Mesomorphic behaviors and charge carrier transport in terthiophene derivatives

Article abstract of DOI:10.1080/15421400490434315

We synthesized dialkylterthiophene derivatives ω-substituted with cyclohexylethyl moiety, which exhibit smectic mesophases at room temperature, and investigated their charge carrier transport properties. In particular, transpentylcyclohexylethyl-propylterthiophene (3-TTP-C2CH-5) held SmB phase at room temperature. Furthermore, a 3:1 mixture of 3-TTP-C2CH-5 and (2-methylbutyl)-dodecylterthiophene (5*-TTP-12) showed the SmB phase between 101°C and -15°C, and the highly ordered smectic phase below -15°C. In the SmB phase of the mixture, hole and electron mobility was 10^-3 cm²/Vs, and independent of temperature and electric field over room temperature. However, below room temperature, the mobility depended on both temperature and electric field, as is often observed in organic amorphous solids and formarized by Gaussian disorder model, suggesting that the carrier transport in the smectic mesophase can be explained by the hopping conduction in disordered molecular sites.

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Mesomorphic behaviors and
charge carrier transport in
terthiophene derivatives
Masahiro Funahashi a & Jun-Ichi Hanna a
a Imaging Science and Engineering Laboratory ,
Tokyo Institute of Technology , Midori-ku, Yokohama,
Japan
Published online: 18 Oct 2010.
To cite this article: Masahiro Funahashi & Jun-Ichi Hanna (2004) Mesomorphic
behaviors and charge carrier transport in terthiophene derivatives, Molecular Crystals
and Liquid Crystals, 410:1, 1-12, DOI: 10.1080/15421400490434315
To link to this article: http://dx.doi.org/10.1080/15421400490434315
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Mol. Cryst. Liq. Cryst., Vol. 410, pp. 1=[529]–12=[540], 2004
Copyright # Taylor & Francis Inc.
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080=15421400490434315
MESOMORPHIC BEHAVIORS AND CHARGE
CARRIER TRANSPORT IN TERTHIOPHENE
DERIVATIVES
Masahiro Funahashi and Jun-ichi Hanna
Imaging Science and Engineering Laboratory, Tokyo Institute of
Technology, Nagatsuta 4259, Midori-ku, Yokohama 226-8503, Japan
We synthesized dialkylterthiophene derivatives x-substituted with cyclohexyl-
ethyl moiety, which exhibit smectic mesophases at room temperature, and
investigated their charge carrier transport properties. In particular, trans-
pentylcyclohexylethyl-propylterthiophene (3-TTP-C2CH-5) held SmB phase at
room temperature. Furthermore, a 3:1 mixture of 3-TTP-C2CH-5 and (2-
methylbutyl)-dodecylterthiophene (5ꢀ-TTP-12) showed the SmB phase between
101ꢁC and ꢂ15ꢁC, and the highly ordered smectic phase below ꢂ15ꢁC. In the
SmB phase of the mixture, hole and electron mobility was 10 ꢂ 3 cm2=Vs, and
independent of temperature and electric field over room temperature. How-
ever, below room temperature, the mobility depended on both temperature
and electric field, as is often observed in organic amorphous solids and formar-
ized by Gaussian disorder model, suggesting that the carrier transport in the
smectic mesophase can be explained by the hopping conduction in disordered
molecular sites.
Keywords: carrier transport; photoconductivity; semiconductor; smectic phase; time-of-flight
INTRODUCTION
Recently fast electronic conduction has been discovered not only discotics
[1] but also smectics [2–4] whose conduction had been believed to be ionic
for a long time. Now, the liquid crystals are being recognized as a new class
of organic semiconductors. The carrier mobility in the liquid crystals is in a
range of 0.1 ꢃ 10ꢂ4 cm2=Vs. This is 100 to 10000 times larger than those of
amorphous organic semiconductors that are practically used in photore-
ceptors for photocopiers and laser printers [5], in addition to the recent
application to organic light emitting diodes [6].
Address correspondence to Masahiro Funahashi, Imaging Science and Engineering Labora-
tory, Tokyo Institute of Technology, Nagatsuta 4259, Midori-ku, Yokohama 226-8503, Japan.
1=[529]

