Syntheses, phase behavior, supramolecular chirality, and field-effect carrier mobility of asymmetrically end-capped mesogenic oligothiophenes

Article abstract of DOI:10.1002/chem.200802470

A novel series of asymmetrically end-capped mesogenic oligothiophenes, with various oligothiophene core lengths, alkoxy tail lengths, and molecular polarities through introducing alkylsulfanyl or alkylsulfonyl functionalities as the terminal group, have b

Full text of DOI:10.1002/chem.200802470

DOI: 10.1002/chem.200802470
Syntheses, Phase Behavior, Supramolecular Chirality, and Field-Effect
Carrier Mobility of Asymmetrically End-Capped Mesogenic Oligothiophenes
Qingwei Meng,[a] Xiao-Hua Sun,[b] Zhengyu Lu,[a] Ping-Fang Xia,[b] Zehua Shi,[a]
Dongzhong Chen,*[a] Man Shing Wong,*[b] Salem Wakim,[c] Jianping Lu,[c]
Jean-Marc Baribeau,[c] and Ye Tao*[c]
Abstract: A novel series of asymmetri-
cally end-capped mesogenic oligothio-
phenes, with various oligothiophene
core lengths, alkoxy tail lengths, and
molecular polarities through introduc-
ing alkylsulfanyl or alkylsulfonyl func-
tionalities as the terminal group, have
been synthesized by palladium-cata-
lyzed Suzuki cross-coupling and
Kumada cross-coupling reactions as
key steps. For the single end-capped
oligothiophenes, CmO-Ar-OT(4)-H in
which m=10, 12, 14, 16, and 18, all of
these oligomers exhibited a broad tem-
perature range of highly ordered smec-
tic E and enantiotropic nematic phases,
apart from the one with the longest oc-
tadecyloxy tail. For the double end-
capped series C10O-Ar-OT(n)-R, R=
Ph-SC6 or Ph-SO2C6 in which n=1, 2,
3, and 4, oligomers with more than one
thiophene ring exhibited smectic A and
smectic C phases, various crystal poly-
morphs and/or unusual low-tempera-
ture condensed phases. In the nonpo-
lar, alkylsulfanylphenyl-substituted oli-
gothiophene series, both the crystal/
solid melting point and mesogenic
clear point increased significantly with
an increasing oligothiophene conjuga-
tion length. In the polar, alkylsulfonyl-
cooling from their preceding fluidic
smectic phases. The unusual twisted
smectic layer structures in the thin
solid films exhibiting distinct supra-
molecular chirality of both handedness-
es, revealed by circular dichroism
measurements, were further confirmed
by XRD analyses characterized by a
sharp layer reflection together with its
higher orders and diffuse wide-angle
scatterings. In addition, initial studies
showed that the highly ordered smectic
phase of the single end-capped oligo-
thiophenes can be utilized to improve
field-effect charge mobility. C10O-Ar-
OT(4)-H showed a hole mobility of
0.07 cm2 Vꢀ1 sꢀ1 when deposited on oc-
tyltrichlorosilane-treated substrates at
1408C and the on/off current ratios
reached 5ꢀ105; on the other hand, its
mobility was only 8ꢀ10ꢀ3 cm2 Vꢀ1 sꢀ1
on the same substrate when deposited
at room temperature.
phenyl-substituted
oligothiophene
series, all the oligomers showed in-
creased melting points, but decreased
mesogenic temperature intervals than
those of their corresponding alkylsul-
fanyl counterparts. Remarkably, two
different helical structures showing dis-
tinct striated textures or striped pat-
terns were observed with a pitch of
several to tens of micrometers under a
polarized optical microscope upon
Keywords:
liquid
crystals
·
materials science · oligothiophenes ·
semiconductors
chirality
·
supramolecular
[a] Dr. Q. Meng, Z. Lu, Z. Shi, Prof. Dr. D. Chen
[c] Dr. S. Wakim, Dr. J. Lu, Dr. J.-M. Baribeau, Dr. Y. Tao
Institute for Microstructural Sciences (IMS)
National Research Council of Canada
Ottawa K1AOR6 ON (Canada)
Key Laboratory of Mesoscopic Chemistry of Ministry of Education
Department of Polymer Science and Engineering
School of Chemistry and Chemical Engineering
Nanjing University, Nanjing 210093 (China)
Fax : (+86)25-8331-7761
Fax : (+1)613-990-0202
E-mail: Ye.Tao@nrc-cnrc.gc.ca
E-mail: cdz@nju.edu.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200802470.
[b] Dr. X.-H. Sun, P.-F. Xia, Prof. Dr. M. S. Wong
Department of Chemistry and Centre for Advanced
Luminescence Materials
Hong Kong Baptist University, Kowloon Tong
Hong Kong (S.A.R. China)
Fax : (+852)3411-7348
E-mail: mswong@hkbu.edu.hk
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Introduction
are of higher order and are solution processable; thus, they
are considered to be realistic candidates for industrial appli-
cations as OFETs for cheap, large area, smart electronic de-
vices.[9b,c,11,14]
Oligothiophenes are an attractive class of organic semicon-
ducting materials for applications as organic field-effect
transistors (OFETs), organic light-emitting diodes
(OLEDs), and photovoltaic (PV) cells because of their high
solid-state ordering and good charge-carrier mobility.[1]
Many nonsubstituted and a,w-substituted oligothiophenes
have been synthesized and applied for OFET applications
recently.[2,3] Most of them exhibit good crystallinity and
form polycrystalline films by vacuum deposition;[4] on the
other hand, some symmetrically and asymmetrically dialkyl-
substituted oligothiophenes exhibited mesophases and
showed high charge-carrier mobility.[3] It is well known that
for OFET applications, proper molecular alignment in the
crystalline solid can give rise to high charge mobility and a
low operating voltage; on the other hand, amorphous mate-
rials often give low charge mobility. Nevertheless, the use of
polycrystalline materials for practical applications is often
hampered by grain boundary effects, which cause electrically
active localized states and dramatically decrease the charge-
carrier mobility.[5] The potential of liquid-crystalline semi-
conductors for OFETs has recently been recognized because
of their self-assembling nature, anisotropic transport, and
high charge-carrier mobility.[6] Discotic[7] and smectic liquid
crystals[3,8–11] are also known for their capabilities in self-
healing structural defects and self-organizing into large
structurally homogeneous domains, as well as exhibiting ex-
cellent charge-carrier transport properties. As a result, there
has been considerable interest in exploring liquid-crystalline
oligothiophenes for various optoelectronic applications.
Most known mesogenic oligothiophenes are generally de-
rived from the symmetrically disubstituted oligothiophenes
with a structural motif of donor–oligothiophene–donor or
acceptor–oligothiophene–acceptor, asymmetrically disubsti-
tuted donor–acceptor-type liquid-crystalline oligothiophene
has rarely been reported.
