Synthesis and characterization of planar five-ring-fused dithiophene-dione

Article abstract of DOI:10.1002/cjoc.201100141

A series of new organic semiconductors based on s-indaceno[1,2-b:5,6- b′]dithiophene-4,9-dione was successfully synthesized and characterized. The electron withdrawing carbonyl group lowers the LUMO energy levels, leading to increased electronegativities, which is beneficial for high photo-stability in air. The n-alkyl substituted compounds, 1c and 1d, crystallize with the rigid coplanar systems packed into slipped face-to-face π-stacks. Interestingly, 1c and 1d also show liquid crystalline behaviors, which give highly ordered molecular packing over large area.

Full text of DOI:10.1002/cjoc.201100141

FULL PAPER
DOI: 10.1002/cjoc.201100141
Synthesis and Characterization of Planar Five-Ring-Fused
Dithiophene-dione
Wang, Jing(王静)
Zeng, Weijing(曾维静)
Xu, Huan(许欢)
Li, Bin(李斌)
Cao, Xiaoping(曹小平)
Zhang, Haoli*(张浩力)
State Key Laboratory of Applied Organic Chemistry (SKLAOC), College of Chemistry and Chemical Engineering,
Lanzhou University, Lanzhou, Gansu 730000, China
A series of new organic semiconductors based on s-indaceno[1,2-b:5,6-b']dithiophene-4,9-dione was success-
fully synthesized and characterized. The electron withdrawing carbonyl group lowers the LUMO energy levels,
leading to increased electronegativities, which is beneficial for high photo-stability in air. The n-alkyl substituted
compounds, 1c and 1d, crystallize with the rigid coplanar systems packed into slipped face-to-face π-stacks. Inter-
estingly, 1c and 1d also show liquid crystalline behaviors, which give highly ordered molecular packing over large
area.
Keywords semiconductors, fused-ring systems, liquid-crystals
Introduction
possesses linear π-conjugated structure similar to pen-
tacene and indenofluorene-6,12-dione. Having the thio-
phene units as the terminal rings and carbonyl groups on
the second ring ensures the conjugated framework to
exist as a rigid coplanar configuration, which is crucial
for high mobility.[17] Ng’s group reported the first syn-
thesis of dithiophene-dione framework and the applica-
tions of thienyl-substituted derivatives in polymer
photovoltaic devices.[18,19]
In this work, we synthesized four new compounds
based on the dithiophene-dione framework. The syn-
thetic route was slightly modified from that developed
by Ng. Moreover, by using different n-alkyl substitu-
tions, we were able to investigate the effects of system-
atic substitution variation on their spectroscopic, elec-
trochemical, thermal properties and molecular packing
in solid states. Liquid crystalline phase and single crys-
tal structures of two new compounds were obtained.
During the preparation of this manuscript, the FET de-
vices of compound 1c were reported by Geng et al.[20]
Significant progress has been made in organic
field-effect transistors (OFETs) in the past few years.[1-4]
A large number of organic semiconductors have been
prepared, and some materials, like pentacene, even ex-
hibited OFET performance comparable to amorphous
silicon.[5-7] However, pentacene and its derivatives often
suffer from rapid degradation in ambient conditions, due
to photoinduced decomposition and formation of
dimeric Diels-Alder adducts on the electron-rich central
ring.[8] Great efforts have been devoted to design new
materials with high charge-carrier mobilities and high
stability.[9-11] It is understood that the enhancement of
stability is possible by regulating the HOMO-LUMO
energy levels by incorporating heteroatoms (e.g., N, S,
O, etc.).[9,12] For instance, the incorporation of fused
thiophene units into heteroacenes widens the
HOMO-LUMO gap and stabilizes the HOMO level.[2]
Meanwhile, design and synthesis of new conjugated
systems for electronically complementary n-type or-
ganic semiconductors is another urgent aspect for the
development of OFETs.[4,13] A common strategy to
achieve n-type organic semiconductors is incorporation
of electron withdrawing groups[4] into the π-conjugated
cores, which could reduce the LUMO energy and facili-
tate electron injection.
Experimental
Reagents and methods
All chemicals and solvents were obtained from
commercial suppliers and used without further purifica-
tion unless otherwise noted. The THF was dried by dis-
tillation over sodium wires/benzophenone. 1H NMR
spectra were recorded on either a 300 or 400 spec-
trometer and calibrated to the residual protonated sol-
The good device properties and desirable longevity
of the indenofluorene-6,12-dione family[14-16] has
stimulated the interest toward dithiophene-dione het-
eroacenes. The five-ring-fused heteroacence framework
*
E-mail: Haoli.zhang@lzu.edu.cn; Tel.: 0086-0931-8912365; Fax: 0086-0931-8912365
Received July 18, 2011; accepted September 17, 2011; published online February 29, 2012.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201100141 or from the author.
