A new pseudo rubrene analogue with excellent film forming ability

Article abstract of DOI:10.1007/s11426-011-4234-x

A novel pseudo rubrene analogue, 6,11-di(thiophen-2-yl)-tetracene-5,12- dione (DTTDO) was synthesized, in which two thienyl groups and two carbonyl groups replacing four phenyl groups in the rubrene molecule were connected to the backbone of tetracene. This compound was characterized by single crystal X-ray structure analysis, thermogravimetric analysis, absorption spectra and electrochemical measurements. Unlike rubrene, DTTDO exhibited excellent film forming ability by normal vacuum deposition, indicating its promising applications in organic thin film transistors.

Full text of DOI:10.1007/s11426-011-4234-x

SCIENCE CHINA
Chemistry
April 2011 Vol.54 No.4: 631–635
doi: 10.1007/s11426-011-4234-x
• ARTICLES •
A new pseudo rubrene analogue with excellent film forming ability
ZHANG XiaoTao, MENG Qing, HE YuDong, WANG ChengLiang,
DONG HuanLi* & HU WenPing*
Beijing National Laboratory for Molecular Science, Key Laboratory of Organic Solids, Institute of Chemistry,
Chinese Academy of Sciences, Beijing 100190, China
Received September 1, 2010; accepted September 26, 2010
A novel pseudo rubrene analogue, 6,11-di(thiophen-2-yl)-tetracene-5,12-dione (DTTDO) was synthesized, in which two
thienyl groups and two carbonyl groups replacing four phenyl groups in the rubrene molecule were connected to the backbone
of tetracene. This compound was characterized by single crystal X-ray structure analysis, thermogravimetric analysis, absorp-
tion spectra and electrochemical measurements. Unlike rubrene, DTTDO exhibited excellent film forming ability by normal
vacuum deposition, indicating its promising applications in organic thin film transistors.
DTTDO, thin films, organic field-effect transistors
process [9–11], developing heteroepitaxial growth meth-
1 Introduction
ods with the buffer layer [12–15], growing homoepitaxy
rubrene thin films on the surface of rubrene single crystals
[16] or incorporating a glass-inducing diluent for control-
ling rubrene thin-film crystallinity [17]. Although signifi-
cant progresses have been made in this field, most of the
techniques always need complicated processes or long
processing time [10, 16], or even arousing new dispute
that what is responsible for the charge transport properties,
the buffer layer or the rubrene [13, 15]? Besides these de-
veloped processing techniques, the development of new
organic semiconductors by virtue of the unique molecular
structure of rubrene with high mobility as well as good
film-forming ability has received increasing interest [18,
19], which is very important for the progress of organic
thin film transistors (OTFTs).
With this aim in mind, herein, a new rubrene analogue,
6,11-di(thiophen-2-yl)tetracene-5,12-dione (DTTDO), in
which two thienyl groups and two carbonyl groups replac-
ing four phenyl groups in the rubrene molecule were con-
nected to the backbone of tetracene (Scheme 1), was de-
signed and synthesized. This compound exhibited good
thermal properties, and more importantly good film-forming
Organic field-effect transistors (OFETs) have attracted
great interest during the last thirty years because of their
unique advantages such as light-weight, low-cost, and
good flexibility [1]. As a core component of OFETs, the
organic semiconductor plays a key role in device per-
formance [2]. Of all recently reported organic semicon-
ductors, fused-ring acene derivatives, such as rubrene,
have been extensively studied both experimentally and
theoretically [3–6] and hold the highest field-effect mobil-
ity as the benchmark for organic semiconductors (up to
15–40 cm2 V1 s1 measured from rubrene single-crystal
transistors [7, 8]). In contrast, thin film transistors based
on rubrene exhibit very low field-effect performance due
to bad film-forming ability [5], which significantly hinders
the practical applications of rubrene in large-area elec-
tronic devices. Recently, many new unconventional proc-
ess techniques have been developed to improve the quality
of rubrene thin films, such as using the special deposition-
*Corresponding author (email: dhl522@iccas.ac.cn; huwp@iccas.ac.cn)
© Science China Press and Springer-Verlag Berlin Heidelberg 2011
chem.scichina.com
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Zhang XT, et al. Sci China Chem April (2011) Vol.54 No.4
3-(thiophen-2-yl)isobenzofuran-1(3H)-one (2) [22, 23]
A hot solution (40 °C) of 2-(2-thenoylbenzoic acid) (11.0 g,
47.7 mmol) in aqueous sodium bicarbonate (5.9 g in 300 mL
of H2O) was treated with NaBH4 (18.0 g, 474 mmol) in por-
tions slowly. After standing overnight, it was heated to re-
fluence for 1 h on the steam bath, cooled to room tempera-
ture, and then cautiously acidified with 12 wt% aqueous
hydro chloric acid to pH ~2. The mixture was stirred for 2 h
at room temperature, and the white solid was filtered to give
compound 2 (8.2 g, 80%).
