Segregated donor-acceptor columns in liquid crystals that exhibit highly efficient ambipolar charge transport

Article abstract of DOI:10.1021/ja203822q

Liquid crystalline donor (i.e., phthalocyanine) was covalently linked to acceptor (i.e, fullerene) to achieve efficient charge-transport properties in a liquid crystalline phase. The columnar structure exhibited highly efficient ambipolar charge-transport

Full text of DOI:10.1021/ja203822q

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Segregated DonorÀAcceptor Columns in Liquid Crystals That Exhibit
Highly Efficient Ambipolar Charge Transport
Hironobu Hayashi,† Wataru Nihashi,† Tomokazu Umeyama,†,^ Yoshihiro Matano,† Shu Seki,*,#,^
Yo Shimizu,*,|| and Hiroshi Imahori*,†,‡,§
†Department of Molecular Engineering, Graduate School of Engineering and ‡Institute for Integrated Cell-Material Sciences (iCeMS),
Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan
^PRESTO, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
#Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1, Yamadaoka, Suita, Osaka 565-0871, Japan
National Institute of Advanced Industrial Science and Technology, Kansai-Center, 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan
§Fukui Institute for Fundamental Chemistry, Kyoto University, Sakyo-ku, Kyoto 606-8103, Japan
S Supporting Information
b
ZnPc 1D column, leading to DÀA bicontinuous structure in
ABSTRACT: Liquid crystalline donor (i.e., phthalocyanine)
was covalently linked to acceptor (i.e, fullerene) to achieve
efficient charge-transport properties in a liquid crystalline
phase. The columnar structure exhibited highly efficient
ambipolar charge-transport character, demonstrating the
potential utility of the strategy in organic electronics.
the LCs.
The synthesis and characterization of ZnPcÀC60 and zinc
phthalocyanine reference (ZnPc-ref) are described in the Sup-
porting Information (SI). The LC properties of ZnPcÀC60 and
ZnPc-ref were examined by a combination of differential scan-
ning calorimetry (DSC), polarized optical microscopy (POM),
and X-ray diffraction (XRD). Upon heating, only one broad
endothermic curve (35À80 ꢀC) with a peak of ca. 53 ꢀC,
corresponding to the LC-like phase-to-LC phase transition, is
seen in ZnPcÀC60, whereas that corresponding to the crystal-to-
LC phase transition is observed at 124 ꢀC in ZnPc-ref (SI, Figure
S1 and Table S1 ). Once ZnPcÀC60 and ZnPc-ref were heated
above the respective transition temperature, they formed a glassy
LC state at 25 ꢀC. Two POM images of ZnPc-ref at 25 and 160 ꢀC
show similar optical textures, while those of ZnPcÀC60 reveal a
birefringence texture upon heating to 160 ꢀC (Figure S2). After
being cooled to 25 ꢀC, both ZnPc-ref and ZnPcÀC60 showed
birefringence textures similar to those at 160 ꢀC. These results
corroborate that ZnPc-ref and ZnPcÀC60 retain the mesophases
consisting of the ordered structures (i.e., glassy LC state) at 25 ꢀC.
These mesophases were identified by XRD (Figure 2 and Table
S2). Upon heating ZnPc-ref to 160 ꢀC, its XRD patterns exhibit
hexagonal columnar (Colh) mesophase (Figure 2a,b). On the
other hand, XRD patterns of ZnPcÀC60 at 25 ꢀC (Figure S3a,b)
and 160 ꢀC (Figure 2c,d) are rather similar, but when the sample
is heated to 160 ꢀC, the peak intensities are remarkably enhanced,
leading to the change from LC-like phase (Colr1) to discotic
rectangular columnar mesophase (Colr2). Additionally, ZnPcÀC60
retains this ordered structure after cooling to 25 ꢀC (Figure S3c,d).
Considering a broad reflection peak of (001) in ZnPcÀC60, the
ZnPc part in ZnPcÀC60 may form a disordered column in the
Colr2 mesophase, although such disordered columns of phthalo-
cyanines are reported to reveal high hole mobility (∼10À1 cm2
ecently liquid crystals (LCs) have drawn much attention as a
R
promising type of molecular semiconductor because of their
potential utility in organic electronics including organic thin-film
transistors, organic light-emitting diodes, and organic solar cells.1
Disc-like molecules functionalized with alkyl groups self-assem-
ble into supramolecular one-dimensional (1D) columns that
further self-organize into hexagonal, rectangular, and nematic
phases as discotic liquid crystals (DLCs).2 Hierarchical self-
assembly from simple disc-like molecules to DLCs through the
formation of 1D columns is a highly appealing strategy to
organize donorÀacceptor (DÀA) molecules,3 i.e., forming
DÀA heterojunction structures exhibiting ambipolar charge-
transport properties4 and efficient photocurrent generation.5
As donors, phthalocyanines are some of the most widely
investigated self-assembling molecules in DLCs.2,6 As acceptors,
fullerenes have been successfully incorporated into LC systems
to give various LC phases.7 Some of the covalently functionalized
fullerenes are known to form different columnar phases.8 There-
fore, a simultaneous application of a phthalocyanine and a fullerene
to LCs is highly attractive to form such DÀA heterojunction
structures. So far there have been only a few examples of
mesogenic phthalocyanineÀfullerene linked dyads.9 As such,
relationships between the LC structures and charge-transport
properties have yet to be examined.
