Photocontrolled manipulation of a microscale object: A rotational or translational mechanism

Article abstract of DOI:10.1002/chem.201001238

In this paper the photocontrolled manipulation of solid materials on the surface of a liquid crystalline thin film is described. Three different types of films namely cholesteric liquid crystal (ChLC), compensated nematic liquid crystal (NLC) and nematic LC were used. The rotational and translational manipulation of the microscale solid object was induced by irradiation of light and mode of manipulation (either translational or rotational) was changed by changing the isomer of the azobenzene compound used to make the film. Rotational motion of the object was observed on the ChLC and compensated NLC films containing chirally pure azobenzene compound. The direction of rotational motion was controlled either by changing the optical isomer of the chiral azobenzene or by changing the irradiating light (from ultraviolet to visible). When racemic mixture of the chiral azobenzene compound was used, a translational motion of the object was observed. Even though the direction of the translational motion can be controlled by controlling irradiation position, more facile and precise manipulation of the objects was possible by spatially controlled irradiation of Ar⁺ laser and diode UV laser.

Full text of DOI:10.1002/chem.201001238

DOI: 10.1002/chem.201001238
Photocontrolled Manipulation of a Microscale Object:
A Rotational or Translational Mechanism
Abu Kausar,[a, b] Hiroto Nagano,[a] Yutaka Kuwahara,[a] Tomonari Ogata,[a] and
Seiji Kurihara*[a]
Abstract: In this paper the photocon-
trolled manipulation of solid materials
on the surface of a liquid crystalline
thin film is described. Three different
types of films namely cholesteric liquid
crystal (ChLC), compensated nematic
liquid crystal (NLC) and nematic LC
were used. The rotational and transla-
tional manipulation of the microscale
solid object was induced by irradiation
of light and mode of manipulation
(either translational or rotational) was
changed by changing the isomer of the
azobenzene compound used to make
the film. Rotational motion of the
object was observed on the ChLC and
compensated NLC films containing
chirally pure azobenzene compound.
The direction of rotational motion was
controlled either by changing the opti-
cal isomer of the chiral azobenzene or
by changing the irradiating light (from
ultraviolet to visible). When racemic
mixture of the chiral azobenzene com-
pound was used, a translational motion
of the object was observed. Even
though the direction of the translation-
al motion can be controlled by control-
ling irradiation position, more facile
and precise manipulation of the objects
was possible by spatially controlled ir-
radiation of Ar+ laser and diode UV
laser.
Keywords:
azobenzene
·
liquid crystals · photochromism ·
surface chemistry
Introduction
groups have reported photocontrolled translational manipu-
lation of a liquid droplet on a glass surface modified with a
photo-switchable molecule, by generating a surface energy
gradient.[5,6] Liquid crystalline elastomers (LCE) with an
azobenzene moiety show the largest photomechanical effect,
and are promising materials for applications as artificial
muscles and actuators.[7,8] Very recently Ikeda et al. exploit-
ed the photomechanical effect of LCE with an azobenzene
moiety to drive a plastic motor by light.[9] Palffy-Mohoray
et al. reported movement of a photoactive LCE disk with a
doped azobenzene compound which changes its shape
during irradiation and can move in response to light.[10] Sev-
eral groups have reported translational manipulation of a
non-photoactive solid object (silica micro-particle or metal
nanorod) on a surface[11] or through a microchannel,[12]
where the motion was induced by conversion of chemical
energy to kinetic energy. However, light driven translational
motion of a solid object on a surface has remained a chal-
lenge for researchers in this field. Photochemical and elec-
trochemical energy is preferable to chemical energy because
of its ease of addressability, fast response time and reversi-
ble external control.[13] In addition, a chemically fuelled
system suffers from problems such as addition of fresh reac-
tants (“fuel”) and removal of waste product: use of light
energy as fuel can overcome these problems.[13]
The miniaturization of components for the construction of
useful devices, which is an essential feature of modern tech-
nology, is currently pursued by a large downward (top–
down) approach. This approach, however, leads scientists to
manipulate progressively smaller pieces of matter. Several
attempts of linear or rotary manipulation of micro-/nano-
scale materials on surfaces have been recently reported.[1,2]
Optical tweezers developed by Ashkin use the forces exert-
ed by a strongly focused beam of light to trap and move ob-
jects ranging in size from tens of nanometres to tens of mi-
crometers.[3] Light driven rotary manipulation of a micro-
scale object by a molecular motor embedded in a liquid
crystalline matrix was reported by Feringa et al.[4] Some
[a] A. Kausar, H. Nagano, Dr. Y. Kuwahara, Dr. T. Ogata,
Prof. S. Kurihara
Department of Applied Chemistry and Bio-chemistry
Kumamoto University, 2-39-1 Kurokami, Kumamoto (Japan)
Fax : (+81)96-342-3679
E-mail: kurihara@gpo.kumamoto-u.ac.jp
[b] A. Kausar
Department of Chemistry, University of Alberta (Canada)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201001238.
