Effect of substituents on the properties of distyrylarylene-based field-effect transistor materials

Article abstract of DOI:10.1016/j.cplett.2009.01.057

Two new distyrylarylene derivatives, 4,4′-bis(2-dibenzothienylvinyl)biphenyl (1) and 4,4′-bis(2-bithienylvinyl)biphenyl (2), were synthesized with good yields. Their properties are compared with 4,4′-bis[2-(3-N-butylcarbazolyl)vinyl]biphenyl (3). Our expe

Full text of DOI:10.1016/j.cplett.2009.01.057

Chemical Physics Letters 470 (2009) 264–268
Contents lists available at ScienceDirect
Chemical Physics Letters
journal homepage: www.elsevier.com/locate/cplett
Effect of substituents on the properties of distyrylarylene-based field-effect
transistor materials
*
Ye-Xin Li, Xu-Tang Tao , Fa-Jun Wang, Tao He, Liang-Liang Zhang, Min-Hua Jiang
State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, China
a r t i c l e i n f o
a b s t r a c t
Two new distyrylarylene derivatives, 4,40-bis(2-dibenzothienylvinyl)biphenyl (1) and 4,40-bis(2-bithie-
nylvinyl)biphenyl (2), were synthesized with good yields. Their properties are compared with 4,40-
bis[2-(3-N-butylcarbazolyl)vinyl]biphenyl (3). Our experiments reveal that the substituents have signif-
icant effects on the optical behaviors and film morphologies. The OFET properties of these materials were
investigated. Both 1 and 2 exhibit hole mobility higher than 0.01 cm2 Vꢀ1 sꢀ1 with good environmental
stability. However, the devices using 3 as active layer show poor carrier-transporting ability due to weak
intermolecular interactions in the film. These results reveal that the substituent has significant effect on
the OFET performance of distyrylarylene-based materials.
Article history:
Received 4 December 2008
In final form 20 January 2009
Available online 27 January 2009
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
far [19,20], probably owing to their weak interchain interaction
and the amorphous nature in the solid state [13].
Recently, organic field-effect transistors (OFETs) have attracted
considerable attention, due to their low cost, processibility and
structure flexibility [1–5]. They show potential uses in electronic
papers, complementary integrated circuits, active-matrix display
and chemical sensors [6–9]. The performance of OFETs is mainly
determined by the carrier mobility in the organic semiconductor
layers. It is generally believed that a good OFET material should
In order to study the effect of the end-capped substituent on the
OFET performance of distyrylarylene(DSA)-based compounds, in
this Letter we designed and synthesized two new OFET semicon-
ductors based on DSA, 4,40-bis(2-dibenzothienylvinyl)biphenyl
(1) and 4,40-bis(2-bithienylvinyl)biphenyl (2) (Fig. 1). For compar-
ison, 4,40-bis[2-(3-N-butylcarbazolyl)vinyl]biphenyl (3), type of
which was usually used in OLEDs, was also synthesized and inves-
tigated. The difference in substituents leads to totally different
intermolecular interaction and morphology in the vacuum-depos-
ited films. Both 1 and 2 display strong intermolecular interaction
and high molecular ordering in the solid state, and the OFET prop-
erties of 1 and 2 are much better than that of 3.
have strong
p–p interaction between neighboring molecules
and can be readily sublimed to form polycrystalline films
[10,11]. Furthermore, these materials should be easily synthe-
sized and be environmentally stable for the practical applications.
Therefore, exploring new simple organic semiconductor materials
and understanding the relationship between the molecular struc-
ture, film morphology and the property are still the major
challenges.
2. Experimental
Distyrylarylene (DSA) derivatives are a kind of multifunctional
materials with many excellent properties: (1) as emitting materials
in efficient and bright electroluminescence (EL) devices in the blue
region [12–14]; (2) as a blue dopant to improve both EL efficiency
and stability of organic light-emitting diodes (OLEDs) [15,16]; (3)
as hole-transporting layer in EL cell [17]; (4) as fluorescent materi-
als exhibiting aggregation-induced emission (AIE) behavior [18].
