Ys oh ri kg ha ta sB i2n . g6 h×a 1m 0toncm /Vs and
5
−4
2
current on/off ratio of I1P. 8: ×1 41 60 .1 v8 i 5a . s2 o0 l 0u t. i3o 5n Op r no :c eF sr si , a0 n9 dJ 1u . n9 ×2 01 10 7 0c 2m :3 /6V : s3 3a nd current on/off ratio
6
Copyright: American Scientific Publishers
of 3.5×10 via vacuum deposition.
Keywords: Organic Thin-Film Transistors, Quinoxaline, Self-Assembled Monolayer.
1
. INTRODUCTION
reported p-channel device characteristics with hole mobil-
−4
2
8
Small molecular organic semiconductors have been widely
studied as active layers of organic thin-film transistors
ities 9.7 × 10 cm /Vs. More recently, Shi et al. syn-
thesized two new quinoxaline derivatives and showed
9
(
OTFTs) to realize inexpensive, large-area, flexible, and
their applications in memory devices. Previous studies
solution-processable organic electronic devices such as
clearly showed the strong electron-withdrawing capability
of quinoxaline moiety and its potential for optoelectronic
applications. Therefore, it is desirable to investigate new
organic semiconductors based on quinoxalines with high
electrical performances.
1
,2
flexible displays, sensors, and electronic papers. Among
organic semiconductors which have been studied widely
over the last few decades, representative examples include
3
4
5
pentacene, oligothiophene, and thiazole derivatives. To
realize various commercial products based on organic elec-
tronic devices, development of new organic semiconduc-
tors with high electrical performance is required. Until
today, several quinoxaline based semiconductors have been
designed and synthesized, and demonstrated their elec-
To this end, we have synthesized three novel quinoxa-
line derivatives and these molecules were tested as poten-
tial organic semiconductors in OTFTs (Fig. 1). All new
compounds were characterized for their thermal, opti-
cal, and electrochemical properties via thermogravimetric
analysis (TGA), differential scanning calorimetry (DSC),
UV-vis spectroscopy, and cyclic voltammetry (CV). Theo-
retical molecular calculation for the new compounds was
performed via density functional theory (DFT) calcula-
tions to obtain molecular structure and HOMO/LUMO
energy level of the compounds. Furthermore, fabrication of
thin-films with functional materials and device application
6
trical device performance. Quinoxaline moiety has been
attracted as an acceptor due to its high electron affinity
which generated from the two symmetric nitrogen atoms
7
in its pyrazine ring. For example, Dutta et al. reported
three new solution-processable quinoxaline molecules and
∗
Authors to whom correspondence should be addressed.
†
10
These two authors contributed equally to this work.
has become important in recent nanotechnology. In this
5530
J. Nanosci. Nanotechnol. 2017, Vol. 17, No. 8
1533-4880/2017/17/5530/009
doi:10.1166/jnn.2017.13841

Kim et al. Synthesis and Characterization of Quinoxaline Derivative as Organic Semiconductors for Organic Thin-Film Transistors
N
N
N
N
N
N
S
S
S
S
3
4
5
Figure 1. Chemical structure of compounds 3, 4, and 5.
study, new compounds were employed as organic semicon-
ductor films via solution process (solution-shearing) and
vacuum-deposition method in a top-contact/bottom-gate
OTFTs. Finally, the resulting devices were characterized
(30 mL), and the reaction mixture was stirred at room
temperature for 24 h. Then the reaction mixture was
washed with water and diethyl ether. The organic layer
was collected and concentrated in vacuo to get crude
product. To the crude product, benzil (630 mg, 3 mmol)
was added in toluene (10 mL) and acetic acid (10 mL)
−4
2
with carrier mobility as high as 1.9×10 cm /Vs and cur-
6
rent on/off ratio of 3.5×10 for thin films of compound 5.
