Light-induced deformation of photoresponsive liquid crystals on a water surface

Article abstract of DOI:10.1002/chem.200802461

The photoinduced deformative behavior of liquid crystals (LCs) composed of an azobenzene chromophore was demonstrated. An azobenzene derivative having a LC phase at room-temperature was used for the study to induce the macroscopic deformation of a LC at a

Full text of DOI:10.1002/chem.200802461

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DOI: 10.1002/chem.200802461
Light-Induced Deformation of Photoresponsive Liquid Crystals on a Water
Surface
Kunihiko Okano,* Masato Shinohara, and Takashi Yamashita*[a]
Materials that can convert input energies into various me-
chanical responses such as displacement, strain, change in
velocity, and stress have been attracting interest. Many of
the materials being developed are based on polymeric mate-
rials such as gels[1] and conducting polymers.[2] Recently, the
development of photoresponsive materials that can convert
photochemical energy into mechanical motion has attracted
interest, because the use of light as an external stimulus
allows the remote control and rapid deformation of various
materials. For example, azobenzene liquid-crystalline elasto-
mers show a mechanical response upon photoirradiation by
the isomerization of the azobenzene moiety.[3] Moreover,
molecular crystals based on anthracene and diarylethene
chromophores exhibit macroscopic changes in shape and
size on photoisomerization.[4,5] To achieve macroscopic
motion of materials, exploitation of the coupling between
molecular order and mechanical strain is one of the efficient
approaches.[6] This idea has been used to create light-driven
materials such as polymers or crystals.[1–5] That is, the photoi-
somerization of constituent molecules gives rise to changes
in molecular alignments, and thereby deforms the material.
Herein we describe and examine the photoinduced defor-
mative behavior of liquid crystals (LCs) composed of an
azobenzene chromophore. Based on the concepts of photo-
mechanical materials, it is expected that a LC having fluidity
and orientation will show macroscopic deformation by a
change in molecular alignment. To induce the macroscopic
deformation of a LC at ambient temperature, we use an
azobenzene derivative that shows a room-temperature LC
phase.
Figure 1. Polarizing optical microscopy (POM) analysis of
all compounds consistently showed characteristic schlieren
textures of a nematic (N) phase. The DSC profiles of com-
pound 3 exhibited the usual two peaks for a LC resulting
from thermal phase transitions at the melting and cleaning
points. On the other hand, compounds 1 and 2, which con-
tain a double bond at the end of the alkyl spacer, showed
suppression of crystallization for a cooling process. Instead
of the crystallization, a glass transition is observed at an ex-
tremely low temperature (ca. ꢀ258C). Based on previous
studies,[7] it is expected that the LC system composed of en-
tirely photochromic molecules shows higher photorespon-
sive behavior than that of the doped system. To the best of
our knowledge, this is a rare example of a photochromic
compound with a room-temperature LC phase, which is
beneficial for various applications. As part of our continuing
studies, we used 1 to examine the photodeformative behav-
ior of a LC having a chromophore.
To investigate the photoinduced deformation of the LC 1,
we placed a LC droplet on a water surface and then irradiat-
ed UV light (366 nm) to induce trans–cis photoisomeriza-
tion. As shown in Figure 2a and in the Supporting Informa-
tion, a LC droplet of 1 continuously underwent a flattening
of its lens shape and became larger upon UV light illumina-
tion (5 s ! 10 s ! 20 s; irradiation intensity, 130 mWcmꢀ2).
The dilatational motion nearly stopped upon irradiation for
20 s as the photostationary state was approached. The final
area of the LC droplet is six to seven times larger than the
initial state after expansion. Upon irradiation with a green
laser beam (wavelength: 532 nm, irradiation intensity:
130 mWcmꢀ2) that induces cis–trans back isomerization, a
recovery process in the LC shape did not occur. However,
after irradiation with the green laser beam, further expan-
sion in the shape occurred with UV irradiation. Further-
more, by monitoring the motion of the floating solid parti-
cles, we confirmed that no change occurred in the shape of
samples. In the next trial, we dropped a mixture of 1 and
30 wt% dimethyl formamide (1/DMF), which exhibits a LC
phase on a water surface, and then illuminated this with UV
light (see Figure 2b and the Supporting Information). Sur-
The structures and differential scanning calorimetry
(DSC) thermograms of compounds 1–3 are shown in
[a] Dr. K. Okano, M. Shinohara, Prof. Dr. T. Yamashita
Department of Pure and Applied Chemistry
Tokyo University of Science
2641 Yamazaki, Noda-shi, Chiba 278-8510 (Japan)
Fax : (+81)4-7124-9067
E-mail: kokano@rs.noda.tus.ac.jp
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200802461.
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nally, this condensation process
was terminated, and the accu-
mulated LC dispersed as soon
as the illumination ceased. We
also observed the condensation
behavior in the mixture of the
LC and other solvents, such as
THF, o-dichlorobenzene, and
toluene. Moreover, upon irradi-
ation with a green laser of the
LC in the photostationary state,
we observed no recovery pro-
cess in terms of the shape.
However, after irradiation with
a green laser beam, further con-
densation occurs upon UV irra-
diation.
To evaluate the microscopic
dynamics of the photorespon-
sive behavior in the LC sample
of 1 and 1/DMF floating on
water, we investigated possible
photoinduced changes in the
molecular alignment at room
temperature by using POM
(Figure 3). Before irradiation
with UV light, both samples
showed a bright schlieren tex-
ture in the polarizing micro-
graphs (left images, Figure 3a
and b), confirming that the
samples had an N phase even
on a water surface. Just after ir-
radiation, dark areas quickly
appeared in the LC phase due
to trans–cis photoisomerization
(right images, Figure 3a and b).
