Cooperative photochemical reaction in molecular crystal induced by intense femtosecond laser excitation: Photochromism of spironaphthooxazine

Photochromism of indolinospironaphthooxazine (SNO) microcrystalline powder was investigated by steady state and time-resolved diffuse reflectance spectroscopy using a femtosecond laser as a light source. When laser intensity is weak, the photoinduced ring-opening reaction in a picosecond time region and subsequent thermal ring-closure one in a nanosecond scale take place in the crystal, not leading to permanent photocoloration of the powder. The same sample shows photochromism upon intense femtosecond laser excitation. The absorption spectra after laser excitation are similar to those of the photocolored form in solution. The spectral shape is independent of the laser fluence, whereas the yield increases nonlinearly with the fluence. To reveal the photocoloration mechanism, we examined the excitation fluence dependence of the transient absorption spectra and elucidated the photocoloration behavior by femtosecond double pulse excitation varying the delay of the two pulses. These experimental results show that cooperative interactions between the excited states and short-lived intermediates are important to generate a long-lived photomerocyanine. Femtosecond induced photocoloration will be discussed in terms of transient and local lattice deformations, and a cooperative photochemical reaction model is proposed.