The mechanism for negative photochromism of spiropyran in silica

Abstract

The mechanism for negative photochromism of spiropyran in silica was investigated. Prior to our study, the chemical origin of the high thermal stability of the photomerocyanine form (PMC‐form) dispersed in perhydropolysilazane (PHPS), which is converted to silica at ambient temperature, had been investigated. The high thermal stability of the PMC‐form is attributed to the protonated PMC‐form (H⋅⋅⋅PMC‐form), which is produced by intermolecular hydrogen bonding between oxide anions generated by the cleavage of the C − O bonds and the partially uncondensed Si − OH and O − H bonds of silica. Furthermore, the H⋅⋅⋅PMC‐form could be thermally isomerized from the SP‐form without UV light irradiation. This specific phenomenon is caused by the so‐called negative photochromism. In this study, we proposed a mechanism for negative photochromism according to the relationship of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). The relationship between the HOMOs was determined using cyclic voltammetry (CV) and ultraviolet photoelectron spectroscopy (UPS). On the other hand, the relationship between the LUMOs was determined from the respective optical bandgap. As a result, the HOMO level of H⋅⋅⋅PMC‐form was −6.1 eV and that of SP‐form was −5.3 eV. Accordingly, the thermodynamic stabilization of H⋅⋅⋅PMC‐form was attributed to the thermal isomerization through negative photochromism from the SP‐form. Copyright © 2011 John Wiley & Sons, Ltd.