Ambipolar charge carrier transport properties in the homologous series of 2,3,6,7,10,11-hexaalkoxytriphenylene

Article abstract of DOI:10.1080/15421400701732324

In this study, ambipolar charge carrier transport properties were investigated for a homologous series of triphenylene by time-of-flight (TOF) technique for the homeotropic domains. Ambipolar electronic charge carrier transport was observed and the both positive and negative charge carrier mobilities decrease with the increase of the alkyl chain length. Ionic conduction was also observed for the negative charge carrier transport except for C4OTP and any remarkable dependency of the alkyl chain length was not seen. It was reasonably found that both the positive and negative charged carrier mobilities by electronic hopping process is affected by the intra columnar order.

Full text of DOI:10.1080/15421400701732324

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Ambipolar Charge Carrier
Transport Properties
in the Homologous
Series of 2,3,6,7,10,11-
hexaalkoxytriphenylene
Hirosato Monobe a , Yo Shimizu a , Shuichi Okamoto a
b & Hiroyuki Enomoto b
a Nanotechnology Research Institute, National
Institute of Advanced Industrial Science and
Technology (AIST) , Ikeda, Osaka, Japan
b Osaka Electro-Communication University,
Hatsumachi , Neyagawa, Osaka, Japan
Published online: 22 Sep 2010.
To cite this article: Hirosato Monobe , Yo Shimizu , Shuichi Okamoto & Hiroyuki
Enomoto (2007) Ambipolar Charge Carrier Transport Properties in the Homologous
Series of 2,3,6,7,10,11-hexaalkoxytriphenylene, Molecular Crystals and Liquid
Crystals, 476:1, 31/[277]-41/[287], DOI: 10.1080/15421400701732324
To link to this article: http://dx.doi.org/10.1080/15421400701732324
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Mol. Cryst. Liq. Cryst., Vol. 476, pp. 31=[277]–41=[287], 2007
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400701732324
Ambipolar Charge Carrier Transport Properties
in the Homologous Series of 2,3,6,7,10,11-
hexaalkoxytriphenylene
Hirosato Monobe
Yo Shimizu
Nanotechnology Research Institute, National Institute of Advanced
Industrial Science and Technology (AIST), Ikeda, Osaka, Japan
Shuichi Okamoto
Nanotechnology Research Institute, National Institute of Advanced
Industrial Science and Technology (AIST), Ikeda, Japan;
Osaka Electro-Communication University, Hatsumachi, Neyagawa,
Osaka, Japan
Hiroyuki Enomoto
Osaka Electro-Communication University, Hatsumachi, Neyagawa,
Osaka, Japan
In this study, ambipolar charge carrier transport properties were investigated for a
homologous series of triphenylene by time-of-flight (TOF) technique for the homeo-
tropic domains. Ambipolar electronic charge carrier transport was observed and
the both positive and negative charge carrier mobilities decrease with the increase
of the alkyl chain length. Ionic conduction was also observed for the negative
charge carrier transport except for C4OTP and any remarkable dependency of
the alkyl chain length was not seen. It was reasonably found that both the positive
and negative charged carrier mobilities by electronic hopping process is affected by
the intra columnar order.
Keywords: ambipolar; charge carrier transport; columnar; triphenylene
The authors gratefully acknowledge the financial support of Ministry of Economy,
Trade and Industry (METI) and New Energy and Industrial Technology Development
Organization (NEDO). The authors thank Mr. Shuichi Shinohara for his support in
preparing the materials.
Address correspondence to Hirosato Monobe, Nanotechnology Research Institute,
National Institute of Advanced Industrial Science and Technology (AIST), 1-8-31
Midorigaoka, Ikeda, Osaka, 563-8577, Japan. E-mail: monobe@ni.aist.go.jp
31=[277]

