Spiropyran-based liquid crystals: The formation of columnar phases via acid-induced spiro-merocyanine isomerisation

Article abstract of DOI:10.1039/b610903a

Hexagonal columnar liquid-crystalline phases are induced for a new fan-shaped spiropyran compound as the result of an acidichromism effect of spiro-merocyanine isomerisation through protonation upon incorporation of 4-methylbenzenesulfonic acid. The Royal Society of Chemistry.

Full text of DOI:10.1039/b610903a

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Spiropyran-based liquid crystals: the formation of columnar phases via
acid-induced spiro–merocyanine isomerisation{
Boon-Hooi Tan,a Masafumi Yoshio,a Takahiro Ichikawa,a Tomohiro Mukai,b Hiroyuki Ohnob and
Takashi Kato*a
Received (in Cambridge, UK) 28th July 2006, Accepted 7th September 2006
First published as an Advance Article on the web 4th October 2006
DOI: 10.1039/b610903a
Hexagonal columnar liquid-crystalline phases are induced for a
new fan-shaped spiropyran compound as the result of an
acidichromism effect of spiro–merocyanine isomerisation
through protonation upon incorporation of 4-methylbenzene-
sulfonic acid.
Columnar liquid crystals forming nano-segregated structures have
drawn enormous interest recently due to their potential applica-
tions, such as for use as one-dimensional ion-, charge-, or energy-
transporting media.1–4 For example, we reported previously one-
dimensional ion conduction of self-organised columnar ionic
liquids.4 For further functionalisation of such ionic materials, an
approach to external stimuli-induced liquid-crystalline self-assem-
bly5,6 is important. Herein, we report the first example of an
Fig. 1 Molecular structures of compounds 1–5.
acid-induced thermotropic columnar liquid-crystalline assembly
for a spiropyran compound, leading to one-dimensional ion
transportation.
To promote mesophase formation through an acidichromism
effect,9 compound 1 was mixed with solutions of 4-methylbenze-
nesulfonic acid (2), 4-dodecylbenzenesulfonic acid (3), 4-methyl-
benzoic acid (4), and 4-methylphenol (5) (Fig. 1) which were then
dried in the air overnight. Compound 2 was selected as it is one of
the simplest aromatic sulfonic acids with a high melting point
(106 uC). Furthermore, both compounds 2 and 3 are strong
protonic acids10,11 for stimulating SP–MC isomerisation.
Compounds 4 and 5 were used to assess the effect of acidity in
SP–MC isomerisation.
Spiropyrans are unique chemo-, thermo-, and photochromic
compounds due to their external stimuli-induced spiro–merocya-
nine (SP–MC) isomerisation.7,8 SP–MC transformation from the
non-planar and non-ionic spiro form to the planar and ionic
merocyanine form is expected to promote aggregation and
formation of ordered nanostructures. Our objective is to
manipulate acid-induced SP–MC isomerisation for: (i) the
formation of columnar phases through self-assembly of merocya-
nine isomers of a spiropyran fan-shaped compound and (ii) the use
of the columnar nanostructures, formed by merocyanine isomers,
as the conduction path for an ion transportation. We have
designed and prepared a spiropyran-based compound (1) with a
fan-shaped trialkoxy benzene group (Fig. 1). To the best of our
knowledge, neither spiropyran core columnar liquid-crystalline
materials nor one-dimensional ion conductors of spiropyran
derivatives have been reported.
As shown in Table 1, compound 1 alone does not show
mesomorphic behaviour as the non-planar spiro form of the
molecule disturbs an ordered stacking of columnar structures.7
Table 1 Thermal properties of 1 and binary mixtures of 1 with 2 and
3
Compound
Phase transition behavioura/uC
A waxy solid of 1 was obtained by etherification of a gallic acid
derivative having a 2-bromoethyl linker with 1,3,3-trimethyl-
69-hydroxy-spiro-[2H-1-benzopyran-2,29-indoline].
1
Cr
40
(50.1)
55
(31.5)
56
(29.2)
56
(81.7)
Iso
1 and 2 (0.50)b Cr
1 and 2 (0.60)b Cr
1 and 2 (0.67)b Cr
Cr + Colh 60
Colh 119
(0.3)
Colh 133
(0.4)
96
Iso
Iso
Iso
Iso
(0.6)
Cr + Colh 62
(2.9)
Cr + Colh 63
(0.3)
29
(8.3)
aDepartment 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
bDepartment of Biotechnology, Tokyo University of Agriculture and
Technology, Koganei, Tokyo, 184-8588, Japan
Colh
Cr3
(0.3)
46
(10.2)
1 and 3 (0.50)b Cr1 23
Cr2
(2.7)
a
Enthalpies of transition (kJ mol21) in parentheses, determined by
{ Electronic supplementary information (ESI) available: Syntheses and
experimental details, binary phase diagram of 1 and 2, UV-vis spectra of
each binary mixture of 1 and 3–5, polarised optical micrographs, X-ray
diffraction patterns, scheme of isomerisation process of 1, and schematic
illustration of a proposed self-assembled columnar structure of an
equimolar mixture of 1 and 2. See DOI: 10.1039/b610903a
DSC (second heating scan, 10 uC min21). Cr = crystalline; Colh
hexagonal columnar; Cr + Colh = a biphasic mixture of crystalline
=
b
and hexagonal columnar; Iso = isotropic. Molar fraction of 1, in
the mixture of 1 and 2, in parentheses.
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Chem. Commun., 2006, 4703–4705 | 4703