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Liquid crystals are ordered soft materials consisting of
self-organized molecules and the orientation of the mole-
cules in a mesophase can be directed by the application of
an external field (light, electric field, etc.). For this reason,
LC materials have received a great attention for manipula-
tion of micro-scale materials.[4,14] Cholesteric (or chiral nem-
atic) liquid crystals (ChLCs) are characterized by large,
supramolecular, chiral organization, the chirality of which is
indicated by the sign and magnitude of the cholesteric pitch.
The pitch (p, the length of one turn of the cholesteric helix)
is dependent on: 1) the concentration (c) of the dopant and
2) the helical twisting power (HTP) of the dopant, following
Equation (1).[15]
menth and l-azomenth was 49.4ꢁ108 mꢁ1 molꢁ1 gꢁ1 E44 and
49.0ꢁ108 mꢁ1 molꢁ1 gꢁ1 E44, respectively. The HTPs of both
d-azomenth and l-azomenth compounds were decreased
upon ultraviolet (UV) light (l=365 nm) irradiation, as a
result of photoisomerization of the azobenzene compound
from trans to cis form. The calculated HTP of d-azomenth
and l-azomenth after irradiation of UV light was 10.4ꢁ
108 mꢁ1 molꢁ1 gꢁ1 E44 and 10.8ꢁ108 mꢁ1 molꢁ1 gꢁ1 E44 respec-
tively. This large difference in HTP of d-azomenth and l-
azomenth between its isomeric forms is essential for effec-
tive switching of liquid crystalline organization, which made
it suitable for rotational manipulation of microscale materi-
als.[4b,15]
p ¼ 1=ðHTP ꢀ cÞ
ð1Þ
When a suitable ratio of chiral compounds possessing the
ability to produce opposite right- and left-helical structures
and to cancel one another is used, the result is known as a
compensated NLC, and does not produce a helical struc-
ture.[16] When one component of a chiral system constituting
a compensated NLC is a photochromic compound such as
an azobenzene, it may be possible to change the state of
compensation through a change in the HTP effected by pho-
toisomerization, so that a ChLC with a helical structure may
be developed. If the racemic mixture of the chiral azo-
benzne compound is used there will be no helix formed as
the helix formed by one isomer will be cancelled with that
of other isomer. In this paper we used ChLC, compensated
NLC and azobenzene doped LC films to study manipulation
of microscopic object.
Scheme 1. Host nematic liquid crystal (E44), photochromic azobenzenes
(d-azomenth, l-azomenth and dl-azomenth) and nonphotochromic com-
pounds (S-811, R-811) used for manipulation of a microscale object.
Rotational manipulation with cholesteric LC: At first we
made cholesteric liquid crystals (ChLC) by chiral azoben-
zene compound d-azomenth/l-azomenth doped in E44.