32=[278]
H. Monobe et al.
INTRODUCTION
Columnar liquid crystals of discotics have been drawing much
attention as novel organic semiconductors due to the fast charge trans-
port phenomena along a columnar axis [1–12] as well as the aniso-
tropic property of conduction along the columns [13] in the recent
decade. In fact, in highly ordered columnar mesophases, fast carrier
mobility (ꢀ10ꢁ1 cm2 Vꢁ1 sꢁ1) by electronic process of carrier transport
was found which is comparable to those of amorphous silicon
[2,5,9,10]. And, thus, the applications as one-dimensional conductors,
photoconductors, molecular wires and fibers, light emitting diodes and
photovoltaic cells have also been exploited. Thereafter, a series of
reinvestigations of rod-like liquid crystals have also revealed
their good performance as a semiconductor with a fast mobility
(ꢀ10ꢁ1 cm2 Vꢁ1
s
ꢁ1) in highly ordered smectic phases [14–16].
Furthermore, columnar liquid crystals are expected as a self-organized
molecular semiconductor to fabricate the related devices by solution
process. In addition, the ambipolar charge carrier transport is another
important property as the organic semiconductors [10–12]. Liquid
crystalline semiconductors are very effective in improving the present
device with their unique features of electrically inactive structural
defects including domain boundaries and high ambipolar mobility.
Recent studies of columnar liquid crystal semiconductors indicate
the electronic transport in columnar mesophase could strongly depend
on the dimension of columnar order [3,6,11,12]. However, there are no
detailed study of charge carrier transport properties in the same
columnar mesophase with different alkyl chain length. It might be
interested in carrier transport phenomena in the similar structure of
p-stacking with different substitution for columnar liquid crystals.
In this work, ambipolar charge carrier transport properties were
investigated for a homologous series of 2,3,6,7,10,11-hexaalkyloxytriphe-
nylene (CnOTP; n ¼ 4–8, alkyl chain length) by time-of-flight (TOF)
technique for the homeotropic domains. Temperature dependence of
positive and negative charge carrier mobilities were also reported for
Colh and Colhp mesophases dependent on the alkyl chain length.
EXPERIMENTAL
1H-NMR spectra were recorded on a JEOL JNM-Alpha 500 MHz
spectrometer using Me4Si as an internal standard. The phase tran-
sition temperatures and enthalpies were detected by a Differential
Scanning Calorimeter (TA Instrument, 2920 MDSC) and the textures
of the mesophases were observed by a polarizing microscope
(Olympus, BH-2) equipped with a hot stage (Mettler, FP80HT).

Ambipolar Electronic Transport in Triphenylene DLCs
33=[279]
Powder X-ray diffraction studies were carried out by using a Rigaku
Geigerflex X-ray diffractometer (CuKa) with a custom hotstage. The
charge carrier mobilities were measured by TOF method on a liquid
crystal film sandwiched between ITO covered glass plates. The cell
was capillary filled in under an inert gas (Ar) after being degassed
under vacuum. By slowly cooling the sample from the isotropic state,
the sample showed a spontaneous homeotropic texture in a certain
area. No surface treatment was used in this study. The cell was set
up in a hotstage equipped with a polarizing microscope and externally
biased by a stabilized DC power supply. A N2 laser (k ¼ 337 nm,
800 ps, / ¼ 1 mm) was used for a pulsed light irradiation. A transient
photocurrent was detected by a digital oscilloscope (Hewlett Packard,
HP54820A) with a handmade preamplifier.
Compounds
The synthesis of compounds were carried out according to the
literatures with slight modifications [17,18]. Alkylation of
1,2-dihydrooxybenzen and the following cyclization reaction of the
resultant 1,2-dialkoxybenzen with FeCl3 gave the homologous series
of CnOTPs. The synthetic route is illustrated in Scheme 1.
1,2-dialkyloxybenzene (1)
The compounds were synthesized by the same method except for
alkyl chain length of alkylbromide. The synthetic procedure of
1,2-dipentyloxybenzene (n ¼ 5) is described below.
1-Bromopentane (15.7 g, 0.10 mol) was slowly added into the etha-
nol solution of 1,2-dihydroxybenzene (4.8 g, 0.043 mol) and K2CO3
SCHEME 1 Synthetic route of 2,3,6,7,10,11-hexaalkyloxytriphenylene
(CnOTP).

