Reverse photochromism of spiropyran in silica

The high thermal stability of the photomerocyanine-form (PMC-form) of spiropyran (SP) dispersed in perhydropolysilazane (PHPS), which converted to silica at ambient temperature, was investigated. PHPS was converted to silica by the de-ammonium reaction with water vapor within approximately 48 h. The structure of the converted silica was not only SiO2, but also partially uncondensed Si–OH and O–H. The PMC-form with high thermal stability was attributed to the protonated form, which was produced by intermolecular hydrogen bonding between oxide anion generated by cleavage of C–O bonds and the partially uncondensed Si–OH and O–H of silica. The protonated PMC-form was directly generated from the SP-form without UV light irradiation, because the singlet ground state of the PMC-form (PMC0) is lower than that of the SP-form (SP0) in silica. In this mechanism, the singlet ground state of the protonated PMC-form (H⋯PMC0), which is significantly stabilized by silica, had polarity similar to water. The stabilization of the PMC-form was attributed to hydrogen bonding between silica and direct generation from the SP-form by reverse photochromism.