When a thin film was made on the surface of glass slide
having unidirectionally rubbed polyimide layer polygonal
fingerprint texture was observed, that is typical for align-
ment of the cholesteric helix axis parallel to the surface
(Figure 1).[19] The pitch of the helix is depended on the con-
centration of the chiral compound, the higher the concentra-
tion the lower the pitch. Thus the lines on the surface
become narrower as the concentration of the chiral com-
pound increases which is shown in the Supporting Informa-
tion (Figure S3).
During irradiation of UV light (l=365 nm) under the mi-
croscope the polygonal texture reorganized in a rotational
(clockwise) fashion in response to the isomerization of the
azobenzene compound and the subsequent modification of
its helical twisting power. As the irradiation continues the
width of the lines become wider and eventually faded out.
To observe the movement of solid, few glass rods of typical
diameter around 7 mm (with varying length) were sprinkled
on the surface of the film, then UV and visible lights were
alternatively irradiated on the film. In this case UV-visible
light was irradiated homogenously on the film.
Results and Discussion
The chiral photochromic azobenzene compounds, non-pho-
tochromic compounds and host nematic liquid crystal (LC;
Scheme 1) were used to study the manipulation of microme-
ter-sized glass rod on the LC surface. UV/Vis spectra con-
firmed that photoirradiation of the azobenzene compound
with UV light (l =365 nm) results in the formation of the
cis isomer, and the reverse conversion from the cis to trans
isomer occurs by irradiation with visible light (l =436 nm;
see the Supporting Information, Figures S1 and S2). S-811
and R-811 reported to give left- and right-handed helical
structures, respectively.[17] A Ch phase was induced by
mixing each chiral azobenzene compound and the nonpho-
tochromic chiral compound in the host nematic liquid crys-
tal (E44). The helical sense (helical rotatory direction) of
the induced cholesteric liquid crystal was affected by the
chirality of each chiral substituent. The helical pitch of each
chiral azobenzene compounds was determined by Canos
wedge method[18] and the HTP was calculated by use of
Equation (1). The determined HTP of S-811 and R-811 was
34.7ꢁ108 mꢁ1 molꢁ1 gꢁ1 E44 and 34.2ꢁ108 mꢁ1 molꢁ1 gꢁ1 E44.
d-Azomenth and l-azomenth showed right- and left-handed
helical structures, respectively. The calculated HTP of d-azo-
Upon UV light irradiation the rods rotated in clockwise
direction, the same direction as the rotating cholesteric tex-
tures (Figure 1a–g). During visible light (l=436 nm) irradia-
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S. Kurihara et al.
film, Figure S5, and Figure S6
shows the effect of concentra-
tion of l-azomenth and intensi-
ty UV-visible light on angle of
rotation, respectively.
Rotational manipulation with
compensated NLC: Rotational
behavior of the glass rod was
also observed on the compen-
sated nematic liquid crystal film
containing a chiral azobenzene
compound. The compensated
NLC was prepared by mixing
photochromic (chiral azoben-
zene compound) and a nonpho-
tochromic chiral compound
having opposite helical twisting
ability to that of chiral azoben-
zene compound in E44 in the
ratio that compensated the
otherꢂs HTP. Upon irradiation
of UV light, the compensated
nematic phase breaks due to
Figure 1. Optical micrographs of rotation of glass rod on surface of LC film doped with azobenzene com- change in the helical twisting
pound. Rotational motion of the glass rod on the film made by doping d-azomenth in E44, upon UV irradia-
tion (l=365 nm) the rod rotated clockwise (a–g), the rod rotated counterclockwise by visible light (l=
power of the photochromic
compound with respect non-
photochromic compound, con-
sequently phase transition from
436 nm) irradiation (i–o). Composition of the film 2.0 wt% of d-azomenth in E44, intensity of UV and visible
light was 13.0 and 140 mWcmꢁ2, respectively.
