A thermoresponsive photoluminescent smectic liquid crystal: Change of photoluminescent color on the smectic-smectic phase transition

Article abstract of DOI:10.1039/b902848j

An anthracene derivative having forklike side chains shows a change of photoluminescent color from green to light blue at the smectic-smectic phase transition.

Full text of DOI:10.1039/b902848j

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A thermoresponsive photoluminescent smectic liquid crystal: change
of photoluminescent color on the smectic–smectic phase transitionw
Shogo Yamane, Yoshimitsu Sagara and Takashi Kato*
Received (in Cambridge, UK) 10th February 2009, Accepted 15th April 2009
First published as an Advance Article on the web 11th May 2009
DOI: 10.1039/b902848j
An anthracene derivative having forklike side chains shows a
change of photoluminescent color from green to light blue at the
smectic–smectic phase transition.
mesogens induced smectic phases for complex molecular
moieties such as interlocked catenane and rotaxane structures.11
Therefore, compound 1 is expected to show smectic A phases.
Compound 2 was also designed to examine the effects of side
chains on the LC behavior.
Soft materials have attracted much attention because of their
dynamic properties.1 Liquid crystals are among soft materials
combining fluidity and ordered structures.2 Based on such
unique properties of liquid crystals, a number of liquid-
crystalline (LC) photoluminescent materials have been developed
for various applications such as organic light emitting diodes,3
laser devices,4 and sensing devices.5 Our intention here is the
development of a new type of photo-functional smectic liquid
crystals exhibiting change of the photoluminescent color by
external stimuli. In general, photoluminescent properties
observed for molecular materials depend on their assembled
structures.5–7 Therefore, liquid crystals can be good candidates
for stimuli-responsive photoluminescent materials because
LC–LC phase transitions are often accompanied by a dynamic
change of the assembled structures.
The LC properties of compounds 1 and 2 are summarized in
Table 1. Compound 1 exhibits a smectic A (SmA) phase and
three unidentified smectic (Sm1, Sm2 and Sm3) phases. On
heating, this compound shows a highly viscous smectic (Sm1)
phase from À14 to 83 1C. Two subsequent unidentified smectic
(Sm2 and Sm3) phases with low viscosity are observed from 83
to 104 1C. The Sm2–Sm3 phase transition is observed at 94 1C.
On further heating, compound 1 shows an SmA phase from
104 to 132 1C. Focal conic textures characteristic of SmA
phases are observed on cooling from the isotropic phase
(see ESIw). The focal conic textures change slightly on
the SmA–Sm3 and Sm3–Sm2 phase transitions on cooling
(see ESIw).
Under UV irradiation (365 nm), compound 1 shows green
photoluminescence in both the Sm3 (Fig. 2, left) and Sm2
phases. In contrast, light-blue photoluminescence is shown in
the SmA phase (Fig. 2, right). This phenomenon is an example
of thermochromic luminescence in ordered structures.5b–d,7c,d
The absorption and emission spectra (Fig. 3) have been
obtained to examine the origin of the color change. As shown
in Fig. 3(a), the absorption bands of 1 in the Sm2 and Sm3
phases are almost identical to that of 1 in the SmA phase.
These absorption spectra, having well-resolved vibronic struc-
tures, are similar to that of 1 in chloroform (1.0 Â 10À5 M,
see ESIw), indicating that no obvious ground-state interactions
occur between the luminescent cores of 1 in the Sm2, Sm3 and
SmA phases. Compared to the absorption spectral features,
the emission spectra for 1 in the Sm2 and Sm3 phases are
different from that for 1 in the SmA phase. The broad and
structureless emission bands observed for 1 in the Sm2 and
Sm3 phases (Fig. 3(b), green dotted and solid lines) are
ascribed to excimer formation of the anthracene moieties of 1.
