Effects of the changes in the properties of silica cage along the gel/xerogel transition on the photochromic behavior of trapped spiropyrans

The photochromism of a series of spiropyrans trapped in a polymerizing silicon tetramethoxide system (the "sol/gel process") was studied as a function of the reaction time. The continuous environmental changes along the gel/xerogel transition are reflected in the gradual changes of the photochromic behavior of the trapped molecule. In particular, the photochromism changes at a certain point to reversed photochromism. Hammett analysis showed that the reversal point is sensitive to substituent effects. The spectra and photochromic changes are interpreted as reflecting a gradual shift in the equilibrium between the adsorbed and dissolved molecule within the cage, enriching the system with the former as the reaction proceeds. This shift is caused by the change in the composition of the cage-trapped solvent from methanol/water to water, by solvent evaporation, and by the gradual shrinkage in the cage size. We attribute the structure known as isomer "X" to the colored species at the reversal photochromism stage.