In smectic liquid crystals, the charge-carrier mobility is
mainly dependent on the molecular distance within a smec-
tic layer, in which the calamitic molecules address and the
charge carriers hop from one molecule to the other. It is de-
sirable that the domain boundary effects in the smectic
phases are very small.[9a,10] In principle, high carrier mobility
could be achieved from a large p-conjugated system exhibit-
ing highly ordered smectic mesophases; however, ever en-
hanced intermolecular interactions between larger p-conju-
gated molecules usually result in crystallization and the
above-mentioned polycrystalline boundary effects. An asym-
metric molecular structure tends to reduce the transition
temperature and extend the mesophase range in the liquid
crystals.[3f,i] On the other hand, with an extension of p conju-
gation, the hybridized co-oligomers of thiophene and phen-
ylene have been developed as a new class of light-emitting
materials[12] and easily processable OFETs.[11,13] In addition,
compared with their polymer analogues, well-defined oligo-
meric liquid crystalline semiconducting materials with high
purity, precise chemical structure, and conjugation length
Supramolecular chirality formed from the assembly of
achiral molecules through noncovalent interactions is highly
important and intriguing for its crucial role in biological
phenomena, such as molecular recognition, as well as in in-
formation storage and potential applications in materials sci-
ence.[15] Chiral symmetry breaking from achiral molecules is
known to occur in controlled crystals,[15c] bent-core mole-
cules,[15e,f,16] in a unique cooperative stereoregular arrange-
ment or helical supramolecular organization through coordi-
nation with metal ions[17,18a,b] or through hydrogen bon-
ding.[15d,h,18c,d] In addition to the interfacial constraint and
the spontaneous overcrowded packing, the p–p stacking be-
tween the rigid aromatic cores and the van der Waals inter-
actions among the alkyl chains are considered to be impor-
tant interactions for many robust supramolecular assemblies
that give rise to the chiral helical superstructure.[18] Nano-
scopic and mesoscopic order in p-conjugated systems is a
topic of utmost importance, while the detailed understand-
ing of the supramolecular interactions between the p-conju-
gated molecules is still one of the most challenging scientific
research areas.[19]
We report herein the synthesis and investigation of ther-
motropic liquid-crystalline structures and properties of a
novel series of asymmetrically end-capped oligothiophenes
by means of modifying the oligothiophene core length,
alkoxy tail length, and molecular polarity by introducing an
alkylsulfanyl or alkylsulfonyl functionality as the terminal
group. The general design principle of these new mesogens
is based on a compromise between the order introduced by
the p–p stacking of the aromatic cores and the disorder
caused by the flexible alkyl substituents.[3h–j] In the present
studies, we explore two different strategies of tuning the
liquid-crystal phase structure and transition temperatures.
One is to systematically change the oligothiophene core
length with predefined asymmetric end substituents, whereas
the other is to alter the length of the alkoxy chain tethered
to the predefined rigid core, arylene-capped quaterthio-
phene. All of these oligomers are components of three
novel series of asymmetrically end-capped oligothiophenes
as shown in Scheme 1. We have demonstrated for the first
time that unusual layered condensed phases with supra-
molecular chirality have been achieved from some of the
newly synthesized achiral double end-capped oligothio-
phenes upon cooling from their smectic phases. In addition,
the initial OFET measurements on the single end-capped
oligothiophenes with highly ordered smectic phases show
high charge mobility.
Results and Discussion
Synthesis: The asymmetrically end-substituted oligothio-
phenes, CmO-Ar-OT(n)-R’ (Scheme 1), in which R’=H, Ph-
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D. Chen, M. S. Wong, Y. Tao et al.
SC6, or Ph-SO2C6 with various oligothiophene core lengths,
alkoxy tail lengths, and molecular polarities through intro-
ducing alkylsulfanyl or alkylsulfonyl functionalities as the
terminal group were synthesized by palladium-catalyzed
Kumada cross-coupling and Suzuki cross-coupling reactions
as key steps as shown in Scheme 2. Previously, the arylene
end-capped quaterthiophene was synthesized by a linear ap-
proach using stepwise Kumada cross-couplings for chain-
length extension.[20] To improve the operation and efficiency
further, the convergent approach was developed to synthe-
size terthienyl and quaterthienyl skeletons by means of
Suzuki cross-coupling reactions. The previously published
protocols were followed for the synthesis of arylene end-
capped thiophene 1 and bithiophene 2. The palladium-cata-
lyzed Suzuki cross-coupling of arylene end-capped bromo-
thiophene or bromobithiophene with thiopheneboronic acid
or bithiopheneboronic acid using PdACTHNUGTRENNUG(OAc)2/2PACHTUNGTREN(NUGN o-tol)3 as a
catalyst afforded arylene end-capped terthiophene 3 and
quaterthiophene 4, respectively, in good yields. By adopting
the same synthetic approach and using various alkyl halides
Scheme 1. Molecular structures of mesogenic oligothiophenes.
Scheme 2. Synthesis of asymmetrically arylene end-capped oligothiophenes.
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Asymmetrically End-Capped Mesogenic Oligothiophenes
FULL PAPER
including dodecyl, tetradecyl, hexadecyl, and octadecyl bro-
mides for the alkylation of 4-bromo-2,6-dimethylphenol, oli-
gomer 5–8 (CmO-Ar-OT(4)-H, in which m=12, 14, 16, and
18) were prepared in good yields. The Suzuki cross-coupling
of arylene end-capped bromooligothiophenes with 4-(hexyl-
sulfanyl)phenylboronic acid, which was prepared by lithi-
um–bromide exchange at ꢀ788C and quenching with tri-
methyl borate, afforded the corresponding asymmetrically
disubstituted oligothiophenes, 9–12, C10O-Ar-OT(n)-Ph-SC6
in which n=1–4, respectively. Subsequent oxidation of 9–12
with 3-chloroperbenzoic acid (m-CPBA) in CHCl3 gave the
corresponding hexylsulfonylphenyl end-capped oligothio-
phenes, 13–16, C10O-Ar-OT(n)-Ph-SO2C6 in which n=1–4,
respectively. All of the newly synthesized oligothiophenes
(POM). For easy comparison, these oligothiophenes, CmO-
Ar-OT(n)-R’ could be divided into two sub-groups: the
single end-capped series, CmO-Ar-OT(4)-H in which m=10,
12, 14, 16, and 18 and the double end-capped series, C10O-
Ar-OT(n)-R, in which R=Ph-SC6 or Ph-SO2C6 and n=1, 2,
3, and 4 (Scheme 1).
For the single end-capped oligothiophenes with a conju-
gating thiophene unit less than four, such as 2 and 3, only a
sharp crystal melting transition was observed in the DSC
heating traces. On the other hand, quaterthiophenes 4–8 of
the CmO-Ar-OT(4)-H series exhibited rich thermal transi-
tions and multiple mesogenic phases, which indicates a good
balance of rigid core and flexible alkoxy chain is required
for these oligothiophenes to form thermotropic liquid-crys-
talline phases. In general, this group of molecules displayed
a broad temperature range of highly ordered smectic phase,
which will be confirmed in detail in the following section to
be smectic E (SmE), rich crystalline polymorphism and
mostly a nematic (N) phase (Table 1, see the Supporting In-
formation for the DSC thermograms). Their mesogenic
clear points decreased with an extended alkoxy tail from a
decyloxy to an octadecyloxy group. Furthermore, those com-
pounds with an alkoxy tail with ten to fourteen methylene
1
were fully characterized by H and 13C NMR spectroscopy,
high-resolution mass spectrometry, and elemental analysis
and were found to be in good agreement with the expected
structures.