To date, a number of anthracene derivatives have been
reported to show excimer emission.12 As for the SmA phase,
Herein we report on a change of photoluminescent color of
anthracene derivative 1 (Fig. 1) from green to light blue upon
smectic–smectic phase transition. In our previous studies,5a,b
pyrene and anthracene derivatives having dendritic side chains
showed changes of photoluminescent color on the cubic–
columnar phase transitions induced by mechanical shearing
and heating. However, until now, only limited numbers of LC
materials are known to change their photoluminescent colors
on LC–LC phase transitions with dynamic change of the
assembled structures.5a–c Moreover, to the best of our knowledge,
no photo-functional liquid crystals that show smectic–smectic
phase transitions associated with change of the photolumines-
cent colors have been prepared.
Our molecular design is shown in Fig. 1. Compound 1 has a
2,6-diethynylanthracene8 group as a photoluminescent core.
Up to now anthracenes have been incorporated into several
LC molecules.9 Compound 1 also has a block structure
consisting of six side chains of tetra(ethylene oxide) and
p-(4-trans-pentylcyclohexyl)phenyl moieties.
A compound
having a block structure composed of the two moieties has
been reported to exhibit the smectic A phase originating from
the nanosegregation between the two different moieties.10 In
addition, similar forklike side chains consisting of rodlike
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
w Electronic supplementary information (ESI) available: Synthetic
procedures and characterization for compounds 1 and 2 and other
detailed experimental data of 1 and 2. See DOI: 10.1039/b902848j
Fig. 1 Molecular structures of anthracene derivatives 1 and 2.
Chem. Commun., 2009, 3597–3599 | 3597
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Table 1 Liquid-crystalline properties of compounds 1 and 2
Compound
Phase transition behavioura
1
2
GꢁÀ14ꢁSm1ꢁ83ꢁSm2ꢁ94ꢁSm3ꢁ104ꢁSmAꢁ132ꢁIso
GꢁÀ26ꢁSm4ꢁ206ꢁNb
a
Transition temperatures (1C) determined by DSC (second heating;
5 1C minÀ1), POM observation and X-ray diffraction patterns.
G: glass; Sm1, Sm2, Sm3 and Sm4: unidentified smectic; SmA: smectic
b
A; N: nematic; Iso: isotropic. Thermal decomposition occurred
above 270 1C before reaching the isotropic state.
Fig. 2 Photoluminescent images of 1 in the Sm3 phase at 98 1C (left)
and in the SmA phase at 120 1C (right) under UV irradiation (365 nm).
Compound 1 is sandwiched between quartz substrates.
Fig. 4 X-Ray diffraction patterns of 1 in the Sm2 phase at 88 1C (a),
in the Sm3 phase at 98 1C (b) and in the smectic A phase at 120 1C (c).
Fig. 3 Absorption spectra (a) and emission spectra (b) of 1 in the Sm2
phase at 88 1C (green dotted line), in the Sm3 phase at 98 1C (green
solid line) and in the SmA phase at 120 1C (blue line).
the emission peaks and shoulders appear at 430, 454 and
487 nm besides the excimer emission band (Fig. 3(b), blue line).
These peaks are attributed to the monomeric emission of 1,
because the emission spectrum of a chloroform solution of 1
(1.0 Â 10À5 M, see ESIw) displays the peaks at the same
positions. The decrease of relative ratio of the excimer
emission on the Sm3–SmA phase transition results in the
change of the photoluminescent color from green to light blue
on the phase transition. On the other hand, no obvious change
in the relative ratio of the excimer emission occurs on
the Sm2–Sm3 phase transition (Fig. 3(b), green dotted and
solid lines).
Fig. 5 Schematic illustrations of the self-assembled structures of 1 in
the Sm3 (left) and SmA (right) phases.
transitions observed for compound 1. To fill the gap between
the anthracene moieties and fork-like side chains, a part of side
chains may be in the layer of anthracene cores by bending of
the oxyethylene chain. In the self-assembled structure for the
Sm2 and Sm3 phases, the emitting cores are close to each
other. Therefore, excited luminescent cores form excimers with
adjacent luminescent cores and the energy transfer may occur
from most of the excited luminescent cores to the excimer
sites.13 As a consequence, no obvious monomeric emission
appears in the emission spectra of 1 in the Sm2 and Sm3
phases (Fig. 3(b), green dotted and solid lines). On the other
hand, the XRD pattern in the SmA phase (Fig. 4(c)) reveals
that the self-assembled structure of 1 in the SmA phase is
different from the monolayer structure observed for 1 in the
Sm2 and Sm3 phases. As shown in Fig. 4(c), only one peak
appears at 38.2 A in the diffraction pattern obtained at 120 1C.
