4 Stepwise Two-Photon Photochromism
65
nm nanosecond laser pulse (1 mJ/pulse) gives a small amount of the absorption band
ascribable to the quinoidal form, the transient absorption spectrum excited by both
a 355 and a 420-nm nanosecond laser pulses (1 and 5.5 mJ/pulse) shows the clear
absorption band assigned to the quinoidal form. It clearly shows that the excitation
of the biradical form of bisImD with a 420-nm laser pulse amplifies the formation
of the quinoidal form. In contrast, the quinoidal form was not detected when the
wavelength of the second laser pulse was 550 nm or 650 nm, indicating that the
photocleavage reaction of the C–N bond would proceed from the higher excited
state of the biradical form. We define the A as the difference in the absorbance
values at 600 nm after the first 355 nm and the second 420 nm excitation, A 355
and A 420 , respectively (A = A 420 – A 355 ). This value corresponds to the
population of the quinoidal form generated by the 420 nm excitation. The A
decreases depending on the concentration of the biradical form with the increase in
the time interval between the two excitation pulses (dot in Fig. 4.4b). The decay of
the A values shows good agreement with the thermal back reaction profile of the
biradical form. In addition, the A values linearly increase with the increase in the
excitation intensity of the 420-nm laser pulse (slope = 1.04, the inset of Fig. 4.4b).
Therefore, the stepwise 2PA process is involved in the photochromic reaction of
bisImD.
This two-photon photochromic reaction can be initiated by incoherent and weak
light sources (Fig. 4.4c). While a single weak incoherent light (5 mW and 355nm light or a halogen lamp, 420–700 nm as white light) does not cause the twophoton induced photochromism, the irradiation of both lights at the same time clearly
enhances the reaction in spite that they are weak and incoherent light sources. It
clearly shows that the power threshold for the stepwise photochromic reaction is
extremely low (~mW/cm
2 ) as compared to those of conventional nonlinear optical
processes (>MW/cm
2 ). The efficient stepwise photochromic reaction of ImD was
realized based on the two key features: (i) the short-lived transient species of Ttype photochromic compounds as an intermediate state for the stepwise two-photon
photochemical reactions and (ii) the strong electronic coupling between the shortlived photogenerated transient chromophores.
The concept of the stepwise photochromic reactions of bisImD can be expanded
to other radical-dissociation type photochromic compounds such as pentaarylbiimidazole (PABI) [33] and phenoxyl-imidazolyl radical complex (PIC) [34, 35],
which are recently developed radical-dissociation type photochromic compounds.
It is important to note that the half-lives of the thermal back reactions of PIC can
be tuned from tens of nanoseconds to tens of seconds by slight modifications of
the molecular framework. Especially, the substitution of the phenyl ring at the 2position of the imidazole ring to the thiophene ring (TPIC) dramatically decelerates
the thermal back reaction most probably due to the donor effect of the thiophene
ring. Based on PIC and TPIC frameworks, we designed and synthesized stepwise
two-photon induced photochromic compound derivatives: p-bisPIC, m-bisPIC, and
bis(thiophene-coupled PIC) (bisTPIC) (Fig. 4.5) [36]. The photogenerated biradical
species of PIC has a through-bond spin-spin interaction between the imidazolyl and
phenoxyl radicals at the ortho-position of the phenylene ring. In the similar manner,
65
nm nanosecond laser pulse (1 mJ/pulse) gives a small amount of the absorption band
ascribable to the quinoidal form, the transient absorption spectrum excited by both
a 355 and a 420-nm nanosecond laser pulses (1 and 5.5 mJ/pulse) shows the clear
absorption band assigned to the quinoidal form. It clearly shows that the excitation
of the biradical form of bisImD with a 420-nm laser pulse amplifies the formation
of the quinoidal form. In contrast, the quinoidal form was not detected when the
wavelength of the second laser pulse was 550 nm or 650 nm, indicating that the
photocleavage reaction of the C–N bond would proceed from the higher excited
state of the biradical form. We define the A as the difference in the absorbance
values at 600 nm after the first 355 nm and the second 420 nm excitation, A 355
and A 420 , respectively (A = A 420 – A 355 ). This value corresponds to the
population of the quinoidal form generated by the 420 nm excitation. The A
decreases depending on the concentration of the biradical form with the increase in
the time interval between the two excitation pulses (dot in Fig. 4.4b). The decay of
the A values shows good agreement with the thermal back reaction profile of the
biradical form. In addition, the A values linearly increase with the increase in the
excitation intensity of the 420-nm laser pulse (slope = 1.04, the inset of Fig. 4.4b).
Therefore, the stepwise 2PA process is involved in the photochromic reaction of
bisImD.
This two-photon photochromic reaction can be initiated by incoherent and weak
light sources (Fig. 4.4c). While a single weak incoherent light (5 mW and 355nm light or a halogen lamp, 420–700 nm as white light) does not cause the twophoton induced photochromism, the irradiation of both lights at the same time clearly
enhances the reaction in spite that they are weak and incoherent light sources. It
clearly shows that the power threshold for the stepwise photochromic reaction is
extremely low (~mW/cm
2 ) as compared to those of conventional nonlinear optical
processes (>MW/cm
2 ). The efficient stepwise photochromic reaction of ImD was
realized based on the two key features: (i) the short-lived transient species of Ttype photochromic compounds as an intermediate state for the stepwise two-photon
photochemical reactions and (ii) the strong electronic coupling between the shortlived photogenerated transient chromophores.
The concept of the stepwise photochromic reactions of bisImD can be expanded
to other radical-dissociation type photochromic compounds such as pentaarylbiimidazole (PABI) [33] and phenoxyl-imidazolyl radical complex (PIC) [34, 35],
which are recently developed radical-dissociation type photochromic compounds.
It is important to note that the half-lives of the thermal back reactions of PIC can
be tuned from tens of nanoseconds to tens of seconds by slight modifications of
the molecular framework. Especially, the substitution of the phenyl ring at the 2position of the imidazole ring to the thiophene ring (TPIC) dramatically decelerates
the thermal back reaction most probably due to the donor effect of the thiophene
ring. Based on PIC and TPIC frameworks, we designed and synthesized stepwise
two-photon induced photochromic compound derivatives: p-bisPIC, m-bisPIC, and
bis(thiophene-coupled PIC) (bisTPIC) (Fig. 4.5) [36]. The photogenerated biradical
species of PIC has a through-bond spin-spin interaction between the imidazolyl and
phenoxyl radicals at the ortho-position of the phenylene ring. In the similar manner,