Chin. J. Chem. 2012, 30, 681—688
© 2012 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
681

Wang et al.
FULL PAPER
vent at δ 0.00 for deuterated chloroform (CDCl3), 4.79
for water (D2O) and 2.50 for deuterated dimethyl sul-
foxide (DMSO-d6). 13C NMR experiments were carried
out in CDCl3 or DMSO-d6 and calibrated at δ 77.00 or
39.52. UV-Vis absorption spectra were recorded using a
T6 UV-Vis spectrometer. IR spectra were obtained as
KBr discs on a Bruker V70 FT-IR spectrometer. Ele-
mental analyses were obtained at the UBC Microana-
lytical facility. Single crystal X-ray diffraction meas-
urements were made on a Bruker X8 APEX diffracto-
meter with graphite monochromated Mo-Kα radiation (λ
＝0.71073 Å) at 296(2) K.
white amorphous solid, which was used for next step
without further purification (1.37 g, 65% yield). m.p.＞
1
300 ℃; H NMR (D2O, 400 MHz) δ: 7.32 (dd, J＝4.8,
1.2 Hz, 1H), 7.04 (dd, J＝3.6, 1.2 Hz, 1H), 7.01 (dd,
J＝4.8, 3.6 Hz, 1H); IR (KBr) v: 3217, 1454, 1194, 810,
－
1
726, 645, 546 cm .
Compound 4b was prepared from compound 2b
(1.40 g, 12.50 mmol) according to the procedure for
compound 4a to yield a white amorphous solid, which
was used for next step without further purification (1.47
g, 60% yield). m.p.＞300 ℃; 1H NMR (D2O, 400 MHz)
δ: 6.78 (d, J＝3.6 Hz, 1H), 6.65 (d, J＝3.6 Hz, 1H),
2.75 (q, J＝7.6 Hz, 2H), 1.32 (t, J＝7.6 Hz, 3H); IR
The electrochemical measurement was conducted
using a CHI660B electrochemistry work station. All
measurements were carried out at room temperature
with a conventional three-electrode configuration con-
sisting of a platinum disk working electrode, an auxil-
iary platinum wire electrode, and a nonaqueous
Ag/AgNO3 reference electrode. The supporting electro-
－
1
(KBr) v: 3414, 2958, 1443, 1229, 936, 530 cm .
Compound 4c was synthesized by the literature
method.[21]
Compound 4d was prepared from compound 2d
(3.15 g, 12.50 mmol) according to the procedure for
compound 4a to yield a white amorphous solid, which
was used for next step without further purification (2.94
g, 70% yield). m.p.＞300 ℃; 1H NMR (D2O, 400 MHz)
δ: 6.76 (d, J＝3.6 Hz, 1H), 6.64 (d, J＝3.6 Hz, 1H),
2.70 (t, J＝7.6 Hz, 2H), 1.56—1.63 (m, 2H), 1.20—
1.32 (m, 18H), 0.84 (t, J＝6.8 Hz, 3H); IR (KBr) v:
lyte was 0.1 mol•L－ tetrabutylammonium hexafluoro-
1
phosphate (TBAPF6) in THF and the scan rate was set
－
1
to 100 mV•s . The ferrocene/ferrocenium couple was
used as an internal standard, and all potentials are re-
ported vs. SCE.
－
1
Thermogravimetric analysis (TGA) was carried out
3371, 2920, 1465, 1441, 1237, 931, 524 cm .
on a PE TGA7. A heating rate of 10 ℃•min－ under
1
Synthesis of dimethyl 2,5-di(2-thienyl)terephthalate
(6a)
flowing N2 was used with runs being conducted from
room temperature to high temperature. Differential
scanning calorimetry (DSC) was carried out on a
NETZSCH STA 449 C. DSC scans under nitrogen at a
The aryl trihydroxyboronate salt 4a (1.68 g, 10.00
mmol), aryl halide 5 (880 mg, 2.50 mmol), Pd(PPh3)4
(115 mg, 0.10 mmol) and a saturated aqueous solution
of NaHCO3 (840 mg, 10.00 mmol) were placed in a
flame flask under argon. Freshly distilled and de-gassed
THF (10 mL) was added and the mixture heated and
stirred at reflux for 24 h. After cooling to room tem-
perature the mixture was poured into a saturated solu-
tion of ammonium chloride, and extracted with ethyl
acetate (10 mL×3). The combined organic layers were
washed with saturated brine and dried over Na2SO4. The
solution was then filtered, the solvent removed and the
residue purified by column chromatography, using a
mixture of hexane and dichloromethane (1∶1, volume
ratio) as the eluent to yield a kelly solid (358 mg, 40%
yield). m.p. 170—172 ℃; 1H NMR (CDCl3, 400 MHz)
δ: 7.82 (s, 2H), 7.39 (dd, J＝4.8, 1.2 Hz, 2H), 7.11 (dd,
J＝3.6, 1.2 Hz, 2H), 7.08 (dd, J＝4.8, 3.6 Hz, 2H), 3.78
(s, 6H); 13C NMR (CDCl3, 100 MHz) δ: 168.1, 140.3,
133.6, 133.4, 131.9, 127.5, 127.0, 126.7, 52.6; IR (KBr)
v: 3444, 2952, 1729, 1428, 1288, 1247, 1209, 1109, 848,
－
1
scan rate of 10 ℃•min . Liquid-crystal micrographs
were taken on an XPR-300C polarized optical micro-
scope.
