A columnar liquid-crystalline shape-persistent macrocycle having a nanosegregated structure

Article abstract of DOI:10.1039/b908669b

A shape-persistent macrocyclic molecule having glutamic acid and oligooxyethylene moieties exhibits a hexagonal columnar liquid-crystalline phase at ambient temperature through nanosegregation and the formation of intermolecular hydrogen bonding. The Roya

Full text of DOI:10.1039/b908669b

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A columnar liquid-crystalline shape-persistent macrocycle having a
nanosegregated structure†
Harutoki Shimura, Masafumi Yoshio and Takashi Kato*
Received 1st May 2009, Accepted 9th June 2009
First published as an Advance Article on the web 19th June 2009
DOI: 10.1039/b908669b
A shape-persistent macrocyclic molecule having glutamic
acid and oligooxyethylene moieties exhibits a hexagonal
columnar liquid-crystalline phase at ambient temperature
through nanosegregation and the formation of intermolec-
ular hydrogen bonding.
by-products, and separated by gel permeation chromatography.
Linear oligomers were not formed, which was confirmed by H
NMR.
1
Compound 1 exhibited an LC phase from 0 to 89 ◦C upon
heating. In this temperature region, birefringence was observed by
polarizing microscopy. The texture was nonspecific to columnar
phases, and the size of the LC domains was smaller than 10 mm
(Fig. 2a). However, the material could easily be sheared. The
boundaries of the domains disappeared after shearing, which
indicates the uniaxial orientation of the columns (Fig. 2b). Fig. 3
presents the DSC thermogram of 1. The thermogram shows sharp
transition peaks, despite the high molecular weight (8165.8 g
mol-1) of 1.
Liquid-crystalline (LC) molecules forming nanosegregated struc-
tures can be applied to anisotropic functional materials.1–6
A
number of LC molecules with new and exotic shapes have been
prepared.2,7–9 Among the structural motifs of liquid crystals,
macrocycles have attracted attention.10–14 Conjugated, rigidmacro-
cycles are called “shape-persistent macrocycles” (SPMs).13–17 Ions
and molecules can be incorporated into the nanometer-size pores
of SPMs.18–25 If these SPMs can be arranged into mesomorphic
tubular assemblies, mobile materials with internal 1D functional
nano-space will be obtained. Several LC SPMs have already been
reported.2,26–30 However, these compounds exhibited LC phases
at relatively high temperature ranges. Moreover, the phases were
often discotic nematic phases due to weak interactions between
the rings.26–28
To achieve LC 1D stacks of aromatic molecules, introduction of
nanosegregated structures and intermolecular hydrogen bonding
into the molecular design is considered to be an important
approach.2,31–33 Herein we report on an LC SPM having fan-
shaped L-glutamic acid derivatives bearing bis(alkyloxy)phenyl
moieties on its outer part, and oligooxyethylene moieties on its
inner part (Fig. 1). The glutamic acid derivatives can be used as a
molecular building block to produce stable columnar LC materials
via hydrogen bonding.34–40 Stable 1D assemblies are expected to
form through the formation of intermolecular hydrogen bonds.
Oligooxyethylene moieties are introduced to fill the internal
space of the columns. Moreover, oligooxyethylene moieties are
polar and they should be incompatible with the nonpolar alkyl
moieties. The segregation between internal and external sub-
stituents of SPMs may suppress the formation of nematic discotic
phases.
The small angle X-ray diffraction profile for 1 at 25 ◦C is
shown in Fig. 4. The diffraction pattern gives three reflections
of 5.17 (100), 2.97 (110), and 2.63 nm (200). The ratio of the
√
d-spacing is 1 : 1/ 3 : 1/2, indicating a hexagonal columnar (Colh)
arrangement. The intercolumnar distance of 1, which corresponds
to the lattice parameter, is 6.0 nm. By molecular modeling, the
diameter of 1 was estimated to be 6.0 nm, which is in agreement
with the experimental result (Fig. 5). The oxyethylene chains are