Because the layer spacing is less than half of the estimated
molecular length, compound 1 is considered to form an
interdigitated structure in the SmA phase (Fig. 5, right). In
this self-assembled structure, luminescent groups are
surrounded by the side chains, leading to separation of the
luminescent cores from each other (Fig. 5, right). In this case,
the deactivation of excited states with monomeric emission
X-Ray diffraction (XRD) measurements have been
performed to examine the relationship between the self-
assembled structure and the luminescent color. The XRD
patterns of 1 in the Sm2 and Sm3 phases show several
diffractions in the small angle region (Fig. 4(a) and (b)). In
the Sm3 phase at 98 1C, the XRD pattern shows the diffraction
peaks at 86.5, 42.6, 27.7, 20.5 and 16.2 A corresponding to
(100), (200), (300), (400) and (500) reflections, respectively
(Fig. 4(b)). The layer spacing is 8.5 nm, which almost coincides
with the estimated molecular length, 8.2 nm for compound 1 in
its extended form (see ESIw). It is assumed that a monolayer
self-assembled structure (Fig. 5, left) is formed in the Sm3
phase. As the diffraction peaks for the Sm2 and Sm3 phases
are very similar (Fig. 4(a) and (b)), the self-assembled struc-
tures of 1 in the Sm2 phase are similar to those of 1 in the Sm3
phase, leading to small DH compared to the other phase
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(around 454 nm) occurs for some of the excited luminescent
cores, before they form excimers or the energy transfer occurs.
As a consequence, both monomeric and excimer emissions are
observed in the emission spectrum of 1 in the SmA phase
(Fig. 3(b), blue line).
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Based on the XRD measurements and emission spectra of
compound 1 (Fig. 3 and 4), the induction of the Sm3–SmA
phase transition can be explained as follows. In the Sm3 phase,
nanosegregation between the tetra(ethylene oxide) and
p-(4-trans-pentylcyclohexyl)phenyl moieties of the forklike
side chains is the dominant driving force to induce the smectic
LC behavior, leading to the formation of a monolayer self-
assembled structure (Fig. 5, left). Since the occupied volume of
the forklike side chains increases with increase of temperature,
an interdigitated structure forms to complement the difference
of spatial volume between the forklike side chains and the
luminescent cores. Consequently, the Sm3–SmA phase transi-
tion occurs (Fig. 5, left - right). A dumbbell-shaped molecule
having similar structures to compound 1 was reported to also
show Sm–Sm phase transitions.14 Moreover, the phase transi-
tions are observed for various types of LC compounds.15
Compared to compound 1, compound 2 having a linear
molecular shape shows no Sm–Sm phase transition (see ESIw).
In summary, we have developed an anthracene-based
smectic liquid crystal showing the change of photoluminescent
color from green to light blue on the Sm3–SmA phase transi-
tion. The luminescent color change is ascribed to the decreasing
of the ratio of the excimer emission, which arises from the
change of the self-assembled structures on the Sm3–SmA
phase transition. This liquid crystal could be used as a
stimuli-responsive polarized photoluminescent material if it
is uniaxially aligned.
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This work was partially supported by Grant-in-Aid for
Creative Scientific Research of ‘‘Invention of Conjugated
Electronic Structures and Novel Functions’’ (no. 16GS0209;
T. K.) from the Japan Society for the Promotion of Science
(JSPS) and The Global COE Program (Chemistry Innovation
through Cooperation of Science and Engineering) (T. K.)
from MEXT. Y. S. is grateful for financial support from the
JSPS Research Fellowship for Young Scientists.
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