Thermal behavior and liquid-crystalline properties: The
thermal behavior and phase transitions of all of the oligo-
thiophenes were initially examined by differential scanning
calorimetry (DSC) and polarized optical microscopy
Table 1. Phase transition temperatures and enthalpies for all compounds 2–16.[a]
Transition temperature [8C] (enthalpies [Jgꢀ1])
Heating (108Cminꢀ1
)
Cooling (108Cminꢀ1
)
2
65.9 (84.5)
K!I
3
113.9 (70.9)
K!I
77.7 (ꢀ11.1)
I!K
4
97.9 (0.63)
K2!K1
95.2 (1.06)
K2!K1
93.6 (0.82)
K2!K1
91.3 (1.18)
K3!K2
92.4 (1.23)
K3!K2
95.7 (62.4)
K!I
137.5 (26.9)
178.8 (39.4)
SmE!N
194.6 (1.06)
N!I
193.6 (ꢀ1.37)
I!N
175.9 (ꢀ38.5)
N!SmE
137.3 (ꢀ0.80)
SmE!K1
95.0 (ꢀ14.4)
K1!K2
K1!SmE
130.0 (11.3)
K1!SmE
131.3 (37.7)
K1!SmE
125.1 (27.4)
K2!K1
5
176.9 (22.5)
SmE!N
183.6 (0.62)
N!I
182.3 (ꢀ0.62)
I!N
174.1 (ꢀ22.8)
N!SmE
130.5 (ꢀ0.42)
SmE!K1
75.1 (ꢀ2.98)
K1!K2
6
174.9 (31.4)
SmE!N
179.2 (0.84)
N!I
177.1 (ꢀ1.37)
I!N
170.4 (ꢀ32.6)
N!SmE
131.1 (ꢀ0.50)
SmE!K1
66.7 (ꢀ8.61)
K1!K2
7
137.3 (0.15)
K1!SmE
141.2 (0.26)
K1!SmE
172.0 (28.2)
SmE!N
171.0 (38.2)
SmE!I
173[b]
N!I
171[b]
168.9 (ꢀ28.4)
N!SmE
133.4 (ꢀ0.35)
SmE!K1
62.0 (ꢀ8.14)
K1!K2
I!N
8
121.7 (18.0)
K2!K1
167.9 (ꢀ37.3)
N!SmE
72.8 (ꢀ61.7)
I!K
136.4 (ꢀ0.45)
SmE!K1
67.9 (ꢀ17.6)
K1!K2
9
10
11
12
13
14
15
16
118.4 (36.1)
K!SmA
123.5 (2.81)
S2!S1
150.5 (7.16)
SmA!N
147.3 (12.5)
S1!SmC
240.0 (25.5)
S1!SmC
109.8 (51.6)
K1!I
153[b]
153[b]
148.5 (ꢀ7.12)
N!SmA
128[b]
97.5 (ꢀ32.7)
SmC!K
106[b]
N!I
I!N
SmA!SmC
144.3 (ꢀ12.2
SmC!S1
237.0 (ꢀ24.8)
SmC!S1
188[b]
216.7 (6.65)
SmA!I
215.1 (ꢀ6.68)
I!SmA
188[b]
SmC!SmA
289.1 (0.40)
SmC!SmA
SmA!SmC
287.8 (ꢀ0.10)
SmA!SmC
17.6 (ꢀ21.9)
K1!K2
S1!S2
186.4 (4.24)
S2!S1
298.0 (9.56)
SmA!I
296.2 (ꢀ8.33)
I!SmA
162.2 (ꢀ6.03)
S1!S2
28.3 (22.5)
K2!K1
135.5 (50.6)
K!I
54.1 (ꢀ40.4)
I!K1
128.2 (ꢀ6.89)
I!SmA
117.0 (ꢀ40.9)
SmA!K
190.4 (45.5)
K!SmA
190[b]
213.0 (7.57)
SmA!I
211.1 (ꢀ7.57)
I!SmA
180[b]
157.1 (ꢀ40.3)
SmC!K
SmA!SmC
268.7 (ꢀ1.35)
SmA!SmC
247.9 (39.5)
S1!SmC
270.7 (1.26)
SmC!SmA
289.5 (9.49)
SmA!I
286.9 (ꢀ9.39)
I!SmA
240.3 (ꢀ38.0)
SmC!S1
120[b]
S1!S2
S2!S1
[a] The transition enthalpies [Jgꢀ1] are given in parentheses; a positive value stands for endothermic, a negative value means exothermic. Data from the
second heating and first cooling cycles; I stands for isotropic phase; N for nematic; SmA, SmC, and SmE for smectic A, C and highly ordered smectic E
phase, respectively; S1 and S2 represents the high and low temperature layered condensed phase, respectively. K stands for crystal, K1 and K2 for the
high and low melting point crystalline polymorph, respectively. [b] These thermal transition data were obtained from POM observations without detecta-
ble transition peaks or enthalpy changes on the DSC curves.
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D. Chen, M. S. Wong, Y. Tao et al.
units in this series (4–6) exhibited enantiotropic nematic
phases of decreasing temperature range, their nematic
phases were confirmed both by their small (less than
1.5 Jgꢀ1) transition enthalpies and the typical schlieren or
thread textures, as shown in Figure 1a. Despite an undetect-
able transition in the DSC trace, a narrow schlieren texture
of the nematic phase was observed under POM for the hex-
adecyloxy-substituted oligomer 7. On the other hand, the
longest octadecyloxy-substituted oligomer 8 possessed only
an ordered SmE phase with the brightest natural mosaic tex-
ture generated directly from the isotropic black field (Fig-
ure 1b).[21] Less-bright mosaic textures, as representatively
shown in Figure 1c, were all observed in the SmE phases of
compounds 4–7 of this series and were transformed from
the preceding N phase schlieren textures. Upon cooling to
the crystalline phase markedly dimmed mosaic textures with
many disclination black lines were usually obtained, as
shown in Figure 1d for 8 as an example. Such a desirable
phase sequence is reminiscent of the similar phase behavior
exhibited by 5,5’’-bis(5-alkyl-2-thienylethynyl)-2,2’:5’,2’’-ter-
thiophene. The gradual increase in order and the slow tran-
sitions between the phases were demonstrated to be a useful
characteristic for obtaining a large-area liquid-crystal mono-
domain.[9b,c]
For the double end-capped series, C10O-Ar-OT(n)-R (R=
Ph-SC6 and Ph-SO2C6; n=1, 2, 3, 4), apart from the one-
thiophene-containing derivatives 9 and 13, exhibited various
smectic A (SmA) and smectic C (SmC) phases, crystal poly-
morphs, and low-temperature
solid-phase transitions. Interest-
ingly, for those three- or four-
thiophene-containing oligomers,
some special low-temperature
layered condensed phases were
observed. This sub-group pro-
vided a platform to explore the
influences of the rigid oligothio-
phene conjugation length and
the polarity of linking end sub-
stituents. For oligomers with a
nonpolar alkylsulfanyl terminal
group (i.e., 9–12), both the crys-
tal melting point and mesogenic
clear point (cp) increased sig-
nificantly with increasing oligo-
thiophene conjugation length.