2=[530]
M. Funahashi and J. Hanna
On the other hand, the carrier transport does not depend on the
temperature and electric field [1–4], which is often observed in molecular
crystals [7]. Thus, here comes a question, what is the essential difference
in charge carrier transport between the ordered smectic phases and the
crystalline phase. However, the fast carrier transport observed so far is
limited only at a high temperature range for each mesophase over room
temperature due to the crystallization of the materials. Before answering
this question, we have to have a reliable data on the carrier transport in
liquid crystalline phases in such a wide temperature range that we can deal
with the data for analysis. Lowering and extending of the temperature
range of the smectic phases is indispensable not only for practical appli-
cation in the future but also for understanding a basic characteristics of
the carrier transport in the liquid crystalline phases. Thus, we selected
terthiphene derivatives as a model material and synthesized several
derivatives x-substituted with cyclohexylethyl moiety for lowering the
crystallization temperature and characterized their charge carrier transport
properties. In this paper, we report a preparation of the terthiophene liquid
crystal semiconductor which exhibits smectic phases in a wide temperature
range from 100 to ꢂ100ꢁC and its excellent carrier transport properties
determined by time-of-flight technique. In addition, we will discuss the
carrier transport mechanism in the smectic phases on the basis of the
carrier transport data obtained.
MATERIAL DESIGN AND SYNTHESIS
We selected terthiophene core moiety as a model compound on the basis of
previous study. As reported, 2,5-dialkyl-2:5:2:5-terthiophene derivatives
exhibit tilted smectic phases, such as SmC, SmF and SmG, and fast
ambipolar electronic conduction takes place [8,9]. Carrier mobility is on
the order of 10ꢂ4 cm2=Vs in the SmC phase, 10ꢂ3 cm2=Vs in the SmF phase,
and 10ꢂ2 cm2=Vs in the SmG phase.
In order to lower crystallization temperature, methyl group is usually
substituted on one side of aromatic core [10]. This approach is surely
effective to inhibit crystallization, but often instabilize liquid crystal phases.
Introduction of Cyclohexylethyl group on the 2 position of 5-alkyl-2:5:2:
5-terthiophene should be effective because this group works as a part of
rigid core moiety which stabilize liquid crystal phases, as well as bulky
substituent preventing molecular packing.
The cyclohexylethylterthiophene derivatives were synthesized as shown
in Figure 1. 5-Alkyl-2:5:2:5-terthiophene skeleton was constructed by
Ni(0)-catalyzed coupling reaction between thienyl magnesium bromide
and corresponding thienylaryl bromide. After Friedel-Crafts acylation

Mesomorphic Behaviors and Charge Carrier Transport
3=[531]
FIGURE 1 Synthetic route of cyclohexylethylterthiophene derivatives.
reaction of the terthiophene skeleton with cyclohexylacetylmethylcarbonyl
chloride and aluminum chloride, cyclohexylethylterthiophene derivatives
was obtained by reduction of the carbonyl group with aluminum chloride
and lithium aluminum hydride in good yields. The crude product was
purified with silicagel column chromatography and recrystalization from
n-hexane.
PHASE TRANSITION BEHAVIORS
The cyclohexylethylterthiophene derivatives synthesized exhibit SmA and
SmB phases, and those which have small alkyl chain exhibit nematic phase
as well as the smectic phases. The phases of these materials were identified
by the observation of micrographical textures under polarized light
microscope and miscibility test with a standard material characterized well.
Phase transition temperature was determined by differential scanning
caloriemetry. The results are summarized in Table 1. Temperature ranges
of these cyclohexylethyl derivatives are wider than those of di(n-alkyl)
derivatives [9,11].
TABLE 1 Mesomorphic Behavior of Terthiophene Derivatives in Cooling Process
12-TTP-C2CH-5 (R ¼ n-dodecyl) Iso 149ꢁC SmA 129ꢁC SmB 92ꢁC Cryst.
6-TTP-C2CH-5 (R ¼ n-hexyl)
3-TTP-C2CH-5 (R ¼ n-propyl)
Iso 154ꢁC N 153ꢁC SmA 125ꢁC SmB 60ꢁC Cryst.
Iso 160ꢁC N 139ꢁC SmA 112ꢁC SmB 11ꢁC Cryst.
Iso 58ꢁC SmCꢀ 46ꢁC Cryst.
5ꢀ-TTP-12=3-TTP-C2CH-5 (3:1) Iso 128ꢁC Nꢀ 118ꢁC SmA 100ꢁC SmB ꢂ23ꢁC SmX
ꢀ5ꢀ-TTP-12:2-(S)-isoamyl-5-dodecyl-2:5:20:50-terthiophene.