ꢀ
and the reaction mixture was heated up to 100 C and
stirred for another 12 h. After the reaction was completed,
the mixture was washed with water, brine solution and
dichloromethane. The organic layer was concentrated in
vacuo. Finally, the crude product was recrystallized from
ethanol to afford 5,8-dibromo-2,3-diphenylquinoxaline as
2
. EXPERIMENT DETAILS
2
.1. General Methods
Air and/or moisture sensitive reactions were carried out
under an N atmosphere in oven-dried glassware and with
anhydrous solvents. All chemicals were purchased from
commercial sources unless otherwise noted and used with-
out further purification. Solvents were freshly distilled
or dried by passing through an alumina column. Thin
2
11,12 1
white solid (846.4 mg, 56.6%).
CDCl ꢃꢁ 7.90 (s, 2H), 7.65–7.62 (m, 4H), 7.39–7.34
H NMR (400 MHz,
3
(
m, 6H).
Delivered by Ingenta to: State University of New York at Binghamton
layer chromatography was carried out on glass plates
IP: 146.185.200.35 On: Fri ,2 .02 9. 2 J. u Sn y n2 t0h 1e 7s i s0 2o f: 3( 65 :- 3p 3h enylthiophen-2yl)boronic
coated with silica gel SiO 60 F254Cf or op my ri Mg he tr: c Ak ;m ve i rs iuc -a n Scientific Publishers
2
acid (2)
alization with a UV lamp (254 nm) or by staining with
a p-anisaldehyde or potassium permanganate solution.
Flash chromatography was performed with silica gel SiO2
(
5-Phenylthiophen-2-yl)boronic acid was synthesized
according to the literature with slight modification. 2-
Bromothiophene (1.0 g, 6.13 mmol), Pd(PPh ꢃ (354 mg,
3
4
6
0 (0.040–0.063 ꢀm, 230–400 mesh), technical solvents,
0
.30 mmol), phenylboronic acid (853.3 mg, 6.99 mmol),
1
and a head pressure of 0.2–0.4 bar. Proton ( H) and
carbon ( C) nuclear magnetic resonance (NMR) spec-
troscopy was performed on a JEOL ECP-400 spectrom-
eter at 400 MHz ( H) and 100 MHz ( C) at 294 K.
Chemical shifts are reported in ppm relative to the resid-
and 2M Na CO (10 mL, 20 mmol) was dissolved in
2
3
1
3
dimethoxy ethane (20 mL) at room temperature, stirred for
0 min, and then refluxed for 2 h under N atmosphere.
1
2
1
13
After cooling down the reaction mixture to room tempera-
ture, the mixture was washed with water and ethyl acetate.
The organic layer was collected and concentrated in vacuo.
The crude product was purified by column chromatog-
raphy using hexane as eluent to give 2-phenylthiophene
ual non-deuterated solvent (CDCl : ꢁH = 7ꢂ26 ppm, ꢁC =
3
1
3
7
7ꢂ16 ppm). All C NMR spectra are proton decoupled.
The resonance multiplicity is described as s (singlet), d
doublet), t (triplet), q (quartet), p (pentet), dd (doublet
(
1
as colorless solid (948.3 mg, 97%). H NMR (400 MHz,
of doublet), dt (doublet of triplet), td (triplet of dou-
blet), m (multiplet), and br (broad). High-resolution mass
spectrometry (HRMS) was measured on a JEOL JMS-700
spectrometer. Mass peaks are reported in m/z units.
CDCl ) ꢁ 7.61–7.57 (m, 2H), 7.38–7.34 (m, 2H), 7.30–
3
7
.25 (m, 2H), 7.06 (q, 3.3 Hz, 1H).
