Angewandte
Chemie
Figure 2. Mechanical force generated in the LCE laminated film byUV
light irradiation. a) The experimental setup. b) Change in the generated
force in the LCE laminated film upon exposure to UV light on one side
with different intensities (366 nm; 20, 60, 120, and 240 mWcmÀ2) at
308C. The LCE layers were coated on the both sides of the PE film to
make the initial sample flat, thus allowing the generated force to be
measured accurately. Film size: 2.5 mm5 mm, thickness of LCE
laminated film layers: PE, 50 mm; LCE layers, 15 mm. An external force
of 44 mN was loaded initiallyon the film to keep its length
unchanged.
Figure 1. Photoinduced rolling motion of a continuous ring of LCE
film. Series of photographs showing time profiles of the photoinduced
rolling motion of the LCE ring bysimultaneous irradiation with UV
(366 nm, 200 mWcmÀ2) and visible light (>500 nm, 120 mWcmÀ2) at
room temperature. Size of the LCE ring: 18 mm3 mm (diameter:
6 mm). Thickness of the LCE ring: 20 mm.
rolled intermittently toward the actinic light source, resulting
almost in a 3608 roll at room temperature. This is the first
example of this kind of photoinduced motion in a single layer
film, although the rolling of the LCE ring used herein was
slow, and stopped when the ring was broken by irradiation.
The photoisomerization of the azobenzene moieties,
which is a trigger of the photoinduced deformation of LCE
films, occurs only in the surface region of the films facing the
incident light, and in the bulk of the films the trans-
azobenzene moieties remain unchanged owing to the limi-
tation of light absorption by the azobenzene mesogens. Thus,
we assume that a bilayer structure may be simply produced by
light irradiation in a single LCE film: The first layer in the
surface region of the LCE film contracts by photoirradiation
while the second layer in the bulk of the film shows no
photochemical reactions but changes its shape just by
following the first layer. On the basis of this assumption,
photoactive LCE layers are needed only in the surface region
of the films facing light sources, and the rest of the films can
be replaced by other materials.
By preparing a laminated structure composed of an LCE
layer and a flexible plastic sheet, both photoresponsive and
good mechanical properties can be provided simultaneously
in a simple laminated film, enabling us to induce various
movements of these films by light without deterioration or
breakages of the materials. We fabricated the laminated films
composed of an LCE layer and an unstretched low-density PE
film having good flexibility and mechanical properties at
room temperature.
To evaluate the driving force of the LCE laminated films
to change their shapes by photoirradiation, we measured the
internal stress generated in the films upon exposure to UV
light with a thermomechanical analyzer. Both ends of a film
were clamped (Figure 2a), and a force of 44 mN was initially
loaded on the film at 308C. Figure 2b shows the change in the
load on the film upon irradiation with UV light at different
light intensities. As the length of the film was kept unchanged,
the increase in the load indicates the generation of mechan-
ical force by photoirradiation. These results clearly indicate
that the maximum force and the increment rate of the
generated force are enhanced with an increase of the light
intensity. It should be mentioned that the LCE laminated
films continued to generate the force during photoirradiation
without breaking, whereas the LCE single-layer films cracked
after short light irradiation at high intensities, owing to
insufficient mechanical strength. The above results demon-
strate the possibility of inducing three-dimensional motion
with the LCE laminated films by light irradiation at room
temperature.
A motor device is one of the most useful energy
conversion systems that can convert input energy directly
into a continuous rotation. Although chemomechanical
motors[20,21] and light-switchable molecular machines that
can move objects by light[22,23] have been demonstrated, light-
driven plastic motors converting light energy directly into a
rotation have not yet realized. We prepared a plastic belt of
the LCE laminated film by connecting both ends of the film,
and then placed the belt on a homemade pulley system as
illustrated in Figure 3a. By irradiating the belt with UV light
from top right and visible light from top left simultaneously,
we induced a rotation of the belt to drive the two pulleys in a
counterclockwise direction at room temperature, as shown in
Figure 3b (see also Movie 1 in the Supporting Information).
A plausible mechanism of the rotation is as follows: Upon
exposure to UV light, a local contraction force is generated at
the irradiated part of the belt near the right pulley along the
alignment direction of the azobenzene mesogens, which is
parallel to the long axis of the belt. This contraction force acts
on the right pulley, leading it to rotate in the counterclockwise
direction. At the same time, the irradiation with visible light
produces a local expansion force at the irradiated part of the
belt near the left pulley, causing a counterclockwise rotation
of the left pulley. These contraction and expansion forces
produced simultaneously at the different parts along the long
axis of the belt give rise to the rotation of the pulleys and the
belt with the same direction. The rotation then brings new
parts of the belt to be exposed to UV and to visible light,
which enables the motor system to rotate continuously.
Reverse rotation of this belt could also be induced just by
changing the irradiation positions of the UV and visible light.
Angew. Chem. Int. Ed. 2008, 47, 4986 –4988
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Yamada, Munenori
