Effect of end-capping functional groups on the optoelectronic properties of oligothiophene derivatives

Article abstract of DOI:10.1002/cjoc.201200487

Five novel oligothiophene derivatives end-capped by different functional groups (R=ethoxyl (EtOP3T), methylsulfanyl (MSP3T), acetyl (AcP3T), methylsulfonyl (MSO₂P3T) and biphenyl (BP3T) groups) were synthesized. They were characterized by Hnuclear magnetic resonanceH (P ¹PH NMR), Hmass spectrometryH (MS) and Fourier transform Infra-red spectra (IR). The relationship between end-capping functional groups and optoelectronic properties of them was investigated. It was found that the compound with sulfonyl group in the molecular structure (MSOR₂RP3T) shows the highest oxidation stability (also supported by theoretical calculations) and best thermal stability among the five compounds. The results of scanning electron microscope (SEM) interpret that MSOR₂RP3T displays excellent ability of self-film forming. This reveals that it could be a potential candidate for thin film material. The liquid crystal property of MSOR₂RP3T was characterized by polarized optical microscopy analysis (POM) and X-ray diffraction (XRD). The results of this paper provide useful information for the design of tailored oligothiophene derivatives for devices. The five novel oligothiophene derivatives (MSO₂RP3T, MSP3T, EtOP3T, AcP3T and BP3T) with different functional groups were synthesized. The effect of end-capping groups on optoelectronic properties of the oligothiophene derivatives was explored. This paper provides useful information for design of tailored molecule through searching the relationships between functional groups and optoelectronic properties. Copyright

Full text of DOI:10.1002/cjoc.201200487

FULL PAPER
DOI: 10.1002/cjoc.201200487
Effect of End-capping Functional Groups on the Optoelectronic
Properties of Oligothiophene Derivatives
Zuo, Zhenyu(左振宇)
Sun, Hongjian(孙宏建)
Li, Xiaoyan*(李晓燕)
School of Chemistry and Chemical Engineering, Shandong University, Jinan, Shandong 250100, China
Five novel oligothiophene derivatives end-capped by different functional groups (R＝ethoxyl (EtOP3T), me-
thylsulfanyl (MSP3T), acetyl (AcP3T), methylsulfonyl (MSO2P3T) and biphenyl (BP3T) groups) were synthe-
sized. They were characterized by nuclear magnetic resonance (1H NMR), mass spectrometry (MS) and Fourier
transform Infra-red spectra (IR). The relationship between end-capping functional groups and optoelectronic prop-
erties of them was investigated. It was found that the compound with sulfonyl group in the molecular structure
(MSO
bility among the five compounds. The results of scanning electron microscope (SEM) interpret that MSO
plays excellent ability of self-film forming. This reveals that it could be a potential candidate for thin film material.
The liquid crystal property of MSO2P3T was characterized by polarized optical microscopy analysis (POM) and
R
R
2P3T) shows the highest oxidation stability (also supported by theoretical calculations) and best thermal sta-
R
R
2P3T dis-
R
R
X-ray diffraction (XRD). The results of this paper provide useful information for the design of tailored oligothio-
phene derivatives for devices.
Keywords oligothiophene, optoelectronic properties, liquid crystal
Introduction
ance, they generally show low air-stability and rapid
performance degradation.[17] This could greatly increase
the cost. Therefore, searching and developing novel
materials with both high air-stability and good device
performance is still a great challenge to materials chem-
Oligothiophene derivatives were widely used in
electrochromics,[1] organic thin film field-effect transis-
tors (OFETS),[2,3] organic light-emitting diodes
(OLEDS),[4-5] liquid crystal materials,[6] and solar
cells,[7-9] due to their excellent optical and electrical
properties. In these applications it was crucial that these
materials possess sufficient stability in ambient condi-
tion to maintain functional performance characteris-
ists. It has been reported that conjugative effect[21]
and
P
P
molecular electronic energy levels were not affected by
variations of end-capping alkyls within the same cate-
gory. Furthermore, it was also proved that bulky sub-
stituents evidently impact backbone planarity which
dominates the performance of device directly.[22] Sym-
metrically end-capping oligothiophenes (including do-
tics.[10,11]
It is clear that there is a direct relationship
P
between the structure and the optoelectronic property of
oligothiophene derivatives. Different functional groups
were introduced in the molecular backbone to tailor the
optoelectronic property and stability of the materi-
als.[12-14] But some key factors in molecular design
which lead to optimal optoelectronic properties and high
stabilities have been not known.[15] Until now, it is very
difficult to figure out what kind of the molecular struc-
ture brings optimum performance. In addition, the cate-
gory of novel oligothiophene derivatives with excellent
performance is relatively fewer.[16] In the past years,
alkyl substituents have been successfully introduced
into oligothiophene backbone to enhance solubility and
to improve molecular aggregation which could greatly
influence electronic properties of the device.[17-20] Al-
though these oligothiophene derivatives possess good
self-assembly properties and excellent device perform-
nor-oligothiophene-donor,
acceptor-oligothiophene-
acceptor and donor-oligothiophene-acceptor systems)
have been investigated, while there are still fewer ex-
amples of asymmetrically end-capping oligothiophene
derivatives. Recently, it has been shown that some
asymmetrically end-capping oligothiophene deriva-
tives[23,24] with highly ordered arrange could lead to
better charge transport compared to corresponding
symmetrically end-capping systems.[25] In this paper, we
introduced five different functional groups at the end of
the molecular structure of oligothiophene derivatives.