Synthesis of sodium 2-thienyl boronate (4a)
Thiophene (1.05 g, 12.50 mmol) was dissolved in
dry THF (20 mL) and the solution was stirred at －78
℃ under argon. n-BuLi (6 mL, 15.00 mmol, 2.5 mol•
L－ solution in hexane) was then added dropwise to the
1
solution over a period of 30 min. The reaction mixture
was allowed to warm to room temperature and stirred
for a further 1.5 h. The mixture was then slowly added
over a period of 1 h to a solution of trimethylborate
(2.60 g, 25.00 mmol) in dry THF (15 mL) which had
been pre-cooled to －78 ℃. The resulting mixture was
allowed to slowly warm to room temperature and left
stirring overnight. Dilute hydrochloric acid (5 mL, 2
－
1
mol•L ) was added and the mixture extracted with di-
ethyl ether (10 mL×3). The combined organic layers
were dried, filtered, and the solvent removed under
vacuum to get compound 3a. The crude product 3a was
dissolved in a minimum amount of warm toluene under
stirring and the solution allowed cooling to room tem-
perature. Saturated NaOH solution was added dropwise
until no further precipitate formed. The mixture was
allowed to stir for 30 min. The precipitate obtained was
filtered off, washed with toluene and petroleum ether
and dried under vacuum to give the compound as a
－
1
718 cm ; HRMS (ESI) calcd for C18H14O4S2 358.0406,
found 359.0411 [M＋H]＋.
Compound 6b was prepared by coupling of com-
pound 4b (1.96 g, 10.00 mmol) with 5 (880 mg, 2.50
mmol) according to the procedure for compound 6a and
purified by column chromatography, using a mixture of
hexane and dichloromethane (4∶1, volume ratio) as the
eluent to yield a kelly solid (673 mg, 65% yield). m.p.
86—90 ℃; 1H NMR (CDCl3, 400 MHz) δ: 7.75 (s, 2H),
682
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© 2012 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2012, 30, 681—688

Synthesis and Characterization of Planar Five-Ring-Fused Dithiophene-dione
6.91 (d, J＝3.6 Hz, 2H), 6.75 (d, J＝3.6 Hz, 2H), 3.80
(s, 6H), 2.87 (q, J＝7.6 Hz, 4H), 1.34 (t, J＝7.6 Hz, 6H),
13C NMR (CDCl3, 100 MHz) δ: 168.4, 149.1, 137.5,
133.2, 133.1, 131.5, 126.6, 123.9, 52.5, 23.5, 15.8; IR
(KBr) v: 3440, 2969, 2848, 1727, 1433, 1289, 1238,
1189, 1108, 815 cm － 1. HRMS (ESI) calcd for
C22H22O4S2 414.0852, found 437.0846 [M＋Na]＋.
compound 7a, which was filtered and dried to afford
yellow solid (443 mg, 85% yield). m.p. 210—214 ℃;
1H NMR (DMSO-d6, 400 MHz) δ: 13.38 (brs, 2H), 7.61
(s, 2H), 7.06 (d, J＝3.6 Hz, 2H), 6.87 (d, J＝3.6 Hz,
2H), 2.85 (q, J＝7.6 Hz, 4H), 1.27 (t, J＝7.6 Hz, 6H);
13C NMR (DMSO-d6, 100 MHz) δ: 168.9, 148.3, 136.9,
134.2, 131.1, 129.7, 126.8, 124.6, 22.8, 15.7; IR (KBr) v:
2978, 2848, 2655, 2564, 1683, 1423, 1288, 1244, 1209,
Compound 6c was prepared by coupling of com-
pound 4c (2.52 g, 10.00 mmol) with 5 (880 mg, 2.50
mmol) according to the procedure for compound 6a and
purified by column chromatography, using a mixture of
hexane and dichloromethane (5∶1, volume ratio) as the
eluent to yield a kelly solid (1.10 g, 84% yield). m.p. 88
－
1
1140, 814 cm . HRMS (ESI) calcd for C20H18O4S2
386.0574, found 385.0571 [M－H]－.
