Organic semiconductors with helical structure based on oligothiophene derivatives exhibiting chiral nematic phase

Article abstract of DOI:10.1080/15421400701735723

We have synthesized 2-[5-(4-[(S)-2-methylbutoxy]phenyl)-3-methyl]-2'''- propyl-2:5'-2':5''-2'':5'''-quaterthiophene (10), a liquid crystalline semiconductor that exhibits a chiral nematic phase, and used the time-of-flight technique to determine its carrier mobility behavior. In the chiral nematic phase, the positive carrier mobility was 210-4cm2/Vs at 120C and the negative carrier mobility was 110-4cm2/Vs, both of which we attribute to electronic processes. For the negative carrier, in addition to the fast electronic process, we also observed a slow carrier transport, having mobility on the order of 10-5cm2/Vs, which we attribute to ionic conduction. Chiral dimers containing binaphthyl group exhibited stable glassy cholesteric phase and cholesteric thin films with selective reflection band in visible light area could be fabricated. They emitted circularly polarized light with dichroic parameter of 1.3 when UV light was illuminated.

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Organic Semiconductors with
Helical Structure Based on
Oligothiophene derivatives
Exhibiting Chiral Nematic
Phase
Dr. Masahiro Funahashi a & Dr. Nobuyuki Tamaoki a
a Molecular Smart System Group, Nanotechnology
Research Institute, National Institute of Advanced
Industrial Science and Technology Tsukuba , Ibaraki,
Japan
Published online: 22 Sep 2010.
To cite this article: Dr. Masahiro Funahashi & Dr. Nobuyuki Tamaoki (2007) Organic
Semiconductors with Helical Structure Based on Oligothiophene derivatives Exhibiting
Chiral Nematic Phase, Molecular Crystals and Liquid Crystals, 475:1, 123-135, DOI:
10.1080/15421400701735723
To link to this article: http://dx.doi.org/10.1080/15421400701735723
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Mol. Cryst. Liq. Cryst., Vol. 475, pp. 123–135, 2007
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400701735723
Organic Semiconductors with Helical Structure Based
on Oligothiophene derivatives Exhibiting Chiral
Nematic Phase
Dr. Masahiro Funahashi
Dr. Nobuyuki Tamaoki
Molecular Smart System Group, Nanotechnology Research Institute,
National Institute of Advanced Industrial Science and Technology
Tsukuba, Ibaraki, Japan
We have synthesized 2-[5-(4-[(S)-2-methylbutoxy]phenyl)-3-methyl]-2000-propyl-2:
50-20:500-200:5000-quaterthiophene (10), a liquid crystalline semiconductor that exhi-
bits a chiral nematic phase, and used the time-of-flight technique to determine
its carrier mobility behavior. In the chiral nematic phase, the positive carrier
mobility was 2 ꢀ 10ꢁ4 cm2=Vs at 120ꢂC and the negative carrier mobility was 1 ꢀ
10ꢁ4 cm2=Vs, both of which we attribute to electronic processes. For the negative
carrier, in addition to the fast electronic process, we also observed a slow carrier
transport, having mobility on the order of 10ꢁ5 cm2=Vs, which we attribute to ionic
conduction. Chiral dimers containing binaphthyl group exhibited stable glassy
cholesteric phase and cholesteric thin films with selective reflection band in visible
light area could be fabricated. They emitted circularly polarized light with
dichroic parameter of 1.3 when UV light was illuminated.
Keywords: carrier mobility; cholesteric; electronic conduction; organic semiconductors;
selective reflection; time-of-flight technique
INRTODUCTION
Cholesteric liquid crystals (CLCs) are promising optical materials
for reflective displays [1] and imaging devices [2] because the periodi-
cities of their helical structures are on the order of the wavelengths in
visible light. Circularly polarized light photoluminescence [3] and
We thank the Association for the Progress of New Chemistry, Sumitomo Foundation,
and Grant in Aid for Scientific Research (No. 16750185) for financial support.
Address correspondence to Masahiro Funahashi, Molecular Smart System Group,
Nanotechnology Research Institute, National Institute of Advanced Industrial Science
and Technology, Tsukuba Central 5-2, Higashi 1-1-1, Tsukuba, Ibaraki 305-8563, Japan.
E-mail: Masahiro-funahashi@aist.go.jp
123