Methylsulfonyl and acetyl are strong electron-with-
drawing groups; methylsulfanyl and ethoxyl are elec-
tron-donating groups; biphenyl group can extend con-
jugation of the system. The effect of end-capping
groups on optoelectronic properties of the oligothio-
*
E-mail: xli63@sdu.edu.cn; Fax: 0086-531-88564464
Received May 24, 2012; accepted July 10, 2012.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201200487 or from the author.
Chin. J. Chem. 2012, 30, 2401—2410 © 2012 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Zuo, Sun & Li
FULL PAPER
phene derivatives was explored. The main purpose of
this paper is to provide useful information for design of
tailored molecule through searching the relationships
between functional groups and optoelectronic proper-
ties.
temperature for 24 h and poured into 100 mL of H2
The product was extracted with 60 mL of CH2Cl2 The
organic layer was separated, washed with 30 mL×3
H2O, dried with Na2SO4 and filtered. The solvent was
evaporated to get crude yellow product which was
re-crystallized in Et2O to afford 3TBr (6.89 g, 90%).
R
O.
R
R
R
R
R
R
R
R
Experimental
Synthesis of 2-(4-ethoxylphenyl)-2,2':5',2''-terthio-
phene (EtOP3T)
Materials and instruments
To the stirred mixture of 3TBr (1.20 g, 3.70 mmol),
2-Bromothiophene and 2,5-dibromothiophene were
purchased from the China Medicine (Group) Shanghai
Chemical Reagent Corp (Shanghai, China). All of the
organic boric acid were obtained from Sigma-Aldrich
(St. Louis, USA). All other chemicals were obtained
from commercially available sources and used without
further purification.
4-ethoxyphenylboronic acid (0.69 g, 4.20 mmol), PdCl2
(dppf) (87.48 mg, 0.12 mmol) in 150 mL of EtOH and
15 mL of H2O was added K2CO3 (1.28 g, 9.25 mmol)
under N2. The mixture was refluxed for 12 h and cooled
to room temperature. 100 mL of H2O was added, and
R
R
R
R
R
R
R
R
R
R
R
the mixture was filtered. The precipitation was collected
1H NMR spectra were recorded in chloroform-d1
and re-crystallized with THF to afford EtOP3T (0.99 g,
1
73%). m.p. 214—215 ℃; H NMR (300 MHz, CDCl3
R
R
)
(CDCl3) on a Bruker AVANCE-300 spectrometer with
R
R
δ: 1.43 (t, J＝6.9 Hz, 3H), 4.07 (q, J＝6.9 Hz, 2H), 6.91
(d, J＝8.7 Hz, 2H), 7.02 (dd, J＝3.6, 5.1 Hz, 1H), 7.07
(s, 2H), 7.10 (s, 2H), 7.17 (d, J＝3.6 Hz, 1H), 7.22 (d,
J＝5.1 Hz, 1H), 7.51 (d, J＝8.7 Hz, 2H); IR (KBr) ν:
tetramethylsilane as an internal reference. Mass spectra
were performed on a MALDI-TOF-MS system. Infra-
red spectra were conducted on Nujol mulls between
KBr disks with Bruker ALPHA-T spectrophotometer.