Compound 7c was prepared from compound 6c (710
mg, 1.35 mmol) according to the procedure for com-
pound 7a, which was filtered and dried to afford yellow
solid (645 mg, 96% yield). m.p. 130—132 ℃; 1H NMR
(DMSO-d6, 300 MHz) δ: 13.44 (b, 2H), 7.61 (s, 2H),
7.05 (d, J＝3.3 Hz, 2H), 6.85 (d, J＝3.3 Hz, 2H), 2.80 (t,
J＝7.5 Hz, 4H), 1.58—1.65 (m, 4H), 1.20—130 (m,
12H), 0.86 (t, J＝6.6 Hz, 6H); 13C NMR (DMSO-d6, 75
MHz) δ: 169.1, 146.9, 137.1, 134.2, 131.2, 129.7, 126.8,
125.4, 31.2, 31.0, 29.4, 28.3, 22.1, 14.0; IR (KBr) v:
2923, 2854, 2634, 2558, 1699, 1425, 1289, 1242, 1211,
1
—90 ℃; H NMR (CDCl3, 400 MHz) δ: 7.74 (s, 2H),
6.90 (d, J＝3.6 Hz, 2H), 6.73 (d, J＝3.6 Hz, 2H), 3.79
(s, 6H), 2.82 (t, J＝7.6 Hz, 4H), 1.66—1.73 (m, 4H),
1.30—1.40 (m, 12H), 0.90 (t, J＝6.8 Hz, 6H); 13C NMR
(CDCl3, 100 MHz) δ: 168.5, 147.7, 137.6, 133.2, 133.1,
131.4, 126.4, 124.5, 52.6, 31.6 (overlapping signal),
30.2, 28.8, 22.6, 14.0; IR (KBr) v: 3440, 2924, 2852,
－
1
1725, 1433, 1290, 1235, 1188, 1110, 805 cm . HRMS
(ESI) calcd for C30H38O4S2 526.2284, found 527.2272
[M＋H]＋.
－
1
1125, 803 cm ; HRMS (ESI) calcd for C28H34O4S2
498.1826, found 497.1816 [M−H]−.
Compound 6d was prepared by coupling of com-
pound 4d (3.36 g, 10.00 mmol) with 5 (880 mg, 2.50
mmol) according to the procedure for compound 6a and
purified by column chromatography, using a mixture of
hexane and dichloromethane (7∶1, volume ratio) as the
eluent to yield a kelly solid (1.53 g, 88% yield). m.p. 74
Compound 7d was prepared from compound 6d
(937 mg, 1.35 mmol) according to the procedure for
compound 7a, which was filtered and dried to afford
yellow solid (818 mg, 91% yield). m.p. 318—320 ℃;
1H NMR (DMSO-d6, 300 MHz) δ: 13.44 (brs, 2H), 7.56
(s, 2H), 7.04 (d, J＝3.6 Hz, 2H), 6.81 (d, J＝3.6 Hz,
2H), 2.78 (t, J＝7.5 Hz, 4H), 1.60—1.64 (m, 4H), 1.15
—1.41 (m, 36H), 0.84 (t, J＝6.6 Hz, 6H); 13C NMR
(DMSO-d6, 75 MHz) δ: 169.2, 146.5, 137.3, 134.5,
130.9, 129.5, 126.6, 125.2, 31.4, 31.2, 29.4 (overlapping
signals), 29.1, 29.0 (overlapping signal), 28.8, 28.6,
22.2, 14.0; IR (KBr) v: 2919, 2850, 2639, 2559, 1702,
1
—76 ℃; H NMR (CDCl3, 400 MHz) δ: 7.74 (s, 2H),
6.90 (d, J＝3.6 Hz, 2H), 6.73 (d, J＝3.6 Hz, 2H), 3.79
(s, 6H), 2.82 (t, J＝7.6 Hz, 4H), 1.66—1.73 (m, 4H),
1.26—1.38 (m, 36H), 0.88 (t, J＝6.8 Hz, 6H); 13C NMR
(CDCl3, 100 MHz) δ: 168.5, 147.7, 137.6, 133.2, 133.1,
131.4, 126.5, 124.6, 52.5, 31.9, 31.6, 30.2, 29.7, 29.6,
29.5, 29.4, 29.3, 29.1 (overlapping signal), 22.7, 14.1;
IR (KBr) v: 3440, 2918, 2850, 1726, 1434, 1291, 1236,
1190, 1111, 803 cm － 1. HRMS (ESI) calcd for
C42H62O4S2 694.4162, found 695.4156 [M＋H]＋.
－
1
1405, 1288, 1240, 1209, 1126, 802 cm . HRMS (ESI)
calcd for C40H58O4S2 666.3704, found 665.3715 [M－
H]－.
Synthesis of 2,5-bis(2-thienyl)terephthaloyl chloride
(8a)
Synthesis of 2,5-di(2-thienyl)terephthalic acid (7a)
A solution of compound 7a (178 mg, 0.54 mmol)
and oxalyl chloride (0.5 mL, 5.70 mmol) in dry CH2Cl2
(30 mL) and several drops DMF was stirred for 12 h at
room temperature. The solvent was removed under
vacuum to obtain the corresponding crude terephthaloyl
chloride, which was used for next step without further
purification.