34=[280]
H. Monobe et al.
(28.9 g, 0.21 mol) with vigorous stirring. The reaction mixture was
stirred under reflux for 96 h and filtered using celite after cooling.
The solvent was evaporated in vacuo. The crude product was purified
by column chromatography (silica gel; dichloromethane : hexane ¼ 1:1,
Rf ¼ 0.66) to give 1,2-dipentyloxybenzen (8.6 g, 0.034 mol 79%) as
colorless liquid.
(n ¼ 4)
1H-NMR (CDCl3) dppm: 6.89 (s, 4H, ArH), 4.00 (t, 4H, OCH2) 1.80
(m, 4H, OCH2CH2), 1.51 (m, 4H, CH2CH3), 0.98 (t, 6H, CH3).
(n ¼ 5)
1H-NMR (CDCl3) dppm: 6.88 (s, 4H, ArH), 3.99 (t, 4H, OCH2) 1.82
(m, 4H, OCH2CH2), 1.34 (m, 8H, (CH2)2CH3), 0.93 (t, 6H, CH3).
(n ¼ 6)
1H-NMR (CDCl3) dppm: 6.89 (s, 4H, ArH), 3.99 (t, 4H, OCH2), 1.81
(m, 4H, OCH2CH2), 1.47 (m, 4H, O(CH2)2CH2), 1.34 (m, 8H,
(CH2)2CH3), 0.90 (t, 6H, CH3).
(n ¼ 7)
1H-NMR (CDCl3) dppm: 6.88 (s, 4H, ArH), 3.99 (t, 4H, OCH2), 1.81
(m, 4H, OCH2CH2), 1.48 (m, 4H, O(CH2)2CH2), 1.32 (m, 12H,
(CH2)3CH3), 0.89 (t, 6H, CH3).
(n ¼ 8)
1H-NMR (CDCl3) dppm: 6.89 (s, 4H, ArH), 3.99 (t, 4H, OCH2), 1.81
(m, 4H, OCH2CH2), 1.47 (m, 4H, O(CH2)2CH2), 1.34 (m, 16H,
(CH2)4CH3), 0.88 (t, 6H, CH3).
2,3,6,7,10,11-Hexaalkyloxytriphenylene (CnOTP)
The compounds were synthesized by the same method using a corre-
spond alkyl chain length of 1,2-dialkyloxybenzene. The synthetic pro-
cedure of 2,3,6,7,10,11-hexapentyloxytriphenylene (C5OTP, n ¼ 5) is
described below.
Anhydrous iron (III) chloride (6.0 g, 0.037 mmol) and sulfuric acid (5
drops) were added to a mixture of 1,2-dipentyloxybenzene (2.42 g,
9.7 mmol) in dichloromethane (150 mL) with vigorous stirring in the
ice bath. When the reaction was stirred for 3 hours, approximate
130 mL methanol was slowly add to the mixture. The solvent was eva-
porated in vacuo. The resultant precipitate was filtered off. The crude
product was purified by column chromatography (silica gel; hexane:
ethylacetate ¼ 1:20, Rf ¼ 0.17) and further purification by recrystalli-
zation from the mixture of ethanol and hexane to give C5OTP (1.04 g,
1.38 mmol, 43%) as white solid.
(n ¼ 4)
1H-NMR (CDCl3) dppm: 7.84 (s, 6H, ArH), 4.24 (t, 12H, OCH2), 1.93
(m, 12H, OCH2CH2), 1.61 (m, 12H, CH2CH3), 1.04 (t, 18H, CH3).

Ambipolar Electronic Transport in Triphenylene DLCs
35=[281]
(n ¼ 5)
1H-NMR (CDCl3) dppm: 7.84 (s, 6H, ArH), 4.23 (t, 12H, OCH2), 1.58
(m, 12H, OCH2CH2), 1.45 (m, 24H, (CH2)2CH3), 0.98 (t, 18H, CH3).
(n ¼ 6)
1H-NMR (CDCl3) dppm: 7.83 (s, 6H, ArH), 4.23 (t, 12H, OCH2), 1.94
(m, 12H, OCH2CH2), 1.58 (m, 12H, O(CH2)2CH2), 1.40 (m, 24H,
(CH2)2CH3), 0.93 (t, 18H, CH3).
(n ¼ 7)
1H-NMR (CDCl3) dppm: 7.83 (s, 6H, ArH), 4.23 (t, 12H, OCH2), 1.94
(m, 12H, OCH2CH2), 1.55 (m, 12H, O(CH2)2CH2), 1.33 (m, 36H,
(CH2)3CH3), 0.91 (t, 18H, CH3).
(n ¼ 8)
1H-NMR (CDCl3) dppm: 7.83 (s, 6H, ArH), 4.23 (t, 12H, OCH2), 1.94
(m, 12H, OCH2CH2), 1.57 (m, 12H, O(CH2)2CH2), 1.32 (m, 48H,
(CH2)4CH3), 0.90 (t, 18H, CH3).
RESULTS AND DISCUSSIONS
Phase Transition Behaviors
CnOTPs show some endothermic peaks on heating corresponding to the
crystal to crystal, crystal to mesophase and the isotropic melt. On cooling
process, there are two exothermic peaks corresponding to the isotropic to
mesophase and crystal exclusive of C4OTP. C4OTP shows an additional
peak on cooling. All homologues exhibited at least an enantiotropic
TABLE 1 Phase Transition Temperature [ꢂC] and Transition Enthalpy [kJ=mol]
in Parenthesis from DSC Curves of CnOTPs (Above Table: Experimental Values,
Below: Literature Values )
Compound
C4OTP
n
4
Cr
Colhp
Colh
Iso
ref
.
85.6
[22.8]
88.6
70.1
[33.7]
69
66.7
[42.2]
67
65.5
[57.7]
68.8
.
143.9
[18.7]
145.6
.
[19]
[19]
[17]
[19]
[19]
C5OTP
C6OTP
C7OTP
C8OTP
5
6
7
8
.
.
.
.
.
.
.
.
121.7
[10.8]
122
98.8
[5.8]
99.5
91.6
[5.1]
93
.
.
.
.
66.6
[108.5]
66.8
84.4
[5.5]
85.6