UV-vis spectra were measured by using HITACHI
U-4000 spectrometer. Thermogravimetric analysis
(TGA) measurement was performed under nitrogen at-
mosphere at a heating rate of 10 ℃/min using TA Q500
instrument, and the differential scanning calorimetric
(DSC) measurement was made using TA Q100 instru-
ment operated under nitrogen atmosphere. Polarized
optical microscopy analysis (POM) was conducted on
Olympus BX51-P. Cyclic voltammetry (CV) measure-
ment was performed on an AUTOLAB/PGSTAT12 at
room temperature with a three-electrode cell in a solu-
－
1
2918, 1602, 1513, 1283, 1048, 793 cm ; MS (MALDI-
TOF-MS) calcd for C20
R
R
H16
R
R
OS3
R
368.04, found 368.10.
Synthesis
of
2-(4-methylthiophenyl)-2,2':5',2''-
terthiophene (MSP3T)
To the stirred mixture of 3TBr (1.20 g, 3.70 mmol),
4-(methylthio)phenylboronic acid (0.71 g, 4.20 mmol),
PdCl2(dppf) (87.48 mg, 0.12 mmol) in 150 mL of EtOH
and 15 mL of H2O was added K2CO3 (1.28 g, 9.25
mmol) under N2. The mixture was refluxed for 12 h and
cooled to room temperature. 100 mL of H2O was added,
and the mixture was filtered. The precipitation was col-
lected and re-crystallized with THF to afford MSP3T
tion of 0.1 mol•L－ tetrabutylammonium perchlorate in
1
1
anhydrous chloroform at room temperature under nitro-
gen gas with a scan rate of 50 mV/s. The working elec-
trode was platinum, the counter electrode was a plati-
num wire, and the reference electrode was Ag/AgCl
(1.15 g, 84%). m.p. 245—246 ℃; H NMR (300 MHz,
CDCl3) δ: 2.52 (s, 3H), 7.03 (dd, J＝3.6, 5.1 Hz, 1H),
R
R
7.09 (s, 2H), 7.13 (d, J＝3.9 Hz, 1H), 7.18 (d, J＝3.9
Hz, 1H), 7.20 (d, J＝3.6 Hz, 1H), 7.23 (d, J＝5.1 Hz,
1H), 7.26 (d, J＝8.7 Hz, 2H), 7.52 (d, J＝8.7 Hz, 2H);
－
1
(0.10 mol•L ) which was separated by a diaphragm.
The measurements were calibrated using ferrocene as a
standard. The X-ray diffraction (XRD) were conducted
on a Bruker D8 advanced diffractometer with Cu Kα
radiation (λ＝1.5406 Å). Scanning electron microscope
(SEM) measurements were carried out on a Hitachi
S-4800 field emission scanning electron microscope.
Melting point measurements were performed on the
WRR melting points instrument of SHANGHAI
SHENGGUANG INSTRUMENT CO. LTD.
－
1
IR (KBr) ν: 2922, 1596, 1403, 1053, 790 cm ; MS
(MALDI-TOF-MS) calcd for C19
369.80.
R
H14S4 370.00, found
R
Synthesis of 2-(4-acetylphenyl)-2,2':5',2''-terthio-
phene (AcP3T)
To the stirred mixture of 3TBr (1.20 g, 3.70 mmol),
4-acetylphenylboronic acid (0.68 g, 4.20 mmol),
PdCl2(dppf) (87.48 mg, 0.12 mmol) in 150 mL of EtOH
and 15 mL of H2O was added K2CO3 (1.28 g, 9.25
mmol) under N2. The mixture was refluxed for 12 h and
cooled to room temperature. 100 mL of H2O was added,
and the mixture was filtered. The precipitation was col-
lected and re-crystallized with THF to afford AcP3T
Synthesis of 2,2':5',2''-terthiophene (3T)
According to previously reported method,[26] 3T was
obtained as a yellow powder in the yield of 88%.