-Phenylthiophene (1.0 g, 6.24 mmol) was dissolved
2
in THF (20 mL) and this reaction mixture was cooled
ꢀ
to −78 C under N atmosphere. Then n-BuLi (1.6M,
2
2
.2. Synthesis
3
.9 mL, 6.42 mmol) was added over 15 min to the solution
2
.2.1. Synthesis of 5,8-dibromo-2,
and stirred for 1 h at the same temperature. Triisopropyl
borate (1.44 mL, 6.24 mmol) was added dropwise at
3
-diphenylquinoxaline (1)
ꢀ
5
,8-Dibromo-2,3-diphenylquinoxaline was synthesized
−78 C to the cooled solution for the duration of 10
by modifying previous literature. 4,7-Dibromobenzo[c]
min, and the resulting mixture was allowed to warm up
to room temperature for overnight. The reaction mix-
ture was acidified with 10% HCl, and stirred for 1 h.
The aqueous mixture was extracted with diethyl ether for
[
0
1,2,5]thiadiazole (1.0 g, 3.4 mmol), CoCl ·6H O (10 mg,
2
2
.07 mmol), sodium borohydride (686.2 mg, 18.13 mmol)
was dissolved in ethanol (10 mL) and tetrahydrofuran
J. Nanosci. Nanotechnol. 17, 5530–5538, 2017
5531

Synthesis and Characterization of Quinoxaline Derivative as Organic Semiconductors for Organic Thin-Film Transistors Kim et al.
three times (50 mL) and the organic layer was washed
with water. The resulting solution was concentrated under
reduced pressure. The crude product was recrystallized
from ethanol:water (1:20) to afford compound 2 as light
under N atmosphere under vigorous stirring. After cool-
ing to room temperature, the mixture was extracted with
2
dichloromethane and dried over anhydrous MgSO . The
4
solvent was removed under reduced pressure and the crude
product was purified by column chromatography on silica
gel using hexane:ethyl acetate (10:1) as eluent to give com-
1
3
1
blue solid (810 mg, 63.6%).
H NMR (400 MHz,
DMSO-d ) ꢁ 8.25 (s, 2H), 7.68–7.64 (m, 3H), 7.52
6
1
5 1
(
d, 3.7 Hz, 1H), 7.41 (t, 8.1 Hz, 2H), 7.32 (t, 7.3 Hz, 1H).
pound 5 as dark red solid (120.8 mg, 88.8%). H NMR
400 MHz, CDCl ) ꢁ 8.15 (s, 2H), 7.88 (d, 4.0 Hz, 2H),
(
3
7
7
.78 (dd, 2.6 Hz, 7.7 Hz, 4H), 7.70 (d, 7.3 Hz, 4H),
.40–7.38 (m, 12H), 7.29 (t, 7.3 Hz, 2H). C NMR
2
.2.3. Synthesis of 2,3,5,8-tetraphenylquinoxaline (3)
1
3
Compound 1 (100 mg, 0.22 mmol), phenyl boronic acid
(
100 MHz, CDCl ) ꢁ 152.4,147.0, 138.6, 138.1, 137.1,
(
66 mg, 0.54 mmol), Pd(PPh ꢃ (26.2 mg, 0.022 mmol),
3
3
4
1
1
34.7, 131.0, 130.5, 129.0, 128.9, 128.2, 127.5, 127.4,
2
M Na CO (2 mL, 4 mmol), and aliquot (100 ꢀL)
2 3
+
26.5, 125.7, 122.9. HRMS-EI(m/z): [M+Na ] calcd. for
was dissolved in toluene (10 mL) and refluxed for 12 h
under N atmosphere under vigorous stirring. After cool-
+
C H N S2Na , 621.1430; found, 621.1432.
4
0
26
2
2
ing to room temperature, the mixture was extracted with
2
.3. Theoretical Calculation
dichloromethane and dried over anhydrous MgSO . The
4
Density functional theory (DFT) calculations on the
present semiconductor were performed using the B3LYP
solvent was removed under reduced pressure and the
residue was purified by column chromatography on sil-
ica gel using hexane: ethyl acetate (10:1) as eluent to
(
Becke’s 3 parameters employing the Lee-Yang-Parr) func-
1
4
tional and the 6-31G(d) basis set as implemented in Gaus-
sian 03W program.
give compound 3 as pale yellow solid (55 mg, 55.7%).
