Charge carrier transport properties of biphenyl liquid crystals with a dimer structure

Article abstract of DOI:10.1246/cl.2009.592

In order to have better understanding design of liquid crystalline molecules as organic semiconductors, we synthesized a series of biphenyl liquid crystals with a dimer structure such as 1,8-bis[4-(4'-butylbiphenyl)]octane (4BP8BP4) and 1,8-bis[4-(4'-octylbiphenyl)Joctane (8BP8BP8) as a model material, and characterized their phase-transition behaviors and chargetransport properties. These biphenyl dimers exhibited smectic B (SmB_hex) phase at higher temperatures and higher mobility, i.e., 5-6 × 10^-3 cm² V^-1 s^-1 that does not depend on temperature, compared with that of a corresponding monomer, 4-octyl-4'- butylbiphenyl (8BP4). We discuss the present result in a framework of disorder in the liquid crystal phase for an organic semiconductor. Copyright

Full text of DOI:10.1246/cl.2009.592

592
Chemistry Letters Vol.38, No.6 (2009)
Charge Carrier Transport Properties of Biphenyl Liquid Crystals with a Dimer Structure
Jiang Wu,ꢀ Takayuki Usui, Akira Ohno, and Jun-ichi Hanna
Imaging Science and Engineering Lab, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503
(Received April 1, 2009; CL-090327; E-mail: wu@isl.titech.ac.jp)
Table 1. Phase-transition behaviors of biphenyl dimers
In order to have better understanding design of liquid crys-
talline molecules as organic semiconductors, we synthesized a
series of biphenyl liquid crystals with a dimer structure such
as 1,8-bis[4-(40-butylbiphenyl)]octane (4BP8BP4) and 1,8-
bis[4-(40-octylbiphenyl)]octane (8BP8BP8) as a model material,
and characterized their phase-transition behaviors and charge-
transport properties. These biphenyl dimers exhibited smectic
B (SmBhex) phase at higher temperatures and higher mobility,
i.e., 5{6 ꢁ 10ꢂ3 cm2 Vꢂ1 sꢂ1 that does not depend on tempera-
ture, compared with that of a corresponding monomer, 4-octyl-
40-butylbiphenyl (8BP4). We discuss the present result in a
framework of disorder in the liquid crystal phase for an organic
semiconductor.
Biphenyls
Cryst.
SmBhex
121 ꢃC
Isotropic
4BP8BP4
8BP8BP8
8BP4
136 ꢃC
138 ꢃC
43 ꢃC
94 ꢃC
ꢂ1:5 ꢃC
35
30
25
20
15
10
5
100
10
1
t T=5.6 µs
t
T=6.29 µs
0.1
1
10
100
t
T=7.96 µs
Time/µs
Since the discovery of electronic conduction in discotic and
smectic liquid crystals in the 1990s,1–3 the electronic conduction
has been reported in various classes of liquid crystals including
triphenylenes, porphyrins, naphthalenes, oligothiophenes,
benzothiazoles, and biphenyls.4–8 Their charge carrier transport
properties are characterized by high mobility over 10ꢂ3 cm2
+70V
+60V
t
T=9.77 µs
+50V
+40V
+30V
+20V
t T=12.8 µs
T=18.2 µs
t
V
ꢂ1 sꢂ1 typically and up to 1 cm2 Vꢂ1 sꢂ1
However, there is no guiding principle of how to design liq-
.
0
0
20
40
60
80
100
uid crystalline molecules for an organic semiconductor having a
high mobility. We synthesized a series of biphenyls with a dimer
structure in which two mesogenic moieties are linked via a flex-
ible spacer and studied liquid crystalline behaviors and charge
carrier transport properties in comparison with those of corre-
sponding biphenyl monomers on the basis of a previous study.9
Scheme 1 shows a schematic diagram of a synthetic route
for biphenyl dimers.10 The chromatographically homogeneous
products were further purified by recrystallization from hexane
several times for time-of-flight measurement.
The phase-transition behavior of these biphenyl dimers syn-
thesized was determined by texture observation under a polar-
ized microscope, DSC measurement, and XRD studies. These
biphenyl dimers exhibit SmBhex phase at a higher temperature
range compared with those of corresponding monomers as sum-
Time/µs
Figure 1. Typical transient photocurrent for positive charge
carriers in SmBhex phase (135 ꢃC) of 8BP8BP8 in a 14.1-mm
thick quartz cell at various applied voltages. The inset shows
double logarithmic plot of transient photocurrents as a function
of time.
marized in Table 1. A higher phase-transition temperature from
SmBhex phase to isotropic phase compared with that of the cor-
responding monomer indicates that the liquid crystalline phase is
stabilized in the dimers.
Figure 1 shows typical transient photocurrents for positive
carriers generated by a pulse illumination of 265 nm light from
a YAG laser in the SmBhex phase of 8BP8BP8 in a 14.1-mm thick
quartz cell as a function of applied biases. We found that mole-
cules were homogeneously aligned to the electrodes in the quartz
cell which was confirmed by texture observation under polarized
microscope.11 Each photocurrent in Figure 1 is nondispersive
and shows a clear shoulder, which indicates a transit time when
the positive carriers arrive at the counter electrode. The photo-
current decay before the transit time is due to charge trapping
at deep states probably attributed to trace amounts of chemical
impurities.
The mobility is estimated to be 5 ꢁ 10ꢂ3 cm2 Vꢂ1 sꢂ1 from
the transit time and a given applied bias. The mobility of
4BP8BP4 is almost the same as that of 8BP8BP8 and around
6 ꢁ 10ꢂ3 cm2 Vꢂ1 sꢂ1 in SmBhax phase, which is 5–6 times high-
er than those of their corresponding monomers. Judging from the
O
C
O
AlCl3
Br
CC6H12
Br
Br
2
ClCOC6H12COCl
+
1
2
C H
B(OH)2
n
8
17
, AlCl
NH2BH3
3
Br
C8H16
Br
CH2Cl2
Pd(TPP)4/Na2CO3/H2O-Toluene
3
C8H17
C8H16
C8H17
Scheme 1. Synthetic pathway for 8BP8BP8 and chemical
structure of corresponding monomer of 8BT4.
same intermolecular distance of biphenyl dimers in SmBhex,
Copyright ꢀ 2009 The Chemical Society of Japan