124
M. Funahashi and N. Tamaoki
laser emission from cholesteric liquid crystals have been investigated
recently [4–7], but only optical pumping was possible in these studies
in which fluorescent dyes were doped in the cholesteric matrixes. The
realization of electrical pumping systems, which are more practical,
requires the construction of semiconductive cholesteric systems.
Electronic charge carrier transport has been confirmed in smectic
systems [8–12] and discotic columnar phases [13–16] in which inter-
molecular packing structures promote intermolecular electronic
charge transfer. In contrast, only ionic conduction caused by contami-
nation of ionic impurity has been observed in nematic or cholesteric
phases [17–23], which exhibit more liquid-like fluidity than do smectic
and discotic mesophases. Electronic conduction has also been observed
in the glassy nematic phases of some low- and high-molecular-weight
materials [24,25]. In particular, the glassy chiral nematic phase of a
fluorene derivative exhibited good hole transporting properties, with
mobility on the order of 10ꢁ4 cm2=Vs, in addition to luminescence of
circularly polarized light [25].
Ionic processes should be dominant in achiral and chiral nematic
phases that possess lower viscosities and have intermolecular transfer
integrals smaller than those of smectic phases. Increasing the transfer
integral should be an effective approach to realizing electronic conduc-
tion in nematic and chiral nematic phases. Recently, electronic con-
duction was confirmed in fluidic chiral nematic [26] and achiral
nematic phases [27] of oligothiophene derivatives with extended p-
conjugated system. However the detailed mechanism of electronic
charge carrier transport in these phases has been unclear. In addition,
for practical application of the liquid crystalline semiconductors with
helical structure to opt-electronics devices, fixation of helical order
at room temperature is indispensable and the helical pitch should be
tuned by temperature or other method.
In this article, we report electronic charge carrier transport charac-
teristics in the fluidic chiral nematic phase of an oligothiophene
derivative as well as synthesis and circularly polarized light emission
of dimeric oligothiophene derivatives containing binaphthyl group as a
chiral moiety.
MATERIALS
In this study, we investigated carrier transport characteristics of
[5-(4-[(S)-2-methylbutoxy]phenyl)-3-methyl]-2000-propyl-2:50-20:500-200:5000-
quaterthiophene (10) as shown in Scheme 1. We adopted the 5-phenyl-
2:50-20:500-200:5000-quarter-thiophene skeleton as an aromatic core having

Organic Semiconductors with Helical Structure
125
SCHEME 1 Synthetic route toward 2-[5-(4-[(S)-2-methylbutoxy]phenyl)-3-
methyl]-2000-propyl-2:50-20:500-200:5000-quaterthiophene.
a large p-conjugated system; for alkyl chains that induce mesophases,
we used short propyl and (S)-2-methylbutyl groups.
In the DSC measurements of 10, small exo- and endo-thermal peaks
(DH ¼ 1.2J=g), which are characteristic of isotropic-nematic and iso-
tropic-chiral nematic transitions, appeared at 182ꢂC during the cooling
and heating cycles. At 83ꢂC, a large exothermal peak (DH ¼ 54J=g),
which corresponds to crystallization, appeared during the cooling pro-
cess. In X ray diffraction, only a broad hallo associated with liquid-like
order was observed around 2h ¼ 20 degrees without sharp peaks in
low angle region in the mesophase, indicating the mesophase is
nematic or chiral nematic phase. No cybotactic peaks were not
observed in low angle region, indicating no microscopic smectic-like
order formation in the phase. For observation of optical textures under
a polarized light microscope, we prepared liquid crystal cells (thick-
nesses: 9 mm) on two ITO-coated glass plates. In the absence of an
electric field we observed a Granjan texture, which is characteristic
of a chiral nematic phase, in the mesophase of 10, indicating a planar
alignment of the liquid crystal molecules. After applying an electric
field (applied voltage >10 V) to the 9-mm-thick sample, we observed
a fingerprint texture, indicating that the helical axis was parallel
to the electrode surface. The molecules of the compound 10 have a