1
H NMR (400 MHz, CDCl ) ꢁ 7.88 (s, 2H), 7.86–7.84
3
(
(
ꢁ
m, 4H), 7.58–7.56 (m, 4H), 7.54–7.50 (m, 4H), 7.44–7.40
m, 2H), 7.30–7.25 (m, 6H). C NMR (100 MHz, CDCl3)
1
3
2.4. Device Fabrication
Top-contact/bottom-gate organic thin-film transistors
(OTFTs) were fabricated on a highly n-doped (100)
silicon wafer with a thermally grown 300 nm silicon
151.3, 139.4, 139.0, 138.5, 138.2, 130.9, 130.0, 129.8,
+
1
28.7, 128.1, 127.9, 127.5. HRMS-EI(m/z): [M+Na ]
+
calcd. for C H N Na , 457.1675; found, 457.1680.
3
2
22
2
Delivered by Ingenta to: State Unive rd si iot yx i od ef Na es wd i Ye loe rc kt r ai ct Bl ai yn eg r h. aT mh et o nS i/SiO substrates were
2
IP: 146.185.200.35 On: Fri, 09 Jun 2017 02:36:33
cleaned via sonication in acetone for 10 min, dried
2
.2.4. Synthesis of
,3-diphenyl-5,8-di(thiophen-2-yl)quinoxaline (4)
Copyright: American Scientific Publishers
with N , and cleaned by air plasma for 5 min (Harrick
2
2
Plasma, 18W). The general recipes were employed for
Compound 1 (100 mg, 0.22 mmol), 2-thiophene boronic
acid (70 mg, 0.54 mmol), Pd(PPh3ꢃ (26.2 mg,
0
(
for 12 h under N atmosphere under vigorous stirring.
After cooling to room temperature, the mixture was
extracted with dichloromethane and dried over anhydrous
MgSO . The solvent was removed under reduced pressure
and the residue was purified by column chromatography
on silica gel using hexane:ethyl acetate (10:1) as eluent
the treatment of gate dielectric layers including PS-brush
4
16
17
layer,
PS (polystyrene) polymer layer
and self-
.022 mmol), 2M Na CO (2 mL, 4 mmol), and aliquot
2 3
assembled monolayers (octadecyltrimethoxysilane; OTMS
and hexamethyldisilane; HMDS) on the Si/SiO2 sub-
100 ꢀL) was dissolved in toluene (10 mL) and refluxed
1
8
2
strates. Hydroxyl end-functionalized polystyrene (M = 10
n
kg/mol, Polymer Source) and polystyrene (M = 20
n
kg/mol, Sigma Aldrich) was employed for the forma-
tion of PS-brush layer and PS layer, respectively. Two
4
19
methods including solution-shearing (SS) and vacuum
deposition were employed for the formation of semicon-
ducting layers. For the optimization of solution-shearing
process, various solvents including toluene, chloroben-
zene, and 1,2,4-trichlorobenzene were employed. The
solution-sheared substrates were placed in a vacuum oven
to give compound 4 as yellowish orange solid (42.5 mg,
4
7
2.7%).11 H NMR (400 MHz, CDCl ) ꢁ 8.14 (s, 2H),
1
3
.87 (dd, 1.1 Hz, 3.7 Hz, 2H), 7.74–7.71 (m, 4H), 7.51
(
dd, 0.7 Hz, 5.1 Hz, 2H), 7.38–7.36 (m, 6H), 7.17 (q,
1
3
3
1
1
.7 Hz, 2H). C NMR (100 MHz, CDCl ) ꢁ 151.6,
ꢀ
3
at 80 C overnight to remove the residual solvent. Vacuum
38.6, 138.6, 137.1, 131.2, 130.4, 128.9, 128.8, 128.1,
deposition was performed on various substrates (bare
+
27.0, 126.5, 126.3. HRMS-EI(m/z): [M+Na ] calcd. for
SiO , PS, HMDS and OTMS) at two different substrate
2
+
C H N S Na , 469.0804; found, 469.0808.
