High carrier mobility up to 0.1 cm2/Vs and a wide mesomorphic temperature range of alkynyl-substituted terthiophene and quaterthiophene derivatives

Article abstract of DOI:10.1080/15421400590954722

Smectic semiconductors with high carrier mobility were designed and synthesized. The hole mobility of 3-TTP-yne-4 and 3-QTP-yne-3 at room temperature was 0.06cm²/Vs and 0.1cm²/Vs, respectively. They exhibit highly ordered smectic phases around room temperature, with softness to some extent, which make it possible homogeneous film with low defect density. Intermolecular distance in the smectic phase of 3-QTP-yne-4 is almost same as that of 3-TTP-yne-4. In this material, larger n-conjugate system results in higher carrier mobility.

Full text of DOI:10.1080/15421400590954722

Mol. Cryst. Liq. Cryst., Vol. 436, pp. 225=[1179]–235=[1189], 2005
Copyright # Taylor & Francis Inc.
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400590954722
High Carrier Mobility up to 0.1 cm2/Vs and a Wide
Mesomorphic Temperature Range of Alkynyl-Substituted
Terthiophene and Quaterthiophene Derivatives
Masahiro Funahashi
Jun-ichi Hanna
Imaging Science and Engineering Laboratory, Tokyo Institute of
Technology, Nagatsuta, Midori-ku, Yokohama, Japan
Smectic semiconductors with high carrier mobility were designed and synthesized.
The hole mobility of 3-TTP-yne-4 and 3-QTP-yne-3 at room temperature was
0.06 cm2=Vs and 0.1 cm2=Vs, respectively. They exhibit highly ordered smectic
phases around room temperature, with softness to some extent, which make it poss-
ible homogeneous film with low defect density. Intermolecular distance in the
smectic phase of 3-QTP-yne-4 is almost same as that of 3-TTP-yne-4. In this
material, larger p-conjugate system results in higher carrier mobility.
Keywords: carrier mobility; disorder model; oligothiophene; semiconductor; smectic
phase; time-of-flight technique
INTRODUCTION
Recently development of organic semiconductors is giving rise to
organic electronics aiming at flexible display [1]. In organic single
crystals with extremely high purity, carrier mobility is usually on
the order of 10ꢀ1 cm2=Vs at room temperature [2]. However, in con-
trast to the organic single crystals, practical amorphous organic semi-
conductors which can be applied to thin films with large area, exhibit
very low carrier mobility on the order of 10ꢀ7ꢁ10ꢀ4 cm2=Vs [3]. This is
attributed to energetic and spatial disorder of organic molecules, and
The author (M. Funahashi) thanks to the association for progress of new chemistry,
Sumitomo foundation, and Grant-in-aid for scientific research (No. 16750158) for finan-
cial support.
Address correspondence to Masahiro Funahashi, Imaging Science and Engineering
Laboratory, Tokyo Institute of Technology, Nagatsuta 4259, Midori-ku, Yokohama
226-8503, Japan. E-mail: funa@isl.titech.ac.jp
225=[1179]

