Prominent electron transport property observed for triply fused metalloporphyrin dimer: Directed columnar liquid crystalline assembly by amphiphilic molecular design

Article abstract of DOI:10.1021/ja8030714

A triply fused copper porphyrin dimer, when site-specifically modified on its periphery with hydrophobic and hydrophilic wedges (1_C12/TEG), self-assembles into a columnar liquid crystalline (LC) mesophase over a wide-temperature range from -17 to 99 °C but gives rise to an amorphous solid when modified with only hydrophobic (1_C12/C12) or hydrophilic wedges (1_TEG/TEG). A LC film of 1_C12/TEG displays at 16 °C a top-class one-dimensional electron mobility (0.27 cm²/V·s), as evaluated from its maximum flash-photolysis time-resolved microwave conductivity. Copyright

Full text of DOI:10.1021/ja8030714

Published on Web 09/27/2008
Prominent Electron Transport Property Observed for Triply Fused
Metalloporphyrin Dimer: Directed Columnar Liquid Crystalline Assembly by
Amphiphilic Molecular Design
Tsuneaki Sakurai,† Keyu Shi,† Hiroshi Sato,† Kentaro Tashiro,*,† Atsuhiro Osuka,‡ Akinori Saeki,§
Shu Seki,§ Seiichi Tagawa,§ Sono Sasaki, Hiroyasu Masunaga, Keiichi Osaka, Masaki Takata,
and Takuzo Aida*,†
School of Engineering and Center for NanoBio Integration, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku,
Tokyo 113-8656, Japan, Graduate School of Science, Kyoto UniVersity, Kyoto 606-8502, Japan, Institute of
Scientific and Industrial Research, Osaka UniVersity, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan, and Japan
Synchrotron Radiation Research Institute/SPring-8, Sayo-gun, Hyogo 679-5198, Japan
Received May 1, 2008; E-mail: aida@macro.t.u-tokyo.ac.jp; tashiro@macro.t.u-tokyo.ac.jp
Organic semiconductors have attracted increasing attention in
view of their potential for low-cost alternatives to amorphous silicon
and design flexibility for elaborate electronic devices.1 In particular,
liquid crystalline (LC) semiconductors composed of large π-con-
jugated molecules are attractive in that their columnar assembly
could provide an anisotropic electrical pathway for charge carriers.
Moreover, LC materials are solution-processable into large-area thin
devices.2 Representative examples include LC assemblies of large
π-conjugated molecules such as triphenylenes,3b hexabenzocoro-
nenes,4 perylene diimides,5 porphyrins,3b and phthalocyanines,3
where a molecule with a larger π-conjugated core tends to show a
higher charge-carrier mobility.6 Here we report the first LC material
of a fused metalloporphyrin dimer (1C12/TEG; Chart 1) that displays
a top-class intrinsic electron trasport property among those reported
for n-type LC semiconductors. Triply fused metalloporphyrin dimers
are a new class of extra-large π-conjugated molecules and
potentially interesting for organic electronics. However, the reported
crystal packing diagrams mostly adopt a T-shaped molecular
Figure 1. (a) DSC traces with phase transition temperatures (∆H values
geometry,7a which is unfavorable for electrical conduction. Ac-
in kJ mol-1) of 1C12/TEG (purple), 1C12/C12 (blue), and 1TEG/TEG (pink), on
second heating/cooling at 10 °C/min. (b) Polarized optical micrograph of
cording to a common strategy for liquid crystallization, we initially
incorporated paraffinic (C12) side chains into the periphery of a
triply fused copper porphyrin dimer (1C12/C12; Chart 1). However,
this strategy failed to give any ordered structures. We then prepared
compound 1TEG/TEG having hydrophilic triethylene glycol (TEG)
chains and, just for curiosity, amphiphilic 1C12/TEG. Quite interest-
ingly, only 1C12/TEG formed a room-temperature columnar liquid
crystal.
1
C12/TEG at 50 °C (scale bar, 0.4 mm). Changes in (c) absorption spectrum
and (d) 559-nm absorbance of 1C12/TEG upon cooling from 100 to 20 °C.
an isotropic melt (Figure 1a). On cooling from the isotropic melt,
the resulting LC mesophase in polarized optical microscopy (POM)
showed a clear dendritic texture (Figure 1b)8 characteristic of
columnar LC assemblies. Similar to unassembled 1C12/TEG in CHCl3,
the isotropic melt of 1C12/TEG at 100 °C showed two characteristic
Soret absorption bands at 405 and 559 nm (Figure 1c).8 However,
when the hot melt was cooled to allow the LC phase transition,
the color of the substance changed from dark purple to brown.
Accordingly, the longer-wavelength Soret band, on cooling from
100 to 20 °C, showed a 13-nm blue shift (Figure 1c). Furthermore,
its absorbance decreased abruptly at the phase transition temperature
(Figure 1d). Such a remarkable hypsochromic shift indicates that
the columnar mesophase is composed of a π-stacked array of the
conjugated core of 1C12/TEG. Meanwhile, a metal-free form of
Chart 1
1C12/TEG displayed a LC mesophase, but the phase transition to an
isotropic melt took place at a lower temperature (74 °C), suggesting
that the high planarity of the copper porphyrin moiety7a contributes
to the thermal stability of the LC columns of 1C12/TEG. X-ray
diffraction (XRD) analysis at 30 °C of the LC assembly of
amphiphilic 1C12/TEG showed two distinct diffraction peaks at 2θ
) 0.79 and 1.60°, along with minor diffractions in a wider-angle
region.8 Most of these diffractions were successfully indexed to a
rectangular columnar lattice with lattice parameters a and b of 65.3
Differential scanning calorimetry (DSC) of 1C12/TEG displayed a
phase transition behavior, where a LC mesophase, on second