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However, an equimolar mixture of 1 and 2 leads to the formation
of an enantiotropic columnar phase with the liquid-crystalline
temperature ranging from 60 to 119 uC on heating. The mesophase
formation can be ascribed to the acid-induced SP–MC isomerisa-
tion and the presence of the protonated merocyanine isomers.{
Fig. 2 shows a polarised optical micrograph of the equimolar
mixture of 1 and 2 at 90 uC on cooling. It displays a typical texture
of a columnar phase. The hexagonal columnar arrangement is
isomers, respectively (Fig. 3(b)). The identification of the observed
peaks is further confirmed from the UV spectrum of an equimolar
mixture solution of 1 and 2 in THF, as depicted in Fig. 3(c). The
absorption peak of the merocyanine isomer in the solution state is
observed at 485 nm. The slight shift in the position of the
merocyanine peak between the bulk liquid-crystalline state and the
solution state is due to the interaction of the merocyanine isomer
with the THF solvent.
Merocyanine is known to form stacked molecular aggregates.7,13
As both merocyanine and protonated merocyanine isomers are
present in the equimolar mixture of 1 and 2, we assume that the
columnar nanostructures are induced and stabilised by nano-
segregation2,14 of the lipophilic parts and the hydrogen-bonded15
and ionic moieties.2,16 On the other hand, when 1 is added to 3 in
an equimolar ratio, no mesomorphism is observed, although both
merocyanine and protonated merocyanine isomers are formed.{
The long alkyl chain of 3 may disturb the packing of the
merocyanines into columnar structures. In the cases of binary
mixtures of 1 with 4 and 5, respectively, neither mesomorphism
nor SP–MC isomerisation is observed, which is considered to be
due to their weak acidities.{ These results indicate that both the
acidity and the anion size of the acid are critical in promoting
liquid crystal formation.
˚
further confirmed by three sharp peaks at 49.1, 28.4, and 24.6 A,
corresponding to (100), (110), and (200) reflections, in the X-ray
diffraction pattern obtained for the same sample at 90 uC.{ A
binary phase diagram of 1 and 2 has been prepared to
examine liquid-crystalline behaviour of the mixtures.{ The
Colh–Iso transition curve shows a significant positive deviation,
suggesting the existence of specific interactions between the two
components.12
To confirm the presence of merocyanine and protonated
merocyanine isomers upon the addition of 2, UV-vis spectra have
been obtained (Fig. 3). Prior to mixing with 2, only the peak of the
spiro isomer at 340 nm is detected for compound 1 in the bulk
state (Fig. 3(a)). Upon the addition of 2, two new peaks at 380 and
480 nm are observed for an equimolar mixture of 1 and 2 in the
bulk liquid-crystalline state at 90 uC, corresponding to the
absorptions of the protonated merocyanine and merocyanine
One-dimensional ion conductivities of an equimolar mixture of
1 and 2 have been examined using an alternating current
impedance method with comb-shaped gold electrodes, as reported
previously.4 The self-assembled columnar polydomains are aligned
in the directions perpendicular and parallel to the gold electrode
via mechanical shearing. Fig. 4 indicates anisotropic ion con-
ductivities for the mesomorphic mixture of 1 and 2. The ion
conductivities parallel to the columnar axis (s||) for the equimolar
mixture of 1 and 2 are higher than those perpendicular to the axis
(sH) in the ordered state up to 123 uC. The highest s|| value in the
Fig. 2 Polarised optical micrograph of an equimolar mixture of 1 and 2
in the columnar state at 90 uC on cooling.
Fig. 4 Anisotropic ion conductivities of an equimolar mixture of 1 and 2
as
a
function of temperature on heating: (#) parallel and ($)
perpendicular to the column axis. The broken line denotes the Colh–Iso
Fig. 3 UV-vis absorption spectra of (a) 1 in the bulk state, (b) an
equimolar mixture of 1 and 2 in the bulk liquid-crystalline state at 90 uC,
and (c) an equimolar mixture solution of 1 and 2 in THF (6.8 6 1025 M).
phase transition temperature.
4704 | Chem. Commun., 2006, 4703–4705
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columnar state is 7.7 6 1027 S cm21 at 120 uC. The anisotropy
(s||/sH) in the ion conductivities in the columnar state is ca. 10.
Above the Colh–Iso transition temperature, the anisotropy
disappears. This change indicates the formation of ion conduction
pathways in the columnar state.4 Previous reports indicated that 2
is a good proton donor17,18 whereas 1 could serve as a proton
acceptor,19 forming the protonated merocyanine isomer. Hence,
we suggest that the observed anisotropic ion conduction is due to
proton hopping20 in the phenolic moieties of the merocyanine
stacks in the columnar nanostructures.
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In summary, by manipulating the chemical responsiveness of a
spiropyran derivative in SP–MC isomerisation, we have succeeded
in developing a new hexagonal columnar mesogen. Furthermore,
the new liquid-crystalline material exhibits anisotropic ion
conductivities and, thus, could be developed as a novel stimuli-
responsive one-dimensional ion conductor.
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This study was partially supported by a Grant-in-Aid for
Creative Scientific Research of ‘‘Invention of Conjugated
Electronic Structures and Novel Functions’’ (No. 16GS0209)
(TK) and a Grant-in-Aid for Scientific Research (B) (No.
17350065) (TK) from the Japan Society for the Promotion of
Science (JSPS) and a Grant-in-Aid for The 21st Century COE
Program for Frontiers in Fundamental Chemistry (TK) and
Scientific Research on Priority Areas of ‘‘Science of Ionic Liquids’’
(No. 18045010) (TK) from the Ministry of Education, Culture,
Sports, Science, and Technology.
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Chem. Commun., 2006, 4703–4705 | 4705