the nematic phase to the cho-
tion the texture and the glass rod rotated in the counter-
clockwise direction (Figure 1i–o) (for movie see Supporting
Information video 18X). This reorganization of the texture
generates a torque on the microscopic object that results in
rotary motion. In this case no translational motion of the
rod was observed. The angle of rotation was determined by
measuring the change in angle of the glass rod during irradi-
ation. Images were captured from the movie and the angle
change was measured by comparing images before and after
irradiation. In general, the angular displacement of the rod
was increased with increasing the concentration of the azo-
benzene compounds (Figure 2). Initially the speed of rota-
tion was faster, but the speed decreased as irradiation con-
tinued and eventually the rotation was stopped, due to the
photostationary state of the azobenzene compound. For a
specific concentration the speed of rotation also increased
with increasing intensity of the irradiating light (Figure 3).
When l-azomenth instead of d-azomenth was used as the
doping azobenzene similar rotational behavior was ob-
served. In this case the glass rod rotated to the counter-
clockwise direction during UV light irradiation and clock-
wise direction during visible light irradiation. Thus, direction
of rotational motion of the glass rod can be easily controlled
either by controlling the isomer of the azobenzene com-
pound or by changing the irradiating light (from UV to visi-
ble). In the Supporting Information, Figure S4 shows the ro-
tational behavior of glass rod on the l-azomenth doped E44
lesteric phase takes place (Figure 4).[16] When compensated
NLC made by using d-azomenth and S-811 was placed on a
glass slide having unidirectionally rubbed polyimide layer,
no helix observed on the glass substrate. Irradiation of UV
light causes the changes in the HTP of d-azometh, so the
compensated state of the films was broken, consequently
helix can be observed on the film. The glass rod sprinkled
on the film started to rotate clockwise as soon as the UV
light irradiated on the film (Figure 4a–g). After some time,
the cholesteric texture appeared (in this case after 1 s) the
rotation continues for a while and eventually rotation was
stopped when the photostationary state of the azobenzene
compound was achieved. Again visible light irradiation
caused the compensated NLC state to regenerate that is, the
cholesteric texture disappeared upon visible light, and the
glass rod rotated to the counter clockwise direction (Fig-
ure 4i–o).
In general, the angle of rotation of the rod was increased
with increasing the concentration of the azobenzene com-
pounds (Figure 5). Initially the speed of rotation was faster,
but the speed decreased as irradiation continued and even-
tually the rotation was stopped, due to the photostationary
state of the azobenzene compound. For a specific concentra-
tion the speed of translation also increased with increasing
intensity of the irradiating light (Figure 6).
Similar rotational behavior of the glass rod was observed
when the compensated NLC was made by using l-azomenth
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Photocontrolled Manipulation
FULL PAPER
and S-811. In this case the glass rod rotated to the counter-
clockwise direction during UV-light irradiation and clock-
wise direction during visible light irradiation. Thus, the di-
rection of rotational motion of the glass rod can be easily
controlled either by controlling the isomer of the azoben-
zene compound or by changing the irradiating light (from
UV to visible). Figure S7 shows the rotational behaviour of
glass rod on the l-azomenth and R-811 doped E44 film. Fig-
ures S8 and S9 show the effect of concentration and intensi-
ty UV-visible light on angle of rotation, respectively.
Observation of translational behavior: In the previous sec-
tions, the rotational behaviour of glass rod was observed
when chiral azobenzene compound was used. When the rac-
emic mixture (dl-azomenth) of the chiral compound was
used translational motion of the glass rod was observed. In
the next section, the translation behavior of the glass rod
with irradiation of UV-visible light, Ar+ laser and UV laser
is described.
