Effect of gel-trapping on spectral properties and relaxation dynamics of some spiro-oxazines

The photochromism of three spiro-oxazines was investigated in a gel network (AOT/isooctane/animal gelatin/water), which is transparent to the visible and near UV light, as well as in a microemulsion (AOT/toluene/water) and in the hydrocarbon component of the microheterogeneous gelled system. The spectral shifts of the photomerocyanine color band, which were determined in different media, provided information on the location of the molecules in these microheterogeneous systems. By a photokinetic study of the color-forming and color-fading reactions, the photocoloration quantum yields and bleaching rate constants were determined. By comparing the results obtained in a gel network with those determined in a homogeneous solution, it was observed that for two of the compounds investigated the photocolorability markedly decreases in the gel as a consequence of hastening of the bleaching processes. For the third molecule (an anthraquinone derivative), the colorability greatly increases in a gel network. The different behaviors are explained in terms of the ground-state polarity properties of the photomerocyanines and the consequent solute/solvent interaction effects on the dynamics of thermal bleaching.