Herein we report the synthesis and LC and charge-transport
properties of a zinc phthalocyanine (ZnPc)ÀC60 dyad (Figure 1).
Six 4-dodecyloxyphenoxy groups were introduced into the per-
V
À1 sÀ1).11 We also measured the absorption spectra of the ZnPc-
ref and ZnPcÀC60 films to get information on the packing
geometries of the ZnPc moieties in the film after heating and
cooling treatment (Figure S4). Both spectra show a blue-shift of
iphery of the central core to form the discotic columnar structure
10
of the ZnPc.
C was also tethered to the ZnPc core via a short,
60
semiflexible bridge. We expected that, due to the strong πÀπ
interaction between the C60 molecules and the covalent linkage,
the C60 molecules would be arranged successively along the
Received: April 26, 2011
Published: June 23, 2011
r
2011 American Chemical Society
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|

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Figure 1. Phthalocyanine derivatives used in this study.
Figure 3. TOF current transients observed for ZnPcÀC60 (a) before
heating at an electric field strength (E) of 1.3 Â 105 V cmÀ1 and (b) at 25
ꢀC, after the sample was heated to 160 ꢀC for 10 min and then cooled to
25 ꢀC at E = 7.7 Â 103 V cmÀ1 after photoexcitation with a 355 nm laser
pulse. Curves I and II were observed under positive and negative bias,
respectively. The logÀlog plots of TOF current transients vs time are
also depicted as insets in (a) and (b). (c) Log hole and (d) log electron
mobilities observed for ZnPcÀC60 as a function of E1/2, (9) before and
(b) after the sample was heated, respectively.
a
Table 1. Charge Mobilities of ZnPc-ref and ZnPcÀC60
TOF μh,
cm2 VÀ1 sÀ1
TOF μe,
cm2 VÀ1 sÀ1
TRMC ∑μ,
cm2 VÀ1 sÀ1
compound
ZnPc-ref
2.8 (À)b
0 (À)b
3.0 (7.3)
ZnPcÀC60
0.26 (0.010)
0.11 (0.016)
0.52 (0.10)
Figure 2. XRD patterns of ZnPc-ref in 2θ ranges of (a) 0À35ꢀ and (b)
1À6ꢀ, and of ZnPcÀC60 in 2θ ranges of (c) 0À35ꢀ and (d) 1À6ꢀ. Each
sample was measured at 160 ꢀC. The intensified XRD patterns in 2θ
ranges of 0À35ꢀ are also depicted as insets in (a) and (c). The asterisk in
Figure 2d marks a peak arising from a helical pitch of C60 molecules
along the ZnPc column.
a The values in parentheses were obtained before the heating procedure.
b Could not be measured.
is formed even before heating, due to the plausible self-assem-
bling process in the reprecipitation procedure by toluene and
methanol as reported previously (vide infra).4f,5a,5c
the absorption arising from the ZnPc moiety relative to those in
toluene, supporting the face-to-face stacking of the ZnPc moieties
in the discotic columnar structures.
To correlate the LC structures with the charge-transport
properties, electron mobility (μe) and hole mobility (μh) were
determined by the time-of-flight (TOF) method (Figures 3 and
S6 and SI). Electron and hole mobilities under the electric field
(E = 0) were obtained by extrapolation from the plots of the
corresponding mobilities as a function of E1/2 (Figure 3c,d). The
μe and μh values of ZnPc-ref could not be obtained before the
sample was heated because of the poor film formation. After
heating and cooling treatment, ZnPc-ref had a high μh value of
2.8 cm2 VÀ1 sÀ1 but did not show electron mobility (Figure S6
and Table 1). In contrast, the current transients of ZnPcÀC60
film before and after heating were observed under both positive
and negative bias (Figure 3a,b), suggesting ZnPcÀC60 exhibits
ambipolar charge-transport character with rather comparable μh
and μe values. The electron mobility of the ZnPcÀC60 film
before heating is larger than the hole mobility, showing that an
electron-transporting pathway (i.e., the helical C60 arrangement
along the ZnPc column) is formed before heating (vide supra). It
should be noted here that the respective μh and μe values increase
It is noteworthy that only ZnPcÀC60 reveals a low-angle peak
at 1.59ꢀ (asterisk in Figure 2d) that can be assigned as a helical
pitch of molecules (5.48 nm), 16 times the layer spacing of
0.34 nm.12 These results suggest that, due to the πÀπ interac-
tion, C60 molecules are also arranged linearly, showing helical
alignment along the ZnPc columnar structures. To shed light on
the C60 arrangement, XRD measurements were also performed
for the uniaxially oriented films of ZnPcÀC60 (Figure S5).13
These oriented films were prepared by uniaxial shearing of the
sample surface. When the direction of the X-ray is parallel to the
axis of the ZnPc column, the peak assigned to the helical pitch
becomes more evident, supporting the helical C60 arrangement
along the ZnPc column. Interestingly, the integrated areas of the
peak assigned to helical pitch extracted by peak fitting are rather
comparable before and after heat treatment (Figure S3e,f). This
implies that this helical C60 arrangement along the ZnPc column
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V
À1 sÀ1 before heating and of 0.52 cm2 VÀ1 sÀ1 after heating and
cooling treatment (Table 1). These short-range trends are
consistent with long-range trends observed by the TOF method.
