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 center dot center dot center dot 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. (C) 2010 Elsevier B.V. All rights reserved.