1
Synthesis
(3TBr)[27]
of
5-bromo-2,2':5',2''-terthiophene
(1.17 g, 87%). m.p. 241—242 ℃; H NMR (300 MHz,
CDCl3) δ: 2.61 (s, 3H), 7.03 (dd, J＝3.6, 5.4 Hz, 1H),
R
R
To the solution of 3T (5.80 g, 23.40 mmol) in 100
mL of DMF was added NBS (4.79 g, 26.90 mmol) in
the absence of light. The mixture was stirred at room
7.10 (d, J＝3.9 Hz, 1H), 7.13 (d, J＝3.9 Hz, 1H), 7.18
(d, J＝3.9 Hz, 1H), 7.19 (d, J＝5.4 Hz, 1H), 7.24 (d,
J＝5.4 Hz, 1H), 7.36 (d, J＝3.9 Hz, 1H), 7.66 (d, J＝
2402
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Chin. J. Chem. 2012, 30, 2401—2410

Effect of End-capping Functional Groups on the Optoelectronic Properties of Oligothiophene Derivatives
8.4 Hz, 2H), 7.97 (d, J＝8.4 Hz, 2H); IR (KBr) ν: 2924,
1667, 1598, 1404, 1047, 792 cm ; MS (MALDI-TOF-
Scheme 1 Synthetic route of five oligothiophene derivatives
－
1
NBS
DMF
S
S
MS) calcd for C20
R
H14OS3 366.02, found 365.77.
R
R
Br
S
S
S
R
;
S
Synthesis of 2-(4-methylsulfonylphenyl)-2,2':5',2''-
3T
3TBr
terthiophene (MSO2P3T)
R
R
To the mixture of 3TBr (1.20 g, 3.70 mmol),
4-methylsulfonylbenzeneboronic acid (0.84 g, 4.20
R
B(OH)2
S
S
S
Pd catalyst
mmol) and Pd(PPh3 4
mL of THF was added 50 mL of 2 mol•L－ K2
der N2. The mixture was refluxed for 6 h, cooled to
R
)
R
R
(46.24 mg, 0.04 mmol) in 100
1
R
R
CO3 un-
RP3T
MSP3T
R
O
S
;
;
;
R =
room temperature and filtered. The filtrate was con-
densed and filtered again. All precipitation was col-
73%
84%
EtOP3T
Et
Me
O
O
lected and washed with H
R
R
2O, methanol and acetone to
AcP3T 87%
MSO2P3T
81%
S
Me
Me
get MSO2P3T (1.20 g, 81%). Phase transition tempera-
R
O
1
ture 263—272 ℃; H NMR (300 MHz, CDCl3) δ: 3.08
(s, 3H), 7.04 (dd, J＝3.6, 5.1 Hz, 1H), 7.12 (d, J＝3.9
Hz, 1H), 7.16 (d, J＝3.6 Hz, 1H), 7.19 (d, J＝3.9 Hz,
1H), 7.21 (d, J＝3.6 Hz, 1H), 7.25 (d, J＝5.1 Hz, 1H),
7.38 (d, J＝3.9 Hz, 1H), 7.76 (d, J＝8.7 Hz, 2H), 7.95
(d, J＝8.7 Hz, 2H); IR (KBr) ν: 2925, 1590, 1401, 1298,
1149, 790 cm－1; MS (MALDI-TOF-MS) calcd for
BP3T
77%
Ph
UV-Vis absorption spectroscopy
The photophysical properties of the five oligothio-
phene derivatives were studied by using UV-Vis and
photoluminescence (PL) spectroscopy. UV Absorption
and fluorescence emission spectra of the five oligothio-
C19
R
R
H14
R
R
O2
R
R
S4, 401.99, found 402.00.
R
phene derivatives were determined in CHCl3. The re-
R
R
sults were illustrated in Figures 1 and 2. The maximum
absorption wavelength (λmax) values of these compounds
were listed in Table 1. In comparison with that of 3T
Synthesis of 2-(biphenyl)-2,2':5',2''-terthiophene
(BP3T)
(λmax
R
R
=354 nm),[35] the λmax
values of these derivatives
R
R
To the stirred mixture of 3TBr (1.20 g, 3.70 mmol),
4-phenylbenzeneboronic acid (1.23 g, 4.20 mmol) and
Pd(PPh3)4 (46.24 mg, 0.04 mmol) in 100 mL of THF
red-shifted by at least 25 nm. This demonstrates that the
introduction of phenyl group would greatly enhance
conjugated effect of oligothiophene. Compared to
MSP3T and EtOP3T (with electron-donating groups),
was added 50 mL of 2 mol•L－ K2CO3 under N2. The
1
mixture was refluxed for 6 h, cooled to room tempera-
ture and filtered. The filtrate was condensed and filtered
again. All precipitation was collected and washed with
H2O, methanol and acetone to get BP3T (1.14 g, 77%).