126
M. Funahashi and N. Tamaoki
tendency to align parallel to the electric field because the molecule
has induced dipole moment along the molecular axis although its
permanent dipole is perpendicular to the axis. From the periodicity
of the observed fingerprint texture, the helical pitch was ca. 7 mm,
i.e., it was comparable to the thickness of the sample (9 mm). The
helical twisting power of 10 should not be so large as to induce a
helical structure having a short periodicity. Under strong electric field,
we observed the change of the direction of the helical axis from perpen-
dicular to parallel to the surface when electric field was applied. We
performed the electrical measurements in such an alignment that
the helical axis was parallel to the electrode surface.
CHARACTERIZATION OF CARRIER TRANSPORT
IN THE CHIRAL NAMTIC PHASE
We determined the carrier mobility through the use of a conventional
time-of-flight technique [31]; the apparatus consisted of a pulse laser
(THG of a Nd:YAG laser; wavelength, 355 nm; pulse duration, 1 ns) for
excitation, the sample on a hot stage, a serial resistor, and a digital
oscilloscope. All measurements were carried out under atmospheric
condition. The electric field was applied to the liquid crystal cell con-
sisting of two ITO-coated glass plates. The pulse laser illuminated one
side of the cell. When the absorption coefficient of a sample is suffi-
ciently large, the excitation pulse is absorbed and a sheet of photo-
carriers is generated near the illuminated electrode. In this study,
the neat film of 10 exhibits strong absorption band around 400 and
250 nm in the chiral nematic phase at 120ꢂC; the depth of penetration
of the excitation light at 356 nm was estimated to be less than 0.5 mm,
i.e., it was much smaller than the sample thickness. Under the influ-
ence of the electric field, the photogenerated carriers drifted across
the bulk of the sample, inducing a displacement current through the
serial resistor. When the charge carriers arrived at the counter
electrode, the induced displacement current decreased to zero. There-
fore, a kink point in a transient photocurrent curve corresponds to the
transit time tT. Using equation (1), the carrier mobility (l) can be cal-
culated from the values of tT, the sample thickness d, and the applied
voltage V:
d2
l ¼
ð1Þ
VtT
Illuminating the electrode under a positive or negative bias allows the
positive or negative carrier mobility to be determined, respectively.

Organic Semiconductors with Helical Structure
127
RESULT AND DISCUSSION
For the quaterthiophene derivative 10, we measured the transient
photocurrents for the positive and negative carriers—in both the
isotropic and chiral nematic phases—at temperatures from 220 to 90ꢂC.
For positive carriers, we obtained non-dispersive transient photo-
current curves for the isotropic and chiral nematic phases, as indi-
cated in Figure 1. The kink point in each transient photocurrent
curve shifted according to the applied electric field, indicating that
the transient photocurrents originated through carrier transport in
the bulk of the sample. At 120 ꢂC and under an electric field of
5 ꢀ 104 V= cm, the positive carrier mobility was 2 ꢀ 10–4 cm2=Vs. This
value is approximately one order of magnitude higher than the ionic
mobility in conventional nematic liquid crystals, and it is comparable
to that found in the glassy nematic phases of fluorene derivatives.
For negative carriers, we also obtained transient photocurrent
curves with kink points over the entire temperature range, but the
results presented in Figure 2 are somewhat more complicated. In
the isotropic phase, we observed non-dispersive transient photocur-
rents in the longer time range that had mobilities of 10–5 cm2=Vs. In
the chiral nematic phase, we obtained non-dispersive curves in almost
the same time range at temperatures above 170 ꢂC, but the transient
curves became ambiguous upon decreasing the temperature below
FIGURE 1 Transient photocurrent curves for positive carriers at 120ꢂC
(chiral nematic phase). Sample thickness, 4 mm; wavelength of excitation,
355 nm.