Compound 8b was prepared from compound 7b
(208 mg, 0.54 mmol) and oxalyl chloride (0.5 mL, 5.70
mmol) according to the procedure for compound 8a,
which was used for next step without further purifica-
tion.
Compound 8c was prepared from compound 7c (269
mg, 0.54 mmol) and oxalyl chloride (0.5 mL, 5.70
mmol) according to the procedure for compound 8a,
which was used for next step without further purifica-
tion.
A mixture of compound 6a (483 mg, 1.35 mmol)
and sodium hydroxide (216 mg, 5.40 mmol) in ethanol
(27 mL) and water (3 mL) were refluxed overnight. The
solvent was evaporated under vacuum to about half of
its original volume. Water was added, and the resulting
aqueous layer was treated with HCl to obtain a solid,
which was filtered and dried to afford yellow solid (290
mg, 65% yield). m.p. 308 —310 ℃; 1H NMR
(DMSO-d6, 300 MHz) δ: 13.44 (brs, 2H), 7.70 (s, 2H),
7.67 (d, J＝4.8 Hz, 2H), 7.25 (d, J＝3.6 Hz, 2H), 7.15
(dd, J＝4.8, 3.6 Hz, 2H); 13C NMR (DMSO-d6, 75 MHz)
δ: 169.7, 139.7, 134.5, 131.5, 130.2, 127.9, 127.5, 127.2;
IR (KBr) v: 2880, 2641, 2530, 1685, 1438, 1405, 1285,
－
1
1255, 1211, 1127, 837, 708 cm . HRMS (ESI) calcd
for C16H10O4S2 329.9948, found 328.9952 [M－ H]－.
Compound 7b was prepared from compound 6b
(559 mg, 1.35 mmol) according to the procedure for
Chin. J. Chem. 2012, 30, 681—688
© 2012 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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683

Wang et al.
FULL PAPER
Compound 8d was prepared from compound 7d
(360 mg, 0.54 mmol) and oxalyl chloride (0.5 mL, 5.70
mmol) according to the procedure for compound 8a,
which was used for next step without further purifica-
tion.
C 68.53, H 4.06, S 18.31.
Compound 1c was prepared from compound 8c (289
mg, 0.54 mmol) according to the procedure for com-
pound 1a. The crude product was purified by flash
chromatography, using a mixture of hexane and di-
chloromethane (5∶1, volume ratio) as the eluent to
yield a green solid (174 mg, 70% yield). m.p. 168—170
℃; 1H NMR (CDCl3, 400 MHz) δ: 7.12 (s, 2H), 6.80 (s,
2H), 2.78 (t, J＝7.6 Hz, 4H), 1.64—1.70 (m, 4H), 1.29
—1.40 (m, 12H), 0.90 (t, J＝6.8 Hz, 6H); 13C NMR
(CDCl3, 100 MHz) δ : 186.5, 156.1, 151.5, 140.8, 139.9,
139.8, 118.2, 114.0, 31.5, 31.3, 30.6, 28.6, 22.5, 14.0;
IR (KBr) v: 3392, 2918, 2848, 1729, 1468, 1321, 773,
Synthesis of 4,9-dihydro-s-indaceno[1,2-b:5,6-b']di-
thiophene-4,9-dione (1a)
A solution of compound 8a (198 mg, 0.54 mmol) in
dry CH2Cl2 (30 mL) was added to a suspension of an-
hydrous AlCl3 (216 mg, 1.62 mmol) in dry CH2Cl2 (10
mL) at 0 ℃. The resulting mixture was further stirred
for 20 min, then at room temperature for 3 h. The reac-
tion mixture was poured into ice water then filtered and
washed by hexane, ethanol to yield a blue solid (103 mg,
65% yield). m.p. 350—352 ℃; we can not get its NMR
spectra because of its poor solubility. IR (KBr) v: 3387,
－
＋
1
480 cm ; LRMS (APCI) m/z: 462, found 463 [M＋H] .
Anal. calcd for C28H30O2S2: C 72.69, H 6.54, S 13.85;
found C 72.75, H 6.57, S 13.77.
－
1
3107, 3072, 1711, 1435, 1320, 778, 463 cm ; LRMS
(APCI) m/z: 294, found 295 [M＋H]＋. Anal. calcd for
C16H6O2S2: C 65.29, H 2.05, S 21.79; found C 65.35, H
2.08, S 21.77.