ꢀ
ꢀ
2
8
18
2
2
temperatures (20 C and 50 C). The deposition rate was
−6
0
.2–0.3 Å/s under 5 × 10 torr. Film thicknesses were
2
.2.5. Synthesis of 2,3-diphenyl-5,8-bis(5-
phenylthiophen-2-yl)quinoxaline (5)
characterized by profilometer (DEKTAK-XT, Brucker)
as 55–60 nm (for solution-sheared films) and 40–60 nm
(for vacuum-deposited films). Gold source and drain
electrodes (40 nm) were thermally evaporated through a
shadow mask with various channel lengths and widths (L;
50–100 ꢀm, W; 1000–2000 ꢀm).
Compound 1 (100 mg, 0.22 mmol), compound 2
(
2
111.3 mg, 0.54 mmol), Pd(PPh ꢃ (26.2 mg, 0.022 mmol),
3 4
M Na CO (2 mL, 4 mmol), and aliquot (100 ꢀL)
2 3
was dissolved in toluene (10 mL) and refluxed for 12 h
5532
J. Nanosci. Nanotechnol. 17, 5530–5538, 2017

Kim et al. Synthesis and Characterization of Quinoxaline Derivative as Organic Semiconductors for Organic Thin-Film Transistors
Table I. Physical and electrochemical properties of the compounds 3,
2
.5. Characterization
4
, and 5.
Thermogravimetric analyses (TGA, TA Instrument Q50-
555) were performed on each sample in a platinum
Ered LUMO E op xe ak HOMO Egap
peak
1
DSC
TGA
UV-Vis
ꢀ
crucible heating from 40 to 700 C (heating rate of
2
calorimetry (DSC) analyses (TA instrument Q20-2487)
were performed from 40 to 350 C (heating rate of 20 C
ꢀ
ꢀ
a
b
b
b
b
b
Tm ( C) ( ; 5%) ꢅmax (nm)
(V)
(eV) (V) (eV) (eV)
ꢀ −1
0 C min under N atmosphere). Differential scanning
2
3
4
5
266
221
265
313
264, 356 −0ꢂ95 −3ꢂ19 1ꢂ29 −5ꢂ43 2ꢂ24
174 312, 384, 434 −0ꢂ86 −3ꢂ28 1ꢂ15 −5ꢂ29 2ꢂ01
ꢀ
ꢀ
131
344, 476 −0ꢂ83 −3ꢂ31 1ꢂ14 −5ꢂ28 1ꢂ97
−1
min ꢃ. UV-visible spectra of the compound in dilute chlo-
Notes ꢆ ameasured by UV-vis spectroscopy. bmeasured by cyclic voltammetry in
−5
roform (conc. 1 × 10 M) were obtained using JASCO
V-530 spectrometer with quartz cuvette over the spectral
range of 200–800 nm. Cyclic voltammetry (CV) experi-
ments were performed with a conventional three-electrode
configuration (glassy carbon working electrode, platinum-
wire counter electrode, and Ag/AgCl reference elec-
trode) with supporting electrolyte of tetrabutylammonium
tetrafluoroborate in dry dichlorobenzene on AUT302N
Electrochemical Analyzer (Autolab). All electrochemical
ꢀ
o-C H Cl at 25 C (using ferrocene/ferrocenium as intermal standard at +0.66 V
6
4
2
for all compounds).
8
-bis(5-phenylthiophen-2-yl)quinoxaline
(5)
were
11ꢄ12
synthesized in one step following Scheme 3.
The compounds 3–5 were synthesized using Suzuki
coupling of 5,8-dibromo-2,3-diphenylquinoxaline (1)
with phenylboronic acid, 2-thiopheneboronic acid, and
(
5-phenylthiophen-2yl)boronic acid (2), respectively.