4=[532]
M. Funahashi and J. Hanna
TOF MEASUREMENT SET UP
The charge carrier transport properties were investigated by conventional
time-of-flight (TOF) technique, consisting of N2 pulse laser (k ¼ 337 nm,
pulse duration ¼ 600 ps), digital oscilloscope, and hot stage whose
temperature is controlled by PID thermocontroller and on which liquid
crystal cell made from two ITO coated glass plates, is fixed. In this method,
transit time, tT is determined from in transient photocurrent induced by
drift of photo-generated carriers. In the present measurements, all the
transient photocurrents were non-dispersive, so that we could determined
the transit time from their shoulder . Carrier mobility, l is determined as
drift velocity of carriers per unit electric field, and therefore l is calculated
as Eq. (1).
d2
l ¼
ð1Þ
VtT
where d is sample thickness and V is applied voltage.
The liquid crystal molecules align parallel to the electrode surfaces, so
that the carriers travel within the smectic layers. No change in molecular
alignment was observed under polarized light microscope in the entire
range of applied electric field.
CARRIER TRANSPORT CHARACTERISTICS
In particular, 2-propyl-50-(trans-4-pentyl)cyclohexylethyl-2:5:20:50-terthio-
phene (3-TTP-C2CH-5) shows SmB phase around room temperature.
And a 3:1 mixture of 3-TTP-C2CH-5 and 5ꢀ-TTP-12 retain highly ordered
smectic phase at ꢂ100ꢁC.
Each component of the mixture exhibits ambpolar intrinsic electronic
carrier transport in their smectic phases. Hole and electron mobility of 3-
TTP-C2CH-5 is 1 ꢄ 10ꢂ3 cm2=Vs in the SmB phase. 5ꢀ-TTP-12 also shows
hole and electron mobility of 3 ꢄ 10ꢂ4 cm2=Vs in the SmCꢀ phase. And they
are independent of both temperature and electric field, which is judged to
be their intrinsic characteristic on the basis of the electronic transport in
smectic phases of 2-phenylnaphthalene and dialkylterthiophene derivatives
which we have already reported [3,4,8,9]. Therefore the mixture is
expected to show intrinsic electronic transport in the smectic phases.
Figure 2 shows transient photocurrent curves for positive and negative
carriers in the SmB phase of the mixture at 80ꢁC. All the photocurrents are
non-dispersive for both carriers, as the kink point is clearly seen. From
these curves, mobility was obtained to be 2 ꢄ 10ꢂ3 cm2=Vs for both
carriers, which was field-independent and comparable to those observed

Mesomorphic Behaviors and Charge Carrier Transport
5=[533]
FIGURE 2 Transient photocurrent curves in the SmB phase of the mixture at 80ꢁC.
The cell thickness was 15 lm, excitation wavelength 337 nm. (a) for positive carrier
(b) for negative carrier.
in the SmB phases of 2-phenylnaphthalene derivatives and SmF phases of
dialkylterthiophene derivatives.
Interestingꢁly, carrier mobility depend on neither temperature nor electric
field above 50 C indeed, but it gradually decreased down to 1 ꢄ 10ꢂ3 cm2=Vs
and depended on field and temperature below 50ꢁC. However, the transient
photocurrents were unchanged to be non-dispersive even below 50ꢁC,
indicating that the charges still do not suffer from the inhomogeneity of
the hopping sites and the trapping at impurities and, therefore, exhibit
the intrinsic carrier transport of its own at that temperature range.