AcP3T and MSO2
R
R
P3T (with electron-withdrawing
groups) enjoyed higher[36] λmax
R
R
values (red shifts of
more than 15 nm). This result can be interpreted by
conjugated effect. There is P-Π conjugation between the
substituent and aromatic system for MSP3T and
EtOP3T while there is Π- Π conjugation between the
additional double bonds and aromatic systems for
m.p. 276—278 ℃; 1H NMR (300 MHz, CDCl3
) δ: 7.04
R
R
(dd, J＝3.6, 5.1 Hz, 1H), 7.12 (dd, J＝2.4, 3.9 Hz, 2H),
7.17—7.28 (m, 4H), 7.36 (d, J＝9.0 Hz, 1H), 7.46 (t,
J＝6.0 Hz, 2H), 7.61—7.70 (m, 6H); IR (KBr) ν: 1598,
－
1
AcP3T and MSO2
R
R
P3T. Unexpectedly, the λmax
R
R
values
1406, 1052, 833, 793 cm ; MS (MALDI-TOF-MS)
of both AcP3T and MSO2
R
R
P3T were higher than BP3T,
calcd for C24
R
H16S3 400.04, found 400.00.
R
although they lack a phenyl group in conjugated chain.
It might be inferred that additional phenyl group makes
a small contribution to conjugated effect in backbone of
BP3T. This conclusion may be explained by the hy-
pothesis that there is partial twisting between the two
benzene rings in BP3T. This hypothesis is supported by
geometry for the most stable conformation obtained
from optimization calculations. As shown in Figure 3,
the torsional angle θ (the deviation from coplanarity
between the two benzene rings) was 36.6°. The emis-
sion maximums and Stokes shifts of the five oligothio-
phene derivatives were listed in Table 1. Compared with
Results and Discussion
Synthesis and characterization of five oligothiophene
derivatives
There are several pathways to synthesize oligothio-
phene derivatives. The methods include Fe- or Cu-
catalyzed[28,29] homo-coupling, Pd(II)/AgF-catalyzed[30]
homo-coupling, Suzuki, Kumada,[31] Stille,[32] Negishi[33]
coupling and so on. In this paper, the five compounds
were prepared by Suzuki coupling reaction (Scheme
1).[34] Filtration and re-crystallization were used to
achieve high purity. All of the five products are light
yellow powders and easily dissolve in THF. They were
the data of absorption wavelength (λabs(max)), similar
R
trend (electron-withdrawing groups lead to the higher
λmax) was observed in the data of fluorescence emission
1
completely characterized by H NMR, MS and IR (see
wavelength (λem(max)
R
) except for EtOP3T. EtOP3T ex-
and biggest Stocks shifts. The
the supporting information).
hibited the highest λem(max)
R
Chin. J. Chem. 2012, 30, 2401—2410
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Zuo, Sun & Li
abs(max)), fluores-
FULL PAPER
plausible reasons for this result are as follows. Among
the five substituents, ethoxyl is the most powerful elec-
tron-donating group and it is connected with the aro-
matic system only by σ bond. This endows EtOP3T
with enough flexibility and therefore, increases the
chances of collision between EtOP3T and solvent
molecules. Furthermore, powerful electron-donating
group can also lead to energy loss in the internal struc-
Table 1 Optical absorption wavelength ((λ
R
cence emission wavelength (λ
R
em(max)) and Stokes Shifts for five
oligothiophene derivatives
Compd.
MSO2P3T
AcP3T
λabs(max)/nm
402
λem(max)/nm Stokes shifta/nm
506
509
485
483
521
104
110
97
399
BP3T
388
tural rotation.[37]
These reasons may obviously enhance
P
MSP3T
EtOP3T
383
100
142
the energy loss during the relaxation process and effi-
379
a Stokes shift was the difference between λem(max) and
R
λabs(max).
R
cient fluorescence. Therefore, EtOP3T displayed the
highest λem
R
and exhibited biggest stocks shifts.
MSO2 P3T
0.5
0.4
BP3T
AcP3T
EtOP3T
0.3
0.2
0.1
0.0
MSP3T
Figure 3 rb3lyp/6-31g (d, p) optimized structure of BP3T.