Compound 1d was prepared from compound 8d
(380 mg, 0.54 mmol) according to the procedure for
compound 1a. The crude product was purified by flash
chromatography, using a mixture of hexane and di-
chloromethane (5∶1, volume ratio) as the eluent to
yield a green solid (221 mg, 65% yield). m.p. 146—148
℃; 1H NMR (CDCl3, 400 MHz) δ: 7.11 (s, 2H), 6.79 (s,
2H), 2.78 (t, J＝7.6 Hz, 4H), 1.66—1.69 (m, 4H), 1.19
—1.41 (m, 36H), 0.88 (t, J＝6.8 Hz, 6H); 13C NMR
(CDCl3, 100 MHz) δ: 186.5, 156.1, 151.6, 140.8, 140.0,
139.9, 118.2, 114.0, 31.9, 31.3, 30.6, 29.6 (overlapping
signal), 29.5, 29.4, 29.3, 28.9 (overlapping signal), 22.7,
14.1; IR (KBr) v: 3388, 2951, 2850, 1697, 1466, 1317,
Compound 1b was prepared from compound 8b
(228 mg, 0.54 mmol) according to the procedure for
compound 1a. The crude product was purified by flash
chromatography, using a mixture of hexane and di-
chloromethane (3∶1, volume ratio) as the eluent to
yield a green solid (132 mg, 70% yield). m.p. 256—258
℃; 1H NMR (CDCl3, 400 MHz) δ: 7.11 (s, 2H), 6.81 (s,
2H), 2.83 (q, J＝7.6 Hz, 4H), 1.33 (t, J＝7.6 Hz, 6H);
13C NMR (CDCl3, 100 MHz) δ: 186.5, 156.0, 153.1,
140.8, 140.0, 139.9, 117.5, 114.0, 24.0, 15.6; IR (KBr) v:
－
1
－
1
774, 483 cm ; LRMS (APCI) m/z: 630, found 631 [M
＋H]＋. Anal. calcd for C40H54O2S2: C 76.14, H 8.63, S
10.16; found C 76.23, H 8.55, S 10.10.
3396, 2924, 2854, 1708, 1434, 1315, 776, 474 cm ;
LRMS (APCI) m/z: 350, found 351 [M＋H]＋. Anal.
calcd for C20H14O2S2: C 68.54, H 4.03, S 18.30; found
Scheme 1 Synthesis of compounds 1a—1d
OH
NaOH
OH
Na+
BuLi
-
R
B
R
R
B
S
S
S
OH
B(OMe)3
OH
HO
2
4
3
COOMe
Br
COOMe
R
4, Pd(PPh3)4
S
NaOH
Br
NaHCO3
EtOH, H2O
S
R
MeOOC
MeOOC
S
5
6
COOH
COCl
S
R
S
(COCl)2
DMF
R
AlCl3
S
R
R
HOOC
ClOC
7
8
R
O
S
1a R = H
1b R = C2H5
1c R = C6H13
1d R = C12H25
S
O
R
1
684
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Chin. J. Chem. 2012, 30, 681—688

Synthesis and Characterization of Planar Five-Ring-Fused Dithiophene-dione
Results and discussion
Materials synthesis
The synthesis of compounds 1a—1d is depicted in
Scheme 1. The starting compounds, 2-dodecylthiophene
(2d) and dimethyl 2,5-dibromoterephthalate (5) were
prepared according to the literature methods.[22,23] Com-
pounds 4a—4d were synthesized by a similar process as
reported before.[21] Instead of Still-coupling reaction
used in previous synthesis,[18] Suzuki coupling was used
in our work to avoid using toxic tin compounds. A
two-folded palladium-catalyzed Suzuki cross-coupling
reaction between compounds 4 and 5 furnished di-
thienyl-substituted terephthalate diesters 6. Saponifica-
tion of the diesters 6 smoothly provided 2,5-bis(5-alkyl-
thiophen-2-yl)terephthalic acids (7), which were readily
transformed into the corresponding terephthaloyl chlo-
rides 8. Upon removal of the volatile materials in vacuo,
a Lewis acid-promoted intramolecular Friedel-Crafts
cyclization of compound 8 took place to give the conju-
gated diketones 1. Compound 1a gave blue crystal,
which was only slightly soluble in high polar solvents.
Compounds 1b, 1c were obtained as shining turquoise
crystals and 1d was aquamarine crystal. They were
readily dissolved by most common organic solvents.
Figure 1 (a) Normalized UV-vis absorption recorded in DCM.
(b) Thin-film UV-vis absorption spectra of 1a—1d.
Optical properties
The UV-vis absorption spectra of the new materials
in dichloromethane (DCM) are shown in Figure 1a.
Compounds 1a—1d exhibit intense absorption bands at
about 295 nm, which are attributable to the π-π* transi-
tion of the conjugated backbone. The weak lowest-
energy absorptions at 560—605 nm can be also assigned
to a π-π* transition.[18,24-26] The UV-vis absorption pro-
files for compounds 1a—1d are very similar, suggesting
the length of n-alkyl groups have little effect on the mo-
lecular π-system.[27] When the new materials are depos-
ited as thin films, their high-energy absorption maxima
are observed at around 310—330 nm, red-shifted by ca.