These materials possess good electron-donating ability and reason-
ably high ambient stability, which meet well with the require-
ments for OFET application. However, DSA related materials used
as active layer in OFET devices have not been fully understood so
2.1. Chemicals and instruments
All chemicals were purchased from Aldrich and Acros and used
as received without further purification. N-butylcarbazole-3-alde-
hyde, dibenzothiophene-2-aldehyde and bithiophene-5-aldehyde
were prepared according to the published procedures [21–23]. 1H
NMR and 13C NMR spectra were recorded on a Bruker Avance
400 spectrometer. Elemental analyses were performed using a Ger-
man Vario EL III elemental analyzer. Absorption measurements
were carried out on a TU-1800 spectrophotometer. Photolumines-
cence (PL) measurements were recorded using a Hitachi F-4500
fluorescence spectrophotometer with a 150 W Xe lamp. Thermo-
gravimetric analyses (TGA) were measured on Perkin Elmer Dia-
* Corresponding author. Fax: +86 531 88574135.
E-mail address: txt@icm.sdu.edu.cn (X.-T. Tao).
mond TG/DTA instrument, under nitrogen atmosphere at
a
0009-2614/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2009.01.057

Y.-X. Li et al. / Chemical Physics Letters 470 (2009) 264–268
3. Result and discussion
265
3.1. Synthesis and characterization
All the compounds were prepared by one-step Horner-Emmons
reaction as yellow solids with good yields. Both 1 and 2 are insol-
uble in ordinary solvents such as Et2O, THF and HCCl3. However,
they show limited solubility in hot DMF, and can be recrystallized
from DMF carefully. On the contrary, compound 3 is soluble in THF
and chloroform. The Fourier-transform infrared (FTIR) spectra of
the three materials reveal strong absorption peak in the region of
945–970 cmꢀ1 which can be attributed to the out-of-plane bending
vibration of the trans-vinylene C–H bond [24]. This result indicates
that all the three compounds are in the trans conformation. These
materials show good thermal stability and the decomposition tem-
perature is 483 °C for 1, 429 °C for 2 and 470 °C for 3 as revealed by
thermogravimetric analysis (TGA) measurements.
Fig. 1. Chemical structure of 4,40-bis(2-dibenzothienylvinyl)biphenyl (1), 4,40-
bis(2-bithienylvinyl)biphenyl (2) and 4,40-bis[2-(3-N-butylcarbazolyl)vinyl]biphe-
nyl (3).
heating rate of 20 °C minꢀ1 to 600 °C. IR spectra were recorded on
NEXUS 670 FTIR spectrophotometer. For morphological character-
izations, the materials were deposited onto Si/SiO2 wafers simulta-
neously with the FET devices. The atomic force microscopy (AFM)
images were obtained by using a Digital Instruments (DI) Dimen-
sion 3100 operating in tapping mode. The X-ray diffraction (XRD)
investigations were carried out on a Bruker D8 advanced diffrac-
3.2. Photophysical properties
tometer equipped with CuKa radiation (k = 1.5406 Å). The data
were collected using a Ni-filtered Cu-target tube at room temper-
ature in the 2h range from 2° to 30°.
Optical absorption and fluorescence emission spectra of 1, 2 and
3 were measured both in chloroform solution (concentration
1 ꢁ 10ꢀ5 mol Lꢀ1) and as thin films deposited at about 25 °C. Com-
parison of the absorption spectra of solution with those of the cor-
responding films reveals drastically different behaviors. As shown
in Fig. 2, the film absorption spectrum of 1 shows blue shift com-
pared with that of solution, which suggests H aggregation of adja-
cent molecules having their long axes parallel to each other. Such a
OFET devices were fabricated in the top contact configuration.