36=[282]
H. Monobe et al.
mesophase. The thermodynamic data from DSC measurements of the
CnOTPs used in this study are summarized in Table 1 as compared to
the those of literatures [17, 19]. The phase transition temperatures of
CnOTPs by DSC measurements were corresponded within ca. 2ꢂC.
XRD Analysis
X-ray diffraction measurements for CnOTPs were performed at 95%
reduced temperature of clearing temperature (K) in the mesophase.
Figure 1 shows the X-ray diffraction patterns of these homologues.
These are a typical pattern of XRD for hexagonal columnar (Colh)
mesophase exclusive of C4OTP. A set of reflections corresponding to
p
d
100, d110 and d200 of which ratio in distance is 1:1= 3:1=2 was
observed and this is an evidence that hexagonal arrays of columns
˚
exist. The Additional two reflections were observed at ca. 4.5A (broad
˚
halo) and ca. 3.5A (weak peak for C6 to C8OTP and sharp peak for
C5OTP), which are assigned to the average distance of the molten
alkyl chains and intracolumnar order, respectively. Sharp reflections
˚
near ca. 3.5A strongly indicates the ordered columnar phase for C4
and C5OTP, and these are columnar hexagonal plastic (Colhp) and
FIGURE 1 Powder XRD patterns of CnOTPs at 95% reduced temperature of
clearing point.

Ambipolar Electronic Transport in Triphenylene DLCs
37=[283]
˚
TABLE 2 The d-Values [A] from XRD Results in CnOTPs. The Measurement
Temperature were Equaled to 95% of Relative Temperature from Clearing
Temperature [K]
Compounds
Reflection
C4OTP
C5OTP
C6OTP
C7OTP
C80TP
(100)
(110)
(200)
(210)
16.47
9.41
8.18
6.17
17.42
11.75
8.77
18.28
12.27
9.18
19.49
13.00
9.77
20.11
13.57
10.10
(300)
4.61
6.48
(220), (211)
(400)
(311), (320)
(001)
4.68
3.83
4.38
3.60
(002)
(102)
3.60
3.51
ordered columnar hexagonal (Colho) mesophases, respectively [3]. On
the other hand, C6 to C8OTP show only weak peak corresponding to
the intracolumnar order and this suggests those are disordered colum-
nar hexagonal (Colhd) mesophase. The results of XRD measurements
are summarized in Table 2. Figure 2 shows the temperature depen-
dence of intracolumnar periodicity for C4 and C5OTP. This indicates
the stacking periodicity decreases with decreasing of temperature in
FIGURE 2 Temperature dependence of stacking periodicity for (a) C4OTP
and (b) C5OTP.

