Angewandte
Chemie
As mentioned above, problems in achieving the precise
control of direction of motion in translational motion of the
solid particles were overcome by using an Ar+ (l = 488 nm)
laser. Both the trans and cis isomers absorb light at 488 nm, so
both trans–cis and cis–trans isomerization can be simultane-
ously induced by irradiation with the Ar+ laser, that is, trans
and cis isomers can reversibly be formed by using the Ar+
laser. Consequently, translational motion of the solid particle
was induced over indefinite time without decrease in speed
upon irradiation with the Ar+ laser. Precise control of the
direction of motion was easily achieved by spatially controlled
irradiation of the film with the Ar+ laser. As the laser beam
was narrow (area ꢀ 3 mm2), the position of irradiation, from
which the particles always moved away, could easily be
changed. It was not necessary to directly irradiate the particle
with the laser. Motion of the particle was observed upon
irradiation of regions near the particle, and the direction
could be controlled merely by changing the position of the
irradiation position. When the irradiation position was on the
left side of the particle, the particle moved to the right side.
After 85 seconds of irradiation, changing the irradiation
position to the right side of the particle induced motion to
the left side. The particle moved to the rear upon placing the
irradiation spot to front side, and moved back to the front by
changing the irradiation position to the rear (Figure 2 and
Video 2 in the Supporting Information). Thus, as soon as the
irradiation position was changed, the direction of motion
immediately changed and it was possible to move the particle
in any direction of interest.
Figure 1. Optical micrographs of translational motion of a solid
particle (PS) on the surface of an LC film doped with dl-azomenth (4
wt% in E44). The film was irradiated with UV/Vis light from the right
side. a) Initial position of the particle. b) Upon irradiation with UV
light (l=365 nm), the solid particle moved to the right side, that is,
towards the irradiation position. c) The solid particle moved to the left
side, that is, in the opposite direction, on visible light (l=436 nm)
irradiation. The diameter of the PS particle was about 5 mm. On
average, the speed of translational motion was 88 mmminÀ1 and
105 mmminÀ1 during UV and visible-light irradiation, respectively. The
intensity of UV and visible light was 45 mWcmÀ2 and 65 mWcmÀ2
respectively.
,
The speed of motion was determined by measuring the
distance traveled by the polystyrene particle in one minute.
Images were captured from the movie during irradiation and
the distance traveled was measured by comparing images
before and after irradiation. The speed of motion varied with
the intensity of the laser and the concentration of the doped
azobenzene compound. For a particular film, the speed
increased as the intensity of the laser increased. The speed
of translational motion as a function of the laser intensity is
shown in Figure 3a. No particle motion was observed on pure
E44 (azobenzene-free) film. This indicates that trans–cis
isomerization of the azobenzene compound induced transla-
tional motion of the particle. Consequently, the speed of
motion increased (with few exceptions) as the concentration
of the doped azobenzene compound at a specific laser
intensity increased. The dependence of the speed on the
concentration of the azobenzene compound is shown in
Figure 3b. Polarization of the Ar+ laser had no significant
effect on the speed, that is, little change of speed was observed
in the case of linearly polarized (polarizing angle 08, 458, and
908 with respect to rubbing direction), circularly polarized,
and nonpolarized lasers (see the Supporting Information).
There was also no significant effect of the rubbing direction
on the motion, that is, similar motion behavior (speed and
direction) was observed during irradiation parallel and
perpendicular to the rubbing direction (see the Supporting
Information).
stable isomer, the cis isomer can revert to the trans isomer
upon visible-light irradiation or by liberation of heat. We
determined whether this thermal back reaction could induce
translational motion of the solid particles. Interestingly, we
observed motion of the PS particles but the speed was low
(25 mmminÀ1) compared to the motion induced by visible-
light irradiation. In this case, the direction of motion was same
as that induced by visible-light irradiation (see the Supporting
Information for details of the UV/thermal motion of the
particles). When the film was irradiated from the left side, the
particle moved to the left side upon UV light irradiation and
to the right side upon visible-light irradiation (see the
Supporting Information). Recently, Sen and co-workers[14]
developed the chemically fueled 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 segment
in the Pt–Ni–Au–Ni–Au nanomotor, whereas we have
eliminated the requirement for such a connection to a
nanomotor and also the use of a magnet.
One of the most important challenges in the development
of a practical nanomachine is the development of fast and
repetitive motion over a prolonged time.[2] As the transla-
tional motion of the solid particles during UV/Vis irradiation
arose from the trans–cis isomerization of the azobenzene
compound, motion of the particles ceased when almost all of
trans isomer was converted to the cis isomer, and vice versa.
The controlled motion of materials or molecules within
the micro- or nanometer range is essential in many nano-
technological applications.[15] The results presented above
Angew. Chem. Int. Ed. 2009, 48, 2144 –2147
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2145
Kausar, Abu
