Temperature dependence of the photochromism of naphthopyrans in functionalized sol–gel thin films

A photochromic naphthopyran derivative, 3-(2,4-dimethoxyphenyl)-3-(4-methoxyphenyl)-3H-naphtho[2,1-b]pyran, was embedded in sol–gel prepared phenyl (Ph) and pentafluorophenyl (pFPh) functionalized silica thin films. The functionalization of the matrix surface and the temperature both play important roles in the photochromic properties of the dye molecules as they affect their interaction with the embedding ormosil matrix and, therefore, the equilibrium between coloured and colourless forms of the molecule. The bleaching kinetics of the dye embedded in these ormosil matrices were strongly accelerated by temperature in all samples, particularly in pFPh functionalized matrices, due to the reduced mobility of the molecules at low temperatures. The usage of different amounts of organic functional substituents in the matrix has an important effect on the size of the pores where the photochromic molecules are located affecting the steric hindrance of the mechanism of formation of the coloured forms. The ability to control the dye–matrix interactions allows the design of materials with defined photochromic properties.