The non-covalent assembly of benzene-bridged metallosalphen dimers: Photoconductive tapes with large carrier mobility and spatially distinctive conduction anisotropy

Article abstract of DOI:10.1039/b904337c

A newly synthesized benzene-bridged metallosalphen dimer tailored with aliphatic chains is demonstrated for the solution-processed assembly of a π-electronic tape, which shows a large intrinsic carrier mobility, is spatially anisotropic in conduction, for

Full text of DOI:10.1039/b904337c

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The non-covalent assembly of benzene-bridged metallosalphen dimers:
photoconductive tapes with large carrier mobility and spatially
distinctive conduction anisotropyw
Long Chen,a Lu Wang,b Xingfa Gao,b Shigeru Nagase,b Yoshihito Honsho,c Akinori Saeki,c
Shu Seki*cd and Donglin Jiang*ad
Received (in Cambridge, UK) 3rd March 2009, Accepted 23rd March 2009
First published as an Advance Article on the web 17th April 2009
DOI: 10.1039/b904337c
A newly synthesized benzene-bridged metallosalphen dimer
tailored with aliphatic chains is demonstrated for the solution-
processed assembly of a p-electronic tape, which shows a large
intrinsic carrier mobility, is spatially anisotropic in conduction,
forms p- or n-type semiconductors tunable upon doping, is
photoconductive and is capable of repetitive switching with large
on–off ratios.
the tape is highly responsive to visible light irradiation, triggers
prominent photocurrent generation, shows on–off ratios as
high as 9.0 Â 104 and is capable of repeated on–off switching
without deterioration. The molecular design of bridged
dinuclear metallocomplexes has led to findings of unusual
physical properties and utilities such as catalysts,5a the
mimicry of biological enzymes,5b molecular magnets,5c
building blocks for coordination polymers5d and MOFs.5e
However, photo functions have remained unexplored to date.6
1 exhibits absorption bands at 457, 489 and 522 nm
(Fig. 1(a), red curve), which is 122 nm red-shifted from
mononuclear reference 2 (Fig. 1(a), black curve). In addition,
1 shows a significantly enhanced molar absorption coeffi-
In photoconduction, photogenerated charge carriers move
towards electrodes and produce an electric current.1 Studies
on such photoresponsive molecules have been an important
part of progress in the field of solar energy conversion.2 Single
crystals of certain p-conjugated molecules, e.g., arenes, have
been reported to become photoconductive as a result of
exciton migration and charge separation at the molecule–
electrode interface.3 However, small arenes absorb photons
only in the ultraviolet region, while large arenes with absorp-
tion bands extending to the visible region require complicated
synthesis, tedious purification and are difficult to process.
Moreover, the undesirable photoinduced dimerization and
bleaching of arenes deter their application in optoelectronics.4
During a study on the assembly of conjugated multinuclear
metallocomplexes, we serendipitously discovered that these
p-electronic metalloconjugates are photoresponsive, robust
against irradiation and may avoid the above drawbacks.
Herein, we report a photoconductive tape assembly from
a benzene-bridged dinuclear metallosalphen (Chart 1; 1) via a
simple solution process (see the ESIw). The tape of 1 absorbs a
wide range of visible photon wavelengths efficiently, exhibits
an extremely high intrinsic carrier mobility and forms paths
for charge carrier transportations with a distinctive spatial
anisotropy. It also affords p- or n-type semiconductors that
are tunable upon oxidation or reduction. We emphasize that
cient (e457
= 7.53 Â 104 MÀ1 cmÀ1) that is about 2.6
nm
times as large as that of mononuclear 2 (e400 nm = 2.87 Â
104 MÀ1 cmÀ1). These results clearly suggest that the
p-conjugation is extended over the molecular skeleton,
including the salphen subunits. Cyclic voltammetry of 1 in
THF (0.5 mM) shows two quasi-reversible redox pairs
at À0.48/À0.26 and 0.39/0.67 V vs. Ag/AgCl (Fig. 1(b)),
indicating that 1 can be both reduced and oxidized at relatively
low potentials.6 In contrast to 1, 2 displays only oxidation
potentials (0.25/0.45 V; Fig. S2, ESIw), while no reduction
signals are observed. Calculation at the spin-unrestricted
B3LYP/6-31G(d,p) level with the Gaussian 03 package (see
the ESIw) revealed that 1 has HOMO–LUMO gaps of 2.6 and
2.8 eV for its a-(spin up) and b-(spin down) electrons, respec-
tively, while 2 has the same a- and b-HOMO–LUMO, with
gaps as high as 3.4 eV (Fig. 1(c); Fig. S3, ESIw).