R
R
200 250 300 350 400 450 500 550 600
Wavelength/nm
Table 2 Optical and electrochemical data for five oligothio-
phene derivatives
Optical energy band
Figure 1 UV-Vis spectra of five compounds in CHCl3.
R
R
Compd. HOMO/eVLUMO/eV
Eox(onset)/V
gap (Eg)/eV
2.67
EtOP3T
－2.94
－2.88
－2.69
－2.62
－2.60
1.23
1.20
1.05
1.01
0.99
MSO2P3T －5.61
600
500
400
300
200
100
0
MSO P3T
2.70
AcP3T
BP3T
－5.58
－5.43
－5.39
－5.37
2
2.74
AcP3T
2.77
MSP3T
EtOP3T
BP3T
2.77
phase transition behaviors and their melting points were
about 216 ℃ (EtOP3T), 245 ℃ (MSP3T), 241 ℃
(AcP3T), 263 ℃ (MSO2P3T) and 275 ℃ (BP3T).
These data are consistent with those measured with
melting apparatus in experimental part. Except for
MSP3T
R
R
400
450
500
550
600
650
700
AcP3T and MSO2P3T, the other three compounds ex-
R
R
Wavelength/nm
hibited reversible peaks in DSC curves. BP3T enjoyed
the highest melting point probably resulting from addi-
tional conjugated phenyl group. Compared to AcP3T
and EtOP3T, MSO2P3T and MSP3T exhibited the
higher melting points possibly resulting from the larger
atomic weight of sulfur. It was found out that, as for
Figure 2 Fluorescence spectra of five compounds in CHCl3.
According to the literature,[38] the optical energy
band gaps (Eg) of the five oligothiophene derivatives
were calculated from data of the absorption edge of
R
R
MSO2
process and only one exothermic peak in the cooling
cycle. It is inferred that MSO2P3T may be a liquid
R
P3T, the two peaks were observed in heating
their UV-Vis spectra in Figure 1. Their Eg values are
R
R
listed in Table 2.
R
R
Thermal properties
crystal material and exhibits liquid crystal properties.
This was confirmed by polarized optical microscopy
analysis (POM) (see also later section). In addition, the
exothermic peak of MSO2P3T was relatively sharp and
strong. This demonstrates good crystallizability. This is
a key factor in the formation of well-ordered thin
The thermal properties of the five oligothiophene
derivatives were characterized by thermogravimetric
analysis (TGA) and differential scanning calorimetry
(DSC) under nitrogen atmosphere. The DSC thermo-
grams (Figure 4) of the derivatives presented different
R
2404
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Chin. J. Chem. 2012, 30, 2401—2410

Effect of End-capping Functional Groups on the Optoelectronic Properties of Oligothiophene Derivatives
Figure 4 DSC thermograms of five compounds: (a) BP3T; (b) MSP3T; (c) EtOP3T; (d) AcP3T; (e) MSO2P3T.
R
R
films.[39] As showed in Figure 5, the decomposition
Electrochemical properties
temperatures of five compounds were 308 ℃ (EtOP3T),
312 ℃ (MSP3T), 326 ℃ (AcP3T), 342 ℃ (MSO2P3T)
The electrochemical measurements of the five oligo-
thiophene derivatives were carried out in chloroform
with ferrocene as internal reference. As shown in Figure
6, the five derivatives exhibited different cycle plots. It
is evident that the five derivatives all exhibited irre-
versible redox peaks. Therefore it is impossible to ac-
curately determine their HOMO and LUMO energies.
The HOMO energies were calculated with the em-
pirical equation (EHOMO＝4.38＋eEox(onset)).[40] The
HOMO energies were estimated through the combina-
tion of HOMO energies and optical energy band gaps
(Eg). These data were listed in Table 2. Compared to the
R
R
and 330 ℃ (BP3T) respectively. It was known that
large molecule may enhance its thermal stability,[39] and
the onset decomposition temperature of linear
α,ω-dihexylsexithiophene (DH6T) was at around 309
℃.[21] By comparing the data, it can be inferred that the
substituted groups effectively improve the thermal sta-
bilities of oligothiophene derivatives. Compared to
BP3T, MSO2P3T exhibits higher onset decomposition
R
R
temperature although it lacks a phenyl group in the
conjugated unit of molecular chain.