6=[534]
M. Funahashi and J. Hanna
In further down below room temperature, that is, in highly ordered
smectic phase (SmX), mobility did depend on temperature and electric
field clearly. Figure 3 shows transient photocurrent curves for positive car-
rier in SmX phase of the mixture at ꢂ50ꢁC. The negative carrier transport
was no longer observed and the current signal was lowered for positive
carrier at this temperature range. In addition, temperature- and electric-
field-dependence of carrier mobility became more remarkable.
Figure 4 shows the carrier mobility as a function of temperature in the
entire range measured. As is described above, above 50ꢁC, carrier mobility
is independent of electric field and temperature, but it starts to depend on
temperature and its dependence becomes remarkable below room tem-
perature, and so does field-dependence obvious with decreasing tempera-
ture. This field- and temperature-dependence is quite similar to that of the
hopping conduction in amorphous organic semiconductors, which can be
described by Gaussian disorder model [12] where the hopping sites are
distributed in Gaussian shape in terms of hopping distance and energy
level. Thus, we adopted this Gaussian disorder model to analyze the field-
dependence of hole mobility below room temperature.
ANALYSIS OF CARRIER TRANSPORT CHARACTERISTICS
BASED ON DISORDER MODEL
In Gaussian disorder model [12], assuming the hopping transport of the
carriers among the hopping sites Gaussian distributed in energy level with
the aid of thermal activation, carrier mobility l is formalized by following
equation
h
i
h
i
pffiffiffi
2
l ¼ l0 ꢅ exp ꢂðar^Þ ꢅ exp Cðr^2 ꢂ R2Þ F
R ꢆ 1:5
R ꢇ 1:5
h
i
h
i
ð2Þ
pffiffiffi
2
l ¼ l0 ꢅ exp ꢂðar^Þ ꢅ exp Cðr^2 ꢂ 2:25Þ F
where F is a given electric field, T temperature, C the constant, and m0
pre-exponential factor. Disorder parameter, r is a width of the Gaussian
distribution of energy levels of the hopping sites and R corresponds to
the spatial disorder. In three dimensional transport as observed in
amorphous organic solids, the coefficient, a is 2=3, however, a should be
between 1 and 2=3 in the two dimensional transport. In fact, the carrier
transport takes place pre-dominantly within smectic layers because of a
big difference in the molecular distance between intre- and intersmectic
layers, which has been confirmed experimentally by evaluating the effects
of dilution and impurity doping on the carrier mobility as a function of
concentration.

Mesomorphic Behaviors and Charge Carrier Transport
7=[535]
FIGURE 3 Transient photocurrent curves in the SmX phase of the mixture at
ꢂ50ꢁC. The cell thickness was 15 lm, excitation wavelength 337 nm. (a) linear plot
(b) double logarithmic plot.

8=[536]
M. Funahashi and J. Hanna
FIGURE 4 Carrier mobility as a function of (a) electric field and (b) temperature
(electric field ¼ 1 ꢄ 105 V=cm).