13—33 nm versus the corresponding solution absorption
maxima. Meanwhile, the lowest-energy absorption
edges at longer wavelengths appear at 605—670 nm,
showing significant red-shifts of ca. 45—67 nm. Inter-
estingly, the red-shifts of the lowest-energy absorption
edges increase with the lengthening of the n-alkyl chains
(Table 1, Figure 1b), revealing strongly enhanced mo-
lecular interaction in the solid phase with the increasing
of the side-chain length. Solution and solid state optical
band gaps are estimated from the lowest-energy absorp-
tion edges of their optical spectra,[28] and the results are
listed in Table 1. The optical band gaps of compounds
1a—1d are smaller than that of pentacene. In solution,
the n-alkyl substituted compounds 1b—1d give very
similar optical band gap. However, the thin film band
gaps of 1a—1d gradually decrease with the lengthening
of n-alkyl chains.
In contrast to the solution of pentacene, which dete-
riorated in a few minutes,[8,29] the solutions and solids of
the new materials show much higher stability in air. The
photo-stability of compounds 1a—1d were investigated
by monitoring the reduction in the absorbances at λmax in
an air-saturated 1.0×10－ mol•L－ toluene solution
under ambient light at 25 ℃ (Figure 2). Compound 1c,
whose long wavelength absorption diminished only 20%
in a one week period (t1/2≈19.4 d), possessed the high-
est stability. Compound 1b, whose absorption dimin-
ished 80% in 7 d, showed the lowest stability.
4
1
Figure 2 Photo-stability of 1a—1d monitored by recording
their absorption decrease at λmax upon exposure to ambient light
and air at 25 ℃.
Chin. J. Chem. 2012, 30, 681—688
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685

Wang et al.
FULL PAPER
The observed stability order is: 1c＞1d＞1a≈1b. The
significantly improved stabilities of the new compounds
compared to pentacene could be attributed to their much
lower HOMO levels.[27]
Electrochemical properties
Cyclic voltammograms (CV) were recorded in THF
to investigate the electrochemical properties. The CV of
compound 1a could not be recorded because of
insufficient solubility. In THF, compounds 1b—1d
exhibit two reduction peaks but no clear oxidation peak
in the electrochemical window (Figure 3). The high
reversibility of the two redox pairs in the potential range
of －0.8 to －1.6 V demonstrates the high stability of
the diketone structure upon electron injection. From the
first half-wave reduction potentials, the LUMO energies
can be estimated by taking the SCE energy level to be
－4.44 eV below the vacuum level,[16] and the results
are listed in Table 1. The low LUMO energies (≤－3.4
eV) for compounds 1b—1d suggest that they have high
electron affinities. The HOMO levels were estimated
from the LUMO levels and the optical band gaps of the
solutions (Table 1). The experimental HOMO, LUMO
energies indicate that the lengthening of n-alkyl chains
results in a gradual increase of the HOMO, LUMO
energies and reduction of the HOMO-LUMO energy
gaps, consistent with a more effective conjugation
between the heteroacene cores.[30] The LUMO levels of
the new compounds are approximately 0.5 eV lower
than that of pentacene.
Figure 3 Cyclic Voltammograms of 1b—1d.
temperatures at about 340, 295, 360 and 375 ℃, respec-
tively; comparable to pentacene, which decomposes at
330 ℃. Comparing the n-alkyl substituted compounds
1b—1d, the longer n-alkyl chain results in a higher
decomposition temperature.
All of the present compounds exhibit reversible
thermal features in DSC scans, in accordance with the
aforementioned good thermal stabilities (Figure 4). It
was found that the n-alkyl chains and the core structures
had a significant effect on the phase transition. Com-
pound 1a shows no phase transition or melting at tem-
peratures lower than the decomposition point. All the
n-alkyl substituted compounds, 1b—1d exhibit one en-
dothermic peak at the second heating cycle and one
exothermic peak at the first cooling cycle, correspond-
ing to phase change temperatures. With the lengthening
of n-alkyl chain, the transition temperatures decrease
significantly. The molecular organizations of all the four
compounds were further examined by polarized optical
microscope (POM). No texture was observed on
Thermal properties
The thermal properties of compounds 1a—1d were
investigated by thermogravimetric analysis (TGA) and
differential scanning calorimetry (DSC) (Table 1).