Chin. J. Chem. 2012, 30, 2401—2410
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Zuo, Sun & Li
FULL PAPER
X-ray diffraction patterns (XRD) of MSO2P3T were
R
R
BP3T
shown in Figure 9. Before annealing there were only
some peaks in the range of wide angle (2θ＞18°). This
demonstrates only crystal-like 3-D structures. After an-
nealing a single diffraction peak appeared in small-an-
gle region as well as several peaks in wide-angle region.
The Bragg reflection (a diffraction peak at 2θ＝4.36°)
corresponds with a d-spacing of 20.22 Å demonstrating
the lamellar ordering. It was noted that the diffraction
peaks became relatively sharp and strong after the ther-
mal annealing treatment from liquid crystal state. This
demonstrates the better packing arrangement. This
property is favorable for charge transport in organic thin
film field-effect transistors.
100
80
60
40
20
0
MSO P3T
2
AcP3T
MSP3T
EtOP3T
-20
100
200
300
Temperature/OC
400
500
In order to estimate effects of substituents on the
charge transport, it is useful to calculate HOMO and
LUMO of the five compounds. The orbital calculations
were carried out by the density functional theory (DFT).
The geometry optimization was calculated at rb3lyp/
6-31g (d, p). All the calculations were conducted with
GAUSSIAN 03 suite of programs. The optimized
structures and frontier orbital of the five oligothiophene
derivatives were shown in Figure 10. The calculated
energy values of LUMO, HOMO and energy band gap
were listed in Table 3. By comparing the data, there are
Figure 5 TGA plots of oligothiophene derivatives.
other compounds, MSO2P3T with relatively higher
R
oxidation potential exhibits enhanced oxidation stability.
This is favorable for application in optoelectric devices.
Analysis of SEM
The films of the five oligothiophene derivatives were
－
fabricated by dropping 0.01 mL (1×10－ mol•L ) of
4
1
the sample solution (dissolved in CHCl3) onto alumi-
R
R
num foil after the evaporation of the solvent for scan-
ning electron microscope (SEM) (Figure 7). Before
testing, platinum powder was sprinkled onto the surface
of the film. It is clear that the self-aggregation of the
molecules is mainly dominated by ∏-∏ stacking and
van der waals interactions. As shown in Figure 6, the
some significant deviations. (Taking MSO2
example, Eg(experimental)＝2.67 eV, Eg(caculated)＝3.18 eV,
HOMO(experimental)＝－5.61 eV, EHOMO(caculated)＝－5.35 eV)
R
R
P3T for
E
For the five compounds, the energy band gap values of
experimental results are smaller than the corresponding
calculated data. The energy values of LUMO and
HOMO in experimental results are lower than that of
the calculated. These deviations mainly result from two
reasons. On the one hand, theoretical calculations were
conducted on systems in vacuo and on a single freezed
conformation,[43] regardless of intermolecular interac-
tion, solvent effect and conformational mobility; on the
other hand, the energy band gap was an approximate
estimation of the transition energy containing significant
contributions from some two-electron integrals.[44] As
shown in Table 3, the results reflected a trend that the
molecule of MSO2P3T assembled to form big flaky
R
R
films which linked with each other and displayed spiral
fashion. Nevertheless, in the graphs of BP3T, AcP3T
and MSP3T, the films were composed of isolated and
circular feature with some cracks. It is evident that the
film of MSO2
among them. This plays an important role in efficient
charge transport. It can be concluded that MSO2P3T
R
R
P3T is the best in smooth and continuity
R
R
displays stronger self-film forming ability. This proba-
bly results from the effect of methylsulfonyl group.
Liquid crystal properties of MSO2
R
R
P3T
electron-withdrawing groups (MSO2P3T and AcP3T)
R
R
lead to the lower HOMO energy and smaller band gap.
Lower HOMO energy is related to improved elec-
tro-affinity[45] and oxidation stability. The smaller band
gap (a critical parameter determining molecular elec-
tronic transport properties) demonstrates that the con-
The polarized optical microscopy analysis (POM)
and X-ray diffraction (XRD) were utilized to study the
liquid crystal properties of MSO2
R
R
P3T. As shown in
Figure 8, when the liquated MSO2P3T was cooled from
R
R
270 to 265 ℃ at the rate of 1 ℃/min [Figure 8, (a)],
only some defect-like droplets[41] were observed as ini-
tial status of liquid crystal. With the increase of the
cooling rate, well-defined textures of liquid crystal ap-
peared [Figure 8, (b) and (c)]. When cooled at the rate
of 3 ℃/min, the textures were dense and obscure. Thus,
the cooling rate of 2 ℃/min is optimal for formation of
liquid crystal. The liquid crystal mesophases of other
compounds were not observed. This result also confirms
that the terminal methylsulfonyl group (SO2Me) was an
important factor in the formation of liquid crystal
phase.[42]
ductivity of MSO2P3T and AcP3T is greater than that
R
Table 3 The calculated LUMO, HOMO energies and energy
band gaps of five compounds
Compd.