These prompt increases of transient conductivities for the ZnPc-
ref and ZnPcÀC60 films arise from the photoresponse behavior
of these films, in which free charge carriers, defined as charges
escaped from the recombination process, are generated by laser
excitation. In the decay part of the transient conductivity, the
carriers are distributed randomly in the film and subsequently
undergo recombination.16
It should be noted that TRMC signals consist of the sum of
charge mobilities for the negative and positive carriers, and we
cannot extract respective electron and hole mobilities from the
TRMC value accurately. The ZnPc-ref has a much higher TRMC
charge mobility than does the ZnPcÀC60 molecule. The pre-
sence of C60 moiety in the ZnPcÀC60 film may disorder the
ZnPc arrangement in the ZnPcÀC60 column, lowering the
TRMC value, especially the hole mobility.12 In the TOF mea-
surements, the ZnPcÀC60 film exhibits high hole (0.26 cm2 VÀ1
Figure 4. (a) TRMC profiles of ZnPc-ref before heating (curve I) and at
25 ꢀC, after the sample was heated to 160 ꢀC for 10 min and then cooled
to 25 ꢀC (curve II). (b) TRMC profiles of ZnPcÀC60 before heating
(curve I) and at 25 ꢀC, after the sample was heated to 160 ꢀC for 10 min
and then cooled to 25 ꢀC (curve II), after photoexcitation with 355 nm
laser pulse.
by 26 and 7 times after the heating and cooling treatment. This
shows that heating allows the ZnPcÀC60 molecules to adopt
more regular arrangement, especially the intracolumn arrange-
ment of the ZnPc moiety as well as the intercolumn arrangement,
leading to improvement of the ambipolar charge-transport
properties. More importantly, ZnPcÀC60 displays remarkably
high μh = 0.26 cm2 VÀ1 sÀ1 and μe = 0.11 cm2 VÀ1 sÀ1, which are
the highest values ever reported for organic materials with DÀA
heterojunction.4,5a Meanwhile, the log charge mobilities decrease
with increasing electric fields (Figure 3c,d). The negative E-field
dependency of charge mobility indicates that carrier trapping
occurs with a larger probability as the applied voltage is larger and
generally arises when different hopping processes with different
activation energies operate. Given that the macroscopic charge-
carrier transport in the sample should involve both intracolumn
(much less energy-demanding) and intercolumn (much more
energy-demanding) hopping events, the observed negative
E-field dependency of the charge mobility seems reasonable.4f,14
With these contrasting long-range charge-transport properties
in mind, we measured flash-photolysis time-resolved microwave
conductivities (TRMC) of ZnPc-ref and ZnPcÀC60 (Figure 4 and
SI). This electrodeless method allows for evaluating short-range
transient conductivitiesofmaterials.15 Forinstance, upon exposure
to a 355 nm laser pulse at 25 ꢀC, ZnPc-ref reveals a prompt
increase of a transient conductivity, Æφ∑μæ, in which φ is the
quantum efficiency of charge separation and ∑μ is the sum of
mobilities of all the transient charge carriers, thus reach-
ing maximum transient conductivities of 2.9 Â 10À4 cm2 VÀ1 sÀ1
before heating and 1.2 Â 10À4 cm2 VÀ1 sÀ1 after heating and
cooling treatment (Table S3). The φ values were determined to
be 4.0 Â 10À3 % before and after heating and cooling treatment
by current integration of TOF transients after heating. Accord-
ingly, the hole mobilities of ZnPc-ref were obtained as 7.3 cm2
VÀ1 sÀ1 before heating and 3.0 cm2 VÀ1 sÀ1 after heating and
cooling treatment (Table 1). The larger value before heating than
after heating and cooling treatment may result from the crystal-
linity nature of ZnPc-ref. That is, the annealing process of ZnPc-
ref molecules causes transformation of crystal to liquid crystal,
yielding the drops in the TRMC transient and the hole mobility.