226=[1180]
M. Funahashi and J.-i. Hanna
carrier mobility and its field- and temperature-dependence is
described by Gaussian disorder model [4].
In liquid crystal phases, the energetic and spatial disorder, should
be reduced by molecular alignment. We have verified fast electronic
conduction in smectic liquid crystals with aromatic p-conjugated sys-
tems [5–8,10], which has been believed to be ionic for a long time
[11], while fast hole transport was observed in discotic columnar
phases [12–14]. And we have also demonstrated their applications to
opt-electronics devices such as photosensors [15], xerography [16],
and light-emitting diode [17].
For practical application, it is indispensable to establish material
design to realize high carrier mobility as well as wide mesomorphic
temperature range around room temperature. However, lowering
mesomorphic temperature range is usually incompatible with fast
carrier transport. Fast intrinsic carrier transport is observed only in
highly ordered smectic phases, which usually have a tendency to
crystallize above room temperature, due to strong p-p interaction.
Introduction of lateral substitutents is often effective on lowering tem-
perature range of mesophases, however closer molecular packing is
usually inhibited, resulting in disappearance of highly ordered smectic
phases or low carrier mobility, caused by decrease in transfer integral.
We have already reported that cylohexylethylterthiophene deriva-
tive mixture exhibit smectic phases and fairly good carrier transport
in wide temperature range from ꢀ100ꢂC to 100ꢂC because of the pres-
ence of bulky cyclohexylethyl group inhibiting crystallization [10].
However the smectic phases at room temperature was smectic B hexa-
tic, and the hole and electron mobility was at best on the order of
10ꢀ3 cm2=Vs at room temperature [10].
In this report, we report oligothiophene derivatives which exhibit
highly ordered smectic phases in wide temperature range around
room temperature. And we show alkynyl-substituted terthiophene
and quaterthiophene derivatives exhibit excellently high carrier
mobility up to 0.1 cm2=Vs.
MATERIAL DESIGN AND SYNTHESIS
We designed new liquid crystalline semiconductors based on terthio-
phene and quaterthiophene which are structural units of promising
organic semiconductors, oligothiophene derivatives. In general, strong
intermolecular p-p interaction increase isotropic-mesophase transition
temperature, while increasing crystallization temperature. In fact,
dialkylterthiophene [8] and dialkylquaterthiophene [18] crystallize
above room temperature. In order to decrease crystallization

High Carrier Mobility up to 0.1 cm2=Vs
227=[1181]
FIGURE 1 Synthetic route of alkynyl-substituted terthiophene and
quaterthiophene derivatives.
temperature, introduction of moiety, which cause asymmetry in
molecular shape should be effective. Instead of alkyl group, alkynyl
group was introduced on 2 position of terthiophene or quarterthio-
phene skeleton.
As shown in Figure 1, 2-alkyl-5:50:20:200-terthiophene was synthe-
sized by repeated Tamao coupling reaction catalyzed by Ni(dppp)Cl2
[19] between thienyl magnesium reagent and thienyl bromide in
diethyl ether from 2-alkyl-5-bromothiophene in 3 steps. The obtained
alkylterthiophene derivatives was iodinated with iodine after
lithiation of the 500 position of the terthiophene derivatives in THF
at ꢀ78ꢁC. The iodide was coupled with excessive 1-hexyne were
coupled in the presence of triethylamine and catalytic amount of
Pd(PPh3)2Cl2 in THF at room temperature, producing 2-alkyl-500-
hexynyl-3:50-20:200-terthiophene in the yield of 76%. The iodide was
coupled with 2-thenyl magnesium bromide in the presence of catalytic
amount of Ni(dppp)Cl2 in THF under refluxing condition. The
obtained 2-alkylquaterthiophene was iodized in the same way. The
resulting iodide and excessive 1-hexyne were coupled in the presence
of triethylamine and catalytic amount of Pd(PPh3)2Cl2 in THF
at room temperature, producing 2-alkyl-500-hexynyl-2:50-20:500-200:5000-
quaterthiophene in the yield of 70%.
Phase Transition Behaviors and Structural Characterization
The phases of the materials were identified by the observation of
optical textures under polarized light microscope, miscibility test