Irradiation of UV-visible light: During asymmetric[21] irradi-
ation of UV light (l=365 nm) over the azobenzene doped
film under microscope the sprinkled glass rod on the film
always translated (speed 103 mmminꢁ1) towards the irradia-
tion position as a result of trans–cis isomerization. After UV
light irradiation, when visible light (l=436 nm) was irradiat-
ed the same rods translated (116 mmminꢁ1) opposite direc-
tion to that of UV light irradiation that is, in this case the
rods moved away from the irradiation position which was
the result of cis–trans isomerization. Figure 7 shows the
translational motion of the rods during irradiation of UV
and visible light from right side, where the rod translated to
right side during UV light irradiation and to the left side
during visible light irradiation.
Figure 2. Changes in the angle of rotation of glass rod with time at differ-
ent concentration of doped d-azomenth during a) UV light irradiation
(clockwise rotation) and b) visible light irradiation (counterclockwise ro-
tation). Intensity of UV and visible light was 13.0 and 140 mWcmꢁ2, re-
spectively. Each point represents the average angle of at least three glass
rods.
Symmetric irradiation of the film caused the movement of
the rod in an arbitrary direction, so direction of translational
motion cannot be controlled. Increasing concentration of
azobenzene in the film caused more molecules to undergo
photoisomerization; consequently the speed of translation
of the rod was increased with increasing the concentration
of azobenzene compound (Figure S10). Recently, Sen and
co-workers[20] developed the chemically fuelled translational
motion of polystyrene microspheres connected to a catalytic
Pt/Au nanomotor. The direction of motion was controlled
by making use of the magnetic properties of the nickel seg-
ment in the Pt-Ni-Au-Ni-Au nanomotor, whereas we have
eliminated the requirement for such a connection to a nano-
motor and also the use of a magnet.
Precise control of motion using Ar+ laser: One of the most
important challenges to the development of practical molec-
ular machine is the development of fast and repetitive
movement over longer time frames.[2] As the translational
motion of the rods during UV-visible was depended on the
trans-cis isomerisation of the azobenzene compound the
translation of the rods was stopped when almost all trans
isomers change to cis isomer and vice versa. Another prob-
Figure 3. Changes in the angle of rotation of glass rod with time at differ-
ent intensity of a) UV light (clockwise rotation) and b) visible light
(counterclockwise rotation). Composition of the film was 2.0 wt% of d-
azomenth in E44. Each point represents the average angle of at least
three glass rods.
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Figure 4. Optical micrographs of rotation of glass rod on surface of compensated NLC film doped with d-azomenth and S-811. Upon UV light irradiation
(l=365 nm) the rod rotated clockwise (a–g), the rod rotated counterclockwise by visible light (l=436 nm) irradiation (i–o). Composition of the film d-
azomenth/S-811/E44 2.0:2.3:95.7, intensity of UV and visible light was 13.0 and 140 mWcmꢁ2, respectively.
Figure 5. Changes in the angle of rotation of glass rod with time at differ-
ent concentration of doped d-azomenth and S-811 during a) UV light ir-
radiation and b) visible light irradiation. Intensity of UV and visible light
was 13.0 and 140 mWcmꢁ2, respectively. Each point represents the aver-
age angle of at least three glass rods.
Figure 6. Changes in the angle of rotation of glass rod with time at differ-
ent intensity of a) UV light and b) visible light. Composition of the film
d-azomenth/S-811/E44 2.0:2.3:95.7. Each point represents the average
angle of at least three glass rods.
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This indicates that trans–cis iso-
merization of the azobenzene
compound induced translational
motion of the rod. Consequent-
ly, the speed of motion in-
creased (with few exceptions)
as the concentration of the
doped azobenzene compound
at a specific laser intensity in-
creased. The dependence of the
speed on the concentration of
the azobenzene compound is
shown in Figure 9b.
Figure 7. Optical micrographs of translational motion of a glass rod on the surface of an LC film doped with
dl-azomenth (6 wt% in E44). The film was irradiated with UV/Vis light from the right side. a) Initial position
of the rod. b) Upon irradiation with UV light (l =365 nm), the glass rod moved to the right side, that is, to-
wards the irradiation position. c) The glass rod moved to the left side, that is, in the opposite direction, on visi-
ble light (l =436 nm) irradiation. The length of the rod was 30 mm and diameter of the rod was about 7 mm.
