A redox-active columnar metallomesogen and its cyclic voltammetric responses

Article abstract of DOI:10.1039/b709103f

We demonstrate a strategy that appears particularly well suited for the design of redox-active liquid crystals and also constitutes the first demonstration of cyclic voltammetric responses of a metallomesogen in the columnar liquid crystalline phase. The Royal Society of Chemistry.

Full text of DOI:10.1039/b709103f

www.rsc.org/materials
Volume 17 | Number 39 | 21 October 2007 | Pages 4105–4212
ISSN 0959-9428
COMMUNICATION
Ho-Chol Chang et al.
HIGHLIGHT
A redox-active columnar
metallomesogen and its cyclic
voltammetric responses
Shaowei Chen
Discrete charge transfer in
nanoparticle solid films
0959-9428(2007)17:39;1-S

COMMUNICATION
www.rsc.org/materials | Journal of Materials Chemistry
A redox-active columnar metallomesogen and its cyclic voltammetric
responses{
Ho-Chol Chang,*ab Tomoki Shiozaki,a Akiko Kamata,a Keisuke Kishida,a Takeshi Ohmori,a
Daisuke Kiriya,a Takae Yamauchi,a Hirotaka Furukawaa and Susumu Kitagawa*a
Received 19th June 2007, Accepted 24th July 2007
First published as an Advance Article on the web 3rd August 2007
DOI: 10.1039/b709103f
We demonstrate a strategy that appears particularly well suited
for the design of redox-active liquid crystals and also constitutes
the first demonstration of cyclic voltammetric responses of a
metallomesogen in the columnar liquid crystalline phase.
Liquid crystals (LCs) have anisotropic structures with an
enormous variety of functional properties. The self-assembly of
mesogenic units into ordered materials has thus been a useful
Scheme 1
technique in materials science. More recently, new molecular
components are being designed that readily form ordered
supramolecular LC assemblies.1 Here we note that in addition to
the presence of periodic order in LCs, flexibility in physical
structure, correlated with changes in electronic structure, seems to
be regarded as a desirable property. Changes in electronic structure
may be associated with an electron transfer process occurring at a
redox-active functionality of the mesogen. LCs with redox-active
groups have been reported,2 however, studies on their electro-
chemical activity have only been carried out in solution where there
is little or no periodic order.3 To develop this field, (1) a proper
design of a LC consisting of redox-active molecular units having a
melting point (the temperature for the crystal-to-liquid crystal
phase transition) around room temperature and (2) a proper
electrochemical methodology are needed as important initial steps.
Thereby, it would be possible to better understand the correlation
between redox-induced changes in electronic structure with the
macroscopic change in physical structure that would occur within
the ordered assembly. In this communication, we report a first
synthesis and electrochemical properties of a metallomesogen
containing dioxylene ligands, which shows reversible redox steps
sample of 1 prepared by methods reported in the ESI{
transformed to an isotropic liquid (Iso) at 189 uC, and successive
cooling initiated the growth of a dendritic texture with sixfold
symmetry at 180 uC, shown in Fig. 1a. Complex 1 manifests itself
through the XRD pattern shown in Fig. 1a obtained at 30 uC. In
the low angle region, up to five sharp Bragg reflections appear,
with reciprocal spacings in the ratio of 1 : (1/3)1/2 : 1/2 : (1/7)1/2 : 1/3
between o-benzoquinone (BQ), o-semiquinonate (SQ 2), and
?
catecholate (Cat22) electronic forms.4
Our design and synthesis of a redox-active metallomesogen
started with a complex of Pt(II) with 2,29-dipyridyl ligands
substituted by two branched 3-octyltridecyl groups (C8,10bpy)
at the 4,49-positions (Scheme 1). Subsequent reaction with
catechol (CatH2) and [PtCl2(C8,10bpy)] afforded violet-colored
[Pt(Cat)(C8,10bpy)] (1). Differential scanning calorimetry (DSC)
and X-ray diffraction (XRD) measurements{ revealed that a
aDepartment of Synthetic Chemistry and Biological Chemistry,
Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-
ku, Kyoto, 615-8510, Japan. E-mail: chang@sbchem.kyoto-u.ac.jp;
Fax: +81 75 383 2732; Tel: +81 75 383 2735
Fig. 1 a) XRD pattern of 1 at 30 uC and optical polarizing micrographic
texture of a thin film of 1 between two glass slides at 180 uC under crossed
polarizers. The crossed arrows indicate the orientation of the polarization
filters in the microscope. b) The assembled structure of the Colho phase;
top view of the column (left) and the hexagonal packing of the columns
(right). The red and blue circles illustrate the domains shared by the
mesogenic core and the molten alkyl chains, respectively.
bPRESTO, Japan Science and Technology Agency, 4-1-8 Honcho,
Kawaguchi, Saitama, 332-0012, Japan
{ Electronic supplementary information (ESI) available: Experiment
procedures, XRD, DSC, and CV data of 1 and the crystal structure of
a model complex with linear tridecyl chains. See DOI: 10.1039/b709103f
4136 | J. Mater. Chem., 2007, 17, 4136–4138
This journal is ß The Royal Society of Chemistry 2007

