Photochromic behaviour of a spirobenzopyran chemisorbed on a colloidal silica surface

The thermal decolouration of the photoformed merocyanine (MC) of a nitrospirobenzopyran attached onto a colloidal silica surface through two different methylene spacers [-(CH2)n–, n= 2 or 10] has been examined in ethylene glycol, chloroform, carbon tetrachloride and cyclohexane. In ethylene glycol, the visible absorption band of the MC diminished slowly without any change in the spectral shape, and the bleaching reaction was well fitted by first-order kinetics, suggesting that the reaction proceeded in a constant micropolarity without any steric constraints. Elongation of the methylene spacer from C2 to C10 caused an increase in the thermal fading rate. In other water-immiscible solvents, on the other hand, a blue shift of the absorption maximum occurred during the thermal bleaching and the reaction was properly described by the superposition of two first-order rate processes. The blue shift and the bimodal first-order kinetics have been interpreted as a partitioning of the MC moiety between two distinctive residential sites with different polarities: an inner region with the higher polarity facing an adsorbed water layer lying on the silica surface and an outer region exposed to a organic solvent phase.