+
potentials were referenced to an Fc /Fc internal standard.
The current-voltage characteristics of fabricated OTFT
devices were measured at room temperature under vac-
uum and in ambient using a Keithley 4200 SCS. Carrier
mobilities (ꢀꢃ were calculated in the saturation regime by
3
.2. Theoretical Calculation
Theoretical calculation was performed to determine the
molecular structure and HOMO/LUMO energy distri-
bution of quinoxaline derivatives by Gaussian 03W
program (B3LYP with 6-31G(d) basis sets). For all
compounds, HOMOs of all compounds were rather dis-
tributed on the main backbone moieties, while LUMOs
were localized on the quinoxaline moiety. The theo-
retical HOMO/LUMO energy levels were calculated as
2
the standard relationship, ꢀsat = (2I L)/[WC (V –V ꢃ ],
DS
i
G
T
where IDS is the source-drain current, L is the channel
length, W is the channel width, C is the areal capaci-
i
−
2
tance of the gate dielectric (C = 11.4 nF cm ꢃ, V is the
i
G
gate voltage, and V is the threshold voltage. The surface
T
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morphology and film microstruct IuPr e: 1o 4f 6t .h1 e8 5s e. 2m 0i 0c o. 3n 5d u Oc tn- : Fri, 09 Jun 2017 02:36:33
−
5.69/−2.21 eV for compound 3, −5.20/−1.97 eV for
Copyright: American Scientific Publishers
ing films were investigated by atomic force microscope
AFM, NX10, Park Systems) and X-ray diffraction (XRD,
Miniflex, Rigaku), respectively.
compound 4, and −4.96/−2.30 eV for compound 5,
(
respectively (Fig. 2). Consequently, theoretically calculated
HOMO-LUMO energy bandgap were 3.48 eV for com-
pound 3, 3.23 eV for compound 4, and 2.66 eV for com-
pound 5, respectively. Lower HOMO-LUMO bandgaps in
the following order of compound 3, 4 and 5 indicate the
increasing electron-donating character of the substituents.
3
. RESULTS AND DISCUSSION
3
.1. Synthesis
Both 5,8-dibromo-2,3-diphenylquinoxaline (1)8ꢄ9 and
3
(
5-phenylthiophen-2yl)boronic acid (2) were prepared
in two steps as shown in Scheme 1 and Scheme 2.
,3,5,8-Tetraphenylquinoxaline (3), 2,3-diphenyl-5,8-
di(thiophen-2-yl)quinoxaline (4), and 2,3-diphenyl-5,
3.3. Thermal, Optical, and Electrochemical Properties
Thermal properties of new compounds were character-
ized by thermogravimetric analysis (TGA) and differential
2
S
NaBH4
H N
NH2
N
N
2
Toluene
+
CoCl .6H O
2
2
Br
Br
Acetic acid
N
N
Br
Br
THF/EtOH
O
O
Br
Br
1
Scheme 1. Synthetic scheme of compound 1.
OH
Pd(PPh3)4
THF,n-BuLi
S
+
OH
B
S
Br
B
HO
S
2M Na2CO3
DME
triisopropyl borate
10% HCl
OH
2
Scheme 2. Synthetic scheme of compound 2.
J. Nanosci. Nanotechnol. 17, 5530–5538, 2017
5533

Synthesis and Characterization of Quinoxaline Derivative as Organic Semiconductors for Organic Thin-Film Transistors Kim et al.
OH
B
OH
N
N
3
4
OH
OH
S
B
Pd(PPh3)4
2
M Na CO3
2
N
N
N
N
Aliquat
Toluene
S
Br
Br
S
1
OH
OH
S
B
2
N
N
S
S
5
Scheme 3. Synthetic scheme of compounds 3, 4 and 5.
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Copyright: American Scientific Publishers
Figure 2. Molecular orbital surfaces of HOMO and LUMO by DFT calculation of compounds 3,4, and 5.