in agreement with Miller indices (10), (11), (20), (21), and (30),
respectively, for a hexagonal lattice. In addition, a broad reflection
˚
centered at 20.7u with a d-spacing of 4.3 A appears, indicative of
the molten alkyl chains. Of particular interest was a reflection
˚
centered at 26.4u with a d-spacing of 3.4 A, demonstrating a
weakly ordered structure. All these features support the conclusion
that complex 1 forms a hexagonal columnar ordered LC phase
(Colho),3 as schematically illustrated in Fig. 1b. Inter- and
˚
intracolumnar distances were estimated to be 22.5 A (ahex) and
˚
3.4 A (001), respectively, which can be reproduced by assuming a
one-dimensional column constructed from the molecules aligned in
an antiparallel fashion. Because the size of the mesogenic core of 1
5
˚
was estimated to be about 11 A, { the antiparallel alignment is
reasonable, not only from the size balance between the observed
˚
ahex and the mesogenic core (11 A), but also from consideration of
the asymmetric dipolar alignment of the cores.
Further analyses of the variable temperature XRD and DSC
data have revealed that the thermal annealing of the supercooled
Colho phase at 0 uC for 5 h leads to the formation of an
unidentified phase (X), which is characterized by a hexagonal
˚
lattice with an expanded lattice constant of ahex(X) = 27.1 A,
together with four new reflections at 5.96, 15.1, 23.9, and 27.2u,
instead of the (001) reflection.{ Upon cooling, the X phase trans-
forms into a glassy phase (GX) at 262 uC keeping its symmetry,
and a subsequent second heating process proved the sequence
of phase changes summarized as follows: GX 262 uC X 31 uC
(3.9 kJ mol21) Colho 192 uC (1.3 kJ mol21) Iso. The Colho phase
appears over the relatively wide temperature range of 161 uC.
Complex 1 undergoes a quasi-reversible one electron oxidation
and reduction at 20.01 V and 21.87 V versus Fc/Fc+, respectively,
in CH2Cl2 solution (Fig. 2a). These electrochemical couples
correspond to ligand-based processes:6 the oxidation process
Fig. 3 a) A photograph of 1 spin-coated on an ITO electrode (0.8 cm 6
5 cm) at 25 uC. b) An atomic force microscopy image of a spin-coated film
on an ITO electrode. c) The XRD patterns of 1 spin-coated on an ITO
electrode (red line) at 25 uC and the bulk sample of 1 at 30 uC (black line).
A diffraction peak at 21.3u (red line) is that of an ITO electrode.
To establish the direct electrochemical activity of 1 in the LC
phase, we mounted 1 on an indium tin oxide (ITO) electrode by a
spin coating method or gradual cooling method from the Iso
phase.{7 A photograph and an AFM image of the spin-coated film
prepared at 4000 rpm for 30 s are shown in Fig. 3a and b,
respectively. The AFM data collection was performed in a tapping
mode under air. The film is very flat, and has a thickness of 50 nm
(a part of the film was removed using hexane before the AFM
measurement to expose the bare ITO surface. This procedure
formed a film with thick edges). The XRD diffraction pattern of
the film shows that two sharp diffractions of at 2h = 4.42u and
8.84u can be indexed as (10) and (20), respectively, indicating the
formation of the Colho phase on the electrode (Fig. 3c). The film
was then immersed in a DMSO solution containing 0.1 M
n-Bu4NClO4 at 25 uC. Since the Colho phase is insoluble in
DMSO, the film of 1 was not removed via dissolution. Fig. 4a
shows a plot of the peak current (Ip) in cyclic voltammetry at
2 V s21 as a function of the number of potential scans. Here, the
peak current decreases relatively rapidly, then remains almost
unchanged after several scans, indicating a pseudo-steady state was
reached. The transient decrease in current during several of the
initial scans may be due to dissolution or exfoliation of the
oxidized molecules that is the result of neutral molecules attached
metastably onto the ITO electrode. In the pseudo-steady state
region (after the twentieth cycle), the film shows a quasi-reversible
CV response against repetitive scans (Fig. 4b), demonstrating the
formation of an oxidized species and the reformation of the initial
species within the electrochemically generated thin regions. The
slight decrease in current on cycling is due to the dissolution of the
II
+
?
could provide a mono-cationic [Pt (SQ )(C8,10bpy)] species,
while a low-lying p* orbital of the C8,10bpy could accept
II
2
?
an electron to produce a mono-anionic [Pt (Cat)(C8,10bpy )]
species (Fig. 2b).
Fig. 2 a) CV in CH2Cl2 solution at 25 uC (1 mM, 0.1 M n-Bu4NClO4,
N2, 100 mV s21) and b) the redox scheme for 1.
This journal is ß The Royal Society of Chemistry 2007
J. Mater. Chem., 2007, 17, 4136–4138 | 4137