The one-thiophene-ring deriva-
tive 9 only showed a sharp
melting point. On the other
hand, the bithiophene deriva-
tive 10 exhibited enantiotropic
nematic and SmA phases with
typical schlieren and focal conic
fan textures, respectively, and
also a monotropic SmC phase
with schlieren and/or paramor-
photic broken focal conic fan
textures. This SmC phase could
be observed by POM on the
cooling cycle despite lacking a
detectable transition peak in
the DSC curve due to the very
small enthalpy change during
the SmA–SmC transition. With
Figure 1. Representative POM textures with crossed polarizers of a) 4, nematic thread texture at 1858C; b) 8,
SmE natural mosaic texture at 1568C; c) 7, SmE mosaic texture at 1608C; d) 8, mosaic texture decorated with
black disclination lines of crystalline solid state at 608C; e) 12, cooling to 2848C, SmA, focal conic fan and ho-
meotropic texture; f) 12, cooling to 2708C, SmC, paramorphotic broken focal conic fan and schlieren texture;
g) 12, cooling to 2388C, transition from SmC broken focal conic fan to S1 striated texture; h) 12, cooling to
2308C, striated texture; i) 16, cooling to 1508C, thick striped texture; j) 16, cooling to room temperature 258C,
thick striped texture with separation lines; k) 14, cooling to 1168C, crystalline spherulite growing from the
SmA phase; and l) 11, cooling to 1348C, fine striated texture. All scale bars correspond to 100 mm. (This figure
is provided in color in the Supporting Information.)
extending the oligothiophene
core to three or four thiophene
rings as 11 and 12, broad enan-
tiotropic SmA and SmC phases
with typical textures, as repre-
sentatively shown in Figure 1e
and f were observed. When the
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Asymmetrically End-Capped Mesogenic Oligothiophenes
FULL PAPER
polarity of the terminal group increased by oxidizing the al-
kylsulfanyl to an alkylsulfonyl group, the alkylsulfonylphen-
yl-substituted oligomers 13–16 showed significantly in-
creased melting points and diminished mesogenic tempera-
ture intervals than their corresponding alkylsulfanyl counter-
parts. Strikingly, for oligomers with four thiophene rings,
such as 12 and 16, striped textures with equidistant lines
(Figure 1g–j) characteristic of helical structures were unex-
pectedly observed under POM in the low-temperature
phase in sharp contrast to the typical spherulite or mosaic
crystalline textures of those with one or two thiophene rings,
such as in 9, 10, 13, and 14 (Figure 1k). In addition, the
three-thiophene-ring oligomer with an alkylsulfanylphenyl
terminal group (11) showed a fine striated texture upon
cooling from its preceding fluidic smectic phase (Figure 1l),
but not for the corresponding alkylsulfonylphenyl-substitut-
ed oligomer 15. In general, the striped textures appeared on
cooling from the preceding SmC broken focal conic fan tex-
tures at temperatures well matched with those peak temper-
atures extracted from DSC measurements. Such textures
could reversibly change back into an SmC phase upon heat-
ing for 11, 12, and 16, showing a distinct transition between
the SmC and condensed phase S1 on both cooling and heat-
ing cycles, whereas the striated lines became gradually
dimmer when cooling from the SmC phase and then got
brighter upon heating again for 11 and 12. The pitch, deter-
mined as the distance between two adjacent dark lines,[21b]
was almost unchanged with temperature for 11: (4.9ꢁ
0.47) mm at 1408C and (4.8ꢁ0.34) mm at 408C. The pitch re-
duced slightly with decreased temperature for 12, for in-
stance, (6.8ꢁ1.2) mm at 1958C and (6.0ꢁ0.8) mm at 308C.
Alternatively, for the alkylsulfonylphenyl-substituted oligo-
mer 16, the thick striped texture became clearer and clearer
upon cooling with some clear separation lines appearing
upon cooling to the transition temperature (1208C) for the
S1 into S2 phase (Figure 1j) and coagulated again into a
continuous striped texture upon heating above 1908C. De-
spite these remarkable visual changes, the measured pitch
was almost unchanged with temperature, approximately
(22.5ꢁ1.9) mm, but showed observable expansion and coali-
tion when closely approaching the transition temperature
into SmC broken fan texture upon heating. The distinct stri-
ated textures or striped patterns are reminiscent of the clas-
sical textures for twisted smectic phases from chiral com-
pounds[21b,22] or from achiral bent-core molecules.[16,23] This
unanticipated superstructure was attributed to the helical
supramolecular stacking of the asymmetric end-capped ex-
tended conjugating oligothiophenes due to steric packing ef-
fects or to bent-core conformational chirality under con-
straint conditions, which will be discussed in detail in the
next section combined with XRD characterization and opti-
cal spectroscopic investigations. For comparison and clarity,
the phase polymorphic transitions are summarized in
Table 1 and phase thermograms and 28 supplementary POM
plates are provided in the Supporting Information.
Supramolecular structure in the mesophases: To investigate
the packing structures of the mesophases of these two ary-
lene-capped oligothiophene series, variable-temperature
XRD measurements were carried out and compared with
the thermal analysis results from DSC and POM.
For the double end-capped series, the XRD-measured la-
mellar spacing (d) showed comparable dimensions with the
corresponding optimized molecular contour lengths in their
SmA phases and significantly reduced values in the SmC
phases, which indicates an obvious tilting arrangement. For
instance, on cooling from the isotropic state to 1958C, the
XRD pattern of compound 11 showed a sharp peak in the
small-angle region (SAR, 2q<108) with a layer spacing of
39.2 ꢂ, which is in good agreement with the molecular
length of 38.4 ꢂ. The XRD pattern also showed a diffuse
peak in the wide-angle region (WAR), indicative of a liquid-
like in-plane order, which is typical for the SmA phase. Fur-
ther cooling to 1608C, the XRD pattern possessed similar
features with reduced layer spacing (d=36.8 ꢂ), together
with the DSC analysis and POM observation, the SmC
phase could be assigned unambiguously, and the tilting
angle related to the layer normal was around 178 based on
the optimized molecular length calculation. Upon further
cooling, a sharp reflection together with its high orders up
to the fourth order in the SAR and diffuse scatterings in the
WAR were observed (see the Supporting Information), indi-
cating the formation of a unique well-defined lamellar struc-
ture. The d spacing further decreased to 33.4 ꢂ in the 1358C
high-temperature condensed phase S1 and 30.2 ꢂ in the
508C lower-temperature phase S2. These phase-transition
sequences were commonly observed for the double end-
capped oligomers containing three or four thiophene rings,
as representatively listed in Table 2 for compounds 11 and
Table 2. The lattice spacings at various temperatures of representative
double end-capped oligothiophenes compared with their optimized mo-
lecular contour lengths.
