T. Kuwabara et al. / Journal of Photochemistry and Photobiology A: Chemistry 269 (2013) 59–64
63
Table 2
which correspond to the charge transfer band between 6 and the
viologen unit of 2. This indicates that 6 is included in the cavity
of 2 to form the inclusion complex, 6@2. In other words, the -
electron deficient cavity of 2 is occupied by the -electron donating
molecule, 6.
The ꢀmax and ꢁ1/2 for the photoreduced viologen radical cations.
Viologen
derivatives
Added -electron
donating component
Phororeduced viologen
radical cation
ꢀmax (nm)
ꢁ1/2 (min)
Upon photoirradiation of the film containing 6@2, the photogen-
erated absorption peak was observed at 620 nm, which is shifted to
longer wavelength by 10 nm as compared with that of the guest free
film of 2. This result suggests that the addition of the guest caused
the red shift of the photoreduced absorption peak of 2. When irra-
diated to the film of the acyclic analog, 3, in the presence of 6, the
photoinduced absorption peak was observed at 611 nm, which was
the same wavelength to that of the 6 free film of 3. This suggests
that there is no interaction between 3 and 6, which was supported
by no observation of the charge transfer absorption band between
3 and 6 in the solution. The supramolecular structure, in which the
-electron donating guest is included in the cavity of 2, is impor-
The guest-induced red shift of the photoreduced 2 may be resulted
from the same reason as the case of 1.
Furthermore, the addition of 6 to the film of 2 caused the faster
bleaching of the photoreduced viologen of 2 as compared with that
of the film of 2 alone, as shown in Fig. 5a. In the presence of 6,
the bleaching rate of the photoreduced 2 is similar to that of 1,
indicating that the bleaching rate is affected by the presence of the
-electron donating guest. The guest-induced faster reversion of
the photoreduced 2 may be resulted form the same reason as the
case of 1. The half-life of 6@2 is almost same as that of 1 (Table 2).
the photoreduced absorption peak but also the bleaching rate of the
photoreduced 6@2 is similar to that of 1. Furthermore, the ability
of complex formation is the polymer matrix seems to be similar
to that in the solution, although the mobility of the compounds is
restricted as mentioned above [18].
When 7 was used as the guest in place of 6, the photogenera-
ted absorption peak of 2 was observed at 615 nm. Furthermore, the
half-life of the film of 7@2 was observed at 20 min (Table 2). As
compared with 2, the red shift and the faster bleaching of the pho-
toreduced viologen of 7@2 were observed. The smaller effects of 7
on the red shift in absorption peak and the reversion rate as com-
pared with those of 6 may be resulted from the weaker interaction
of 7 with 2 than that of 6. This is coincident with the fact that the
binding constant of 7@2 is smaller than that of 6@2.
Upon addition of benzonitrile, 8, which was used as the -
electron deficient guest in place of the -electron donating one,
into the film containing 2, the absorption peak and the bleaching
rate of the photoreduced viologen of 2 was not affected. This result
indicates that there is no change in the photochromic properties of
2 when the -electron deficient guest was used. It can be concluded
that the red shift and the faster reversion of the photogenerated
viologen radical of 2 are induced by the addition of the -electron
donating guest, which is included in the cavity of the tetracationic
cyclophane, 2, by means of the -donor/-acceptor interaction.
1
2
620
610
620
615
611
611
610
610
15
24
14
20
22
22
22
15
6
7
3
4
6
5
3.3. Photochromism of 2 as compared with 1
When irradiated to the pale yellow film containing 2, the film
showed the similar spectral change to that of 1 (Fig. 4). This indi-
cates that photogenaration of the viologen radical cation and the
reversion to the dication with time proceeding. However, the pho-
togenerated ꢀmax of 2 was observed at 610 nm, which is shifted to
shorter wavelength by 10 nm as compared with that of 1 (620 nm),
while the film containing 3 and 4 exhibited the photogenerated
absorption peak at 611 and 610 nm, respectively. Since the radical
cation dimer is known to show the peak at 360 and 520 nm [20], the
obtained results suggest that there is no interaction between two
viologen units in a molecule of 2 and that the red shift of the photo-
generated absorption peak for 1 is unusual. The following reasons
can be considered for interpreting the red shift of the photoreduced
viologen peak of 1; (i) the difference of the environment around the
viologen unit between 1 and 2 due to the different species included
in the cavity of the tetracationic cyclophane; the dialkoxybenzene
unit for 1 and solvent molecules for 2, (ii) the decrease in the transi-
tion energy due to the interaction of the transition dipole moment
between viologen and dialkoxybenzene unit, or (iii) the change
in the interaction between viologen units by the insertion of the
between 1 and 2 was observed in the bleaching rate of the pho-
toreduced viologen. The bleaching rate of the photoreduced 2 was
slower than that of 1 but was similar to those of 3 and 4, as shown
in Fig. 5. (The data of 3 was not shown.) The faster reversion of
the photogenerated viologen radical of 1 suggests the faster back
reaction as compared with that of 2 as well as 3 and 4. Among two
quenching processes mentioned above, the recombination reaction
of the photogenerated radicals might be affected by making the
catenane structure. The existence of -electron donating dialkoxy-
benzene unit around the photochromic viologen unit may promote
the recombination reaction of the photogenerated radicals. This
may be resulted from the stability of the viologen under the condi-
tion of the -electron donating environment. The viologen included
in the -electron donating cavity may prefer to exist as the dication
rather than the reduced radical cation due to the donor–acceptor
electrostatic interaction. Therefore the photoreduced viologen rad-
ical cation of 1 reverts to the dication with the faster rate than that
of 2.
3.5. Photochromism of 4 in the presence of 5
The effect of the -electron donating cyclic molecule on the pho-
tochromic behavior of the acyclic viologen was also investigated in
the polymer matrix [14]. 4 and 5 were used as the acyclic viologen
and the -electron donating cyclic molecule, respectively.
As mentioned above, the film containing 4 exhibited the pho-
toinduced color development of the maximum at 610 nm and
bleached with the half-life time of 22 min. This photoinduced spec-
tral variation was similar to that of the guest free film of 2.
The film containing 4 in the presence of 5 exhibited the charge
transfer band around 450 nm before photoirradiation, indicating
3.4. Photochromism of 2 in the presence of the guest
To confirm the effect of the -electron donating species on the
absorption peak and the bleaching rate of the photoreduced violo-
gen radical, the photoirradiation of the film containing 2 has been
carried out in the presence of the -electron donating guest such
as 6 and 7.
When 6 was used as the -electron donating guest, the film
containing 6@2 exhibited the orange color before photoirradiation,