On average, the speed of translational motion was 103 and 116 mmminꢁ1 during UV and visible-light irradia-
tion, respectively. The intensity of UV and visible light was 13 and 140 mWcmꢁ2, respectively.
lem with translational motion of glass rod by UV-visible ir-
radiation was that precise control of direction was difficult
to achieve due to difficulty in changing the irradiation con-
dition such as direction, angle, position. These problems
were overcome by irradiation of Ar+ (l=488 nm) laser
beam. Both trans and cis isomers absorb at 488 nm so both
trans–cis and cis–trans isomerisation can be simultaneously
induced by irradiation of Ar+ laser that is, Ar+ laser can re-
versibly form trans and cis isomers. Consequently, transla-
tional motion of the glass rod was caused over infinite time
without decrease in speed upon irradiation of Ar+ laser
beam. Precise control of direction of translation was easily
achieved by spatially controlled irradiation of Ar+ laser on
the film. As the laser beam is narrow (area of the beam
~3 mm2) it was possible to change the irradiation position
easily. In this case the rods always moved away from the ir-
radiation position. It should be noted that it was not neces-
sary to irradiate laser beam on the rod, the movement of
the rod was observed upon irradiation of nearby regions of
the rod and the direction was controlled just by changing
the position of irradiation spot. If we place the irradiation
spot on left side of the rod, the rod moved to the right side,
after 44 s changing the irradiation spot to right side of the
rod caused movement to the left side; the rod moved to the
rear side upon placing the irradiation spot to front side and
moved back to the front side by changing the irradiation
spot to the rear side (Figure 8) (for movie, see Supporting
Information video 2 8X). So it was possible to move the rod
in any direction of interest and as soon as the irradiation po-
Figure 8. Optical micrographs of the precise control of translational
motion of glass rod by spatially controlled irradiation with an Ar+ laser.
a),b) Laser irradiation from the left side of the rod resulted in motion of
the rod to the right side and vice versa. c),d) Laser irradiation from the
front side of the rod resulted in motion to the rear side and vice versa. e)
The final position of the rod. Composition of the film 4 wt% dl-azo-
menth and the intensity of the laser was 240 mWcmꢁ2. On average the
sition was changed the direction of translation was changed
immediately.
The speed of motion was determined by measuring the
distance travelled by the glass rod in one minute. Images
were captured from the movie during irradiation and the
distance travelled was measured by comparing images
before and after irradiation. Several factors affected the
speed of motion, namely, intensity of the laser, the concen-
tration of the doped azobenzene compound. For a particular
film, the speed increased as the intensity of the laser in-
creased. The speed of translational motion as a function of
the laser intensity is shown in Figure 9a. No movement of
the rod was observed on pure E44 (azobenzene-free) film.
speed of translational motion was 173 mmminꢁ1
.
Control of motion with UV laser: Since the absorptivity of
azobenzene molecule at 488 nm wavelength is not so high, it
was necessary to irradiate relatively high intensity of Ar+
laser to induce the movement of the glass rod. However,
very fast movement of glass rod was observed when UV
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S. Kurihara et al.
On the other hand, in the case of translational movement
by irradiation with an Ar+ laser, the movement of the parti-
cle continued without stopping as long as the irradiation was
continued (Figure 8). The absorbance of dl-azomenth at
488 nm is much lower than that at 365 nm, however, not
only the trans form but also the cis form has small absorp-
tion at 488 nm, so the irradiation at 488 nm causes trans–cis–
trans photoisomerization cycle, and the amount of cis form
produced with Ar+ laser is very small compared to that by
irradiation of UV light. On the basis of the results, we ex-
pected that the driving force for the translation motion with
Ar+ laser is different from the movement by UV and visible
light irradiation.