1,3-di(thiophen-2-yl)isobenzo–furan (3) [24, 25]
Scheme 1 Synthesis of DTTDO.
At –78 °C, 2-bromothiophene (3.42 g, 21 mmol) resolved in
THF was stirred under nitrogen, butyllithium (2.5 M in
n-hexane, 9.1 mL, 22.6 mmol) was added, when there was
synthesized thiophen-2-yllithium solution after 1 h, it was
treated with a solution of 3-(thiophen-2-yl)-isobenzofuran-
1(3H)-one (2) (4.4 g, 20.4 mmol) in dry THF, keeping the
temperature at 78 °C for 0.5 h until acetic anhydride was
added, the temperature was raised to reflux the solution for
10 min, ending the reaction. The solution was extracted by
CH2Cl2, combined the organic phase and dried by Na2SO4,
then concentrated in vacuum. The residue was purified by
column chromatography to give compound 3 as red solid
(2.7 g, 47%).
ability under vacuum deposition conditions, indicating its
promising applications in OTFTs.
2 Experimental
2.1 General methods
All chemicals and solvents were obtained from commercial
1
suppliers and used as received unless otherwise noted. H
NMR spectra were taken in CDCl3 with TMS ( 0.00 ppm)
as internal standard at room temperature and were recorded
on Bruker 400 NMR spectrometer. UV-vis spectra were
obtained on a JSCO V-570 UV-vis spectrometer. Cyclic
voltammetirc (CV) measurements were conducted using a
CHI660C electrochemistry station with tetrabutlyammonium
hexafluorophosphate (Bu4NPF6) as electrolyte (0.001 M in
dry CH2Cl2). The working, counter and reference electrodes
were Glassy carbon, Pt wire and Ag/AgCl, respectively.
Thermogravimetric analysis (TGA) was recorded on a
PERKIN ELMER TGA7 apparatus with the scanning rate
of 10 °C/min. X-Ray diffraction measurements were obtained
in reflection mode at 40 kV and 200 mA with Cu K radia-
tion using a 2-kW Rigaku D/max-2500 X-ray diffractometer.
AFM images of films were obtained by using a Digital In-
struments Nanoscope IIIa Multimode atomic force micro-
scope in air.
6,11-di(thiophen-2-yl)tetracene-5,12-dione (DTTDO)[26, 27]
Compound 3 (2.82 g, 10 mmol) and 1,4-naphthaquinone
(1.58 g, 10 mmol) were resolved in dry CH2Cl2 (50 mL)
under nitrogen, then the solution was stirred overnight at
room temperature, another 100 mL dry CH2Cl2 was added,
cooling the solution to –78 °C and BBr3 (2.88 g, 11.5 mmol)
was added, after 0.5 h, solution was heated to reflux for 4 h,
then cooling it to room temperature. The solution was
dumped into water (250 mL) in beaker, extracted with
CH2Cl2 combined organic phase and washed with saturation
brine, concentrated in vacuum. The residue was purified by
column chromatography to give compound DTTDO as yel-
1
low solid (1.77 g, 42%). mp 301 °C, H NMR (400 MHz,
CDCl3, TMS, ppm)  8.11 (d, 2H), 7.83 (d, 2H), 7.70 (d,
2H), 7.61 (d, 4H), 7.30(d, 2H), 7.03 (m, 2H); MS (EI) m/z:
422 (M+).
