314
Y. Haketa et al.
the XRD pattern assignable to Phase B disappeared, and that assignable to Phase A
re-appeared in the small-angle region. In the POM observations, the birefringence
disappeared immediately after visible light irradiation and then new birefringence
appeared. This transient optically isotropic state is probably different from the optically isotropic ionic liquid state induced by heating, by considering the fact that
Phase A was not observed during the cooling process from the ionic liquid state.
As the clearing temperature of 16
C16− ·TBA
+ under UV irradiation (52 °C) was
slightly lower than that without UV irradiation (55 °C), the photo-induced crystal–
crystal phase transition was observed below 52 °C. Thus, the crystalline lamellar
phase mainly comprising the bulky cis form (Phase B) was fairly stable due to the
interactions between the long alkyl chains. In addition, the crystal–crystal phase
transition can be repeated by switching between the two irradiation wavelengths
(365/436 nm) (Fig. 18.12b), although the liquid–crystal phase transition was not
observed at 52–55 °C by visible light (436 nm, 60 mW/cm
2 ) irradiation. Thus,
ion pairs of azobenzene anions possess several advantages for photo-induced phase
transitions: (i) the bulky countercations provide free volumes around the azobenzene
moieties, (ii) isomerization of the azobenzene unit is not significantly restricted by
the packing of alkyl chains on the countercations, because the ion pairs are weakly
connected by noncovalent interactions, and (iii) the assembled structures and their
properties can be controlled by the modifications of the countercations along with
the anionic species [38].
18.2.4 Photo-Responsive Properties of Ion-Pairing
Assemblies Based on Receptor–Anion Complexes
On the basis of the high binding constant (K a ) of 7a for 16
C16− (3,100,000 M
−1 ,
CH 2 Cl 2 ), which is comparable to that for CH 3 CO 2
− (930,000 M
−1 ) [39], the ion pair
7a·16
C16− -TBA
+ was formed from a 1:1 mixture of 7a and 16
C16− -TBA
+ and was
purified by recrystallization from diisopropylether [40]. In the
1 H NMR spectrum of
7a, the NH and bridging CH signals were observed at 9.59 and 6.55 ppm, respectively, whereas, in 7a·16
C16− -TBA
+ , the corresponding signals disappeared with the
appearance of new signals at 14.19 and 8.05 ppm. The formation of the receptor–
anion complex also caused a shift in the signals of the azobenzene carboxylate (e.g.,
H
a in Fig. 18.13a shifted from 8.08 to 8.26 ppm). After UV (365 nm) irradiation, new
signals from the cis form appeared both in 16
C16− and 7a·16
C16− -TBA
+ (e.g., 7.83
and 8.02 ppm for H
a in 16
C16− and 7a·16
C16− -TBA
+ , respectively). The difference
in the
1 H NMR chemical shifts between 16
C16− and 7a·16
C16− -TBA
+ for the trans
forms, 0.18 ppm (H
a ), was slightly smaller than that for the cis forms, 0.19 ppm (H
a ),
suggesting that photo-isomerization from the trans to cis forms slightly affects the
anionic properties of the carboxylate and the interactions in the receptor–carboxylate
complexes. In addition, the trans/cis ratios in the PSS 365 were estimated as 0:100 for
16
C16− and 30:70 for 16
C16− in equilibrium with 7a·16
C16− . This difference can be
Y. Haketa et al.
the XRD pattern assignable to Phase B disappeared, and that assignable to Phase A
re-appeared in the small-angle region. In the POM observations, the birefringence
disappeared immediately after visible light irradiation and then new birefringence
appeared. This transient optically isotropic state is probably different from the optically isotropic ionic liquid state induced by heating, by considering the fact that
Phase A was not observed during the cooling process from the ionic liquid state.
As the clearing temperature of 16
C16− ·TBA
+ under UV irradiation (52 °C) was
slightly lower than that without UV irradiation (55 °C), the photo-induced crystal–
crystal phase transition was observed below 52 °C. Thus, the crystalline lamellar
phase mainly comprising the bulky cis form (Phase B) was fairly stable due to the
interactions between the long alkyl chains. In addition, the crystal–crystal phase
transition can be repeated by switching between the two irradiation wavelengths
(365/436 nm) (Fig. 18.12b), although the liquid–crystal phase transition was not
observed at 52–55 °C by visible light (436 nm, 60 mW/cm
2 ) irradiation. Thus,
ion pairs of azobenzene anions possess several advantages for photo-induced phase
transitions: (i) the bulky countercations provide free volumes around the azobenzene
moieties, (ii) isomerization of the azobenzene unit is not significantly restricted by
the packing of alkyl chains on the countercations, because the ion pairs are weakly
connected by noncovalent interactions, and (iii) the assembled structures and their
properties can be controlled by the modifications of the countercations along with
the anionic species [38].
18.2.4 Photo-Responsive Properties of Ion-Pairing
Assemblies Based on Receptor–Anion Complexes
On the basis of the high binding constant (K a ) of 7a for 16
C16− (3,100,000 M
−1 ,
CH 2 Cl 2 ), which is comparable to that for CH 3 CO 2
− (930,000 M
−1 ) [39], the ion pair
7a·16
C16− -TBA
+ was formed from a 1:1 mixture of 7a and 16
C16− -TBA
+ and was
purified by recrystallization from diisopropylether [40]. In the
1 H NMR spectrum of
7a, the NH and bridging CH signals were observed at 9.59 and 6.55 ppm, respectively, whereas, in 7a·16
C16− -TBA
+ , the corresponding signals disappeared with the
appearance of new signals at 14.19 and 8.05 ppm. The formation of the receptor–
anion complex also caused a shift in the signals of the azobenzene carboxylate (e.g.,
H
a in Fig. 18.13a shifted from 8.08 to 8.26 ppm). After UV (365 nm) irradiation, new
signals from the cis form appeared both in 16
C16− and 7a·16
C16− -TBA
+ (e.g., 7.83
and 8.02 ppm for H
a in 16
C16− and 7a·16
C16− -TBA
+ , respectively). The difference
in the
1 H NMR chemical shifts between 16
C16− and 7a·16
C16− -TBA
+ for the trans
forms, 0.18 ppm (H
a ), was slightly smaller than that for the cis forms, 0.19 ppm (H
a ),
suggesting that photo-isomerization from the trans to cis forms slightly affects the
anionic properties of the carboxylate and the interactions in the receptor–carboxylate
complexes. In addition, the trans/cis ratios in the PSS 365 were estimated as 0:100 for
16
C16− and 30:70 for 16
C16− in equilibrium with 7a·16
C16− . This difference can be