Luminescent bow-tie-shaped decaaryl[60]fullerene mesogens

Article abstract of DOI:10.1021/ja907908m

(Chemical Equation Presented) A pseudo-D_5d symmetric decaaryl[60]fullerene molecule has a rigid bow-tie shape and bears an emissive [10]cyclophenacene chromophore in the middle. It forms a smectic liquid crystalline mesophase that persists over

Full text of DOI:10.1021/ja907908m

Published on Web 11/06/2009
Luminescent Bow-Tie-Shaped Decaaryl[60]fullerene Mesogens
Chang-Zhi Li,† Yutaka Matsuo,*,†,‡ and Eiichi Nakamura*,†,‡
Nakamura Functional Carbon Cluster Project, ERATO, Japan Science and Technology Agency (JST), Hongo,
Bunkyo-ku, Tokyo 113-0033, Japan, and Department of Chemistry, The UniVersity of Tokyo, Hongo, Bunkyo-ku,
Tokyo 113-0033, Japan
Received September 17, 2009; E-mail: matsuo@chem.s.u-tokyo.ac.jp; nakamura@chem.s.u-tokyo.ac.jp
Molecular shape is a primary factor in the formation of
mesogenic superstructures,1 yet it is often unclear whether the bulk
properties, such as optical properties, can be controlled by the shape
of individual molecules. Rod, disk, and bent-core mesogens form
a variety of liquid crystals (LCs) that exhibit anisotropic and
switching properties that are conducive to numerous applications.2
Because of its unique isotropic structure and nanometer size,
[60]fullerene has provided new opportunities for the development
of new classes of mesogens,3 especially those possessing a large
three-dimensional structure, e.g., C5V or pseudo-C5V symmetric
pentaaryl[60]fullerene (C60Ar5X, X ) H, Me, CpFe)4 and Th
symmetric hexa(organo)fulllerenes5 that can hardly be developed
from small molecules. When a C60Ar5X molecule bears large Ar
Figure 1. Structure of compounds 1-4 (left) and a schematic illustration
of the proposed smectic organization of 1 based on XRD data (right).
groups, the molecule resembles a badminton shuttlecock and forms
columnar stacks in the LC mesophase.4 The deca-aryl fullerenes
(Ar5MeC60Ar5Me) 1-4 (Figure 1) contain a pseudo-D5d symmetric
structure6 that represents a head/head superposition of two
C60Ar5Me molecules. These molecules are unique for their central
hoop-shaped [10]cyclophenacene moiety6 (colored orange in Figure
1), which exhibits anisotropic luminescence.7
The XRD data exhibited a 2D smectic periodicity over a wide
temperature range.
Herein we report on the synthesis and properties of a new class
of mesogenssbow-tie-shaped mesogens, which form a smectic
mesophase that emits light anisotropically.8 The anisotropic property
of the light emission can be changed by the uniaxial shearing of
the LC film7c,d,9 because of the change in the long-range spatial
alignment of the emissive [10]cyclophenacene moiety in the bulk.10
The LCs of 1 exhibit luminescence with a quantum yield of Φ )
0.21, which is the highest reported value for fullerene derivatives.6,11
Compounds 1-4 were synthesized in two synthetic operations
from [60]fullerene (see Supporting Information). Differential scan-
ning calorimetry (DSC), polarized optical microscopy (POM), and
X-ray diffraction (XRD) studies indicated that compounds 1 and 2
exhibit a layered mesomorphism, while compounds 3 and 4 form
a crystalline structure.
A DSC analysis of compound 1 (C18) showed only one
mesophase over a wide temperature range (∆T ) 240 °C), in which
Figure 2. Properties of 1 (C18). (a) DSC trace. (b) POM texture under
crossed polarizer (× 20 times) at 241 °C on the second heating process. (c)
XRD data at 195 °C. (d) Absorption and emission of a cyclohexane solution
(blue and orange curves, respectively), and emission of casted film (black
curve). Both samples were excited at 366 nm. Insert: pictures of yellow
photoluminescence of solid 1 on the left and 1 in cyclohexane on the right.
we observed one endothermic transition at 17 °C (79.7 kJ ·mol-1
,
Tcr-lc) and another at 257 °C (27.4 kJ·mol-1, Tlc-iso). During
subsequent heating-cooling cycles at a rate of 10 °C/min, we found