128
M. Funahashi and N. Tamaoki
FIGURE 2 Transient photocurrent curves for negative carriers at 50V at
various temperatures. Sample thickness, 9 mm; wavelength of excitation,
355nm.
170 ꢂC, and another transit, at a shorter time range, in addition to that
at the longer time range, appeared at temperatures below 150 ꢂC. The
both kink points appeared at a shorter and longer time range shift to
shorter time when electric field increased and these kink points should
be therefore attributed to carrier transport. The negative carrier
mobility in the shorter time range was 2 ꢀ 10ꢁ4 cm2=Vs and that in
longer time range was 4 ꢀ 10ꢁ5 cm2=Vs. The former value is compara-
ble to the positive carrier mobility of this material at the same
temperature and electric field. The latter value is approximately one
order of magnitude lower than the positive carrier mobility of this
compound, but it is on the same order of magnitude of the ionic carrier
mobilities of conventional nematic liquid crystals.
Figure 3 displays a plot of the carrier mobility as a function of the
electric field and temperature. The positive carrier mobility increased
and eventually became saturated upon increasing the temperature,
but then a discontinuous decrease in the positive carrier mobility
occurred at the temperature of the transition from the chiral nematic
to the isotropic phase.
In contrast to the positive carrier, the negative carrier mobility
at the longer time range increased monotonically upon increasing the
temperature. This change was continuous at the phase transition tem-
perature, indicating that the positive and negative carriers undergo
different transport mechanisms. The negative carrier mobility at the

Organic Semiconductors with Helical Structure
129
FIGURE 3 Positive carrier mobilities plotted as functions of the (a) electric
field and (b) temperature.
shorter time range exhibited a positive dependence on the tempera-
ture, indicating a thermal activation process.
The viscosity of compound 10 at 130 ꢂC, which we measured using a
capillary viscometer, was 0.6 poise, almost the same as that in the
nematic phase of 5CB at room temperature; in that case, ionized
5CB molecules or smaller metal ions were assumed to be the charge
carriers [21]. The carrier mobility in the nematic phase of 5CB, as

130
M. Funahashi and N. Tamaoki
determined through TOF and AC conductivity measurements, is on
the order of 10ꢁ6 to 10ꢁ5 cm2=Vs [20,21]. Therefore, the ionic mobility
in the chiral nematic phase of 10 should be on the order of 10ꢁ5 or
smaller, assuming that the ionic charge carriers are the ionized liquid
crystal molecules. The measured positive carrier mobility, and the
negative one at shorter time range, was 1.5 ꢀ 10ꢁ4 cm2=Vs, which is
one order of magnitude larger than the predicted value; the negative
carrier mobility at the longer time scale was on the same order as
the predicted value. This result implies that the positive and negative
carrier transport at the shorter time range was electronic in nature
and that the negative carrier transport at the longer time range was
ionic in nature if the size of the ionic species was on the same order.
The positive carrier mobility decreased during the transition from
the chiral nematic phase to the isotropic phase upon increasing the
temperature. In this study, the helical axis of the chiral nematic phase
was aligned parallel to the electrode surface under the applied electric
field, and the helical pitch was comparable in size to the sample thick-
ness. Therefore, we regarded the molecular alignment as being in a
nematic-like state consisting of areas in which the molecular axes
were vertical and parallel to the electrode surface. For ionic transport
in the nematic phase, the carrier mobility along and perpendicular to
the director was measured using the time-of-flight method and
through AC conductivity measurements in samples aligned with the
magnetic field. The carrier mobility along the director increased
almost continuously, or decreased slightly, at the point of transition
from the nematic to the isotropic phase upon increasing the tempera-
ture, whereas the carrier mobility perpendicular to the director
increased discontinuously at that point upon increasing the tempera-
ture [17–23]. The mobility along the director was larger than that per-
pendicular to the director, but the anisotropy was small (at 1.5–2, at
best) [17,21]. The discontinuous decrease in the measured positive car-
rier mobility at the transition point should be attributed to an increase
in the disorder and a decrease in the transfer integral based on elec-
tronic, rather than ionic, conduction. The change at the phase tran-
sition temperature was, however, only 1.7, which is much smaller
than the changes observed at the temperatures of transition between
the smectic and isotropic phases of 2-phenylnaphthalene and terthio-
phene derivatives [9,10]. This phenomenon arises because these
phases are more liquid-like than are the smectic phases.
The negative carrier mobility at the longer time range increased
continuously at the phase transition temperature upon increasing
the temperature; this behavior is typical for ionic mobility along the
director in the nematic phase [17,21,22]. Therefore, ionic conduction

