4 Stepwise Two-Photon Photochromism
71
laser. The slope of the concentration of CL-CL against the incident laser power
was estimated to be 1.8. This indicates that bisBN-PIC generates colorless CL-CL
in a stepwise manner and shows the nonlinear photoresponse to the incident light
intensity (Fig. 4.10c).
In conclusion of this section, we demonstrated the stepwise negative
photochromism of bisBN-PIC which consists of electronically coupled two fastswitchable negative photochromic units. bisBN-PIC nonlinearly responds to incident light intensity. The two transient species C-C (initial state) and CL-C (onephoton product) have absorption bands at around 450 nm which is attributable to
the π–π* transition on the cyclohexadienone unit of C. In contrast, CL-CL generated by the two-photon negative photochromic reaction has no absorption band in
the visible light region. Because the thermal back reaction rate from CL-CL to
CL-C is 10-times faster than that from CL-C to C-C, the concentration of CL-CL
is not accumulated upon weak light irradiation, leading to the excitation intensitydependent photochromism in which only the intense light irradiation makes the
solution of bisBN-PIC transparent. This visible light intensity-dependent negative photochromism will give an important strategy to design the organic molecule
applicable to nonlinear optical materials such as saturable absorber.
4.5 Stepwise Photochromism of Bisnaphthopyrans
Exhibiting Excitation Intensity-Dependent Color
Change
Our strategy to realize stepwise photochromic reactions depending on the excitation
intensity is not limited to the radical complex system. Naphthopyran derivatives are
an important class of T-type photochromic molecules for practical applications such
as photochromic lenses because of their high colorability and tunable spectrokinetic
properties [50–54]. The pyran ring of naphthopyran shows the ring-opening reaction
upon UV light irradiation to form the two colored open-ring isomers, transoid-cis
(TC) and transoid-trans (TT) forms (Fig. 4.11). Although the slow thermal back
reaction of the TT form was one of the issues to be solved for real-time switching,
we have recently succeeded to decrease the generation amount of the TT form.
The 8H-pyrano[3,2-f ]quinazoline (8H-PQ) reported by Pozzo and co-workers is an
attractive molecular framework to accelerate the thermal back reaction of the TC
form [55]. We recently demonstrated that the introduction of an alkoxy group to
the 1-position of 8H-PQ effectively reduces the generation amount of the undesired
long-lived TT form because of the C–H···O intramolecular hydrogen bond in the TC
form [56].
In this study, we developed a bisnaphthopyran system showing excitation light
intensity-dependent photochromism. We designed and synthesized bisnaphthopyran
derivatives bridged by a dihydroanthracene moiety, bis-NP, and bis-PQ, which
are composed of 3H-naphtho[2,1-b]pyran (3H-NP) and 8H-PQ units, respectively
71
laser. The slope of the concentration of CL-CL against the incident laser power
was estimated to be 1.8. This indicates that bisBN-PIC generates colorless CL-CL
in a stepwise manner and shows the nonlinear photoresponse to the incident light
intensity (Fig. 4.10c).
In conclusion of this section, we demonstrated the stepwise negative
photochromism of bisBN-PIC which consists of electronically coupled two fastswitchable negative photochromic units. bisBN-PIC nonlinearly responds to incident light intensity. The two transient species C-C (initial state) and CL-C (onephoton product) have absorption bands at around 450 nm which is attributable to
the π–π* transition on the cyclohexadienone unit of C. In contrast, CL-CL generated by the two-photon negative photochromic reaction has no absorption band in
the visible light region. Because the thermal back reaction rate from CL-CL to
CL-C is 10-times faster than that from CL-C to C-C, the concentration of CL-CL
is not accumulated upon weak light irradiation, leading to the excitation intensitydependent photochromism in which only the intense light irradiation makes the
solution of bisBN-PIC transparent. This visible light intensity-dependent negative photochromism will give an important strategy to design the organic molecule
applicable to nonlinear optical materials such as saturable absorber.
4.5 Stepwise Photochromism of Bisnaphthopyrans
Exhibiting Excitation Intensity-Dependent Color
Change
Our strategy to realize stepwise photochromic reactions depending on the excitation
intensity is not limited to the radical complex system. Naphthopyran derivatives are
an important class of T-type photochromic molecules for practical applications such
as photochromic lenses because of their high colorability and tunable spectrokinetic
properties [50–54]. The pyran ring of naphthopyran shows the ring-opening reaction
upon UV light irradiation to form the two colored open-ring isomers, transoid-cis
(TC) and transoid-trans (TT) forms (Fig. 4.11). Although the slow thermal back
reaction of the TT form was one of the issues to be solved for real-time switching,
we have recently succeeded to decrease the generation amount of the TT form.
The 8H-pyrano[3,2-f ]quinazoline (8H-PQ) reported by Pozzo and co-workers is an
attractive molecular framework to accelerate the thermal back reaction of the TC
form [55]. We recently demonstrated that the introduction of an alkoxy group to
the 1-position of 8H-PQ effectively reduces the generation amount of the undesired
long-lived TT form because of the C–H···O intramolecular hydrogen bond in the TC
form [56].
In this study, we developed a bisnaphthopyran system showing excitation light
intensity-dependent photochromism. We designed and synthesized bisnaphthopyran
derivatives bridged by a dihydroanthracene moiety, bis-NP, and bis-PQ, which
are composed of 3H-naphtho[2,1-b]pyran (3H-NP) and 8H-PQ units, respectively