228=[1182]
M. Funahashi and J.-i. Hanna
FIGURE 2 X-ray diffraction patterns of (a) 3-TTP-yne-4 and (b) 3-QTP-yne-4
at 30ꢂC.
and X-ray diffraction. Phase transition temperature was determined
by differential scanning caloriemetry.
These materials precipitate as crystals at room temperature, how-
ever highly ordered smectic phases, which are characterized by mosaic
optical textures under observation of polarized light microscope, when
they are cooled down from isotropic melting liquids. In DSC measure-
ment, only one exothermal peak was observed at 98ꢂC for 3-TTP-yne-4
and at 196ꢂC for 3-QTP-yne-4 cooling from isotropic liquids to ꢀ50ꢂC,
indicating no signs of crystallization, therefore these three compounds
exhibit single smectic phases respectively around room temperature in
cooling process.
As shown in Figure 2, diffraction peaks in low angle region associated
with layer structures and two weak peaks in high angle region related to
molecular order in the smectic layers appear in X-ray diffraction curves
of these three compounds. Molecular order within the smectic layers of
these three compounds at room temperature is hexagonal, since the
pffiffiffiffiffiffiffiffi
ratio of lattice constants in high angle region is approximately 3=2.
In the cases of 6-TTP-yne-4 and 3-TTP-yne-4, lattice constants of [100]
are identical with molecular lengths with all-trans conformation of alkyl
chains. Therefore the smectic phase of 6-TTP-yen-4 and 3-TTP-yne-4 can
be identified to be smectic B crystal phase. In the case of 3-QTP-yne-4,
the constant is shorter than molecular length, and therefore the phase
should be characterized to be tilted hexagonal smectic phase, such as
smectic G or smectic J phase. The diffraction angles, Miller indices,
and lattice constants are listed in Table I.
TOF MEASUREMENT SET UP
Carrier mobility was determined by conventional time-of-flight
(TOF) technique, consisting of N2 pulse laser (k ¼ 337 nm, pulse

High Carrier Mobility up to 0.1 cm2=Vs
TABLE I X-ray Diffraction Measurement of 3-TTP-yne-4 and 3-QTP-yne-4
3-QTP-yne-4 3-TTP-yne-4
229=[1183]
2h
h
[h k l]
d(A)
2h
h
[h k l]
d(A)
3.67
1.83
3.45
9.85
001
002
100
110
24.11
12.80
4.50
4.20
8.00
19.46
23.30
2.10
4.00
9.73
001
002
100
110
21.04
11.05
4.56
6.90
19.70
22.97
11.48
3.87
11.65
3.82
duration¼600 ps), digital oscilloscope, and hot stage whose tempera-
ture 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 a kink point in transient photocur-
rent induced by drift of photo-generated carriers. 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 aligned parallel to the electrode sur-
faces, and no change in molecular alignment was observed under
polarized light microscope when electric field was applied to the cells.
Therefore carrier transport within smectic layers should be observed
under the condition.
CARRIER TRANSPORT CHARACTERISTICS OF 3-TTP-YNE-4
First, carrier transport characteristics in the SmBcryst phase of
3-TTP-yne-4 is studied with TOF technique from 90ꢂC to ꢀ100ꢂC.
The hole mobility which was clearly determined in the whole tempera-
ture range, and its field- and temperature-dependence is discussed.
Figure 3 shows transient photocurrent curves for hole at 30ꢂC. Non-
dispersive transient photocurrent is observed. From these curves,
field-independent mobility is obtained to be 6 ꢃ 10ꢀ2 cm2=Vs, which
was larger than those observed in the highly ordered SmE and SmG
phases of 2-phenylnaphthalene and dialkylterthiophene derivatives
and the hole mobility in the discotic columnar plastic phase of
hexaalkoxytriphenylene derivatives [12,13].

230=[1184]
M. Funahashi and J.-i. Hanna
FIGURE 3 Transient photocurrent curves for hole in the SmBcryst phase of
3-TTP-yne-4 at 15ꢂC. The cell thickness was 25 mm, excitation wavelength
337 nm. (a) linear plot (b) double logarithmic plot.
And the hole mobility is independent of electric field and tempera-
ture above ꢀ20ꢂC, which is similar to those of the smectic phases of
these materials above room temperature. This result indicates that
fast electronic conduction is dominant in the SmBcryst phase of this
compound.
However, the carrier mobility decreases with decrease in tempera-
ture, being field- and temperature-dependent below ꢀ20ꢂC. As shown
in Figure 4, non-dispersive transient photocurrent curves are still
observed, suggesting intrinsic carrier transport even below room
temperature, however carrier transport mechanism might become
different from those above ꢀ20ꢂC. In addition, the temperature-and
electric-field-dependence of the hole mobility becomes more remark-
able with decrease in temperature.
FIGURE 4 Transient photocurrent curves in SmBcryst phase of 3-TTP-yne-4
at ꢀ50ꢂC. The cell thickness was 25 mm, excitation wavelength 337 nm.
(a) linear plot (b) double logarithmic plot.

