Solid-state materials containing photochromic dithienylethene derivatives were prepared by the sol-gel process in the form of thin films of a few mu m-thick. The effect of substituents (pyridyl or methoxyphenyl) of the optically active molecules on photochromic properties was investigated. Thicknesses and refractive indices either at 633 nm or 785 nm were determined in the colored and in the discolored state by using the Attenuated Total Reflexion method. Sol-gel films containing methoxyphenyl substituted molecules show a large refractive index change, Delta n approximate to 3.10(-3) at 785 nm. This variation is important considering the low doping level (0.7, wt%).