11
16
Phase assigned
d [ꢂ]/T [8C]
39.2/195
36.8/160
33.4/135
30.2/50
38.4
45.5/285
43.3/265
39.8/200
37.8/50
44.6
SmA
SmC
S1
S2
–
Molecular length[a] [ꢂ]
[a] Measured in optimized molecular geometry by MM+ calculations in
HyperChem 7.5.
16. Further molecular tilting or conformational twisting and
alkyl chain interdigitation should account for these ever-
shortened layer spacings. The clear lamellar structure and
equidistant striped POM textures are reminiscent of those
features exhibited by polar SmCP or B2 phases of bent-core
or banana-shaped molecules.[16,23]
For the single end-capped series, compound 4, for in-
stance, after annealing at the isotropic temperature of
1958C for five minutes to eliminate the effect of thermal
history, formed an imperfect crystalline phase upon rapid
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D. Chen, M. S. Wong, Y. Tao et al.
Figure 2. Variable temperature XRD patterns of 4 a) cooling to 508C,
b) heating to 1608C, c) heating to 1908C, d) cooling to 1708C, e) cooling
to 1108C, f) cooling to 508C, and g) cooling to room temperature (238C).
Figure 3. Spacings (d) measured by XRD versus the number of methyl-
ene units (n) of the alkoxy tail in the single end-capped oligothiophenes
*
for the layered crystalline state ( ) and for the ordered smectic phase in
~
!
&
*
), and the molecular lengths ( )
the SAR ( ) and in the WAR (3
calculated by MM+ simulation for comparison.
cooling down to 508C, as shown in Figure 2 (curve a). Then
upon heating to the mesophase temperature of 1608C
(Figure 2, curve b), sharp higher-order reflections were ob-
served in the SAR and several marked reflection peaks su-
perposed on the broad band in the WAR consistently indi-
cated the formation of a highly ordered smectic pha-
se.[3f,9b,c,24,25] All the peaks could be indexed with a centered
rectangular smectic E (SmE) phase as 39.4 (001), 19.72
(002), 9.90 (004), 4.67 (110), 3.99 (200), and 3.29 ꢂ (210)
with a layer spacing of 39.4 ꢂ. Upon further increasing the
temperature to 1908C, it displayed no SAR reflections, but
only a diffuse peak in the WAR that confirmed nematic
phase formation (Figure 2, curve c). Upon cooling back to
1708C, the SmE phase recovered with relative weak reflec-
tions in the WAR (Figure 2, curve d). Further cooling to
1108C led to the formation of a crystalline phase with a
layer spacing of 33.2 ꢂ (Figure 2, curve e) and then cooling
to 508C resulted in another crystalline phase with a slightly
reduced spacing of 31.5 ꢂ (Figure 2, curve f). A complex
crystalline polymorphism was achieved after the sample was
further cooled to 238C and left to stand overnight with two
main lattice spacings of 35.0 and 31.3 ꢂ (see Figure 2, curve
g and also the Supporting Information).
Other compounds in this series exhibited quite similar
XRD patterns to those of compound 4, along with even
stronger crystallization inhibition upon cooling from the
melt. The d spacing obtained from the ordered smectic
phases (1608C) and the crystalline state (508C) of the whole
single end-capped series is plotted versus the methylene
number (n) of the alkoxy tail in Figure 3. For the ordered
smectic phase, the powder XRD patterns usually contain
three to four sharp reflections in the SAR corresponding to
the one to four orders of the smectic layer period. Among
them, the intensity of the third order reflection is significant-
ly diminished or even absent. The smectic period d grows
linearly with the length of the alkoxy tail (Figure 3), accord-
ing to Equation (1) (R=0.996):
ð1Þ
d ½ꢂꢂ ¼ ð23:52 ꢁ 1:14Þ þ ð1:60 ꢁ 0:08Þn
In the WAR, in addition to the diffuse band at 2q around
208 related to the molten state of the alkyl chains, all com-
pounds in this series showed three medium to strong intensi-
ty peaks (curves b and d in Figure 2 for 4 as an example)
with essentially constant spacings as shown in Figure 3 (3,
~
!
,
). These observations are in agreement with those an-
ticipated for an ordered smectic phase with a thickness in-
creasing with the length of the molecules, but lateral pack-
ing common to all members.[25] According to MM+ geome-
try optimization, the single end-capped oligothiophenes
often adopt an energy-favored stretched conformation as
shown, for example, for 6, in Figure 4a. The calculated d
spacing of the period layers is equal to 1.2–1.3ꢀthe molecu-
lar length (L) simulated by MM+ calculations, an interdigi-
tated antiparallel packing arrangement as shown in Fig-
ure 4b was proposed, in which the aromatic–aliphatic di-
block molecules oriented alternatively up and down adhered
by the p–p interactions between the conjugated oligothio-
phene part and by the weak van der Waals interactions be-
tween alkyl chains forming a two-dimensional ordered lat-
tice structure. Here the dimethyl-substituted phenyl ring
may adopt a staggered position for the steric hindrance and
contribute to the increased layer spacing. The rotation-limit-
ed bladelike molecules adopt a herringbone packing struc-
ture, as shown in Figure 4c. Deduced from the y intercept of
d(n) under the assumption of constant molecular area,
which is perfectly legitimate here because they share the
same rectangular lateral arrangement independent of the
alkoxy chain length, the thickness of the rigid aromatic part
d0 =23.52 is in good agreement with the value of 23.42 ꢂ
calculated by molecular modeling, assuming the quaterthio-
phene units are reversely adhered by p–p stacking and the
phenyl rings are staggered due to the steric hindrance of the
two methyl groups (see the Supporting Information). The
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Asymmetrically End-Capped Mesogenic Oligothiophenes
FULL PAPER
whereas in the high-tempera-
ture ordered smectic phase the
predominant interaction is be-
tween the oligothiophene rings.
Although we cannot determine
the precise crystal structure
from the limited efficient peaks
due to crystallization inhibition,
it can be deduced from the
slope of 1.08 ꢂ that the mole-
cules only need to slightly tilt
away from the rigid core paral-
lel to layer normal arrangement
by an angle of less than 108; the
fully stretched alkyl chain has
an angle of approximately 1558
with the rigid part according to
results of the MM+ optimiza-
tion (see the Supporting Infor-
mation).
CD/UV-visible spectroscopic in-
vestigations and supramolecular
chirality: For those double end-
capped oligothiophenes exhibit-
ing unique striped POM tex-
tures at temperatures lower
Figure 4. Molecular models of representative compound 6 based on molecular mechanics (HyperChem MM+
Force field) calculations (a) and the proposed packing structure of the single end-capped oligothiophene series
in the ordered SmE phase, b) side view and c) top view.
slope of the d(n) straight line (1.60 ꢂ), which represents the
growth rate of the lamellar thickness with the number of
methylene groups, is significantly larger than the length of
one zigzag in a paraffin chain fully extended interdigitated
single layer (1.27 ꢂ) and smaller than the increment
(2.54 ꢂ) in an end-to-end double-layer structure, further
verified the partially interdigitated arrangement. All XRD
data of this series at 1608C could be accounted for by an or-
thogonal-centered rectangular lattice, SmE, in which a=
7.94 ꢂ and b=5.72 ꢂ for compound 6 as an example.
