Electron transport and electrochemistry of mesomorphic fullerenes with long-range ordered lamellae

Article abstract of DOI:10.1021/ja803593j

Fullerenes, C₆₀, modified with long alkyl chains form long-range ordered lamellar mesophases permitting a high C₆₀ content. The mesomorphic fullerenes feature reversible electrochemistry and a comparably high electron carrier mobilit

Full text of DOI:10.1021/ja803593j

Published on Web 06/27/2008
Electron Transport and Electrochemistry of Mesomorphic Fullerenes with
Long-Range Ordered Lamellae
Takashi Nakanishi,*,†,‡,§ Yanfei Shen,‡ Jiaobing Wang,‡ Shiki Yagai,§, Masahiro Funahashi,|
Takashi Kato,| Paulo Fernandes,‡ Helmuth Mo¨hwald,‡ and Dirk G. Kurth†,‡
National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba 305-0047, Japan, Max Planck Institute of
Colloids and Interfaces, 14424 Potsdam, Germany, PRESTO, Japan Science and Technology (JST), 4-1-8 Honcho
Kawaguchi, Saitama, Japan, Chiba UniVersity, 1-33 Yayoi-cho, Inageku, Chiba 263-8522, Japan, and
The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyoku, Tokyo 113-8656, Japan
Received May 14, 2008; E-mail: nakanishi.takashi@nims.go.jp
One of the great challenges in organic-based materials science
is the creation of soft materials such as supramolecular materials1
and mesomorphic materials2 with tunable morphologies of the
molecular organization and properties. Wet-chemical approaches
for thin-film (TFT) and field-effect transistors (FET) using meso-
morphic materials like liquid crystalline organic semiconductors
are examples for practical devices, since organic single crystals
are usually impractical.3 The basic requirements to achieve high
carrier mobilities in liquid crystals involve close molecular packing,
highly ordered structures, and extended π-conjugation leading to
semiconductor features.3b Fullerene C60 is one kind of promising
electron-transport material as an n-type semiconductor exhibiting
relatively high carrier mobility of approximately 1.0 cm2 V-1 s-1
in the single crystal and polycrystalline thin films.4 Although various
morphologies of C60-containing liquid crystals such as nematic,
smectic, and columnar phases have been studied in the past decade,5
to the best of our knowledge, the carrier mobility of fullerenes in
the liquid crystalline state has not been evaluated so far. Typically
the C60 content (based on molecular weight) in liquid crystals is
only a few percent because of their bulky appendage and is up to
Figure 1. (a) Structure of C60 derivatives (1-6); (b) polarizing optical
42% at maximum.5 Herein, we utilize fulleropyrrolidines function-
micrographic texture of mesophase of 1 at 190 °C on cooling from the
isotropic phase at a rate of 0.1 °C min-1; (c) XRD patterns of 1 at 185 °C;
and (d) illustration for the proposed lamellar organization of 1.
alized with a multi(alkyloxy)phenyl group (Figure 1a) to satisfy
the requirements for a high carrier mobility in the C60-containing
mesomorphic materials: a high C60 content (up to 50%), a compact
packing, and a highly ordered mesophase. Moreover, fulleropyr-
Table 1. Phase Transition Behavior of 1, 2, and 4
derivatives
phase transition temperature (°C)a
rolidines form stable species under electrochemical reduction of
the C60 cage6 and thus act as redox-active mesomorphic materials.
We earlier reported supramolecular fullerene nano- and microar-
chitectures utilizing the intermolecular forces introduced by C60
(π-π) and alkyl chain interactions (van der Waals).7 In this
Communication, the formation of a highly ordered lamellar
mesophase from simple fullerene derivatives bearing long alkyl
chains providing high C60 content and a comparably high electron
mobility of ∼3 × 10-3 cm2 V-1 s-1 as well as electrochemical
properties is reported.
Fullerenes 1-6 were synthesized following our same strategy
as reported previously and unambiguously characterized.7 The
motivation here is to study the interplay of alkyl chain and fullerene
packing on structure formation. Fullerene derivatives 1, 2, and 4
exhibit thermotropic polymorphism. In the other cases of derivatives
3, 5, and 6 as well as all of the corresponding benzaldehydes as
precursors of the derivatives were found to be nonmesomorphic.8
The thermal properties of 1, 2, and 4 have been examined by a
1
2
4
Cr
Cr
Cr
62 (72.9)
ML
ML
ML
193 (31.3)
226 (12.8)
223 (10.7)
Iso
Iso
Iso
44 (37.5)b
33 (31.4)
a Transition temperature (°C) and enthalpies (kJ mol-1
,
in
;
parentheses) determined by DSC on the second heating; 10 °C min-1
cr, crystalline; ML, mesomorphic lamella; iso, isotropic. b Determined by
DSC on the first cooling.
combination of differential scanning calorimetry (DSC), polarized
optical microscopy (POM), and X-ray diffraction (XRD). The
results of DSC analysis are summarized in Table 1. Upon heating,
mainly two endothermic peaks corresponding to the crystalline-to-
mesomorphic and mesomorphic-to-isotropic phase transitions were
observed in 1, 2, and 4. As the alkyl chains become longer and the
