photostationary state under a 500 W super high-pressure Hg lamp [162], while that
in octadecyltrimethylammonium cation (C 18 3C 1 N
+
)-TSM was about 35% under a
100 W high-pressure Hg lamp [349]. The difference of the fraction of the cis-isomer
is thought to be due to the molecular packing of the surfactants in the interlayer
space. The thermal cis- to trans-isomerization of AZ in C 18 3C 1 N
+ -TSM took 2 days
[349] which was faster than the half-life of cis-AZ in a benzene solution (5 days)
[308]. The thermal isomerization of cis-AZ in the solution followed a first-order
kinetics while in such polymers as poly(methyl methacrylate) (PMMA) [350] and
poly(ethyl methacrylate) (PEMA) [351] and a silica gel synthesized by sol-gel
method did not follow the first-order kinetics [350], indicating that the azobenzene
molecules were in several environments in these solid-state materials.
The photochromism of cationic azobenzenes in montmorillonites [62, 161, 344],
saponites [163, 337, 352], a fluorohectrite [337], taeniolite [163], and magadiite
[164, 165, 186, 346] was reported [167]. Organically modified fluoro-tetrasilicic
mica [345], montmorillonite [353], and beidellite [353] were also used. As summarized in Table 2, the fraction of the cis-isomer of AZC 2 N
+ C 2 OH in mesoporous silica
(MCM-41) with the pore size of 3.2 nm was 70% [333], and those of AZ in a zeolite
NaY (pore size: 0.74 nm) and a sodium mordenite (pore size: 0.7 Â 0.65 nm) were
80 and 50%, respectively [313]. The fraction of cis-isomer of the cationic
azobenzene (AZC 2 N
+
C 2 OH, Table 3) in magadiite at room temperature was 80%,
similar to AZ in a cyclohexane solution [165]. It was thought that the structural
change of azobenzenes was accommodated by the change of the basal spacing to
achieve the relatively high yield of cis-isomer. The thermal isomerization of cisisomer of AZC 2 N
+
C 2 OH in magadiite followed the first-order kinetics, indicating
that AZC 2 N
+
C 2 OH was homogeneously distributed in magadiite. The fraction of
cis-isomer at the photostationary state decreased in KF [161] and magadiite [165] at
low temperatures. The fraction of cis-isomer of the cationic azobenzenes
(C 8 AZC 10 N
+ and C 12 AZC 5 N
+
, in Table 3) in KF at room temperature was about
50%, while cis-isomer was practically not detected at the temperature lower than
200 K, suggesting that the molecular motion was suppressed in the interlayer space
of KF.
There are several examples of the suppression of the trans- to cis-isomerization.
In a fluorohectorite (obtained from Corning Inc.) in both suspensions and films,
photoisomerization of N
+
C 1 AZC 1 N
+ (Table 3) was suppressed, while AZC 1 N
+
isomerized as shown by the change in the absorption spectrum (Fig. 14) [337]. It
was explained that attractive electrostatic forces between the silicate layers and the
dicationic N
+
C 1 AZC 1 N
+ hindered the isomerization. Photoisomerization of
N
+
AZN
+ (Table 3) was suppressed by the adsorption on SA [348]. Both transand cis-isomers of N
+ AZN
+ were exchanged on SA. The cis-isomer showed
photoisomerization to the trans-isomer on SA by 420 nm light irradiation with a
higher quantum yield than that in an aqueous solution (without clay), while transisomer on SA did not show photoisomerization. The suppression of the trans- to cisisomerization was thought to be due to the matching of the intercharge distance of
the trans-isomer and that of the adjacent negative surface charge of the silicate layer.
The trans-isomer interacted with the silicate layer with both of two cationic moieties
Photofunctions of Dye-Clay Hybrids: Recent Developments
279
in octadecyltrimethylammonium cation (C 18 3C 1 N
+
)-TSM was about 35% under a
100 W high-pressure Hg lamp [349]. The difference of the fraction of the cis-isomer
is thought to be due to the molecular packing of the surfactants in the interlayer
space. The thermal cis- to trans-isomerization of AZ in C 18 3C 1 N
+ -TSM took 2 days
[349] which was faster than the half-life of cis-AZ in a benzene solution (5 days)
[308]. The thermal isomerization of cis-AZ in the solution followed a first-order
kinetics while in such polymers as poly(methyl methacrylate) (PMMA) [350] and
poly(ethyl methacrylate) (PEMA) [351] and a silica gel synthesized by sol-gel
method did not follow the first-order kinetics [350], indicating that the azobenzene
molecules were in several environments in these solid-state materials.
The photochromism of cationic azobenzenes in montmorillonites [62, 161, 344],
saponites [163, 337, 352], a fluorohectrite [337], taeniolite [163], and magadiite
[164, 165, 186, 346] was reported [167]. Organically modified fluoro-tetrasilicic
mica [345], montmorillonite [353], and beidellite [353] were also used. As summarized in Table 2, the fraction of the cis-isomer of AZC 2 N
+ C 2 OH in mesoporous silica
(MCM-41) with the pore size of 3.2 nm was 70% [333], and those of AZ in a zeolite
NaY (pore size: 0.74 nm) and a sodium mordenite (pore size: 0.7 Â 0.65 nm) were
80 and 50%, respectively [313]. The fraction of cis-isomer of the cationic
azobenzene (AZC 2 N
+
C 2 OH, Table 3) in magadiite at room temperature was 80%,
similar to AZ in a cyclohexane solution [165]. It was thought that the structural
change of azobenzenes was accommodated by the change of the basal spacing to
achieve the relatively high yield of cis-isomer. The thermal isomerization of cisisomer of AZC 2 N
+
C 2 OH in magadiite followed the first-order kinetics, indicating
that AZC 2 N
+
C 2 OH was homogeneously distributed in magadiite. The fraction of
cis-isomer at the photostationary state decreased in KF [161] and magadiite [165] at
low temperatures. The fraction of cis-isomer of the cationic azobenzenes
(C 8 AZC 10 N
+ and C 12 AZC 5 N
+
, in Table 3) in KF at room temperature was about
50%, while cis-isomer was practically not detected at the temperature lower than
200 K, suggesting that the molecular motion was suppressed in the interlayer space
of KF.
There are several examples of the suppression of the trans- to cis-isomerization.
In a fluorohectorite (obtained from Corning Inc.) in both suspensions and films,
photoisomerization of N
+
C 1 AZC 1 N
+ (Table 3) was suppressed, while AZC 1 N
+
isomerized as shown by the change in the absorption spectrum (Fig. 14) [337]. It
was explained that attractive electrostatic forces between the silicate layers and the
dicationic N
+
C 1 AZC 1 N
+ hindered the isomerization. Photoisomerization of
N
+
AZN
+ (Table 3) was suppressed by the adsorption on SA [348]. Both transand cis-isomers of N
+ AZN
+ were exchanged on SA. The cis-isomer showed
photoisomerization to the trans-isomer on SA by 420 nm light irradiation with a
higher quantum yield than that in an aqueous solution (without clay), while transisomer on SA did not show photoisomerization. The suppression of the trans- to cisisomerization was thought to be due to the matching of the intercharge distance of
the trans-isomer and that of the adjacent negative surface charge of the silicate layer.
The trans-isomer interacted with the silicate layer with both of two cationic moieties
Photofunctions of Dye-Clay Hybrids: Recent Developments
279