For the crystalline state, aside from some peaks indicative
of a three-dimensional crystal lattice, the XRD patterns usu-
ally show three reflection peaks with reciprocal spacings in
the ratio of 1:2:3 in the SAR, as normally observed with
long alkyl-chain derivatives,[24,25] revealing that the mole-
cules are arranged in a lamellar fashion. The period of the
lamellar structure increases linearly with the number of the
methylene units (Figure 3), according to Equation (2) (R=
0.995):
than fluid smectic phases, we performed circular dichroism
(CD) measurements and spectroscopic investigations to ex-
plore further their packing structure and supramolecular
chirality. Figure 5a shows that the CD and UV/Vis absorp-
tion spectra of solid films of 12, which contains the nonpolar
alkylsulfanylphenyl terminal group, have striped POM tex-
tures. Interestingly, strong CD signals exhibiting remarkable
bisignate Cotton effects with the crossing wavelength basi-
cally coinciding with the absorption maximum of UV/Vis
spectra of the films were observed, which suggests the for-
mation of a well-defined helical stacking sequence and the
existence of supramolecular chirality. The three-thiophene-
containing analogue 11, which showed a fine striated tex-
ture, also exhibited a distinct CD signal in a similar fashion
(see the Supporting Information). Moreover, solid films of
16, which has a polar alkylsulfonylphenyl terminal group,
showed a thick striped pattern and exhibited strong CD sig-
nals with a different, almost unimodal, Cotton effect essen-
tially consistent with the UV/Vis absorption spectra of the
films, as shown in Figure 5b. Such unimodal CD response is
reminiscent of that of a helical structure formed by banana-
shaped molecules.[26] On the other hand, compound 15 and
other shorter oligothiophene analogues showed no signifi-
cant optical activity. For those samples showing optical ac-
tivity, we noted that the sign of the CD signals altered from
overall positive (right-handedness, P, black curve) to nega-
tive (left-handedness, M, grey curve) on different parts of
the same sample or upon different sample preparations, al-
though the spectral wavelengths scarcely changed, as shown
in Figure 5a and b. This suggests that the observed CD
ð2Þ
d ½ꢂꢂ ¼ ð20:56 ꢁ 0:95Þ þ ð1:08 ꢁ 0:07Þn
From the y intercept of d(n), the thickness of the rigid ar-
omatic part, d0 =20.56, is in agreement with the value of
20.53 ꢂ calculated by molecular modeling for the aromatic
rigid core (see the Supporting Information). Therefore, in
the crystalline state, the single end-capped molecules may
adopt a fully interdigitated single-layer packing structure
thanks to the ever-enhanced interactions among the fully
stretched alkyl chains and the steric packing requirement,
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D. Chen, M. S. Wong, Y. Tao et al.
Figure 5. CD and UV/Vis spectra of solid films at room temperature of a) 12; b) 16, the overall positive (black) and negative (grey) curves were mea-
sured on different locations of the same film; and c) UV/Vis absorption spectra and d) CD spectra of compound 12 in chloroform/methanol mixed at dif-
ferent ratios.
effect resulted from an unbalanced formation of the two do-
mains of opposite chirality. The right- and left-handed heli-
ces occurred randomly, which is similar to those observed
previously from achiral bent-core molecules,[16b,26,27] coordi-
nation polymer crystals,[17] and supramolecular helical as-
semblies from achiral amphiphiles in an air/water inter-
face.[18] Furthermore, to shed light on the packing behavior
in the aggregated state, the optical properties of 12 were in-
vestigated by changing from the molecularly dissolved state
in chloroform to a gradually aggregated state in poorly dis-
solved solvent, that is, methanol as shown in Figure 5c. With
an increase in the volume ratio of methanol, the UV/Vis ab-
sorption spectra from the molecularly dissolved state, with a
principal broad absorption band maximum at l=433 nm in
CHCl3, blueshifted to around 410 nm, particularly when the
methanol volume ratio was more than 45%. A sharp transi-
tion of the spectral shape with a distinct blueshift and the
appearance of new fine vibronic shoulders at l=460 and
505 nm strongly support the formation of aggregates. The
large blueshift in the UV/Vis spectrum for aggregate forma-
tion suggests that the supramolecular organization formed
in this case is tightly packed, which results in a strong exci-
tonic coupling similar to those of oligothiophenes with oli-
go(ethyleneoxide) tails[19,22b] that have been demonstrated to
arrange in an H-type aggregate.[29] The UV/Vis spectra of
the aggregated form of 12 in solution is quite similar to that
of its thin solid film; nevertheless, the CD measurements
did not show any distinct signals (Figure 5d and for 16 in so-
lution see the Supporting Information), which suggests that
either the aggregate formed is achiral or the chiral aggre-
gates in solution are racemic with which no CD response is
observed. Furthermore, the CD signals of the thin films
result from not only the aggregation packing, but also the
constraint in the solid-state phase, which plays a key role for
the generation of optical activity.
It has been proposed that p-stacking interactions play a
central role in the supramolecular assembly of poly-/oligo-
thiophene derivatives into an aggregation in solution. Mean-
while, the interplay between the conjugated molecules and
solvent molecules is also of primary importance.[30–32] In the
solid state, interactions with solvent molecules are absent
and the driving forces for the supramolecular organization
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Asymmetrically End-Capped Mesogenic Oligothiophenes
FULL PAPER
are the intermolecular interactions between the conjugated
species and the substrate surface, the intrinsic constraints,
and the noncovalent interactions, such as p stacking and van
der Waals interactions, which are crucial in determining
thin-film properties.[19,33] Determination of the origin of the
helicity and chirality from achiral molecules has been an in-
triguingly important research topic in the liquid-crystal
field.[15,16,26,27] Recently, the achiral (without the asymmetric
carbon) banana-shaped bent-core molecules have been ex-
tensively studied because they exhibit various kinds of un-
usual smectic phases (banana phases) with chiral and/or hel-
ical smectic structures.[16b,26,34] The twisting conformation,
the tilting of molecules in a smectic layer, and the escape
from the macroscopic polar order within a layer have been
considered as origins of the chirality or helicity and the
mechanism of spontaneous desymmetrization in such smec-
tic mesophases from banana-shaped achiral mole-
by the simple hockey-shaped molecules[27,37,38] provides us
strong hints at the least deviation from the conventional
straight-core mesogen geometry required to form a banana-
shaped molecule, as well as the fact that the longer alkyl
chain can help to realize the desymmetric superstructure or-
ganization and thus exhibit supramolecular chirality. The
asymmetric structures with donor and acceptor substituents
in the double end-capped oligothiophenes thus possessing
an intrinsic dipole can provide a polar order. Furthermore,
the cis conformation of the double end-capped quaterthio-
phene derivatives 16 (pseudo-banana shaped) has a very
similar total energy as the trans conformation (Z shaped) as
optimized by MM+ (see the Supporting Information). With
the hexyl and decyl alkyl chain on either end, the formation
of a true banana-shaped molecule with twisting conforma-
tion leading to a tilted chiral molecular arrangement to
escape from spontaneous polarization should be reasonable.