The formation of dark areas is
attributed to the photochemical
N-to-Iso phase transition and
the vanishing of birefringence,
as reported previously. We
assume that this result provides
us with a key to one of the
mechanisms of light-induced
deformation, because the pho-
tochemical phase transition in-
Figure 1. a) Chemical structures of the LC compounds used in this study; Iso, isotropic; N, nematic; G, glassy;
Cr, crystalline. b) DSC thermograms of the compounds (recording rate: 108C minꢀ1).
Figure 2. a) The expansion of a ~2 mL LC droplet on the surface of water at 258C occurred as a result of irra-
diation with a perpendicular beam of 366 nm light (130 mWcmꢀ2) focused on the center of the drop. b) The
condensation of a ~2 mL LC droplet with 30 wt% DMF under the same conditions. The white dashed lines
show the edges of the LC droplet.
prisingly, the LC droplet of 1/DMF exhibited condensation
behavior near the spot of UV illumination. The condensa-
tion processes of the sample were observed from a fully dis-
persed LC state (0 s) and snapshots of the LC shape during
the subsequent photoisomerization (1 s ! 5 s ! 20 s; irradi-
ation intensity, 130 mWcmꢀ2). That is, upon UV irradiation,
the bulge was generated immediately due to accumulation
of the LC sample in the center of the irradiation region.
Further irradiation enlarged the size of this bulge from the
center of the bright spot to the surrounding bright area. Fi-
duced a significant change in chemical or physical proper-
ties.
Next, we discuss the effect of light intensity on the rate of
deformation. Figure 4a and b show the maximal speed of ex-
pansion and contraction versus illumination power for the
LC droplet on a water surface, respectively. In the expansion
process, the maximal rate of change in diameter can be as
high as 6.8 mmsꢀ1 for light with an intensity of
130 mWcmꢀ2. In the case of condensation, the maximal rate
of a change in diameter was 2.8 mmsꢀ1 using light of the
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Photodeformative Liquid Crystals
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transition of a LC decreases its elastic energy, which leads
to expansion of the droplet. Next, considering the process of
condensation in the 1/DMF droplet, we propose the follow-
ing mechanism of transport. When the sample is irradiated
locally with UV light, photoisomerization generates a gradi-
ent of cis isomer concentration. In a previous study, glycerol
colloids in the LC doped with an azobenzene moiety caused
microscopic accumulation toward the UV illumination,[10]
because the LC–glycerol interfacial tension is reduced to a
greater extent in the cis-rich region. Based on these results,
the mechanism of condensation behavior can be interpreted
as follows. The DMF solution in the LC droplet gathers
around the region of higher cis concentration, which then
drives the condensation of the LC droplet toward the center
of the illuminated area.
In conclusion, we propose that a photochemical process
can induce expansion and condensation behavior in pure
azobenzene LC materials. In general, fluid material tends to
disperse on a liquid surface due to thermodynamic effects,
but we have succeeded in causing the LC material to gather
in an irradiated region. The photodeformation reported in
this study may serve as a versatile approach for the develop-
ment of optofluidic devices, and
Figure 3. Polarizing micrographs observed during the photochemical
phase transition of the LC droplet on a water surface. a) Before (left)
and after (right) photoirradiation in a LC of 1 with UV light (365 nm)
for 1 s. Irradiation induced an N-to-Iso phase transition on a water sur-
face. b) Before (left) and after (right) UV irradiation (365 nm) in a LC
with DMF 30 wt% for 1 s.
provides an improved under-
standing of the relationship be-
tween interface phenomena and
photoinduced alignment change
in LC materials.
Experimental Section
The thermotropic properties of LCs
were determined by using a DSC (Shi-
mazu Corporation DSC-60 and
TA60WS). To measure the deforma-
tion behavior of the LC sample on
water, a droplet of the LC samples
was placed on a water surface, and
then subjected to UV light irradiation
from the bottom of a petri dish using a
Figure 4. Maximal deformation rate of a) 1 and b) 1/DMF of ~2 mL LC samples on a water surface caused by
irradiation at various light intensities. The error bars in both plots represent the standard deviation of the four
measurements for each data point.
Keyence UV-400 UV-LED (wave-
length: 365 nm). Movies and photo-
graphs of the deformation behavior
were captured by using
a digital
camera (Cannon PowerShot S5IS, 8.0
megapixels). We captured a movie of
the texture of the LC samples using a digital camera (moticam2000) with
a POM (OLYMPUS BX-51). For the characterization of the compounds
1–3 see the Supporting Information.
same intensity. These deformation rates can be further in-
creased by increasing the intensity of illumination. There-
fore, the effect of light intensity on deformation behavior
should be considered one of the factors affecting the rate of
the light-induced motion of a LC sample.
In the expansion process of 1, we assume that two factors
affect deformation behavior: i) trans–cis photoisomerization
of an azobenzene moiety leads to an increase in the dipole
moment, which generates a higher affinity of the cis isomer
to the water surface.[8] The change in surface energy causes
expansion of LC droplets on the water surface. Therefore,
we assume that this phenomena is highly relevant to the
concept of command surfaces.[9] ii) The photochemical phase
Acknowledgements
The present study was supported by CLUSTER (second stage) and En-
couragement of Young Scientists from the Ministry of Education, Cul-
ture, Sports, Science and Technology, Japan (No. 19850024).
Keywords: azobenzenes · light-induced deformation · liquid
crystals · photochemistry · photoisomerization
Chem. Eur. J. 2009, 15, 3657 – 3660
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www.chemeurj.org
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Published online: February 27, 2009
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