1 Advanced Control of Photochemical Reactions …
15
1.2.3 Off-Resonant Simultaneous Two-Photon
Cycloreversion Reactions
Off-resonant simultaneous multiphoton absorption is one of representative
phenomena under the photo-irradiation of high peak power lasers. Generally, simultaneous multiphoton absorption pumps the molecule into the electronic state which
is optically forbidden by the one-photon absorption. The difference in the initial
electronic state may lead to the different relaxation and reaction. Not only from the
importance in the selection of the initial electronic state but also from the viewpoint
of the laser processing, the simultaneous multiphoton absorption can provide new
application of photochromic systems. Because the high peak power of lasers can
induce higher-order (≥3) multiphoton absorption, we can induce both isomerization reactions of photochromic molecules with one-color laser light [21]. That is,
the high intensity laser induces the three-photon absorption resulting in the forward
reaction, and the irradiation with weak intensity induces the backward reaction via
the two-photon absorption. Accordingly, we can prepare the spatial patterning of the
photochromic reactions by controlling the beam profile of the one-color laser light
[21]. In this section, we introduce the reaction dynamics of the closed-ring isomer
of a diarylethene derivative, PT(c) in Fig. 1.2, in higher excited states attained by the
simultaneous off-resonant excitation at 730 nm and discuss the detailed reactions by
comparing these results with the dynamics observed by the one-photon excitation at
365 nm [22].
Before the discussion on the dynamics, we first show the excitation power dependence of transient absorbance and the relation between the time interval between
the two laser pulses at 730 nm and the signal intensity in Fig. 1.16. As shown in
Fig. 1.16a, the transient absorbance increases quadratically in proportion with the
increase in the excitation power. This result clearly shows that the two-photon absorption is responsible for the transient signals. In Fig. 1.16b, we exhibit the correlation
trace of the permanent bleaching signal as a function of time intervals between the
two 730-nm excitation pulses. In this figure, the ordinate is the transient absorbance
at 400 ps after the irradiation. As shown in Fig. 1.16, the maximum value of the
cycloreversion reaction amount is observed at t = 0 with the correlation width
of ca. 130-fs FWHM. Because this value is identical with the autocorrelation width
of the excitation pulse monitored by SHG and the transient absorbance increases
quadratically in proportion with the laser intensity, the cycloreversion reaction is
induced by the simultaneous two-photon absorption. Efficient excitation of PT(c) is
due to the large two-photon cross section, 380 GM at 730 nm [22].
Figure 1.17a shows the transient absorption spectra of PT(c) in n-hexane solution, excited with a femtosecond 365-nm laser pulse. The excitation at 365 nm pumps
PT(c) into the excited state which is ca. 11,000 cm
−1 higher than the relaxed S 1 state.
Immediately after the excitation, an absorption peak appeared at 590 nm with broad
absorption in the entire spectral window of 400–1000 nm. This 590-nm maximum
is ascribable to the absorption of highly excited state (S n state), because it was not
15
1.2.3 Off-Resonant Simultaneous Two-Photon
Cycloreversion Reactions
Off-resonant simultaneous multiphoton absorption is one of representative
phenomena under the photo-irradiation of high peak power lasers. Generally, simultaneous multiphoton absorption pumps the molecule into the electronic state which
is optically forbidden by the one-photon absorption. The difference in the initial
electronic state may lead to the different relaxation and reaction. Not only from the
importance in the selection of the initial electronic state but also from the viewpoint
of the laser processing, the simultaneous multiphoton absorption can provide new
application of photochromic systems. Because the high peak power of lasers can
induce higher-order (≥3) multiphoton absorption, we can induce both isomerization reactions of photochromic molecules with one-color laser light [21]. That is,
the high intensity laser induces the three-photon absorption resulting in the forward
reaction, and the irradiation with weak intensity induces the backward reaction via
the two-photon absorption. Accordingly, we can prepare the spatial patterning of the
photochromic reactions by controlling the beam profile of the one-color laser light
[21]. In this section, we introduce the reaction dynamics of the closed-ring isomer
of a diarylethene derivative, PT(c) in Fig. 1.2, in higher excited states attained by the
simultaneous off-resonant excitation at 730 nm and discuss the detailed reactions by
comparing these results with the dynamics observed by the one-photon excitation at
365 nm [22].
Before the discussion on the dynamics, we first show the excitation power dependence of transient absorbance and the relation between the time interval between
the two laser pulses at 730 nm and the signal intensity in Fig. 1.16. As shown in
Fig. 1.16a, the transient absorbance increases quadratically in proportion with the
increase in the excitation power. This result clearly shows that the two-photon absorption is responsible for the transient signals. In Fig. 1.16b, we exhibit the correlation
trace of the permanent bleaching signal as a function of time intervals between the
two 730-nm excitation pulses. In this figure, the ordinate is the transient absorbance
at 400 ps after the irradiation. As shown in Fig. 1.16, the maximum value of the
cycloreversion reaction amount is observed at t = 0 with the correlation width
of ca. 130-fs FWHM. Because this value is identical with the autocorrelation width
of the excitation pulse monitored by SHG and the transient absorbance increases
quadratically in proportion with the laser intensity, the cycloreversion reaction is
induced by the simultaneous two-photon absorption. Efficient excitation of PT(c) is
due to the large two-photon cross section, 380 GM at 730 nm [22].
Figure 1.17a shows the transient absorption spectra of PT(c) in n-hexane solution, excited with a femtosecond 365-nm laser pulse. The excitation at 365 nm pumps
PT(c) into the excited state which is ca. 11,000 cm
−1 higher than the relaxed S 1 state.
Immediately after the excitation, an absorption peak appeared at 590 nm with broad
absorption in the entire spectral window of 400–1000 nm. This 590-nm maximum
is ascribable to the absorption of highly excited state (S n state), because it was not