Gold electrodes were deposited using shadow masks with width-
to-length ratio (W/L) of ca. 60/1. Organic semiconductors were
deposited at
a a pressure of about
rate of 0.1 Å sꢀ1 under
5.0 ꢁ 10ꢀ4 Pa. OFET characteristics were obtained at room temper-
ature in air on Keithley 4200 SCS.
behavior is generally associated with strong p-conjugated systems
in the solid state [25,26,11]. In the case of compound 2, its solid
state absorption spectrum is drastically broadened and has signif-
icant red shifted features. The broadening implies that there exist
different intermolecular interactions in the film. In contrast, the so-
lid state absorption spectrum of 3 is essentially identical to the
solution spectrum with a slight broadening of the absorption band,
indicating weak intermolecular interaction in the solid of 3. The
HOMO-LUMO gaps obtained from the end-absorptions are 3.0 eV
for 1, 2.7 eV for 2 and 2.9 eV for 3, respectively.
2.2. Synthesis
2.2.1. 4,40-bis(2-dibenzothienylvinyl)biphenyl (1)
At 0 °C, t-BuOK (0.85 g, 7.6 mmol) in THF (20 mL) was added
dropwise into
yl)biphenyl (0.35 g, 0.77 mmol) and dibenzothiophene-2-aldehyde
(0.4 g, 1.9 mmol) in THF (20 mL) under nitrogen. The reaction mix-
ture was stirred overnight at room temperature. After filtration,
the filtrate was recrystallized from DMF to give yellow solid
(0.34 g, yield 77.4%). Anal. Calcd for C40H26S2: C, 84.17; H, 4.59;
S, 11.24. Found: C, 84.12; H, 4.59; S, 11.66.
a
mixture of 4,40-bis(diethylphosphonometh-
3.3. Structural investigation on the thin films
2.2.2. 4,40-bis(2-bithienylvinyl)biphenyl (2)
The OFET performance is closely related to the film morphology
[10,27]. Substitution of dibenzothienyl or bithienyl for the carbaz-
olyl in DSA derivatives exerts a dramatic influence on molecular
ordering. The orientation of the vapor-deposited films was investi-
gated by X-ray diffraction (XRD). Fig. 3a shows the XRD patterns of
compounds 1 and 3 deposited on bare and octyltrichlorosilane
(OTS)-pretreated SiO2 at 50 °C, respectively. In order to improve
the OFET performance, the surface pretreatment with OTS is usu-
ally used to reduce the surface energy between the semiconductor
and the SiO2 interface. For compound 1, the peaks in the XRD pat-
terns indicate the existing crystalline microstructure in the films.
In comparison with the film on bare SiO2, the XRD pattern of the
film on OTS-pretreated SiO2 shows more well-resolved reflections
with sharp and strong peaks, indicative of a high degree of ordering
and crystallinity. As for polycrystalline films, such structure is
known to be a favorable structure in terms of achieving high
mobility. The d-spacing obtained from the first reflection
(2h = 3.2°) is 27.6 Å, indicating that the molecules of 1 potentially
adopt an edge-on orientation on the substrate. The XRD pattern
of compound 3 shows no discernible diffraction peaks no matter
it is deposited on bare or OTS-pretreated SiO2, suggesting that
the films are amorphous and the molecules are randomly oriented.
Microstructural aspects of compound 2 are also very interest-
ing. Fig. 3b shows the XRD patterns of the vacuum-deposited films
Compound 2 was prepared according to the same procedure as
that for 1 from bithiophene-5-aldehyde (0.91 g, 4.69 mmol), 4,40-
bis(diethylphosphonomethyl)biphenyl (0.83 g, 1.83 mmol) and t-
BuOK (1.7 g, 15.18 mmol). The product was recrystallized from
DMF to give yellow solid (0.70 g, yield 71.6%). Anal. Calcd for
C32H22S4: C, 71.87; H, 4.15; S, 23.98. Found: C, 71.97; H, 4.14; S,
23.97.