5534
J. Nanosci. Nanotechnol. 17, 5530–5538, 2017

Kim et al. Synthesis and Characterization of Quinoxaline Derivative as Organic Semiconductors for Organic Thin-Film Transistors
Figure 4. Optical UV-vis spectra of the new compounds 3–5 in chlo-
roform solution.
temperature. The compounds 3, 4, and 5 exhibited oxida-
tion peaks at +1.29 V, +1.15 V, and +1.14 V and reduc-
tion peaks at −0.95 V, −0.86 V, and −0.83 V, respec-
tively, using ferrocene/ferrocenium as internal standard at
+
0.66 V. Hence, HOMO energy level calculated by the
conversion of oxidation potential using ferrocene as the
standard was −5.43 eV, −5.29 eV, and −5.28 eV, respec-
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tively. In the same way, LUMO energy was obtained as
IP: 146.185.200.35 On: Fri, 09 Jun 2017 02:36:33
−
3.19 eV, −3.28 eV, and −3.31 eV, respectively (Fig. 5).
Copyright: American Scientific Publishers
The HOMO-LUMO energy gaps of the compounds 3, 4,
and 5 were calculated as 2.24 eV, 2.01 eV, and 1.97 eV,
respectively, showing same trend as the values determined
optically (Table I).
Figure 3. (a) Thermogravimetric analysis and (b) differential scanning
calorimetry graphs of the new compounds 3–5.
scanning calorimetry (DSC) (Fig. 3). For new compounds
3
3
5
–5, weight loss (∼5%) temperatures were observed above
ꢀ ꢀ ꢀ
13 C, 174 C, and 131 C, respectively. Compound
showed much larger thermal degradation compared
to compounds 3 and 4. Furthermore, sharp endotherms
ꢀ
for the quinoxaline derivatives exhibited above 221 C
(
Table I). It indicates the possible temperature for thermal
treatment of each compounds in this study.
The optical absorption spectra of the compounds 3 (ꢅmax
=
264, 356 nm), 4 (ꢅmax = 312, 384, 434 nm) and 5
(
ꢅ
max
= 344, 476 nm) in dilute chloroform solution are
shown in Figure 4. It exhibited slightly red-shifts in the
following order of compound 3, 4 and 5. Increasing the
extent of conjugation leads to red-shifts of the absorp-
tion spectra and decreases HOMO-LUMO bandgap. The
HOMO-LUMO energy gaps calculated from the onset of
the experimental optical absorption were 3.06 (3), 2.47 (4),
and 2.23 eV (5), respectively. Compound 5 with largest
conjugation among all compounds in this study showed
smallest energy gap compared to other compounds.
2
0
Cyclic voltammetry (CV) measurements of the new
compounds were performed in o-C H Cl at room
Figure 5. Cyclic voltammograms of the new compounds 3–5 in
o-C H Cl .
6
4
2
6
4
2
J. Nanosci. Nanotechnol. 17, 5530–5538, 2017
5535

Synthesis and Characterization of Quinoxaline Derivative as Organic Semiconductors for Organic Thin-Film Transistors Kim et al.
Table II. TFT device performance parameters based on thin films of compounds 3, 4, and 5 employed in this study (ꢀ: maximum carrier mobility,
a
ꢀavg : average carrier mobility, I /I : current on/off ratio, V : threshold voltage).
on off
T
2
Material
Method
Substrate
ꢀ(ꢀavg ꢃ (cm /Vs)
I /I
on off
VT (V)
3
4
5
Not active
Not active
2.6×10− (2.0×10
5
−5
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
ꢃ
1.8×10
2.4×10
1.1×10
8.3×10
8.3×10
1.3×10
3.5×10
8.6×10
1.6×10
5
5
5
5
5
5
6
5
5
−16
−49
−29
−52
−51
−34
−31
−22
−27
SS
PS-brush
Bare
ꢀ
−5
−5
−5
−5
−5
−5
−4
−5
−5
Vacuum deposition (TD = 50 C)
3.2×10 (3.2×10
−5
PS
9.6×10 (5.4×10
−5
OTMS
HMDS
Bare
PS
OTMS
HMDS
6.7 ×10 (5.3×10
−5
3.9×10 (1.7×10
ꢀ
−5
Vacuum deposition (TD = 50 C)
4.6×10 (4.5×10
−4
1.9×10 (1.7×10
−5
5.7 ×10 (4.2×10
2.8×10− (2.7×10
5
Notes ꢆ aDevices were measured under vacuum. The average carrier mobilities are averaged values of three devices made under identical conditions.