number of alkyl chains becomes greater in these derivatives, the
crystalline-to-mesomorphic transition temperature becomes higher,
while the mesomorphic-to-isotropic transition temperatures become
lower. The optical texture of 1 observed with a polarized optical
microscope indicates ordered mesomorphic domains (Figure 1b),
which exhibit birefringence and confirmed the fluid nature of the
material. The optical textures of 2 and 4 also indicate ordered
mesomorphic domains (Supporting Information, Figure S2).
† National Institute for Materials Science (NIMS).
‡ Max Planck Institute of Colloids and Interfaces.
§ Japan Science and Technology Agency (JST).
Chiba University.
| The University of Tokyo.
9
9236 J. AM. CHEM. SOC. 2008, 130, 9236–9237
10.1021/ja803593j CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
mobility of C60 in the fluid-mesomorphic state was investigated
for the first time. The simple modification of C60 by two- (2) or
three-long alkyl chains (1 and 4) permits a high C60 content in the
mesomorphic materials. The materials are electrochemically active
and possess a comparably high electron carrier mobility. Our
findings are of interest for the design of fullerene containing soft
materials with electrochemical and optoelectronic functions. More-
over, the long-range ordered lamellae described here can be
employed as “soft template” materials12 for the synthesis of two-
dimensional polymers or layered material with conductive or optical
properties.
Figure 2. (a) Cyclic (scan rate 0.1 V s-1) and (b) differential pulse (pulse
amplitude 25 mV) voltammograms of a cast film of 4 on a glassy carbon
electrode (0.1 M aqueous n-Bu4NCl solution, at 60 °C (black and red) and
at 15 °C (blue)). (c) Transient photocurrent curves as a double logarithmic
plot for a negative carrier of 1 in the mesophase at 120 °C.
Acknowledgment. This work was supported, in part, by a
Grand-in-Aid from the Ministry of Education, Sciences, Sports and
Culture, Japan, and PRESTO, JST, Japan (T.N.). We thank Drs.
K. Ariga and M. Takeuchi (NIMS) and Dr. T. Michinobu (Tokyo
Inst. Technol.) for useful discussion.
The XRD pattern for 1 at 185 °C shows a strong peak at 2θ )
1.58° assigned (0 0 1) (d spacing ) 5.59 nm) accompanied by higher
order peaks up to (0 0 14) at 2θ ) 23.44° (Figure 1c). In addition, a
broad halo centered at 2θ ≈ 19° is assigned to the molten alkyl chains.
The XRD analyses of 2 and 4 in the mesophase also showed similar
patterns with the peaks assigned from (0 0 1) to (0 0 12), from which
their interlayer distances are calculated as 4.75 and 4.80 nm,
respectively (Figure S3). These higher-degree peaks in the XRD
patterns, together with the fluid and birefringent mesophase in the
POM, reveal a long-range ordered lamellar mesophase comparable to
an ordered smectic phase. A schematic model of the supramolecular
organization of 1 is depicted in Figure 1d.
Supporting Information Available: Thermal analyses, electro-
chemical data as well as TOF analyses. This material is available free
of charge via the Internet at http://pubs.acs.org.
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It is advantageous if the electrochemical9 and optoelectronic10
properties of fullerenes in the mesomorphic state are maintained
when practical applications are considered. The derivatives display
electrochemical activity when measured as cast films on a glassy
carbon electrode above the solid/mesomorphic phase transition
temperatures. A cast film of 4 (or 1: see Figure S4) in 0.1 M aqueous
n-Bu4NCl solution at 60 °C showed the first and second redox
events corresponding to the generation of C60 monoanion and
dianion at potentials of Ered,1 ) -0.70 and Ered,2 ) -0.87 V,
respectively (Figure 2a, black, and 2b, red lines). In contrast, no
redox response was observed for a cast film of 4 at 15 °C in the
crystalline state (Figure 2a blue line). The fluid-mesophase evidently
facilitates the diffusion of molecules in this system such as
supporting electrolyte, solvent, and the fullerene derivative itself.2c
We also found that the materials possess a relatively large charge
carrier mobility. Figure 2c shows the transient photocurrent curves
(log-log plots) of 1 for a negative carrier at 120 °C measured using
a conventional time-of-flight setup (λex ) 356 nm). From the kink
points in the log-log plots, which correspond to the transit time,
the electron mobility was estimated to be ∼3 × 10-3 cm2 V-1
s-1 at an electric field of 2 × 105 V cm-1. This value is comparable
to those of smectic liquid crystalline phases of organic semiconduc-
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This value is considered a lower limit because the sample is
composed of random domains and consequently one expects a
higher mobility for a uniformly oriented sample.11d To better
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In summary, long-range ordered lamellar mesophases of fullerene
derivatives bearing long alkyl chains were presented and the carrier
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