10
H. Miyasaka et al.
As shown in this figure, the cycloreversion yield by the second laser pulse in the
wavelength region shorter than 680 nm is ca. 50–90%. The cycloreversion yield in
higher excited states under the multiple excitations of PT(c) by a picosecond 532nm laser pulse was estimated to be ca. 50% [6]. Because the picosecond laser pulse
with 15-ps FWHM excites the 1B and 2A state during the pulse duration. In addition,
unrelaxed 2A state is also excited in addition to the relaxed 2A state. Accordingly, the
reaction yield in the high state estimated in the picosecond experiment was slightly
smaller than that obtained for the relaxed 2A state in the femtosecond experiment,
70%. In anyhow, present results are almost in the same order of the picosecond
pulsed excitation. On the other hand, the reaction yield is much smaller, less than
1%, in the wavelength region ≥730 nm. It is worth noting for the correlation curve
with the second pulse ≥730 nm in Fig. 1.8 that the amount of the cycloreversion
reaction induced by the second photon absorption showed the monotonous decrease
with an increase in t and no selectivity depending on the initial electronic state or
the geometry of the molecule for the second photon absorption was found. These
results strongly suggest the contribution of the vibrationally excited state in the lowest
electronically excited state, which was produced by the rapid internal conversion from
the highly excited states pumped by Pump 2 in the wavelength region ≥730 nm. The
excess energy of the S 1 state may assist the molecules in overcoming the activation
barrier on the 2A potential energy surface, leading to the slight increase in the reaction
yield of the cycloreversion. Actually, the cycloreversion reaction yield of diarylethene
derivatives generally increases with increasing temperature [19].
By summarizing above results and discussion, we could conclude that the character of the electronic state, the Franck–Condon geometry of molecules in highly
excited states, and the level of the final state are key factors for the marked increase
of the cycloreversion reaction by the stepwise two-photon excitation.
1.2.2 Stepwise Two-Photon Cycloreversion Reactions
in Fulgide Derivatives
To elucidate the generality of these reactions in 6π electron systems, we have studied
the reaction dynamics of fulgide derivatives by the stepwise two-photon excitation
of femtosecond laser pulses [20].
Figure 1.10 shows the photochromic reaction of F1, together with the absorption
spectra of the open-ring isomer, F1(o), and closed-ring one, F1(c), in toluene solution.
The fluorescence spectrum of F1(c) is also exhibited. As shown in Fig. 1.2, F1(o) has
absorption bands only in the UV region while F1(c) shows the absorption bands also
in the visible region. The fluorescence spectrum of F1(c) shows a very broad band
with rather large Stokes shift in nonpolar solution, strongly suggesting that the relaxed
excited state of F1(c) in the lowest excited state has a geometry largely different from
the ground state as observed for the diarylethene derivative. The cyclization yield
upon the irradiation at 366 nm and the cycloreversion yield upon the 505-nm exposure
were, respectively, 0.39 and 0.11 in toluene solution at 295 K [20].
H. Miyasaka et al.
As shown in this figure, the cycloreversion yield by the second laser pulse in the
wavelength region shorter than 680 nm is ca. 50–90%. The cycloreversion yield in
higher excited states under the multiple excitations of PT(c) by a picosecond 532nm laser pulse was estimated to be ca. 50% [6]. Because the picosecond laser pulse
with 15-ps FWHM excites the 1B and 2A state during the pulse duration. In addition,
unrelaxed 2A state is also excited in addition to the relaxed 2A state. Accordingly, the
reaction yield in the high state estimated in the picosecond experiment was slightly
smaller than that obtained for the relaxed 2A state in the femtosecond experiment,
70%. In anyhow, present results are almost in the same order of the picosecond
pulsed excitation. On the other hand, the reaction yield is much smaller, less than
1%, in the wavelength region ≥730 nm. It is worth noting for the correlation curve
with the second pulse ≥730 nm in Fig. 1.8 that the amount of the cycloreversion
reaction induced by the second photon absorption showed the monotonous decrease
with an increase in t and no selectivity depending on the initial electronic state or
the geometry of the molecule for the second photon absorption was found. These
results strongly suggest the contribution of the vibrationally excited state in the lowest
electronically excited state, which was produced by the rapid internal conversion from
the highly excited states pumped by Pump 2 in the wavelength region ≥730 nm. The
excess energy of the S 1 state may assist the molecules in overcoming the activation
barrier on the 2A potential energy surface, leading to the slight increase in the reaction
yield of the cycloreversion. Actually, the cycloreversion reaction yield of diarylethene
derivatives generally increases with increasing temperature [19].
By summarizing above results and discussion, we could conclude that the character of the electronic state, the Franck–Condon geometry of molecules in highly
excited states, and the level of the final state are key factors for the marked increase
of the cycloreversion reaction by the stepwise two-photon excitation.
1.2.2 Stepwise Two-Photon Cycloreversion Reactions
in Fulgide Derivatives
To elucidate the generality of these reactions in 6π electron systems, we have studied
the reaction dynamics of fulgide derivatives by the stepwise two-photon excitation
of femtosecond laser pulses [20].
Figure 1.10 shows the photochromic reaction of F1, together with the absorption
spectra of the open-ring isomer, F1(o), and closed-ring one, F1(c), in toluene solution.
The fluorescence spectrum of F1(c) is also exhibited. As shown in Fig. 1.2, F1(o) has
absorption bands only in the UV region while F1(c) shows the absorption bands also
in the visible region. The fluorescence spectrum of F1(c) shows a very broad band
with rather large Stokes shift in nonpolar solution, strongly suggesting that the relaxed
excited state of F1(c) in the lowest excited state has a geometry largely different from
the ground state as observed for the diarylethene derivative. The cyclization yield
upon the irradiation at 366 nm and the cycloreversion yield upon the 505-nm exposure
were, respectively, 0.39 and 0.11 in toluene solution at 295 K [20].