3
.4. Thin-Film Transistor Characterization
To test the new quinoxaline derivatives as organic semi-
conductors, top-contact/bottom-gate OTFT devices were
fabricated via two different methods: solution-shearing
and vacuum deposition. The OTFT data are summarized
in Table II and representative transfer and output plots
are shown in Figure 6. Overall, only thin films of com-
pound 5 exhibited p-channel activity with carrier mobil-
−
4
−5
2
ities of 10 –10 cm /Vs and a current on/off ratio
5
6
of 10 –10 . Vacuum-deposited thin films showed slightly
higher device performance than those formed via solution-
shearing. Among vac uDu em l i-vd ee rp eo ds i bt e yd I nfi gl me ns ,t afit lom : sS toa nt e PUS niversity of New York at Binghamton
IP: 146.185.200.35 On: Fri, 09 Jun 2017 02:36:33
or OTMS substrates showed better device performance
Copyright: American Scientific Publishers
than others, exhibiting highest carrier mobility of 1.9 ×
−
4
2
6
1
0
cm /Vs and a current on/off ratio of 3.5×10 on PS-
ꢀ
coated substrate at T = 50 C. Furthermore, all devices
D
based on thin films of compound 5 were measured in
ambient and showed slightly lower device performance
(
<50%) than those measured under vacuum.
3
.5. Thin-Film Microstructure and Morphology
Thin-film microstructure and morphology of solution-
sheared and vacuum-deposited films were characterized by
AFM and wide-angle ꢇ-2ꢇ XRD to evaluate device perfor-
mance. Surface morphologies based on compound 5 were
characterized by AFM, as shown in Figure 7. As shown,
different film-forming methodologies afforded different
film morphologies, Solution-sheared organic semiconduc-
tor thin films based on compound 5 exhibited skein-like
morphology with a surface roughness of ∼40.9 nm
(
Fig. 7(a)), while vacuum-deposited films at deposition
ꢀ
temperatures (T s) of 20 and 50 C showed small grains
D
with a surface roughness of ∼21.9 nm and 54.5 nm,
respectively (Figs. 7(b and c)). Higher deposition tem-
perature affored larger grain sizes for thin films of 5
on PS-coated substrates. Furthermore, conventional ꢇ–2ꢇ
XRD scans of thin films were characterized to analyze
microstructural order in semiconductor films. All of the
films did not exhibit any significant Bragg reflections, indi-
cating poor film texture (not shown).
Figure 6. Representative (a) transfer and (b) output characteristics of
the OTFT device based on thin films of compound 5.
5536
J. Nanosci. Nanotechnol. 17, 5530–5538, 2017

Kim et al. Synthesis and Characterization of Quinoxaline Derivative as Organic Semiconductors for Organic Thin-Film Transistors
quinoxaline-based organic semiconductors for the realiza-
tion of applications in organic electronics.
Acknowledgments: This study was supported by a
Research Grant of Pukyong National University (2016).
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J. Nanosci. Nanotechnol. 17, 5530–5538, 2017
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Received: 26 July 2016. Accepted: 23 August 2016.
Delivered by Ingenta to: State University of New York at Binghamton
IP: 146.185.200.35 On: Fri, 09 Jun 2017 02:36:33
Copyright: American Scientific Publishers
5538
J. Nanosci. Nanotechnol. 17, 5530–5538, 2017