Chemistry Letters Vol.38, No.6 (2009)
593
10−1
1.2
1
8BP4
0.8
0.6
0.4
0.2
0
8BP8BP8
(Sm B Phase)
10−2
4BP8BP4
Ea=0.012eV
10−3
Ea=0.108eV
0
2
4
6
8
8BP4
Normalized Time t/τt (µs)
(Sm B Phase)
Figure 3. Normalized transient photocurrents of positive
charge carriers in SmBhex phases of biphenyl monomer, 8BP4
and dimer, 4BP8BP4 as a function of time normalized by their
transit times.
10−4
6.4 10−6 7.2 10−6 8 10−6 8.8 10−6 9.6 10−6 1.04 10−51.12 10−5
1/T2 (1/K2)
Figure 2. Temperature dependence of positive charge carrier
mobility of 8BP8BP8 and 8BP4 in SmBhex
dimers, 4BP8BP4 and 8BP8BP8 in comparison with those of
corresponding monomers. We conclude that the liquid crystal
phase of dimers is stabilized by a dimer structure, leading to a
higher temperature range of liquid crystal phase and tempera-
ture-independent mobility, which is a few times higher than
those of corresponding monomers. This gives an insight into
how to design a liquid crystalline molecule for a quality organic
semiconductor.
.
˚
5.2 A, which is determined by X-ray diffraction, as those of
biphenyl monomers in the same phase, the enhanced mobility
is not attributed to a change of the intermolecular distance of
smectic layers.
Figure 2 shows the mobility of 8BP8BP8 and 8BP4 as a
function of a square inverse of temperature. We can see that
the mobility in SmBhex phase of 8BP8BP8 and 8BP4 exhibit dif-
ferent behaviors, which relate to distribution of the density of
states (DOS) responsible for hole conduction:12 a shallow slope
of the dimer, which is typical behavior of mobility in smectic
mesophases,5,6 indicates a narrow distribution of the DOS, while
a steep slope of the monomer, which is the same trend as biphen-
yl derivatives reported previously,9 does a broad distribution of
the DOS.13 Taking account of no structural difference between
8BP8BP8 and 8BP4 except for a linkage of two monomer units,
we can conclude that the stabilized liquid crystalline phase by
the linked structure contributes to the reduction of the DOS.
In general, the current decay of transient photocurrent after
the transit time reflects a dispersion of carrier velocity for the
carriers experiencing hopping events, including diffusion, trap-
ping and thermal releasing at shallow states, and variable range
hopping of carriers in distributed DOS. Therefore, the shape of
transient photocurrent provides us with additional information
about the DOS discussed above.
Figure 3 shows the comparison of transient photocurrents of
4BP8BP4 and 8BP4 as a function of time normalized by their
transit times and current. It is very clear that the transient photo-
current of 4BP8BP4 decays more sharply than that of 8BP4.
Taking account of a nondispersive electronic transient photo-
current of both the dimers and the monomer that assures a high
purity of these materials,14,15 in addition to a low diffusivity at
lower temperature, a sharp decay of transient photocurrent in
the dimers support a narrow distribution of their DOS.12
Furthermore, the present results suggests that the different
charge transport-properties in biphenyl monomers originate
from less ordered molecular alignment attributed to a small core
moiety of biphenyl.
The authors would like to thank Dr. Hiroaki Iino for helpful
discussions regarding experimental analysis. The study was
partly supported by a Core Research for Evolutional Science
and Technology (CREST) program.
References and Notes
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11 Supporting Information is available electronically on the CSJ-
Journal Web site, http://www.csj.jp/journals/chem-lett/index.
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In summary, we studied liquid crystalline behavior and
charge carrier transport properties of liquid crystalline biphenyl
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Published on the web (Advance View) May 16, 2009; doi:10.1246/cl.2009.592