a quick and reversible phase transition (Figure 2a) that took place
much faster than the transition observed for the corresponding
C60Ar5X molecules.4d Upon the second heating to 241 °C, we
observed a fan-shaped POM texture (Figure 2b), which persisted
for several months at room temperature. The XRD pattern at 195
°C showed a strong (01) reflection at 2θ ) 2.54° (d spacing )
3.48 nm), with a (02) reflection (2θ ) 5.02°, Figure 2c). In addition,
a broad halo centered at 2θ ≈ 19.4° indicated molten alkyl chains.
Compound 2 (C12) exhibited a 3D smectic mesophase.12 Unlike
the 2D smectic phase where the molecules are ordered to form a
layer but disordered within the layer, LCs of 2 exhibited periodicity
within the lamella layer as indicated by the reflections of the
positional order (d010) and a long-range ordering of lamellas (d001
to d006) (Figures S1 and S2). It also showed a higher Tlc-iso of 310
°C (22.6 kJ ·mol-1, Figure S5). Compounds 3 (C8) and 4 (C4), which
bear much shorter alkyl chains, showed only a crystalline phase.
XRD indicated that the fullerene moiety is aligned in a plane with
† JST.
‡ The University of Tokyo.
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10.1021/ja907908m CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
the longer molecular axis perpendicular to this plane (cf. Figure
1). The interlayer distance in the LCs and crystals of 1-4 ((001)
reflections at 195 °C) increases linearly as the alkyl chain length
increases (from 1 to 4 and 3.48 to 1.80 nm; Figure S3). We
therefore conclude that the alkyl chains are sandwiched between
the fullerene layers, as illustrated in Figure 1. We ascribe this
microphase segregation to the mesogens’ bow-tie molecular
shape and the incompatibility of the alkyl chains with the
fullerene core. Note that the latter contributed to the formation
of one-dimensional stacking of the conical C60Ar5X molecules.4
Compounds 1-4, which bear an emissive 40π-electron [10]cy-
clophenacene system, exhibit photoluminescence (PL).6 As shown
in Figure 2d, compound 1 exhibits intense absorption bands at <400
nm, with a shoulder at 340 nm, and some weak absorption in the
visible ranges (maxima at 428 and 456 nm) that are characteristic
of the [10]cyclophenacene moiety. Upon excitation of compound
1 at 366 nm in cyclohexane, we observed two intense emission
peaks with a maximum at 568 nm and with a PL quantum yield of
Φsol ) 0.21 (absolute measurement), which is the highest value
thus far reported for fullerene derivatives. For the drop-casted film,
we also observed an emission at 563 nm with a comparable quantum
yield of Φfilm ) 0.18, which strongly suggests that the cyclophen-
acene chromophores are spatially separated from each other in the
LCs and do not suffer much from self-quenching in the excited
state.
once a way is found to improve the ordering, the macroscopic
emissive property of 1-4 will generate interesting applications
when combined with organic light-emitting diode technology.14
Acknowledgment. We thank MEXT (KAKENHI to E.N., No.
18105004) and the Global COE Program for Chemistry Innovation.
Supporting Information Available: Synthesis, DSC, optical tex-
tures, XRD, and anisotropic photoluminescence. This material is
available free of charge via the Internet at http://pubs.acs.org
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In conclusion, decaaryl[60]fullerene molecules (Ar5MeC60Ar5Me)
1-4 display a unique bow-tie shape, which is found in the central
fullerene part forming a layered structure whose longer molecular
axis is perpendicular to the plane. This molecular orientation causes
the alignment of the emissive [10]cyclophenacene rings in the plane
of the layer and allows the bulk LC phase to emit light anisotro-
pically due to the presence of ordered lamellas in the smectic phase.
It is apparent that such long-range ordering is far from perfect,
and hence, the ratio of anisotropic emission remains low. However,
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