Photochromism and Thermochromism of some Spirooxazines and Naphthopyrans in the Solid State and in Polymeric Film

In this work the photochromic behaviors of four Reversacol compounds from James Robinson Ltd., previously investigated in solution, have been studied in a microcrystalline solid phase and embedded in a poly(methyl methacrylate) matrix (PMMA). The compounds studied belong to the classes of spirooxazines and naphthopyrans. In solution, they showed photocoloration and thermocoloration. Embedded into polymeric films, as well as in the microcrystalline phase, they have maintained their photochromic and thermochromic behavior. The photocoloration quantum yields in PMMA were determined and found to be fairly high. Compared to solution, the kinetic and thermodynamic activation parameters of thermal bleaching significantly changed. The solid environment has the effect of slowing down the bleaching rates in films as well in the pure crystalline phase and introduces multiexponential terms in the photocoloration and thermal bleaching kinetics. Activation enthalpy and entropy decrease, compared to solution, leaving the activation free energy substantially unchanged. Thermochromism was characterized through the equilibrium constant and the thermodynamic standard parameters, ΔH0, ΔS0, and ΔG0, of the thermal reactions. Time-dependent processes were also analyzed according to the maximum entropy method (MEM). Based on the data from the least-squares and MEM treatments, the occurrence of multiexponential kinetics in polymers was attributed to the effects of the inhomogeneous distribution of free volume in the matrix.