19 Photoinduced Morphological Transformation and Photodriven …
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the change in morphology itself and that in the physical properties of the suspension. Here, we demonstrate the transduction of the morphological changes of the
supramolecular architecture into the movement of objects based on the photoinduced
LCST transition using the ether-linked diarylethene 1, which forms long nanofibers.
19.3.1 Photoinduced Shrinking of Bundled Fibers
by Depletion Force
The shrinking of the nanofiber 1b can be used for unidirectional movement of an
external object. However, as described above, since the fibers are generated radially
from the microsphere surface (Fig. 19.4c), it is necessary for the nanofibers to align
and bundle to control the direction of motion. Therefore, we considered using a
depletion force.
Depletion force is the attractive interaction between colloidal particles dispersed
in a polymer solution [21]. Since polymer chains form a random coil structure in
solution, i.e., a spatially extended structure, an excluded volume, equal to the thickness of the radius of the solute polymers, is generated on the surface of each colloidal
particle. This region is known as the depletion layer. The attractive force acts between
the colloidal particles in the direction of reduction of the depletion layer exposing
to the solvent. The depletion force acts in the order of hundreds of nanometers,
whereas hydrophobic interaction acts between molecules. As a result, the depletion
force leads to form a coagulated structure of sub-millimeter size. In particular, fibers
coagulate and form a bundled structure because the overlap of the depletion layer in
parallel alignment is much larger than that in the orthogonal case [22]. Therefore,
photoresponsive bundle is expected to be obtained by the use of the supramolecular
architecture of amphiphilic diarylethene. Here, we examined the coagulation and
photoinduced morphological change of ether-linked 1, when it was dispersed into an
aqueous solution of methylcellulose (MC). The molecular weight of MC is 40,000
and 1–5 wt% aqueous solution was prepared.
The photoinduced morphological change of 1a showed different behavior
depending on the size of assembly and the concentration of the depletant [13]. In the
case of pure water, the red-purple hazy assembly is observed, consisting of radially
generated nanofibers 1b as shown in Fig. 19.10a. In contrast, few micrometer-long
rod-like structures were observed around the large microsphere in the MC solution
(Fig. 19.10b). When the size of the supramolecular architecture is small, the suspension without the depletant shows reddish-purple color upon the photoirradiation due
to the homogeneously dispersed nanofibers (Fig. 19.11(a)) whereas in the suspension
with MC (4.1 wt%), bundles are generated in the entire field of view (Fig. 19.11b)
and the coagulation of the bundles is also observed (Fig. 19.11c). The length of the
bundle reached several tens of micrometers, and by stirring the suspension with a
pipette, the length of bundle exceeded 1 mm (Fig. 19.12).
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the change in morphology itself and that in the physical properties of the suspension. Here, we demonstrate the transduction of the morphological changes of the
supramolecular architecture into the movement of objects based on the photoinduced
LCST transition using the ether-linked diarylethene 1, which forms long nanofibers.
19.3.1 Photoinduced Shrinking of Bundled Fibers
by Depletion Force
The shrinking of the nanofiber 1b can be used for unidirectional movement of an
external object. However, as described above, since the fibers are generated radially
from the microsphere surface (Fig. 19.4c), it is necessary for the nanofibers to align
and bundle to control the direction of motion. Therefore, we considered using a
depletion force.
Depletion force is the attractive interaction between colloidal particles dispersed
in a polymer solution [21]. Since polymer chains form a random coil structure in
solution, i.e., a spatially extended structure, an excluded volume, equal to the thickness of the radius of the solute polymers, is generated on the surface of each colloidal
particle. This region is known as the depletion layer. The attractive force acts between
the colloidal particles in the direction of reduction of the depletion layer exposing
to the solvent. The depletion force acts in the order of hundreds of nanometers,
whereas hydrophobic interaction acts between molecules. As a result, the depletion
force leads to form a coagulated structure of sub-millimeter size. In particular, fibers
coagulate and form a bundled structure because the overlap of the depletion layer in
parallel alignment is much larger than that in the orthogonal case [22]. Therefore,
photoresponsive bundle is expected to be obtained by the use of the supramolecular
architecture of amphiphilic diarylethene. Here, we examined the coagulation and
photoinduced morphological change of ether-linked 1, when it was dispersed into an
aqueous solution of methylcellulose (MC). The molecular weight of MC is 40,000
and 1–5 wt% aqueous solution was prepared.
The photoinduced morphological change of 1a showed different behavior
depending on the size of assembly and the concentration of the depletant [13]. In the
case of pure water, the red-purple hazy assembly is observed, consisting of radially
generated nanofibers 1b as shown in Fig. 19.10a. In contrast, few micrometer-long
rod-like structures were observed around the large microsphere in the MC solution
(Fig. 19.10b). When the size of the supramolecular architecture is small, the suspension without the depletant shows reddish-purple color upon the photoirradiation due
to the homogeneously dispersed nanofibers (Fig. 19.11(a)) whereas in the suspension
with MC (4.1 wt%), bundles are generated in the entire field of view (Fig. 19.11b)
and the coagulation of the bundles is also observed (Fig. 19.11c). The length of the
bundle reached several tens of micrometers, and by stirring the suspension with a
pipette, the length of bundle exceeded 1 mm (Fig. 19.12).