heating, appeared at -17 °C and then disappeared at 99 °C to form
† The University of Tokyo.
‡ Kyoto University.
§ Osaka University.
SPring-8.
9
13812 J. AM. CHEM. SOC. 2008, 130, 13812–13813
10.1021/ja8030714 CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
Figure 3. (a) FP-TRMC responses of film samples of 1C12/TEG (red) and
1C12/C12 (blue), and (b) transient absorption spectrum of a film sample of
Figure 2. Schematic representations of rectangular 2D molecular packing
diagrams plausible for liquid crystalline 1C12/TEG
1C12/TEG, at 16 °C upon photoirradiation at 355 nm.
.
nm, the LC film of 1C12/TEG showed a transient absorption at 620
nm (Figure 3b), whose rise and decay profiles were nicely correlated
with those of the TRMC response.8 By reference to chemically
and 37.2 Å, respectively (Figure 2). In sharp contrast with 1C12/TEG
,
hydrophobic 1C12/C12 and hydrophilic 1TEG/TEG showed neither a
phase transition behavior (Figure 1a) nor X-ray diffractions.8 The
absorption spectral patterns of these compounds resembled those
in CHCl3 and remained virtually unchanged over a wide temperature
range from 100 to 20 °C.8
Why does the amphiphilic molecular design of 1C12/TEG enhance
the π-stacking interaction? This is most likely due to a nanoscale
phase separation caused by an incompatibility between the hydro-
phobic and hydrophilic side chains of 1C12/TEG. Figure 2 shows
rectangular 2D molecular packing diagrams, which are most
plausible in that they involve only a minimum contact between the
hydrophobic and hydrophilic nanodomains. Diagram I with a dense
molecular packing is considered more likely than diagram II having
many vacant sites. In fact, the XRD patterns8 and lattice parameters,
observed for sheared and unsheared samples of liquid crystalline
•-
•+ 8
generated radical anion 1C12/TEG and cation 1C12/TEG
, we
noticed in the transient absorption spectrum a clear sign of the
formation of a radical anion species. Namely, liquid crystalline 1C12/
TEG serves as an n-type semiconductor. This finding was beyond
our expectation, while the first reduction potential (-0.90 V) of
1
C12/TEG is close to that of C60 (-1.06 V),12 a representative n-type
semiconductor. From the <φΣµ> value in TRMC along with the
quantity of photochemically generated 1C12/TEG•-, the one-
dimensional electron mobility µ1D of the LC film was evaluated8,11
as 0.27 cm2/V·s at 16 °C. Noteworthy, this is the largest electron
mobility among those reported for room-temperature columnar LC
materials, studied by TRMC.5
While amphiphilic design has been utilized for obtaining 2D
lamellar structures,13 the present work demonstrated that it is useful
for constructing 1D columnar structures. Since high-performance
n-type organic semiconductors are very rare,14 the prominent
electron transport capability, unveiled for π-stacked 1C12/TEG, is
noteworthy. Along with an easy tunability of the redox properties
by central metal ions, the new LC semiconductor with a large
absorptivity for visible light has the potential for solution-
processable photovoltaic materials.
1
C12/TEG, are satisfied only with diagram I. Considering the
rectangular lattice geometry, the LC mesophase is most likely
formed by piling up this 2D layer on top of each other. When the
neighboring 2D layers are disordered along the a-axis, the
hydrophobic and hydrophilic domains would have to contact one
another. Likewise, disordering of the 2D layers along the b-axis
could destabilize the LC, since unfavorable mixing of the rigid core
and flexible side chains would result. Consequently, the π-conju-
gated core is forced to stack up by this nanoscale phase separation.8
Again, the amphiphilic molecular design is essential, since mixing
of hydrophobic 1C12/C12 and hydrophilic 1TEG/TEG resulted in a
macroscopic phase separation without any particular structural
features.
Square-wave voltammetry of amphiphilic 1C12/TEG in CH2Cl2
showed first oxidation and reduction potentials of 0.25 and -0.90
V vs Fc/Fc+, respectively.8 Similar to reported examples,7c the
calculated HOMO-LUMO gap of 1.15 V is obviously smaller than
those of copper porphyrin monomers (2.2-2.4 V),7c and even
smaller than those of perylene diimides (1.4-2.2 V)9 and phtha-
locyanines (1.4-1.8 V).10 This feature is quite beneficial for the
efficient transport of charge carriers. To evaluate the intrinsic
charge-carrier mobility with a minimum grain-boundary effect, we
measured the flash-photolysis time-resolved microwave conductivity
(FP-TRMC)11 of the LC state of 1C12/TEG. Upon exposure to a laser
pulse of λ ) 355 nm at 16 °C,8 the sample showed a prompt rise
of a TRMC signal to furnish in 2.6 µs a maximum transient
conductivity <φΣµmax> of 2.4 × 10-4 cm2/V·s (Figure 3a, red).
For the observed TRMC response, the π-stacking of the 1C12/TEG
core in the columnar LC assembly is crucial, since an amorphous
film of 1C12/C12 exhibited <φΣµmax> of only 0.3 × 10-4 cm2/V·s
(Figure 3a, blue), which is roughly an order of magnitude smaller
than that of liquid crystalline 1C12/TEG. We conducted in situ
transient absorption spectroscopy to identify and quantify the charge
carrier responsible for the TRMC signal. Upon excitation at 355
Supporting Information Available: Synthesis and analytical data
of 1C12/TEG, 1C12/C12, and 1TEG/TEG. This material is available free of
charge via the Internet at http://pubs.acs.org.
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