4 Stepwise Two-Photon Photochromism
61
Fig. 4.2 a Normalized transient absorption spectra of ImD in benzene excited at 355 nm of the
nanosecond laser pulse, and that of ImD-ZnTPP in benzene excited at 425 and 550 nm of the
nanosecond laser pulse at 20 ms. b Normalized transient absorption spectra of ImD-ZnTPP excited
at weak (blue line) and intense (red line) pump powers with a 532-nm picosecond laser pulse at 300 ps
and the differential spectrum between the normalized spectra (bottom). c Plausible photochromic
reaction pathway of ImD-ZnTPP upon excitation with visible light [18]
moiety. The time constant of the spectral evolution is 2.0 ns, which is almost the
same as the fluorescence lifetime of free ZnTPP. Therefore, no effective quenching
of the ZnTPP moiety took place in ImD-ZnTPP in the fluorescent state, and no
photochromic reactions occured under the weak visible light excitation.
On the other hand, the transient absorption spectrum of ImD-ZnTPP in benzene
under the intense excitation condition (1.2 mJ mm
−2 ) is quite different from that
under the weak excitation condition (Fig. 4.2b). The differential spectrum between