Acknowledgements
This work was supported by the PRESTO project, Japan
Science and Technology Agency and a Grant-In-Aid for
Science Research from the Ministry of Education, Culture,
Sports, Science and Technology, Japan. The authors thank Dr
Naoya Nishi (Kyoto Univ.) and Dr Ken Kawata (Fuji Film
Co. Ltd.) for their help and discussions.
Notes and references
{ XRD measurements were carried out with Cu Ka radiation equipped
with a Rigaku RINT-2000 diffractometer using Al sample pans.
Differential scanning calorimetry measurements were performed on a
METTLER DSC822e. Electrochemical measurements were carried out
with an ALS model 650A electrochemical analyzer. A standard three-
electrode system (a glassy carbon working electrode, platinum-wire counter
electrode, and Ag/Ag+/CH3CN electrode as reference) was used for CV
studies in solution. Liquid crystalline 1 was spin-coated (4000 rpm, 30 s)
from 100 ml of a 10 mM toluene solution on an indium tin oxide electrode
purchased from Sanyo Vacuum Industries Co. Ltd. (5.7 V h21, A =
2.4 cm2). The AFM image was taken with a NANOSCALE hybrid
microscope VN-8000 series (Keyence) in a tapping mode.
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H.-W. Spiess and V. Vill, Wiley-VCH, Weinheim, Germany, 1998; (b)
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Fig. 4 a) Anodic (blue line) and cathodic (red line) peak currents as a
function of the number of potential scans and b) repetitive CVs after the
twentieth cycle for a spin-coated film of 1 on an ITO electrode.
oxidized species into the electrolyte solution.8 The observed I–V
curve is characterized by diffusion tails with a peak separation
of 0.21 V (the twentieth scan), suggesting a diffusion-controlled
electron transfer process. The Emid values are linearly dependent
on the logarithm of the concentration of n-Bu4NClO4,{ suggesting
anion insertion/expulsion processes upon oxidation and reduction
accompanied by the redox processes described in eqn (1).8
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for several days.
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2
2
1Colho + ClO4
P 1+?ClO4
+ e2
ITO
(1)
DMSO
In summary, we demonstrate a strategy for the design of
electrochemically active LC materials. A significant stabilization of
the Colho phase is brought out by the interplay of the branched
alkyl chains and the stacking interactions of the central planar core
comprised of the asymmetric mixed ligands. The results reported
here also characterize the electrochemical activity of a metallo-
mesogen, promising to provide significant insight into the
electronic functions of LC materials. Further work is in progress
to control both molecular properties and macroscopic bulk
phases of liquid crystalline compounds by means of electro-
chemical stimulus.
4138 | J. Mater. Chem., 2007, 17, 4136–4138
This journal is ß The Royal Society of Chemistry 2007