On the other hand, ZnPcÀC60 exhibits maximum transient
conductivities of 5.3 Â 10À5 cm2 VÀ1 sÀ1 before heating and
of 1.5 Â 10À4 cm2 VÀ1 sÀ1 after heating and cooling treatment.
s
À1) and electron (0.11 cm2 VÀ1 À1) mobilities, the hole
s
mobility being ∼2 times larger than the electron mobility. Thus,
both charge mobilities would contribute to the TRMC value of
ZnPcÀC60 because of the comparable electron and hole mobi-
lities of ZnPcÀC60 obtained by the TOF measurements as
well as the expected short-range TRMC charge mobilities
(g0.11 cm2 VÀ1 sÀ1), which are typically higher than the long-
range TOF charge mobilities. Some previous papers suggested
that co-assembly of a DÀA linked dyad with corresponding D or
A molecules allows better packing of the molecular units and
results in larger charge mobilities.9b,17 We may be able to
improve the charge-transport properties of ZnPcÀC60 by blend-
ing suitable phthalocyanine or fullerene derivatives.
Considering that TRMC and TOF measurements provide
short-range and long-range charge-transport properties, respec-
tively, TRMC values depend on the degree of alignment for
ZnPcÀC60 molecules in the ZnPcÀC60 intracolumn, whereas
TOF values correlate with that between ZnPcÀC60 intercol-
umns as well as in the ZnPcÀC60 intracolumn. Note that the μh
and μe values measured by TOF after heating are 26 and 7 times
higher than those before heating, respectively. On the other
hand, the total charge mobility measured by TRMC after the
heating and cooling treatment is 5 times higher than that before
heating. It is evident that the enhancement in the total charge
mobility obtained by TOF is larger than that obtained by TRMC.
More well-ordered alignment of the respective ZnPcÀC60
columns in the LC after heating may facilitate intracharge
transport between the ZnPcÀC60 columns, leading to the
improvement of the macroscopic charge mobilities.
In conclusion, we have successfully prepared a promising type
of DÀA linked dyad that is self-assembled to form segregated
DÀA columns in liquid crystals. The DÀA heterojunction
structure of ZnPcÀC60 molecules was found to exhibit highly
efficient ambipolar charge-transport properties. This relationship
between the LC structures and charge-transport properties will
provide basic and fundamental information on the rational design
of high-performance LC materials for use in organic electronics.
’ ASSOCIATED CONTENT
From the φ values (5.2 Â 10À2 % before heating, 2.9 Â 10À2
%
S
Supporting Information. Complete ref 5a, experimental
b
after heating and cooling treatment) of ZnPcÀC60 (Table S3),
the charge mobilities of ZnPcÀC60 were obtained as 0.10 cm2
details, schemes, tables, and additional figures. This material is
available free of charge via the Internet at http://pubs.acs.org.
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’ AUTHOR INFORMATION
(10) Ichihara, M.; Suzuki, A.; Hatsusaka, K.; Ohta, K. Liq. Cryst.
2007, 34, 555.
Corresponding Author
seki@chem.eng.osaka-u.ac.jp; yo-shimizu@aist.go.jp; imahori@
scl.kyoto-u.ac.jp
(11) (a) Iino, H.; Takayashiki, J.; Hanna, J.; Bushby, R. J. Jpn. J. Appl.
Phys. 2005, 44, L1310. (b) Iino, H.; Hanna, J.; Bushby, R. J.; Movaghar,
B.; Whitaker, B. J.; Cook, M. J. Appl. Phys. Lett. 2005, 7, 132102.
(c) Miyake, Y.; Shiraiwa, Y.; Okada, K.; Monobe, H.; Hori, T.; Yamasaki,
N.; Yoshida, H.; Cook, M. J.; Fujii, A.; Ozaki, M.; Shimizu, Y. Appl. Phys.
Express 2011, 4, 021604.
(12) On the basis of the Colr2 structure for ZnPcÀC60, the ZnPc
plane is tilted by 7.4ꢀ against the axis of the ZnPc column. Given that
the C60 helical pitch is 16 times the layer spacing, the separation distance
between the C60 (0.7À0.9 nm) is estimated to be smaller than the
shortest one between the C60 (∼1.0 nm). Such enhanced packing of the
C60 may be achieved by the disordered wedge-shaped open stacked
ZnPc in the ZnPc column.
’ ACKNOWLEDGMENT
This work was supported by Grant-in-Aid (No. 21350100 to
H.I.) and WPI Initiative, MEXT, Japan. H.H. is grateful for a JSPS
fellowship for young scientists.
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