38=[284]
H. Monobe et al.
the mesophase. The textures of mesophase were similar to a typical
texture for Colh mesophase for CnOTPs exclusive of C4OTP which
shows highly ordered Colhp mesophase.
Charge Carrier Transport Property
The transient photocurrent measurements have been performed by
TOF technique for the homeotropically aligned area to investigate
the charge transport along the columnar axis. Figure 3(a) and (b) show
a typical transient photocurrent of C6OTP (90ꢂC, thickness 15 mm) for
positive and negative carriers in the mesophase under various electric
fields, respectively. Figure 4 shows alkyl chain length and tempera-
ture dependence of the charge carrier mobilities of CnOTPs for posi-
tive and negative carriers. The ambipolar carrier transports were
observed for the all of CnOTPs. However, in fact, in order to obtain
the ambipolar electronic carrier transport, it was needed careful puri-
fication of compound by means of repeated recrystalization. A clear
fast transit was observed which corresponded to the transport of posi-
tive and negative charge carriers by electronic process and the mobili-
ties of 10ꢁ4 ꢀ 10ꢁ2 cm2 Vꢁ1 sꢁ1 at a short time range of microseconds
without electric field dependence. For negative carriers, there existed
an additional slow transit at a longer time scale of millisecond as
shown in Figure 3(b), which corresponded to ionic conduction and a
mobility of 10ꢁ6 ꢀ 10ꢁ5 cm2 Vꢁ1 sꢁ1 exclusive of C4OTP probably
due to its high viscosity. For electronic transport, the charge carrier
mobility of C4, C5 and C6OTP depends on the columnar mesophases
FIGURE 3 Transient photocurrent curves taking by TOF method of C6OTP
15 mm cell at 90ꢂC under various electric field for (a) positive and (b) negative
charge carriers.

Ambipolar Electronic Transport in Triphenylene DLCs
39=[285]
FIGURE 4 Temperature dependence of charge carrier mobilities of CnOTPs
for (a) positive and (b) negative charge carriers.
(Colhp, Colho and Colhd, respectively) according to previous reports
[3,6,11,12]. According to the XRD measurements, C4OTP and
˚
C5OTP have a high intracolumnar order in the vicinity of 3.5A, in contrast
to the undetectable intracolumnar order of the longer homologues.
It was reasonably found that both the positive and negative charged
carrier mobilities by electronic hopping process is affected by the intra-
columnar order. On the other hand, the electronic carrier mobilitites of
C6, C7 and C8OTP in Colhd mesophase decrease with increasing of
alkyl chain length although XRD patterns show no difference in the
wide angle region which corresponds to the intracolumnar order. It
was reasonably found that both the positive and negative charged car-
rier mobilities by electronic hopping process is affected by the intra-
columnar order in the same manner. Temperature dependence of the
charge carrier mobility was also seen in Figure 4. For C4 and C5OTP,

40=[286]
H. Monobe et al.
FIGURE 5 The reduced temperature dependency of the negative charge car-
rier mobility of CnOTPs. The open circles presented for electronic transport
and the closed ones indicate for the slower ionic migration.
the electronic carrier mobility were almost constant and slightly
increased with decreasing of temperature. These are consistent with
the the temperature dependence of intra columnar periodicity as shown
in Figure 2. On the contrary, the charge carrier mobility decreases with
decreasing of temperature for C6 to C8OTP. This implies that the
thermal activation hopping process were the dominant factor for carrier
transport in Colhd phase. Figure 5 shows temperature dependence of
the slower ionic carrier mobility by using the reduced temperature from
the clearing point for CnOTPs exclusive of C4OTP. No dependency of
alkyl chain length was observed for the ionic carrier mobility. Consider-
ation of micro-segregation in the mesophase materials, it is very reason-
able that the ions migrate to a fluid-like region which consist of
substituted alkyl chains aggregating loosely, while the core tripheny-
lene part was stacked rigidly. This implies that there were no signifi-
cant difference in the state of alkyl side chains in the columnar
structure for C6 to C8OTP in spite of those of intracolumnar p-stacking
was slightly decrease with increasing of the alkyl chain length.
CONCLUSION
Ambipolar electronic charge carrier transport was observed for
CnOTPs and the both positive and negative charge carrier mobilities

Ambipolar Electronic Transport in Triphenylene DLCs
41=[287]
decrease with the increase of the alkyl chain length. Ionic conduction
was also observed for the negative charge carrier transport exclusive
of C4OTP and any remarkable dependency of the alkyl chain length
was not seen. It was reasonably found that both the positive and nega-
tive charged carrier mobilities by electronic hopping process is affected
by the intracolumnar order. The electronic carrier mobilities of C6 to
C8OTP decrease with increasing of alkyl chain length in spite of
XRD patterns show no difference in the wide angle region with alkyl
chain length. These results imply that the intracolumnar order of
C6 to C8OTP decreases with increasing of alkyl chain length. It would
be possible to estimate the order of intra columns of columnar liquid
crystals from its electronic carrier mobility in the mesophase.
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