Compounds 1a—1d showed high thermal decomposition
Table 1 Optical absorption wavelength (λ), caculated energy gap (Eg), electrochemical properties, decomposition temperatures and DSC
measurements for compounds 1a—1d
λ/nm
Energy gap
T
DSC/℃
EHOMO
ELUMOb/eV
/
Half-life
timec/d
Compound
E1/2red b/V
TTGA/℃
Eg/eV
Eg/eV
Second
heating
First
Film Soln
(Film)a
(Soln)a
cooling
310
605
327
626
310
655
330
670
670
292
560 1.58
297
1.86
340
295
360
2.5
1a
1b
1c
—
—
—
—
－5.18/
－3.48
－5.17/
－3.45
－5.12/
－3.41
—
596 1.57
297
1.70
−0.96
—
251
170
220
151
129
1.9
598 1.53
297
1.72
19.4
603 1.48
577 1.85
1.71
2.15[31]
−1.03
147
375
330
5.1
5
1d
pentacene
—
—
—
b
a Optical HOMO−LUMO gaps determined from the onset of lowest-energy visible absorption band. Recorded E1/2 values vs SCE in
THF with TBAPF6 as supporting electrolyte. LUMO energies determined from the equation: ELUMO＝－E1/2red－4.44. c Half-life time was
obtained by fitting the data in Figure 2 according to unimolecular first-order kinetics mechanism.
686
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© 2012 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2012, 30, 681—688

Synthesis and Characterization of Planar Five-Ring-Fused Dithiophene-dione
compounds 1a and 1b. However, both 1c and 1d ex-
perienced obvious texture changes at temperatures
where the phase changes occurred in the DSC diagrams.
Figure 4b and 4c show the POM images of compounds
1c and 1d in their mesophases. Both compounds showed
a clear dendritic texture during cooling from the iso-
tropic melt. Our results reveal that flexible n-alkyl
chains with suitable length are needed to obtain liquid
crystalline phase. For device applications, it is of im-
portance that the organic semiconductor molecules can
self-assemble into long-range ordered structures, and the
alignment of conjugated liquid crystals at the interface
with the dielectric layer is critical to the OFET per-
formance.[32] The liquid crystal behaviours of com-
pounds 1c and 1d are advantageous for future device
applications.
distances for 1c and 1d are 3.255 and 3.427 Å,
respectively; lower than the sum of van der Waals
radii,[35] indicating strong π-π interactions between the
stacks. Remarkably, marked S…O short contacts (3.118
Å) were found between the neighboring columns of 1d.
The strong intraplanar S …O interactions link the
columns into layered structures. Bao[36] and Zhu[37,38]
have demonstrated that such double-channel fashion of
molecular packing could facilitate carrier transport and
result in high device performance.
Figure 4 (a) Differential scanning calorimetry (DSC) of the
compounds 1a—1d at a temperature ramp of 10 ℃•min－ under
1
N2. (b) Optical textures, as taken by polarized optimal microscopy
of 1c (at 156 ℃). (c) Optical textures, as taken by polarized op-
timal microscopy of 1d (at 145 ℃).
Figure 5 (a) X-ray crystal structure of compound 1c. (b) X-ray
crystal structure of compound 1d. n-Alkyl group and hydrogen
atoms have been omitted for clarity in packing.
X-ray single-crystal analysis
Single crystals of compounds 1c and 1d, which were
successfully obtained by slow evaporation of solution in
a chloroform/ethanol mixture, have been analyzed by
X-ray diffraction. 1c forms a triclinic unit cell and be-
longs to a P-1 space group with the crystallographic
data a＝5.1056(4) Å, b＝7.8568(6) Å, c＝15.7812(12)
Å, α＝85.276(4)°, β＝87.953(4)°, γ＝76.426(4)°, V＝
613.18(8) Å3, Z＝2, Dx＝1.253 Mg/m3, and CCDC ref-
erence number 751385. 1d forms a triclinic unit cell and
belongs to a P-1 space group with the crystallographic
data a＝5.4953(16) Å, b＝7.0700(19) Å, c＝24.359(8)
Å, α＝94.948(16)°, β＝92.776(17)°, γ＝106.352(14)°, V
＝902.1(5) Å3, Z＝2, Dx＝1.161 Mg/m3, and CCDC
reference number 751386. The main five-ring-fused
backbone shows planar structure in both crystals, while
the n-alkyl chains on the α-position of thienyl groups lie
outside of the skeleton plane (Figure 5). Both 1c and 1d
form slipped face-to-face π-stacking motifs in the
crystals. This type of molecular arrangement is expected
to give stronger electronic coupling between molecules
than the herringbone arrangement.[33,34] The interplanar
Conclusions
In conclusion, we have designed and synthesized a
new family of dithiophene-dione heteroacenes materials.
These materials exhibit much higher solubility, along
with higher thermal and photo-stability compared to
pentacene. The low LUMO energies and high
reversibility for electron injection revealed that they
have high electron affinities and are potentially
candidates for n-channel liquid-crystal semiconductors.
Further works including device fabrication are
underway in our group.
Acknowledgement
This work was supported by National Natural
Science Foundation of China (Nos. 20621091,
20872055, J0730429), Chunhui project and “111”
project.
Chin. J. Chem. 2012, 30, 681—688
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www.cjc.wiley-vch.de
687

Wang et al.
FULL PAPER
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