2.2 Materials synthesis
3 Results and discussion
2-(2-thienyl)benzoic acid (1) [20, 21]
A suspension of pthalic anhydride (20 g, 0.135 mol) in dry
CH2Cl2 (200 mL) was stirred in the flask under nitrogen for
0.5 h, and then anhydrous AlCl3 (37.8 g, 0.284 mol) was
added slowly. The mixture was stirred for 0.5 h at room
temperature and was treated drop wise with a solution of
thiophene (12.5 g, 0.149 mol) in CH2Cl2 (100 mL) in 40
min. The mixture was stirred for 3 h at 30 °C. Standard
workup yielded the 2-(2-thienyl)benzoic acid (compound 1)
as a white brown solid (24.7 g, 79%).
3.1 Thermal, optical and electrochemical properties
The thermal, optical and electrochemical properties of
compound DTTDO were measured by thermal gravimetric
analysis (TGA), UV-vis spectrum, and cyclic voltammetry
(CV). The TGA curve of DTTDO (Figure 1) measured with
a heating rate of 10 °C/min under nitrogen demonstrated
that this compound had a good thermal stability with the
decomposition temperature of 310 °C which was near to

Zhang XT, et al. Sci China Chem April (2011) Vol.54 No.4
633
Figure 3 Cyclic voltammogram of DTTDO in DMF solution using Ag/
AgCl as the reference electrode.
Figure 1 TGA curve of DTTDO with a heating rate of 10 °C/min under
nitrogen.
that of rubrene (315–332 °C) [28, 29].
Figure 2 shows the absorption spectra of DTTDO in
CH2Cl2 solution (105 M) and in thin film form (50 nm) va-
cuum deposited on quartz glass substrate. Compared the
absorption spectra of DTTDO in thin film with that in solu-
tion, it is clearly noted that the maximum absorption in the
longer wavelength was red shifted ~17 nm, from 395 to 412
nm, which was induced by the strong intermolecular inter-
actions of DTTDO in the thin films. Additionally, the
HOMO-LUMO energy gaps of DTTDO in solution esti-
mated from the initial absorption was 2.90 eV, which was a
little higher than the energy gaps (2.6 eV) of rubrene [6].
The cyclic voltammetry of compound DTTDO was per-
Figure 4 Energy level diagram among rubrene, Au and DTTDO.
formed in dry DMF solution with a concentration of 103
M
3.2 Crystallographic analysis
(as shown in Figure 3), it is noted that the reduction curve
of DTTDO is reversible. While the HOMO level of rubrene
was 4.69 eV [6], the LUMO level of DTTDO estimated
from cyclic voltammogram was 3.77 eV, combining the
LUMO level and the energy band gap the HOMO could be
calculated is 6.67 eV, which is much lower than that of
rubrene though they have similar energy gaps, just as we
can see from Figure 4.
Single-crystal X-ray crystallography was performed to ex-
amine the molecular packing and the intermolecular interac-
tions of DTTDO molecules. The large single-crystals of
compound DTTDO used for the single-crystal measurement
were obtained from CH2Cl2 solution through slow evapora-
tion at room temperature. Single-crystal X-ray diffraction
patterns (XRD) of DTTDO crystals demonstrated that this
compound crystallized in space group P-1 of triclinic sys-
tem with unit-cell dimensions of a = 7.7731(16), b =
11.926(2), c = 12.758(3),  = 78.057(7),  = 87.724(9),  =
76.074(8). As shown in Figure 5(c), in the DTTDO single
crystal one unit cell consists of two CH2Cl2 solvent mole-
cules and two DTTDO molecules, which further connect
together with their neighbour molecules through the S··O
interactions with distance of 3.23 Å. Different from the
phenyl groups in rubrene, thiophene group has an asymmet-
ric structure because of the presence of sulphur atom which
results in two types of DTTDO molecular structure, trans-
and cis-structure. Additionally, the thiophene planes are
twisted strongly out of the backbone plane with an angle of
approximate 88.8° and the carbonyl oxygen atoms are also
out of the plane of tetracene backbone 9° (with ∠OCC an-
gle of 171°, Figure 5(a)). The backbone exhibited a planar
structure with very slightly twisted structure between the
Figure 2 UV-vis spectra of DTTDO. The dashed line represents the
solution spectrum with 105 M in CH2Cl2, and the solid line represents the
thin film spectrum vacuum deposited on quartz glass.

