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K. Uchida et al.
isomers. The size of the crystals is inversely proportional to the molecular size. For
example, 1c and 3c form needle-shaped crystals. Since the molecular size of 1c is
smaller than 3c, the size of needle-shaped crystals of 1c is much larger than that of
3c (Fig. 24.3).
24.3 Photoinduced Topographical Changes
of a Diarylethene Derivative Providing Control
of the Superhydrophilicity
Photoresponsive superhydrophobic surfaces with the lotus effect were prepared in
the previous section. In contrast, it was difficult to prepare photoresponsive superhydrophilic surfaces; in order to construct a superhydrophilic surface, it is required
to prepare a derivative of diarylethene that is hydrophilic enough to reduce a contact
angle of less than 65° on its flat surface, but not dissolve in water. Here, the critical
contact angle of 65° is based on the report that a contact angle less (more) than 65°
on flat polymer surfaces decreased (increased) when the surface roughened [9].
We prepared the photoresponsive superhydrophilic surface of microcrystalline
films of diarylethene derivatives with ionic liquid structures [10]. A series of
diarylethene derivatives that we have synthesized has pyridinium salt moieties. We
found that a derivative 5o (upper panel in Fig. 24.4) shows photoinduced reversible
topographical changes upon alternate irradiation with UV and visible light and that
a superhydrophilic surface was generated (Fig. 24.4) [10].
The molecular structures, the estimated crystal shapes from the crystal units, and
the phase diagram of 5o and 5c are shown in Fig. 24.4. The melting points of 5o
and 5c were 203.2 and 190.2 °C, respectively. The eutectic temperature was 161.1
°C, where the ratio of 5o and 5c was 66: 34. The T g of 5o was measured to be 64.4
°C by DSC when heating the glassy state of 5o, which was formed by cooling down
the melted 5o after reaching the melting temperature. As we have discussed for the
photoswitching of hydrophobic surfaces, photoinduced crystal growth took place at
temperatures above T g , and in fact, the current photoinduced topographical changes
were observed at 70 °C, which is slightly higher temperature than the T g .
The microcrystalline film of 5o was prepared by coating with an acetonitrile solution on a glass substrate and by evaporating the solvent. The surface was irradiated
with UV light for 5 min, and the surface was melted and became flat (Fig. 24.5 right
panel) where the ratio of 5o and 5c was nearly 4: 6. It is the hydrophilic surface,
whose contact angle was less than 65°. Thus, a superhydrophilic surface can be realized by roughening the surface [9]. After visible-light (λ > 430 nm) irradiation for
5 min onto the surface followed by maintaining the temperature of the film at 70 °C
for 24 h, bumpy crystals of 5o grew on the surface, and the contact angle became 0°
(Fig. 24.5 left panel) [10].
The reversible formations of the flat and the superhydrophilic surfaces were
observed by alternate irradiation with UV and visible light at least three times.
K. Uchida et al.
isomers. The size of the crystals is inversely proportional to the molecular size. For
example, 1c and 3c form needle-shaped crystals. Since the molecular size of 1c is
smaller than 3c, the size of needle-shaped crystals of 1c is much larger than that of
3c (Fig. 24.3).
24.3 Photoinduced Topographical Changes
of a Diarylethene Derivative Providing Control
of the Superhydrophilicity
Photoresponsive superhydrophobic surfaces with the lotus effect were prepared in
the previous section. In contrast, it was difficult to prepare photoresponsive superhydrophilic surfaces; in order to construct a superhydrophilic surface, it is required
to prepare a derivative of diarylethene that is hydrophilic enough to reduce a contact
angle of less than 65° on its flat surface, but not dissolve in water. Here, the critical
contact angle of 65° is based on the report that a contact angle less (more) than 65°
on flat polymer surfaces decreased (increased) when the surface roughened [9].
We prepared the photoresponsive superhydrophilic surface of microcrystalline
films of diarylethene derivatives with ionic liquid structures [10]. A series of
diarylethene derivatives that we have synthesized has pyridinium salt moieties. We
found that a derivative 5o (upper panel in Fig. 24.4) shows photoinduced reversible
topographical changes upon alternate irradiation with UV and visible light and that
a superhydrophilic surface was generated (Fig. 24.4) [10].
The molecular structures, the estimated crystal shapes from the crystal units, and
the phase diagram of 5o and 5c are shown in Fig. 24.4. The melting points of 5o
and 5c were 203.2 and 190.2 °C, respectively. The eutectic temperature was 161.1
°C, where the ratio of 5o and 5c was 66: 34. The T g of 5o was measured to be 64.4
°C by DSC when heating the glassy state of 5o, which was formed by cooling down
the melted 5o after reaching the melting temperature. As we have discussed for the
photoswitching of hydrophobic surfaces, photoinduced crystal growth took place at
temperatures above T g , and in fact, the current photoinduced topographical changes
were observed at 70 °C, which is slightly higher temperature than the T g .
The microcrystalline film of 5o was prepared by coating with an acetonitrile solution on a glass substrate and by evaporating the solvent. The surface was irradiated
with UV light for 5 min, and the surface was melted and became flat (Fig. 24.5 right
panel) where the ratio of 5o and 5c was nearly 4: 6. It is the hydrophilic surface,
whose contact angle was less than 65°. Thus, a superhydrophilic surface can be realized by roughening the surface [9]. After visible-light (λ > 430 nm) irradiation for
5 min onto the surface followed by maintaining the temperature of the film at 70 °C
for 24 h, bumpy crystals of 5o grew on the surface, and the contact angle became 0°
(Fig. 24.5 left panel) [10].
The reversible formations of the flat and the superhydrophilic surfaces were
observed by alternate irradiation with UV and visible light at least three times.