High Carrier Mobility up to 0.1 cm2=Vs
231=[1185]
FIGURE 5 Carrier mobility as a function of (a) electric field and (b) tempera-
ture.
Figure 5 exhibits temperature- and electric-field-dependence of
carrier mobilities. Above ꢀ20ꢂC, carrier mobility is independent of
electric field and temperature. However the dependence is remarkable
below room temperature, and field-dependence becomes larger with
decreasing temperature. This behavior is quite similar to those of
hopping conduction in amorphous organic semiconductors, which
can be described by Gaussian disorder model [4]. Therefore the
field-dependence of hole mobility below ꢀ20ꢂC is analyzed based on
Gaussian disorder model.
ANALYSIS OF CARRIER TRANSPORT CHARACTERISTICS
BASED ON DISORDER MODEL
In Gaussian disorder model [4], assuming gaussian distribution of
energy levels of hopping sites, and thermal activation of carriers
relaxed in the distribution to transport level, based on carrier hopping
between molecules, carrier mobility l is expressed as shown in Eq. (2),
h
i
h
i
pffiffiffiffi
2
l ¼ l0 ꢄ exp ꢀðar^Þ ꢄ exp Cðr^2 ꢀ R2Þ F
R ꢅ 1:5
R ꢆ 1:5
h
i
h
i
ð2Þ
pffiffiffiffi
2
l ¼ l0 ꢄ exp ꢀðar^Þ ꢄ exp Cðr^2 ꢀ 2:25Þ F
where F electric field, T temperature, and C is constant. Preexpon-
ential factor l0 corresponds to transfer integral. Disorder parameter
r is width of the gaussian distribution of orbital levels. R is associated
with spatial disorder. In three dimensional transport as observed in
amorphous organic solids, coefficient a is 2=3, however a should be
0.8 in this two dimensional transport restricted within smectic layers
[20] which has been confirmed by trapping effect.

232=[1186]
M. Funahashi and J.-i. Hanna
FIGURE 6 l0 and b as a function of 1=kT2.
In contrast to carrier mobility independent of electric field and
temperature above ꢀ20ꢂC, logarithm of hole mobility is proportional
to square root of electric field, and their dependence increases with
decrease of temperature below ꢀ20ꢂC.
Figure 6 exhibits l at zero field and the coefficient b ¼ @l=@F
¼ Cðr2 ꢀ R2Þ, the dependence of carrier mobility on electric field as a
function of temperature. In the phase, good fits are obtained in these
two plots, although data are slightly scattered at low temperature. b
increases with decrease of temperature, which is typical behavior of
disordered organic semiconductors. From this plot, l0, r, R, and C were
determined to be 6.5 cm2=Vs, 47 meV, 1.9, and 2.85 ꢃ 10ꢀ3 V0.5cmꢀ0.5
,
respectively.
In the smectic phase, 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 molecular alignment in these
smectic phases, which should cause field- and temperature-inde-
pendent carrier mobility above room temperature. Carrier mobility
without disorder, l0, is on the order of 10ꢀ0 cm2=Vs, which is much lar-
ger than amorphous organic semiconductors including moleculary
doped polymers and vacuum evaporated triphenylamine derivatives
[21]. This large l0 should be due to close packing structure and
resulting large intermolecular orbital overlap in the highly ordered
smectic phase.