Chono et al. reported that the movement of a small parti-
cle which was achieved by applying electric field to the LC
system.[14] This phenomenon was explained as velocity gradi-
ent induced by combination of anchoring effect of glass sub-
strate and rotation of LC molecules under electric field. We
guess that the translation with Ar+ laser is related to the re-
orientation of LC molecules due to the trans–cis–trans pho-
toisomerization cycle, and gradient of molecular motion be-
tween irradiated and dark regions. That is, the molecular
motion of LC molecules in the irradiation site is activated
due to the trans–cis–trans photoisomerization cycle, so we
think the gradient of molecular motion is driving force of
the translation motion of the glass rod.
Figure 9. a) Effect of intensity of Ar+ laser on translational motion. The
speed of motion increased linearly with increasing intensity of Ar+ laser.
Composition of the film was 4 wt% of dl-azomenth in E44. b) Effect of
concentration on speed of motion. The speed of translation increases
almost linearly with increasing concentration of doped azobenzene com-
pound. Intensity of laser was 163 mWcmꢁ2. Each point represents the
average speed of at least three particles.
Conclusion
laser (375 nm) was irradiated on the films. Unlike Ar+ laser/
visible light irradiation the glass rods always translated to-
wards the irradiation position during UV laser irradiation.
The directional behavior is similar to that of UV light irradi-
ation. Figure 10 shows the translational behaviour of glass
rod during irradiation of UV laser. On average, the speed of
translation of the rod was 466 mmminꢁ1.
Controlled movement of materials or molecules within the
micro/nanometer range is essential in many applications of
nanotechnology.[23] Herein we showed the manipulation of a
microscale object, this process may be applicable for biologi-
cal material DNA, RNA, bacteria, nanoparticle. More pre-
cise control of the materials will be achieved if laser beam
comparable/narrower to the size of the rods can be used.
Even though continuous translational motion of the glass
rod was achieved with Ar+ laser, rotational motion was con-
tinued for a short period of time, so it is necessary to design
molecules for which continuous and faster rotation can be
observed. Future studies with different liquid crystals, azo-
benzene compounds and irradiation conditions will allow us
to get clear insight of the origin of the driving force for ma-
nipulation.
Possible mechanism: Liquid crystals are elastic medium, so
that the volume of the LC can be easily changed by varying
the temperature. When UV light was irradiated on the LC
film, the photoisomerization of azo molecules from trans-
form having rod shape to cis form having bent shape was
caused. The change in the molecular shape disrupts the mo-
lecular orientation of LC, and consequently photochemical
phase transition occurs. It is known that the elasticity of a
nematic LC decreases with increasing temperature in a LC
phase near the isotropization temperature,[22] so the elastici-
ty of the area irradiated becomes lower than that of dark
area. As a result, the glass rods move forward to irradiation
position with UV light/laser. The recovery of the well or-
dered orientation of the LC molecules by subsequent visible
light irradiation results in the movement of objects to the in-
itial position. Therefore, the rate of movement became
slower during long time irradiation, and came to a stop fi-
nally due to the photostationary state of the azobenzene
molecule.
Experimental Section
The azobenzene compounds were synthesized following the previously
described method.[21] The films were made by dissolving an accurately
weighed mixture (various wt% in E44) in THF. The solution was drop-
ped onto a glass slide (25ꢁ20 mm) with a unidirectionally rubbed poly-
imide alignment layer. Evaporation of the solvent gave the azobenzene
doped liquid crystalline film. Use of polyimide-rubbed glass allowed uni-
form films to be made, whereas attempts to make film on glass without
polyimide/rubbing were not successful. To observe the translational
motion of solid objects a few glass rods of typical diameter about 7 mm
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Acknowledgements
This work was supported by a Grant-in-Aid for Scientific Research in
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Received: May 8, 2010
Published online: November 12, 2010
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