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J. Abe et al.
the normalized spectra at 300 ps is very similar to the transient absorption spectrum excited at 2 ms after 550-nm nanosecond laser pulse (the bottom spectrum
of Fig. 4.2b), which is assigned to the biradical form of the ImD unit. Therefore, this result indicates that the photochromic reaction is induced by the visible
light depending on the excitation intensity. The signal of the generated biradical
form shows the quadratic dependence on the excitation intensity, indicating that the
photochromic reaction is induced by the 2PA process. Because the photon flux density
is small for the simultaneous 2PA process, this nonlinear process is most probably due
to the stepwise 2PA process. Further investigations by transient absorption, electrochemical, and emission decay measurements revealed that the whole photochromic
reaction pathway is written as Fig. 4.2c.
When the 550-nm light is irradiated to ImD-ZnTPP, the S 1 state of the ZnTPP
unit (Q band) is formed, and the photochromic reaction does not occur from the S 1
state of the ZnTPP unit. When the excitation intensity increases, the stepwise 2PA
occurs to produce the higher excited state of the ZnTPP unit. After the formation of
the higher excited state of the ZnTPP unit, the electron is transferred to the ImD unit
with ultrafast time scale. The formation of the charge transfer state was confirmed
by detecting the cation radical of ZnTPP. Once the electron is injected to ImD,
the ImD anion spontaneously cleaves the C–N bond between two imidazole rings,
and an imidazole radical anion and an imidazolyl radical are generated [26]. The
back electron transfer occurs with a time scale of 30 ps, and finally, the biradical
of ImD is formed. The generated biradical form reverts to the initial closed form
with a half-life of 40 ms. By estimating the energy level of the charge separated
state between the ZnTPP and ImD units, it was revealed that the energy level of
the charge separated state is lower than the higher excited state of the ZnTPP unit
attained by the stepwise 2PA process. Thus, the charge transfer from the higher
excited state is a trigger to induce the further photochromic reaction. More recently,
it was demonstrated that the electron transfer occurs from higher excited states even
between noncovalently bound two chromophores [27]. The stepwise 2PA process is
becoming more important process to selectively produce the higher excited state and
induce the advanced photochemical reactions beyond the one-photon process.
4.3 Stepwise Two-Photon Photochromism
of Imidazole-Based Biphotochromic Systems
The power threshold of the stepwise 2PA process depends on the lifetime of the intermediate transient state. In organic molecular systems, because the lifetime of the T 1
state (~microseconds) is usually much longer than that of the S 1 state (~nanoseconds), the power threshold for the stepwise 2PA process can be reduced if the T 1
state is used for the intermediate state of the stepwise 2PA process instead of the
S 1 state. However, it is still difficult to induce the stepwise 2PA process by weak
J. Abe et al.
the normalized spectra at 300 ps is very similar to the transient absorption spectrum excited at 2 ms after 550-nm nanosecond laser pulse (the bottom spectrum
of Fig. 4.2b), which is assigned to the biradical form of the ImD unit. Therefore, this result indicates that the photochromic reaction is induced by the visible
light depending on the excitation intensity. The signal of the generated biradical
form shows the quadratic dependence on the excitation intensity, indicating that the
photochromic reaction is induced by the 2PA process. Because the photon flux density
is small for the simultaneous 2PA process, this nonlinear process is most probably due
to the stepwise 2PA process. Further investigations by transient absorption, electrochemical, and emission decay measurements revealed that the whole photochromic
reaction pathway is written as Fig. 4.2c.
When the 550-nm light is irradiated to ImD-ZnTPP, the S 1 state of the ZnTPP
unit (Q band) is formed, and the photochromic reaction does not occur from the S 1
state of the ZnTPP unit. When the excitation intensity increases, the stepwise 2PA
occurs to produce the higher excited state of the ZnTPP unit. After the formation of
the higher excited state of the ZnTPP unit, the electron is transferred to the ImD unit
with ultrafast time scale. The formation of the charge transfer state was confirmed
by detecting the cation radical of ZnTPP. Once the electron is injected to ImD,
the ImD anion spontaneously cleaves the C–N bond between two imidazole rings,
and an imidazole radical anion and an imidazolyl radical are generated [26]. The
back electron transfer occurs with a time scale of 30 ps, and finally, the biradical
of ImD is formed. The generated biradical form reverts to the initial closed form
with a half-life of 40 ms. By estimating the energy level of the charge separated
state between the ZnTPP and ImD units, it was revealed that the energy level of
the charge separated state is lower than the higher excited state of the ZnTPP unit
attained by the stepwise 2PA process. Thus, the charge transfer from the higher
excited state is a trigger to induce the further photochromic reaction. More recently,
it was demonstrated that the electron transfer occurs from higher excited states even
between noncovalently bound two chromophores [27]. The stepwise 2PA process is
becoming more important process to selectively produce the higher excited state and
induce the advanced photochemical reactions beyond the one-photon process.
4.3 Stepwise Two-Photon Photochromism
of Imidazole-Based Biphotochromic Systems
The power threshold of the stepwise 2PA process depends on the lifetime of the intermediate transient state. In organic molecular systems, because the lifetime of the T 1
state (~microseconds) is usually much longer than that of the S 1 state (~nanoseconds), the power threshold for the stepwise 2PA process can be reduced if the T 1
state is used for the intermediate state of the stepwise 2PA process instead of the
S 1 state. However, it is still difficult to induce the stepwise 2PA process by weak