High Carrier Mobility up to 0.1 cm2=Vs
233=[1187]
The transition from the temperature region where the mobility is
field- and temperature-dependent to that where it is independent of
field and temperature is observed as the case of cyclohexylethyl-
terthiophene mixture. One possibility is that most of photo-generated
carriers are thermally excited to transport level in gaussian density of
states in high temperature region, resulting in field- and temperature-
independent hole mobility. However, the transition temperatures are
different between these two systems in spite of almost same r’s, distri-
bution of the density of states. Freezing of molecular movement may
be associated with the transition.
The comparison between this result and that of cyclohexylethyl-
terthiophene mixture, which exhibits less highly ordered smectic B
hexatic phase, should be noted. In spite of the difference in molecular
packing and order between two systems, disorder parameters, r and R
are almost same, however l0 of the alkynyl-substituted terthiophene,
3-TTP-yne-4 is quite lager than that of the cyclehexylethylterthio-
phene mixture. This fact indicates that increase of transfer integral
by closer molecular packing in highly ordered smectic phases should
be more effective on increase of carrier mobility, in contrast to the case
of the mixture in which bulky substituent inhibits close intermolecular
packing.
HIGH HOLE MOBILITY OF 3-QTP-YNE-4 UP TO 0.1 CM2/VS
According to the result of 3-TTP-yne-4, the disorders of the highly
ordered smectic phase dose not decrease so remarkable. Increase of
l0 is effective on increase of carrier mobility rather than decrease of
disorders. Extension of p-molecular orbital should increase transfer
integral, resulting in increase of l0, and thus carrier transport of 3-
QTP-yne-4 is investigated.
Figure 7 shows transient photocurrent curves for hole in the
smectic phase of 3-QTP-yne-4 with larger p-conjugate system at room
temperature. In the case of this compound, slightly dispersive curves
with clear kink point are obtained, indicating fast carrier movement
with small distribution. The hole mobility at room temperature
reaches 0.1 cm2=Vs which is comparable to that of molecular crystals
of aromatic compounds [2] and helical phase of hexahexylthiotriphe-
nylene [14], and the highest value among smectic and other discotic
semiconductors.
Above room temperature, hole mobility is independent of electric
field and temperature also in this compound. This feature is similar
to those in the smectic phases of other calamitic semiconductors. This

234=[1188]
M. Funahashi and J.-i. Hanna
FIGURE 7 Transient photocurrent curves in the highly ordered smectic
phase of 3-QTP-yne-4 at 30ꢂC. The cell thickness was 25 mm, excitation
wavelength 337 nm. (a) linear plot (b) double logarithmic plot.
result should also be attributed to small energetical and structural
disorder in the highly ordered smectic phase.
These compounds exhibit quite high hole mobility at room tempera-
ture. In general, carrier mobility both in hopping process and band
conduction is determined by transfer integral between molecules,
which increases with decrease in intermolecular distance and increase
in size of molecular orbitals. The intermolecular distances within
smectic layers of these compounds are 3.9 A, which is smaller than
those in smectic phases of other smectic liquid crystalline semicon-
ductors such as 2-phenylnaphthalene [7] and dialkylterthiophene
derivatives [8]. Therefore this result should be attributed to close
molecular packing structure in the smectic phases, in other words,
small intermolecular distance within the smectic layers.
In comparison with discotic columnar phases, it should be remark-
able that intermolecular distance within the smectic layers is not so
shorter than those of discotic columnar phases. The distance in the
smectic phases of 6-TTP-yne-4 and 3-QTP-yne-4 is 3.9 A, however
the typical value of intermolecular distance within columns is 3.6
and 3.5 A in columnar ordered phase and plastic phase of hexaalkoxy-
triphenylene derivatives respectively [13]. The distance is merely 3.5 A
even in the helical phase of hexahexylthiotriphenylene which exhibits
the highest hole drift mobility of 0.1 cm2=Vs determined by TOF tech-
nique [14,21]. In these highly ordered smectic phases, intermolecular
charge transfer proceeds more efficiently than discotic columnar
phases. The reason is not clear now, however sulfur atom with
large van der Waals radius may increase transfer integral, or the
two dimensional nature of the carrier transport in these smectic
phases may causes different behaviors from one dimensional columnar
phases.

High Carrier Mobility up to 0.1 cm2=Vs
235=[1189]
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