considered to be assembled in the center of the hexagonal columns,
because of incompatibility with alkyl chains.
The diameter of the conjugated macrocycle part was estimated
to be 1.8 nm by molecular modeling. The diffraction of compound
1 also exhibited a broad halo around 0.5 nm (see ESI†). No sharp
peaks ascribed to the order of the rings in the direction of the
columnar axis were seen in the diffraction pattern.
It is of interest that the precursor 2, which is not macrocyclic,
showed only a crystalline phase. It was reported that imidazolium
salts having similar fan-shaped L-glutamic acid derivatives showed
columnar LC phases.40 The polarity of oligooxyethylene chains is
lower than that of imidazolium salts, which would be the reason
why the precursors were not mesomorphic. However, with the
help of the macrocycle core, compound 1 formed stable columns
in which the oligooxyethylene moieties were segregated from the
alkyl chains at the periphery.
Macrocycle 1 was synthesized from a 3,5-diethynylbenzene
derivative of the glutamic acid moiety (2). The dimeric compound
was prepared using oxidative Hay coupling41 of 2. The Eglinton–
Glaser coupling42 of the dimer under high dilution conditions
gave cyclic hexamer 1 as a main product (for detailed synthetic
scheme, see ESI†). Cyclic tetramer and octamer were obtained as
The infrared spectra for 1 were obtained in the neat state to
examine the molecular interactions between the stacked macrocy-
cles. The broad N–H stretching band was observed at 3300 cm-1 at
◦
=
25 C in the Colh state. The C O stretching band of the glutamic
acid moieties was seen at 1639 cm-1 at 25 ◦C. These results show the
formation of intermolecular hydrogen bonds. The hydrogen bonds
between macrocycles should contribute to the induction of the
one-dimensional columnar assembly (Fig. 5). It is considered that
the rotation of the molecules along the columnar axis is restricted
by the formation of hydrogen bonds. However, rotational disorder
of the assembled single column in the hexagonal lattice may be
possible.
Department of Chemistry and Biotechnology, School of Engineering, The
University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan. E-mail:
kato@chiral.t.u-tokyo.ac.jp; Fax: (+81) 3-5841-8661
† Electronic supplementary information (ESI) available: Synthetic scheme,
wide angle X-ray diffraction pattern, and IR spectrum of compound 1;
experimental details for compounds 1–6; 1H and 13C NMR spectra for
compounds 3–6. See DOI: 10.1039/b908669b
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Fig. 2 Polarizing optical photomicrographs of 1 in the Colh phase at 87 ◦C
(a) on simple cooling, (b) after applying mechanical shear. Directions are
A: analyzer; P: polarizer; S: shearing.
Fig. 3 DSC thermogram of 1 on the 2nd heating.
Fig. 1 Molecular structures of liquid-crystalline macrocycle 1 and its
precursor 2.
Fig. 4 Small angle X-ray diffraction pattern of 1 at 25 ◦C. “Kapton” is
the diffraction peak of the polyimide film used in the measurement.
In summary, a macrocyclic columnar liquid crystal was
prepared from a shape-persistent macrocycle. For the design
of LC assemblies, the combination of the molecular shape,
nanosegregation of the building blocks, and the intermolecular
interactions is important.2–4,43,44 The molecule formed colum-
nar structures guided by the nanosegregation between the oli-
gooxyethylene chains inside the macrocycle and the alkyl chains
at the periphery, and the formation of hydrogen bonds be-
tween the amide groups. The 1D LC assembly may incorporate
ions into it, therefore it has potential as an ion-conductive
channel.2,3,45–50
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This work was partially supported by a Grant-in-Aid for Scientific
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Structures and Novel Functions” (no. 16GS0209; T.K.) from the
Japan Society for the Promotion of Science (JSPS) and a Grant-
in-Aid for the Global COE Program for Chemistry Innovation
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