Organic Semiconductors with Helical Structure
131
should be valid for negative carrier transport at the longer time range
in the chiral nematic and isotropic phases.
In addition, the temperature dependence of the positive carrier
mobility within the chiral nematic phase of 10 also indicates that elec-
tronic process should be dominant for positive carrier transport in this
phase. In the nematic and chiral nematic phases, the viscosity
decreases monotonically upon increasing the temperature and,
therefore, the ionic carrier mobility should increase monotonically
[17,20,21]. As measured using a capillary viscometer, we found
that the viscosity increased monotonically upon increasing the tem-
perature within the chiral nematic phase of 10. From the Arrhenius
plot of the logarithm of inverted viscosity against 1=T, we estimated
the activation energy to be 0.3 eV. For negative carrier transport,
the Arrhenius plot confirmed that linearity existed between the logar-
ithm of the negative carrier mobility at longer time range and 1=T; the
activation energy of 0.3 eV is identical to that of the viscosity. This
result supports the notion that negative carrier transport at the longer
time range is based on an ionic process. The ionic carriers in this pro-
cess are possibly impurities or liquid crystal molecules that were
ionized upon illumination with the laser. In contrast, the positive
carrier mobility exhibits saturation upon increasing the temperature,
possibly because of competition between thermally activated inter-
molecular electron transfer and structural disorder caused by molecu-
lar movement in the electronic carrier transport process, but not in
the ionic case. Such saturation in hole mobility with respect to
temperature has been observed for discotic columnar and smectic
phases [12,14].
The results of this study indicate that extending the p-conjugated of
a system is an effective means of realizing electronic conduction in
chiral nematic phases. In the nematic and chiral nematic phases of
conventional liquid crystals, however, the p-conjugated systems are
not sufficiently large and, therefore, they exhibit small intermolecular
transfer integrals and low viscosities. Consequently, only ionic carrier
transport has been observed previously in nematic and cholesteric
phases.
SYNTHESIS AND PHASE TRANSITION BEHAVIOR
OF CHIRAL DIMERS 14 AND 15
Compound 10 crystallizes at room temperature and cannot retain
the chiral nematic phase. Dimeric compounds which exhibit glassy
cholesteric phase below room temperature have been reported by N.
Tamaoki [2]. In this study, chiral dimeric oligothiophene derivatives