2.2.3. 4,40-bis[2-(3-N-butylcarbazolyl)vinyl]biphenyl (3)
Compound 3 was prepared according to the same procedure as
that for 1 from N-butylcarbazole-3-aldehyde (1.86 g, 7.32 mmol),
4,40-bis(diethylphosphonomethyl)biphenyl (1.12 g, 2.47 mmol)
and t-BuOK (2.2 g, 19.64 mmol). The product was recrystallized
from ethyl acetate/chloroform to give yellow solid (1.3 g, yield
81.2%). 1H NMR (CDCl3, 400 MHz) d: 0.96 (t, J = 7.3 Hz, 6H), 1.42
(m, 4H), 1.88 (m, 4H), 4.32 (t, J = 7.1 Hz, 4H), 7.20 (d, J = 16.2 Hz,
2H), 7.23–7.27(m, 2H), 7.36–7.42 (m, 6H), 7.46–7.50 (m, 2H),
7.63–7.72 (m, 10H), 8.14 (d, J = 7.6 Hz, 2H), 8.26 (s, 2H). 13C NMR
(CDCl3, 100.62 MHz) d: 13.89, 20.58, 31.18, 42.99, 108.93, 109.40,
118.73, 119.03, 120.31, 120.46, 120.76, 122.92, 123.27, 124.47,
125.57, 125.82, 126.68, 127.05, 128.58, 129.74, 137.07, 139.29,
140.33, 140.92. Anal. Calcd for C48H44N2: C, 88.85; H, 6.83; N,
4.32. Found: C, 88.83; H, 6.59; N, 4.21.

266
Y.-X. Li et al. / Chemical Physics Letters 470 (2009) 264–268
Fig. 3. (a) XRD patterns of films of 1 and 3; (b) XRD patterns of films and powder of
2. All the films were deposited at 50 °C on bare and OTS-pretreated SiO2.
behavior has been observed in the vacuum-deposited TiOPc films
[28].
Fig. 4 shows the atomic force microscopy (AFM) images of the
thin film of 1 and 2 deposited on the same condition. Both materi-
als can form high crystalline films. The film of 1 on OTS-pretreated
SiO2 consists of roll-like grains in good connectivity. With the in-
crease of the substrate temperature during deposition, substantial
increase in the size of the grains was observed. As for the morphol-
ogy of 2, there are distinctly two kind of crystal forms, nanowire
and small grain with a spherical size, in the film deposited on bare
SiO2. However, only small and homogeneous grains are observed in
the film on OTS-pretreated SiO2. This result is in good agreement
with the XRD study. In contrast, the microstructure of 3 displays
amorphous characteristics, which may explain the low mobility
measured under the same condition.
Fig. 2. Normalized optical absorption (UV/vis) and emission (PL) spectra in
chloroform solution and as vacuum-deposited films of 1 (a), 2 (b) and 3 (c). The PL
spectra were obtained by exciting at 370 nm for 1, 410 nm for 2 and 383 nm for 3.
3.4. OFET properties
and powder. Compared with the powder pattern, the strong dif-
fraction peak at 2h = 18.58° indicates there exists a new crystal
structure or polymorph when the film was deposited on bare
SiO2 at 50 °C. However, as for the film on OTS-pretreated SiO2,
the absence of peaks at 2h = 19.66° and 23.76° shows that this
new gas phase is the main ingredient. The X-ray patterns of the
films deposited at high temperature also give similar results. It is
obvious that the OTS-pretreatment can exert remarkable effect
on the molecular alignment of vacuum-deposited films. A similar
OFET devices of 1, 2 and 3 were fabricated using top contact
geometry. After sublimating the molecules onto substrate, gold
was deposited as source and drain electrodes using shadow mask
with W/L = 60. The device performance was measured in air and
the results are shown in Table 1. All materials behave as p-type
semiconductors. The hole mobility was extracted from the satura-
tion regime of the transfer curve by using the classic equation
describing field-effect transistor.

Y.-X. Li et al. / Chemical Physics Letters 470 (2009) 264–268
267
Fig. 4. AFM images of films deposited on bare (upper) and OTS-pretreated (lower) SiO2 at 50 °C (3 lm ꢁ 3 lm): (a) 1 and (b) 2.
Table 1
the great differences in film morphology as shown by XRD patterns
and AFM micrographs. The stronger intermolecular interaction in
organic crystalline films is more helpful for carrier transport [10].
The typical field-effect transistor transfer and output characteris-
tics of 1 on OTS-pretreated SiO2 at 50 °C are presented in Fig. 5.