Two types of helical structure with remarkably different
pitches and brightnesses were observed as described above.
For the nonpolar alkylsulfanyl end-capped 11 and 12, the
striated patterns are superimposed on the fan-shaped tex-
tures. It is believed that the p–p stacking of the aromatic
rigid core and the cooperative van der Waals interactions
among alkyl chains provide the driving force for the associa-
tion of somewhat bent molecules under confined conditions.
In addition, the two methyl groups substituted at the phenyl
ring together with steric-packing requirements may arouse
the twisting and thus the helical superstructure formation in
which the helical axis is perpendicular to the layer, similar
to the chiral SmC* phase as shown in Figure 6a. On the
other hand, the polar alkylsulfonyl end-capped 16 showing a
much larger pitch exhibited a dark field under crossed polar-
izers for a very short time, which implies that a transient ho-
meotropic state occurs before the arrival of the thick striped
pattern texture. Therefore, the helical twisting of molecules
takes place along the layer and the helical axis lies in the
layer similar to that observed in the twisted grain boundary
(TGB) phase as shown in Figure 6b. The polar order in-
duced by the polar alkylsulfonyl group clearly provides the
driving force for twisting to escape from macroscopic polari-
zation. The supramolecular chirality produced by the steric
interfacial packing of achiral amphiphilic molecules provid-
ed a simple two-dimensional model for our system.[18]
cules.[16b,26,34] In poly-/oligothiophenes, each C C bond is a
ꢀ
potential center of chirality by atropisomerism depending
on various intrinsic and extrinsic factors, such as a subtle
change in the solvent, has significantly altered or inverted
the chirality of chiral aggregates thus formed in solu-
tion.[30,35] For the asymmetrically double end-capped oligo-
thiophenes, in view of the distinct striped texture, the strong
CD effects and the well-defined smectic layer structure with
marked shortened spacing and diffuse WAR scatterings
from XRD analyses, a helical packing model comparable to
that from banana-shaped achiral molecules[16b,23] is proposed,
as illustrated in Figure 6. The preference of the bent-core
banana-shaped conformation and the helix formation can be
triggered by lowering of the temperature and the constraint
thus produced in the solid phase, which results in the chiral
symmetrical breaking and optical activity in the solid films,
as reversibly demonstrated that it could not realize in the or-
dered fluidic mesophases.[36] The phase behavior exhibited
Some further work such as electro-optic experiments may
be needed to gain a detailed understanding of the supra-
molecular chirality exhibited by the alkyl-substituted,
almost-straight aromatic core achiral molecules; however,
we can currently propose that the chirality achieved in this
system originates from the supramolecular organization of
the uniquely designed asymmetric molecules in helical
stacking structures of somewhat twisted comformers. These
asymmetrically double end-capped oligothiophenes also rep-
resent the first achiral, almost-straight, aromatic-core-based
mesogens that exhibit supramolecular chirality.
Figure 6. Schematic illustrations of the molecular packing in the banana-
shaped conformation and the smectic-like layer structures. a) The helical
structure along the layer normal in the nonpolar alkylsulfanylphenyl end-
capped oligothiophene derivatives 11 and 12, and b) the helical axis lying
in the layer for the polar alkylsulfonylphenyl end-capped oligothiophene
derivative 16, similar to the TGB phase.
FET properties: To explore the potential applications of the
single end-capped oligothiophenes Cm-Ar-OT(4)-H with a
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3483

D. Chen, M. S. Wong, Y. Tao et al.
highly ordered smectic phase in OFET, top-contact OFETs
using 4 as a semiconductor layer were fabricated on plain
and octyltrichlorosilane (OTS)-treated (nonmodified or oc-
tylsilane-modified) SiO2/Si substrates by high-vacuum evap-
oration according to a published procedure.[39] Different sub-
strate deposition temperatures (Td) were investigated to
evaluate the effect of mesophases on the OFET performan-
ces. The electrical measurements were carried out at room
temperature under ambient conditions. Compound 4 was
found to exhibit typical p-type organic semiconductor char-
acteristics, as shown in Figure 7. The hole mobilities were
spectively, m is the field-effect mobility, Ci is the capacitance
per unit area of the SiO2 layer, and VG and VT are the gate
voltage and threshold voltage, respectively. In our case, the
channel length is 45 mm and the channel width is 2.5 mm.
Compound 4 deposited at Td =258C on plain and OTS-
treated substrates showed hole carrier mobilities of 4.6ꢀ
10ꢀ3 and 8ꢀ10ꢀ3 cm2 Vꢀ1 sꢀ1, respectively, and on/off current
ratios of about 105. When the substrate temperature was in-
creased to Td =1408C during film deposition, the hole mobi-
lities on plain and OTS-treated substrates improved and
reached 0.052 and 0.07 cm2 Vꢀ1 sꢀ1, respectively. The on/off
current ratios also slightly increased to 5ꢀ105. Both the hole
mobilities and on/off current ratios are comparable to the
highest values reported for the series of symmetrical hexyl-
phenyl double end-capped phenylene-thiophene co-oligo-
mers by Marks and co-workers.[11] This strong dependence
of the mobility of 4 on the substrate temperature during
evaporation is in good agreement with the DSC result show-
ing the formation of a highly ordered smectic E phase
above 137.58C.
As further evidenced by XRD analysis, the molecules in
the vacuum-deposited films at both 25 and 1408C adopt an
organization in which the p–p stacking direction is parallel
to the substrate surface. This kind of organization is known
to be favorable for high mobility in OFETs.[8,9] In addition,
Figure 8 shows some difference in the XRD patterns for the
films deposited at different temperatures. The w/2q scan of
the film of 4 deposited at a substrate temperature of 1408C
shows a diffraction peak at 2.88 and a second order line at
5.68, corresponding to a repeat distance (periodicity) of
3.20 nm. This d-spacing value suggested that the molecules
are almost perpendicular to the substrate. The curve also
shows nice thickness oscillations, indicating that the film is
quite smooth. The periodicity of the fringes suggests that
the molecules self-organized into ten repeats of vertically
aligned chains at this deposition temperature. For the film
deposited at 258C, XRD measurements also showed two
diffraction peaks at 2.83 and 5.658, respectively, correspond-
ing to a slightly shorter periodicity of ꢃ3.15 nm. However,
the curve does not display the thickness fringes. This sug-
gests that the molecules are again predominantly stacked
perpendicular to the surface, but the stacking is disordered.
This explains the strong diffraction peak, but absence of pe-
riodic fringes and thus relatively lower hole mobility.
Figure 7. a) Source-drain current (IDS) versus source-drain voltage (VDS
)
at various gate voltages (VG) of 4 based top-contact OFETs deposited at
1408C on plain SiO2/Si substrates. b) The transfer characteristics in the
saturation regime at a constant source-drain voltage of ꢀ100 V.