132
M. Funahashi and N. Tamaoki
SCHEME 2 Synthetic route toward chiral dimer 14 and 15.
containing binaphthyl group as a chiral moiety 14 and 15 were
designed and syntheized. They were synthesized by Pd(0)-catalyzed
Suzuki coupling reaction between propylquaterthienyl boric acid 2,2-
propanediyl ester 13 and binaphthyl derivatives 17 as shown in
Scheme 2.
PHASE TRANSITION BEHAVIOR OF CHIRAL DIMMERS
Phase transition behaviors of these chiral dimmers were characterized
by optical texture observation under polarized light microscope and
differential scanning caloriemetry (DSC). In DSC measurement,
dimer 14 and 15 exhibited a single transition peak corresponding to
the isotropic to cholesteric phase at 163 ꢂC and 155 ꢂC respectively.
Below the transition peak, they exhibited glass transition at 72 ꢂC
and 57 oC respectively. From X ray diffraction, only broad hallo around
2h ¼ 20 degree was observed in the cholesteric phase of these com-
pounds, indicating they have no smectic order. Under polarized light
microscope, Granjan texture was observed, which is characteristic
for cholesteric phase.
REFLECTION SPECTRA OF THE DIMERIC COMPOUNDS
Dimer 14 and 15 exhibited selective reflection band in visible light
wavelength area. And the selective reflection band of these compounds
can be tuned from near infrared to ultraviolet area by temperature
and mixing with monomeric compound. Figure 4 shows reflection
spectrum of the mixture of dimer 15 and monomeric compound 10 at
various temperature. It should be noted that the wide reflection band

Organic Semiconductors with Helical Structure
133
FIGURE 4 Selective reflection spectra of the 1:1 mixture of 10 and 15 at
various temperature.
width exceeding over 100 nm and broad tunability from near infrared
to ultraviolet area. This prominent properties should be attributed to
large anisotropy in refractive index due to long extended p-conjugated
system. Therefore we can fabricate cholesteric thin films with various
reflection colors.
CIRCULARLY POLARIZED PHOTOLUMINESCENCE
FROM THE CHOLESTERIC FILMS
Circularly polarized light photoluminescence from cholesteric films
was reported by Chen et al. [33]. and M. O’Neill who reported circu-
larly polarized light photoluminescence from glassy cholesteric
semiconductor with wide wavelength range [3].
The mixture of dimer 15 and monomeric compound 10 form well
aligned cholesteric thin film could be fabricated on substrates on
which rubbed polyimide film was coated. Circularly polarized photo-
luminescence was observed through a filter consisting of k=2 and k=4
plates, illuminating UV light (wavelength ¼ 356 nm) from the back
side of the film sample. Figure 5 shows circularly polarized light spec-
tra from the 1:1 mixture. These compounds have photoluminescence
band between 500 and 700 nm and exhibit orange color. Within the
reflection band, circularly polarized luminescence with different heli-
city from the cholesteric film was suppressed, while circularly polar-
ized luminescence with same helicity was emitted with no change.

134
M. Funahashi and N. Tamaoki
FIGURE 5 Circularly polarized photoluminescence of the 1:1 mixture of 10
o
and 15 at 102.5C.
The circularly polarization parameter reached up to 1.3. At the band
edge, distortion in the spectrum of with different helicity from the cho-
lesteric film was observed. This distortion is associated with laser
emission. These cholesteric film can transport hole and they should
be applied electrically-pumped opt-electronics devices.
In conclusion, we have synthesized a liquid crystalline semicon-
ductor, 2-[5-(4-[(S)-2-methylbutoxy]phenyl)-3-methyl]-2000-propyl-2:50-20:
500-200:5000-quaterthiophene (10), which exhibits a chiral nematic
phase. We used the time-of-flight technique to determine its carrier
mobility. In the chiral nematic phase, the positive carrier mobility
was 2ꢀ 10–4 cm2=Vs at 120 ꢂC and the negative carrier mobility was
1ꢀ 10–4 cm2=Vs; they are both attributed to electronic processes. For
the negative carrier, in addition to the fast electronic process, we also
observed a slow carrier transport having mobility on the order of 10–
5 cm2=Vs, which we attribute to ionic conduction.
Chiral dimers 14 and 15 containing binaphthyl group exhibited
glassy cholesteric phase around room temperature and cholesteric
thin films with selective reflection band in visible light area could be
fabricated. They emitted circularly polarized light when UV light
was illuminated. The dichroic parameter reached up to 1.3.
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