a Department of Materials Molecular Science, Institute for Molecular
Science, 5-1 Higashiyama, Myodaiji, Okazaki 444-8787, Japan.
E-mail: jiang@ims.ac.jp
b Department of Theoretical and Computational Molecular Science,
Institute for Molecular Science, 38 Nishigo-naka, Myodaiji,
Okazaki 444-8585, Japan
c Department of Applied Chemistry, Graduate School of Engineering,
Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan.
E-mail: seki@chem.eng.osaka-u.ac.jp
Chart 1 (a) Schematic representations of benzene-bridged copper
salphen metallodimer 1 and its mononuclear control 2. (b) The
possible molecular alignment in the tape: top and side views.
Molecules of 1 in green are in the same layer and overlap the molecule
layer in red. x, y and z are the width, length and thickness directions of
the tape, respectively.
d Precursory Research for Embryonic Science and Technology
(PRESTO), Japan Science and Technology Agency (JST),
Chiyoda-ku, Tokyo 102-0075, Japan
w Electronic supplementary information (ESI) available: Experimental
procedures, spectra and simulation data. See DOI: 10.1039/b904337c
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Fig. 2 (a) The I–V profile of the tapes between a 10 mm-wide Pt gap
(black curve, without tapes; blue curve, with tapes; red curve, with
iodine-doped tapes). (b) The electric current when a 2 V bias voltage is
turned on or off. (c) The I–V profile of the tapes between a 10 mm-wide
Pt gap (black curve, without tapes; blue curve, with tapes; red curve,
with hydrazine-doped tapes). (d) The electric current when a 2 V bias
voltage is turned on or off.
Fig. 1 (a) Electronic absorption spectra of 1 (red curve) and 2 (black
curve) in CH2Cl2 at 25 1C (9.0 Â 10À6 M; at this concentration, no
aggregation was observed). (b) CV profile of 1 in THF. (c) HOMO and
LUMO levels of 1 and 2. (d) FE-SEM image. (e) HR-TEM image
(inset: SAED image of a single tape). (f) AFM image (10 Â 10 mm).
electrical conductivity using a two-probe method with a
10 mm-wide Pt gap. The tape shows an almost linear I–V profile
(Fig. 2(a), blue curve), while the gap itself is silent (black curve).
Upon oxidization with iodine, the electrical conductivity
increases (Fig. 2(a), red curve), characteristic of a p-type
semiconductor. The conductivity can be switched many times
without deterioration (Fig. 2(b)). Out of interest, exposure to
hydrazine vapor increased the electric current, suggesting an
n-type semiconductor character (Fig. 2(c), red curve).9 The
electric current can be on–off switched repeatedly (Fig. 2(d)).