The mobility was found to be 1.7 ꢁ 10ꢀ2 cm2 Vꢀ1 sꢀ1 with an on/
off current ratio of 105. The performance is higher than that of de-
vice fabricated on bare SiO2 for improved crystallinity. The OFET
performance of 1 varied with substrate temperature, due to the
change in morphological features. The devices fabricated from 2
also show good hole-transporting property. The mobility reaches
2.0 ꢁ 10ꢀ2 cm2 Vꢀ1 sꢀ1 at 80 °C. As for 2, the performance of films
on OTS-pretreated SiO2 is much better than those of films on bare
SiO2 probably due to the reduction of the density of hole traps after
surface pretreatment.
Furthermore, the devices made from 1 and 2 show good envi-
ronmental stability. As shown in Table 1, the carrier mobility and
on/off ratio were still on the same order of magnitude as those of
the freshly prepared devices after the transistors were exposed to
ambient environment for one month. In comparison with 2, the de-
vices made from 1 exhibit much better stability. The devices made
from 1 deposited at 50 °C were examined for several months. No
obvious decrease in mobility as well as on/off ratio was found in
air. After 140 days, the mobility was 1.9 ꢁ 10ꢀ2 cm2 Vꢀ1 sꢀ1 with
an on/off current ratio over 105. To further test the reproducibility,
we dipped the bare devices, which were made from 1 deposited on
OTS-pretreated SiO2 at 80 °C, into water. After dried under ambient
environment, the devices still show OFET performance with the
carrier mobility of 1.4 ꢁ 10ꢀ2 cm2 Vꢀ1 sꢀ1 and the on/off ratio of
105. The good environmental stability is probably due to the
The performance of OFETs based on 1, 2 and 3 fabricated with different surface
treatments and under different substrate temperatures (Tsub).a
Compound
Tsub
(°C)
SiO2
Mobility
Ion/Ioff ratio
(cm2 Vꢀ1 sꢀ1
)
1
50
Bare
OTS
Bare
OTS
Bare
OTS
5.1 ꢁ 10ꢀ4
103 (103)
105 (105)
104 (104)
105 (105)
104 (104)
105 (105)
(4.2 ꢁ 10ꢀ4
1.7 ꢁ 10ꢀ2
(2.2 ꢁ 10ꢀ2
2.1 ꢁ 10ꢀ3
(3.4 ꢁ 10ꢀ3
2.0 ꢁ 10ꢀ2
(2.1 ꢁ 10ꢀ2
4.0 ꢁ 10ꢀ3
(4.3 ꢁ 10ꢀ3
1.4 ꢁ 10ꢀ2
(1.6 ꢁ 10ꢀ2
)
)
)
)
)
)
80
110
2
50
80
Bare
OTS
Bare
OTS
4.5 ꢁ 10ꢀ3
(3.4 ꢁ 10ꢀ3
6.9 ꢁ 10ꢀ3
(8.0 ꢁ 10ꢀ3
6.7 ꢁ 10ꢀ3
(3.9 ꢁ 10ꢀ3
2.0 ꢁ 10ꢀ2
(1.4 ꢁ 10ꢀ2
104 (104)
105 (105)
104 (104)
105 (105)
)
)
)
)
3
50
80
Bare
OTS
Bare
OTS
7.1 ꢁ 10ꢀ6
2.3 ꢁ 10ꢀ4
1.9 ꢁ 10ꢀ5
2.0 ꢁ 10ꢀ4
10
103
102
103
a
Values in parentheses were measured using the same devices after about one
month.
As shown in Table 1, the OFET properties of 1 and 2 are much
better than those of 3 under the same conditions, mainly due to

268
Y.-X. Li et al. / Chemical Physics Letters 470 (2009) 264–268
Fig. 5. OFET characteristics of 1 on OTS-pretreated substrate (Tsub = 50 °C): (a) transfer characteristic at VDS = ꢀ80 V and (b) output characteristic.
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compounds 1 and 2 display strongly intermolecular interaction
and can form ordered structure in the thin films. Polymorphism
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