Conclusion
A novel series of asymmetrically end-capped mesogenic oli-
gothiophenes with various oligothiophene core lengths,
alkoxy tail lengths, and molecular polarities through intro-
ducing an alkylsulfanyl or alkylsulfonyl functionality as the
terminal group have been synthesized by palladium-cata-
lyzed Suzuki cross-coupling and/or Kumada addition reac-
tions as key steps. For the single end-capped oligothio-
phenes, CmO-Ar-OT(4)-H, in which m=10, 12, 14, 16, and
18 (corresponding to molecules 4–8, respectively), all oligo-
calculated in the saturation regime at VDS =ꢀ100 V by using
Equation (3):
2
ð3Þ
IDS ¼ ðW=2LÞmCiðVGꢀVTÞ
in which VDS is the source-drain voltage, IDS is the source-
drain current, W and L are the channel width and length, re-
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Asymmetrically End-Capped Mesogenic Oligothiophenes
FULL PAPER
increased melting points and thus diminished mesogenic
temperature intervals than those of their corresponding al-
kylsulfanyl counterparts. Interestingly, upon cooling from
the melted state, crystallization was remarkably inhibited
and unusual layered, low-temperature phases formed, par-
ticularly for those oligomers with three or four thiophene
rings. Two different types of helical structures showing dis-
tinct striated textures or regular striped patterns were ob-
served with a pitch of several to tens of micrometers under
POM at the low-temperature condensed phases. For nonpo-
lar alkylsulfanyl end-capped 11 and 12, the striated patterns
are superimposed on the fan-shaped textures with a pitch of
several micrometers, the helical axis is perpendicular to the
layer similar to the chiral SmC* phase. On the other hand,
for the polar alkylsulfonyl end-capped 16, which shows a
thick striped pattern with a much larger pitch of around 22
micrometers, the helical twisting of molecules develops
along the layer and the helical axis lies in the layer similar
to the TGB phase.
The twisted chiral, smectic phase-like helical structures
were further confirmed by XRD analysis and showed well-
defined smectic layer structures with markedly shortened
layer spacing, and directly evidenced by the distinct CD sig-
nals measured from the thin solid films. Furthermore, the
optical property investigations of compounds 12 and 16, by
means of changing from the molecularly dissolved state to a
gradually aggregated state in mixed solvents, suggested the
supramolecular aggregation structures formed with no sig-
nificant optical activity. So it is believed that the p–p stack-
ing of the aromatic rigid core and the cooperative van der
Waals interactions among alkyl chains provide the driving
force for the association of the somewhat bent molecules
under confined conditions in the thin solid films. In addition,
the two methyl groups substituted at the phenyl ring togeth-
er with steric packing requirements may generate the twist-
ing and thus form the helical superstructure. The polar
order introduced by the polar alkylsulfonyl group may fur-
ther drive the in-plane twisting to escape from macroscopic
polarization and result in the TGB-like helical structure for-
mation.
Figure 8. XRD patterns of thin films of 4 vacuum deposited at a) 140 and
b) 258C.
mers exhibited enantiotropic nematic phases and a broad
temperature range of highly ordered SmE phases except for
the octadecyloxy-substituted oligomer 8, which did not show
a nematic phase, were confirmed by DSC thermal measure-
ments, POM observations, and XRD analyses. Our initial
studies indicated that a high charge mobility of OFET with
hole mobility up to 0.07 cm2 Vꢀ1 sꢀ1 could be achieved with
C10O-Ar-OT(4)-H by taking advantage of its highly ordered
SmE liquid-crystalline arrangement.
For the double end-capped series C10O-Ar-OT(n)-R, in
which R=Ph-SC6 or Ph-SO2C6 and n=1, 2, 3, and 4 (com-
pound 9–16), all compounds with two or more thiophene
rings were found to exhibit SmA and SmC phases, various
crystal polymorphs, and/or unusual low-temperature, layered
condensed phases. This series provides a platform to investi-
gate the effects of the rigid oligothiophene conjugation
length and the polarity exerted by the end-linking group.
For oligomers with an alkylsulfanyl terminal group (com-
pounds 9–12), both the crystal melting point and mesogenic
cp increased significantly with an increase in the oligothio-
phene conjugation length. Oligomers with a polar alkylsul-
fonyl terminal group (molecules 13–16) showed significantly
This asymmetrically double end-capped oligothiophenes
represent the first almost-straight aromatic core achiral mol-
ecules that exhibit supramolecular chirality. Our findings
could stimulate the search for chiral behavior and further
ferroelectricity in a new class of organic semiconducting ma-
terials.
Experimental Section
Detailed synthetic procedures and characterization for all compounds
can be found in the Supporting Information. The DSC thermograms
were recorded on a Perkin–Elmer Pyris 1 instrument equipped with a
cooling accessory and under a nitrogen atmosphere. The temperatures
and heat flows were calibrated by employing an indium standard sample.
Typically, approximately 4 mg of powdered sample was encapsulated in a
sealed aluminum pan with an identical empty pan as the reference. The
heating and cooling rate was 108Cminꢀ1. The phase transitions and
Chem. Eur. J. 2009, 15, 3474 – 3487
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3485

D. Chen, M. S. Wong, Y. Tao et al.
liquid-crystal textures were observed and photographed as melt-pressed
preparations sandwiched between a glass slide and a cover glass by using
a polarized optical microscope equipped with a Leitz-350 heating stage
and a digital CCD camera. The film samples for UV/Vis and CD meas-
urements were melt-pressed and sandwiched between quartz glass. The
solutions for CD measurements were prepared from a stock solution of
12 or 16 in chloroform (5 mg/5 mL), then stock solution (0.5 mL) was
transferred to a 1 cm quartz cell, with an appropriate volume of methanol
and chloroform diluent added according to the predefined ratio so as to
keep the total volume (3.0 mL) and the oligomer concentration (0.5 mg/
3 mL) constant. For the UV/Vis measurement, the concentration was re-
duced to 20% with the predefined solvent ratio. The CD signals for the
film and the solution were recorded on a JASCO A-810(S) spectropo-
larimeter with a scanning wavelength range from 190 to 1080 nm. The
UV/Vis spectra were performed on a Shimadzu UV-2401 instrument.
XRD experiments were performed on a D/max-2550 PC diffractometer
with a programmed temperature-controlling oven system using CuKa
(1.5406 ꢂ) as the radiation source with 40 kV, 300 mA power. The film
samples cast from a solution in dichloromethane with a thickness of ap-
proximately 0.1 mm on clean glass substrate were set on the sample stage
and first heated to the isotropic state and then cooled to the liquid-crystal
state or the programmed temperatures.
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Acknowledgements
We gratefully acknowledge financial support of this work from the Na-
tional Natural Science Foundation of China (20874044 and 50273013),
the Research Grants Council, Hong Kong (Earmarked Research Grant:
HKBU2020/06P) and the Hong Kong Baptist University (FRG/06-07/II-
53).
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Received: November 26, 2008
Published online: February 13, 2009
Chem. Eur. J. 2009, 15, 3474 – 3487
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