4 Stepwise Two-Photon Photochromism
59
because these materials show the photoresponses that cannot be achieved by a onephoton reaction of a single chromophore. These cooperative interactions of multiple
photons and molecules are recently termed as the “photosynergetic” effects, and
the understanding and utilization of these effects are becoming important research
subjects. Photochromism is defined as a reversible transformation of a chemical
species between two isomers having different absorption spectra induced in one
or both directions by irradiation of light. If the photogenerated chemical species
is thermally unstable and the back reaction to the initial structure occurs by heat
(and light), this photochromism is classified as “T-type.” Stepwise multiple-photon
absorptions could occur when the excitation light is continuously irradiated to T-type
photochromic compounds to reach the photostationary state (PSS). However, most of
the photogenerated colored species of T-type photochromic compounds do not induce
additional photophysical and photochemical reactions when they absorb a photon
except the photodegradation. If the photogenerated colored species of photochromic
compounds has a property to induce another photochemical reaction by the absorption of the second photon, this photochromic reaction becomes a reversible stepwise 2PA system [14]. The longer lifetime of the photogenerated colored species
increases the probability to absorb the second photon, and therefore, reduces the
power threshold of the stepwise 2PA. We have applied the fast T-type photochromic
molecules to the stepwise 2PA process and succeeded in realizing the extremely low
power threshold of the stepwise 2PA process, which can be initiated even by weak CW
LEDs. This type of the stepwise two-photon photochromism opens up novel potential
applications that cannot be achieved by conventional photofunctional materials.
4.2 Stepwise Two-Photon Photochromism of Porphyrin
Substituted Bridged Imidazole Dimer Via Higher
Excited States
In the electronically excited states, stepwise 2PA processes create a higher excited
state. It is usually difficult to utilize higher excited states to induce photochemical
reactions because of ultrafast nonradiative relaxations to the lowest excited state.
However, there are several exceptions to the limitation. For example, it was reported
that the photochromic reaction yields of the ring-opening reactions of diarylethene
derivatives are usually at most ~1% in the one-photon process [15]. On the other
hand, the ring-opening reactions are dramatically enhanced by a ten to a thousand times by the stepwise 2PA process [16]. Similar characteristic reactions by
stepwise 2PA processes have been reported in fulgide, retinal, and cyanine dyes
[17]. However, stepwise two-photon induced photochemical reactions via the higher
excited states were reported in limited molecular frameworks within a single chromophore. To investigate the reaction from higher excited states between two chromophores, we synthesized a zinc tetraphenylporphyrin (ZnTPP) substituted bridged
imidazole dimer (ImD, ImD-ZnTPP, Fig. 4.1) [18]. Bridged imidazole dimer is one
59
because these materials show the photoresponses that cannot be achieved by a onephoton reaction of a single chromophore. These cooperative interactions of multiple
photons and molecules are recently termed as the “photosynergetic” effects, and
the understanding and utilization of these effects are becoming important research
subjects. Photochromism is defined as a reversible transformation of a chemical
species between two isomers having different absorption spectra induced in one
or both directions by irradiation of light. If the photogenerated chemical species
is thermally unstable and the back reaction to the initial structure occurs by heat
(and light), this photochromism is classified as “T-type.” Stepwise multiple-photon
absorptions could occur when the excitation light is continuously irradiated to T-type
photochromic compounds to reach the photostationary state (PSS). However, most of
the photogenerated colored species of T-type photochromic compounds do not induce
additional photophysical and photochemical reactions when they absorb a photon
except the photodegradation. If the photogenerated colored species of photochromic
compounds has a property to induce another photochemical reaction by the absorption of the second photon, this photochromic reaction becomes a reversible stepwise 2PA system [14]. The longer lifetime of the photogenerated colored species
increases the probability to absorb the second photon, and therefore, reduces the
power threshold of the stepwise 2PA. We have applied the fast T-type photochromic
molecules to the stepwise 2PA process and succeeded in realizing the extremely low
power threshold of the stepwise 2PA process, which can be initiated even by weak CW
LEDs. This type of the stepwise two-photon photochromism opens up novel potential
applications that cannot be achieved by conventional photofunctional materials.
4.2 Stepwise Two-Photon Photochromism of Porphyrin
Substituted Bridged Imidazole Dimer Via Higher
Excited States
In the electronically excited states, stepwise 2PA processes create a higher excited
state. It is usually difficult to utilize higher excited states to induce photochemical
reactions because of ultrafast nonradiative relaxations to the lowest excited state.
However, there are several exceptions to the limitation. For example, it was reported
that the photochromic reaction yields of the ring-opening reactions of diarylethene
derivatives are usually at most ~1% in the one-photon process [15]. On the other
hand, the ring-opening reactions are dramatically enhanced by a ten to a thousand times by the stepwise 2PA process [16]. Similar characteristic reactions by
stepwise 2PA processes have been reported in fulgide, retinal, and cyanine dyes
[17]. However, stepwise two-photon induced photochemical reactions via the higher
excited states were reported in limited molecular frameworks within a single chromophore. To investigate the reaction from higher excited states between two chromophores, we synthesized a zinc tetraphenylporphyrin (ZnTPP) substituted bridged
imidazole dimer (ImD, ImD-ZnTPP, Fig. 4.1) [18]. Bridged imidazole dimer is one