Along these lines, we further investigated the photo-
conductivity of the tape under visible light irradiation. Since
the molecular orientations parallel and perpendicular to the
long axis of the tape are different, we utilized two types of
electrodes, having different configurations to show the effect of
molecular alignment on photocurrent generation. The tapes
laid on the Pt gap electrodes show a response to visible
light irradiation (4 400 nm), with an increase in electric
current (Fig. 3(a), red curve). The photocurrent can be
on–off-switched at negative (Fig. 3(b), red curve) and positive
(Fig. 3(b), blue curve) bias voltages, with on–off ratios of
about 1.6 Â 102 and 1.9 Â 102, respectively. In contrast, tapes
sandwiched between Al/Au electrodes under otherwise iden-
tical conditions show a more substantial photocurrent genera-
tion (Fig. 3(c)). In this case, the on–off ratio is as large as
9.0 Â 104 (Fig. 3(d)),10 which is 50 times larger than that on Pt
gap electrodes. The significant difference in photocurrent
generation between the two configurations shows that the
molecular alignment has an effect on the conduction. 1 stacks
perpendicular to the xy plane of the tape at a p–p distance of
about 3.46 A, while in the xy plane, 1 is separated by much
larger center-to-center spacings (3.48 and 2.30 nm). Thus, the
conduction path perpendicular to the xy plane of the tape is
most likely to be superior to the parallel one, leading to a
spatially distinctive conduction anisotropy.
2 is highly soluble and scarcely aggregates in THF. In
contrast, 1 assembles in THF (1.0 mg mlÀ1) to form
tape-shaped aggregates in 83% yield by a heating-and-cooling
process. FE-SEM images reveal that the 1D tapes have lengths
reaching hundreds of micrometers (Fig. 1(d)). HR-TEM
shows that the width of an elementary tape is about 100 nm
(Fig. 1(e), inset: the SAED pattern shows the crystalline
molecular order). The thickness ranges from 10 to 100 nm
(estimated from AFM images; Fig. 1(f)). No other shapes,
such as spheres, sheets or 3D objects, were observed. Powder
X-Ray diffraction (PXRD) measurements show sharp diffrac-
tions at 2y = 3.83, 5.07, 6.46, 10.28, 15.73, 20.61 and 25.711
(Fig. S4, blue curve; ESIw). Simulation with molecular
modelling and Pawley refinement using Reflex implemented
in the Materials Studio7 package reveals that the monoclinic
unit cell of a = 34.76, b = 22.96 and c = 6.92 A with a = b =
90 and g = 1301 gives a PXRD pattern that mostly fits
the experimental observations (Fig. S4b, red curve; ESIw).
Accordingly, 1 is most likely to be interlocked via aliphatic
chain interdigitation to form molecular tapes, and p–p
stacking leads to the growth in the thickness direction
(Chart 1(b)). Indeed, FT-IR spectroscopy displays CH2
stretching vibrations at 2849 (nsym) and 2919 (nasym) cmÀ1
(Fig. S5, ESIw), demonstrating that the aliphatic chains adopt
a stretched conformation and are in a crystalline state. Time-
dependent absorption spectroscopy shows a spectral change
typical of J-aggregation (Fig. S6, ESIw).8
The highly ordered structure of p-conjugated 1 in the tape
suggests a high probability of it being electronically semi-
conducting. We investigated this possibility by measuring its
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In summary, we have demonstrated a newly synthesized
benzene-bridged metallosalphen dimer that can be used to
form high quality sub-millimeter-length tapes with large aspect
ratios by a high throughput solution process. The p–p inter-
actions, together with van der Waals forces, result in a
unidirectional molecular order in the tapes, favoring the
formation of carrier paths with a significant anisotropic
character. The tapes are capable of harvesting visible light
and trigger photocurrent generation with a large on–off ratio.
Moreover, their redox nature enables the formation of either
p- or n-channel semiconductors. These unique functions,
together with their large carrier mobility feature, suggest the
great potential of p-conjugated metallocomplexes in developing
photofunctional materials and molecular optoelectronics.
This research was partially supported by a Grant-in-Aid for
Scientific Research (B) (20350059) from MEXT (Ministry of
Education, Culture, Sports, Science and Technology, Japan).
D. J. thanks the JSPS Asian core program.
Notes and references
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0.068 cm2 VÀ1 À1, which is comparable with or higher than
s
those of large arenes, such as assemblies of hexabenzocoronene12a
and sumanene,12b conjugated polymers,10 and liquid crystalline
systems.12c
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Chem. Commun., 2009, 3119–3121 | 3121