Mesomorphic Behaviors and Charge Carrier Transport
9=[537]
In contrast to the carrier mobility independent of electric field and
temperature above 50ꢁC, logarithm of hole mobility is proportional to
square root of electric field, and their dependence becomes obvious with
a decrease in temperature below room temperature. This tendency is same
in the present two smectic phases, i.e., SmB and SmX phases, although a
discontinuity is observed at the phase transition temperature.
Next, disorder parameters were determined from measured data. Figure
5(a) and (b) exhibit l at zero field and the coefficient, b ¼ C(r2 ꢂ R2) in
the Eq. (2) as a function of temperature for SmB and SmX phases, respect-
ively. In these two phases, all data points are almost on a line, although data
are slightly scattered at low temperature due to small photosignals.
b increases with a decrease in temperature, which is the typical behavior
in disordered organic semiconductors. From this plot, we can determine l0,
r, R, and C and they are listed in Table 3.
In these two smectic phases, disorder parameter r and R are much
smaller than those in organic amorphous solids, in which r is usually
100 meV and R is 2.5. This should be attributed to fairly homogeneous
hopping sites in terms of hoping levels and distance originated from the
molecular alignment in these smectic phases. Furthermore, with taking into
account of the thermal energy of 23 meV at room temperature, this can
explain why the mobility does not depend on electric field- and tempera-
ture above room temperature. Carrier mobility without disorder, l0, that
corresponds to carrier mobility in non-disorder system is on the order of
10ꢂ2 cm2=Vs, which is larger than moleculary doped polymer with small
organic semiconductor molecules [13], but it is almost the same order as
vacuum evaporated triphenylamine derivatives [14]. This large l0 should
be due to closely packed in layers resulting in large intermolecular orbital
overlap among the neighboring molecules in these phases. Comparing
these two smectic phases, r and R are almost same although l0 in the
SmX phase is larger than that in the SmB phase by the factor of 2. Struc-
tural difference between the two phases is considered to be very small,
judging from a small transition enthalpy and small change of dielectric
anisotropy, and therefore, the disorder parameter should be same as
determined. Only intermolecular packing becomes closer at the phase
transition from the SmB to the SmX phase.
DISCUSSION ON CARRIER TRANSPORT MECHANISM
IN SMECTIC PHASES
As mentioned above, hole transport in the smectic phases below 50ꢁC was
clearly described by Gaussian disorder model, which is similar to amorph-
ous organic semiconductors. This result indicates that hole transport in

10=[538]
M. Funahashi and J. Hanna
FIGURE 5 (a) l0 and (b) b as a function of temperature.

Mesomorphic Behaviors and Charge Carrier Transport
11=[539]
TABLE 2 Disorder Parameters in the Two Smectic Phase of the Mixture
l0 (cm2=Vs)
r (meV)
R
C (cmꢂ0.5V ꢂ0.5
SmB phase
SmX phase
2.4 ꢄ 10ꢂ2
4.3 ꢄ 10ꢂ2
50.7
50.1
1.77
1.67
1.02 ꢄ 10 ꢂ 3
9.07 ꢄ 10 ꢂ 4
the smectic phases can be explained as hopping transport in two-dimen-
sional disordered system, with a smaller disorder originated from crystal-
like molecular alignment and close packing structure in smectic phases.
Above 50ꢁC, most of photo-generated carriers are thermally activated to
transport level, giving rise to temperature- and electric field-independent
carrier mobility. Not only this mixture, but also the smectic phases of other
liquid crystalline semiconductor which we have previously reported, exhi-
bit the same temperature- and field-independent carrier mobility at their
mesophases over room temperature, which are understood in the same
framework of a narrow distribution of hopping site energy, i.e., a small r
of about 50 meV. Molecular alignment and small structural disorder in the
smectic phases result in large l0 and small r and R, leading to fast carrier
mobility independent of temperature and electric field above room
temperature.
In summary, we synthesized a new terthiophene derivative exhibiting
smectic mesophases below room temperature and made a mixture exhibit-
ing a highly ordered smectic mesophase down to –100ꢁC and investigated
its carrier transport properties. According to the analysis of their transport
properties at different temperature and electric field, we have deduced
that the carrier transport process in these smectic phases is described by
charge carrier hopping in disordered system with a small disorder
attributed to crystal-like ordered structure in smectic mesoiphase. It is very
intriguing that even small disorder in the smectic phases causes amorph-
ous-like carrier transport, and this clearly distinguishes between the crystal
phases and highly ordered smectic phases from a view point of the carrier
transport.
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