MSO2P3T
AcP3T
HOMO/eV LUMO/eV Energy band gap/eV
－5.35
－5.24
－5.04
－4.97
－4.91
－2.17
－2.15
－1.84
－1.77
－1.67
3.18
3.09
3.20
3.20
3.24
BP3T
MSP3T
EtOP3T
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Chin. J. Chem. 2012, 30, 2401—2410

Effect of End-capping Functional Groups on the Optoelectronic Properties of Oligothiophene Derivatives
Figure 6 Cyclovoltammograms of five compounds in 0.1 mol•L－ [Bu4
R
R
N]＋[ClO4
] solution in chloroform at a scan rate of 50 mV/s
－
R
R
1
respectively: (a) EtOP3T; (b) AcP3T; (c) MSO2P3T; (d) MSP3T; (e) BP3T.
R
R
of the other compounds.[46] This trend is also in agree-
ment with that in experimental data in Table 2. In ex-
in their ground states.[47] The electron density of the
LUMO of the five compounds exhibits bonding charac-
ter between the subunits. There is a clear charge transfer
in particular a change on the charge density around the
C—C bonds connecting the thiophene rings. This indi-
cates that the LUMO shows more planar and enhanced
delocalization. Obviously, the electron density of the
HOMO and LUMO show some overlap. It is a key fac-
tor[48] for effective intramolecular charge transfer. As for
perimental section, MSO2
R
P3T (EHOMO＝－5.61 eV,
HOMO＝－5.58 eV, Eg
E
R
g＝2.67 eV) and AcP3T (E
R
R
＝
2.70 eV) also exhibited the lower HOMO energy and
smaller band gap compared to other three compounds.
As shown Figure 10, the electron density of HOMO is
mainly distributed in carbon double bonds backbone of
the conjugated aromatic rings. A very small contribution
from the five substituents is observed. The HOMO of
the five compounds presents antibonding character be-
tween the subunits. This indicates nonplanar character
AcP3T and MSO2P3T, the effects of substituents are
R
R
obvious on the LUMO because the bonds connecting
the substituents and conjugated systems are surrounded
Chin. J. Chem. 2012, 30, 2401—2410
© 2012 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.cjc.wiley-vch.de
2407

Zuo, Sun & Li
FULL PAPER
Figure 7 SEM micrographs of five oligothiophene derivatives: (a) BP3T; (b) MSP3T; (c) EtOP3T; (d) AcP3T; (e) MSO2P3T.
R
R
Figure 8 POM textures of MSO2P3T, cooled from 270 to 265 ℃. (a) 1 ℃/min.; (b) 2 ℃/min.; (c) 3 ℃/min.
R
R
Figure 9 XRD of MSO2P3T: (a) before annealing at room temperature; (b) after annealing at room temperature.
R
R
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Chin. J. Chem. 2012, 30, 2401—2410

Effect of End-capping Functional Groups on the Optoelectronic Properties of Oligothiophene Derivatives
Theoretical calculations
and Qingbin Xue for CV, XRD and POM analysis.
Compd.
HOMO
LUMO
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stituents are not observed on the LUMO of the other
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R
R
candidate for thin film materials.
Conclusions
In summary, the five novel oligothiophene deriva-
tives (MSO2P3T, MSP3T, EtOP3T, AcP3T and BP3T)
R
R
with different functional groups were synthesized. The
relationship between end-capping groups and optoelec-
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R
R
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R
R
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R
R
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R
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This work was supported by the academic program
of Shandong Province (No.2011GGX1023). We ac-
knowledge the support of Prof.s Houyi Ma, Xiuling Jiao
Chin. J. Chem. 2012, 30, 2401—2410
© 2012 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Chin. J. Chem. 2012, 30, 2401—2410