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Chemistry Letters Vol.38, No.11 (2009)
Thermotropic Columnar Liquid Crystal of a C6-Symmetric Hydrogen-bonded
Hexakis(phenylethynyl)benzene Derivative with Amino Acid Pendant Groups
Koichi Sakajiri,ꢀ Hironori Yoshida, Keiichi Moriya, and Shoichi Kutsumizu
Department of Chemistry, Faculty of Engineering, Gifu University, 1-1 Yanagido, Gifu 501-1193
(Received August 19, 2009; CL-090765; E-mail: sakajiri@gifu-u.ac.jp)
The thermotropic liquid crystalline C6-symmetric hydro-
tion in n-alkane suggests a possibility of forming a thermotropic
LC if density of alkyl chains is increased. Hence, in the present
study, we synthesized an analogous compound, L-2, bearing chi-
ral didodecyl L-glutamate (Figure 1).
gen-bonded disk-like molecule has been obtained in a hexakis-
(phenylethynyl)benzene derivative bearing chiral didodecyl L-
glutamates, which forms a hexagonal columnar phase over a
wide temperature range including room temperature, where each
ꢀ–ꢀ stacking between the large central cores is reinforced by the
six intermolecular hydrogen bonds.
First of all, the self-assembly of L-2 was confirmed by
measuring UV–visible absorption (UV–vis) and circular dichro-
ism (CD) spectra in n-heptane (4:20 ꢂ 10ꢃ5 M) at 25 ꢁC
(Figure S1).8 L-2 shows clear Cotton effects and the spectral fea-
tures are similar to those of the analogous L-1 reported previous-
ly.7 This result indicates that L-2 also forms a supramolecular
helical columnar structure even in such a dilute solution. More-
over, in the drying process of this solution, L-2 exhibits a lyo-
tropic LC. This fact further supports that columnar structure is
being constructed.3g,5c,7
The thermal behavior of L-2 in the bulk state was investigat-
ed by differential scanning calorimetry and polarizing microsco-
py (Figures S2a and S2b, respectively).8 The combined results
revealed that L-2 forms a Col LC phase in a wide temperature
range from ca. 0 to ca. 170 ꢁC, where a dendritic growth texture
is visible (Figure S2b).8
To get information about the molecular aggregation state
such as the peripheral alkyl chain motion and the hydrogen
bonding, temperature-dependent infrared (IR) spectra were
measured on the second heating. Figure S3 shows representative
IR spectra of L-2.8 The methylene C–H antisymmetric
(ꢁas(CH2)) and symmetric (ꢁs(CH2)) frequencies of L-2 are ob-
served at 2924 (at 30–180 ꢁC) and 2853 (at 30 ꢁC)–2855 cmꢃ1
(180 ꢁC), respectively, and these two bands are almost independ-
ent of temperature. The values indicate that the conformational
structure of the alkyl chains is liquid-like in the Col LC state.9
Information about the hydrogen-bonding state was obtained
from the N–H stretching (amide A) and the C=O stretching
vibrations (amide I).5b,5e At 30 ꢁC in the Col LC state, the vibra-
tions of amide A and amide I are observed at 3264 and 1633
cmꢃ1, respectively, indicating the presence of a strong hydrogen
bonding within the column formed by L-2. When temperature
is elevated from 30 to 165 ꢁC in the Col LC state, both amide
A and amide I bands are gradually shifted to the higher wave-
numbers, 3300 and 1642 cmꢃ1, respectively (Figure 2). This re-
sult indicates that the hydrogen bonding within the column is
maintained over the whole temperature range of the Col LC
state but gradually weakened as a result of thermal motion.
The significant changes of these two amide bands are observed
at the LC–Iso (isotropic liquid state) transition temperature,
170 ꢁC, where the vibrations of both amide A and amide I bands
change drastically to the much higher wavenumbers, 3336 and
1671 cmꢃ1, respectively. These changes imply that the hydrogen
bonding within the column is broken upon the LC–Iso transition.
The liquid crystalline structure was investigated by X-ray
diffraction (XRD) measurements (Figure 3). In the temperature
range from 30 to 165 ꢁC of the Col LC state, several sharp reflec-
Supramolecular chiral assemblies constructed using various
types of noncovalent bonding interaction, as utilized in biologi-
cal systems, are of great interest from fundamental and biologi-
cal viewpoints and offer potential applications in functional ma-
terials exhibiting liquid crystal (LC), charge transport, lumines-
cence, and other combined properties.1 Among such supramo-
lecular assemblies, most of the ordered structures have been
obtained from disk-shaped molecules, which offer the possibility
of forming highly ordered columnar (Col) LC.2 To stabilize the
ꢀ–ꢀ stacking among aromatic groups and reinforce the intraco-
lumnar stacking order, it is important to enlarge the disk size3
and/or to introduce hydrogen bonds.3d,4 The majority of studies
on hydrogen-bonded helical columns have been performed on
1,3,5-benzene tricarboxamide derivatives, which are C3-sym-
metric molecules consisting of a central benzene ring and three
peripheral chiral segments connected via amide bonds.3f,3g,5 In
this system, the ꢀ–ꢀ interactions of the central benzene cores
are assisted by three fold intermolecular hydrogen bonding. Ami-
no acid units are also widely used as effective structuring elements
which form hydrogen bonds in self-assembled architectures such
as liquid crystalline dendritic peptides.6 From this background, we
previously designed a C6-symmetric hydrogen-bonded disk-like
molecule, L-1, and succeeded in further enhancing the stability
of the supramolecular helical columnar structure (Figure 1).7
The L-1 compound consists of a large hexakis(phenylethynyl)-
benzene central core bearing chiral L-alanine parts and peripher-
al hydrophobic dodecyl chains. The stacking among large cen-
tral cores of L-1 is enforced by six intermolecular hydrogen
bonds, leading to an exceptionally stable supramolecular helical
columnar structure, which is maintained even at 100 ꢁC in dilute
n-alkane solutions. The rigidity of the helical column also pro-
duced lyotropic LC properties at a relatively low concentration,
ca. 6 wt % in n-alkane but not a thermotropic LC. The LC forma-
Figure 1. Molecular structures of L-1 and L-2.
Copyright Ó 2009 The Chemical Society of Japan
Sakajiri, Koichi