634
Zhang XT, et al. Sci China Chem April (2011) Vol.54 No.4
Figure 6 Optical microscopy images of DTTDO thin films vacuum-depo-
sited on substrates at different temperatures: at (a) 20 °C, (b) 50 °C on bare
Si/SiO2 substrates and at (d) 20 °C, (e) 50 °C on OTS-treated Si/SiO2 sub-
strates. And AFM images of 100-nm-thick thin films at 20 °C, (c) on bare
Si/SiO2 substrate, and (f) on OTS treated Si/SiO2 substrate.
polar of the compound, DTTDO is polar organic compound
which was expected to form low-energy interfaces with
SiO2 substrate, while DTTDO and OTS, rubrene and SiO2
were converse in polar, so the good thin films of DTTDO
suggest its promising applications in OTFTs.
Figure 5 Molecular structure of DTTDO seeing along the (a) tetracene
backbone plane and (b) long axis of tetracene backbone, (c) unit-cell of
DTTDO single crystal and molecular packing motifs of DTTDO with view
of (d) bc plane and (e) (011) plane.
4 Conclusions
end phenyl rings (Figure 5(c)). Interestingly, it is noted that
DTTDO molecules in crystals exhibited typical lamellar
packing motifs with one-dimensional - stacking (with
distance of ~3.5 Å) along the a axis (as shown in Figure 5(d)
and 5(e)) probably due to the reduced steric effect of thio-
phene and carbonyl substitutions compared to that of phen-
yls in rubrene [6].
A new semiconductor with thienyl groups and carbonyl
groups on the backbone of tetracene was synthesized. Cor-
responding characterizations demonstrated that DTTDO
compound exhibited good thermal and thin-film forming
properties. Single crystal X-diffraction analysis indicated
that DTTDO molecules in crystals exhibited typical lamel-
lar packing motifs with one-dimensional - stacking (with
distance of ~3.5 Å) along a axis. Good thin-film forming
properties as well as the typical - stacking motifs of
DTTDO molecules indicate its promising applications in
organic thin-film transistors.
3.3 Thin films of DTTDO
To examine the film-forming properties of DTTDO mole-
cules, the morphologies of vacuum-deposited thin films
both on the bare Si/SiO2 substrates and on the octadecyl-
trichlorosilane (OTS) treated Si/SiO2 substrates under dif-
ferent substrate temperatures were investigated by optical
microscopy and atomic force microscopy (AFM). The mi-
croscopy images of DTTDO films (Figure 6) showed that at
lower substrate temperature of 20 °C and 50 °C, good thin
films of DTTDO semiconductor could be obtained on both
Si/SiO2 and OTS-modified substrates although there were
still existing some large crystallize grains on bare Si/SiO2
substrate at 20 °C. Figure 6(c) and 6(f) show the AFM im-
ages of DTTDO films on substrate at 20 °C. These results
demonstrated that good thin films of DTTDO could be ob-
tained on bare Si/SiO2 substrates under vacuum deposition
conditions (Figure 6(c)), which is different from rubrene
thin films in which isolands form and isolated from each
other by large areas of exposed SiO2 [30]. So DTTDO is
easier to get good thin films, maybe these are because of the
The authors thank for the support of the National Natural Science Founda-
tion of China (60771031, 60736004, 20571079, 20721061 and 50725311),
National Basic Research Program of China (973 Program, 2006CB806200
& 2006CB932100) and Chinese Academy of Sciences.
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