Photochromism and thermochromism of phenanthrospirooxazine in poly(alkyl methacrylates)

The decay of the photomerocyanine (MC) formed after flash irradiation of a phenanthrospirooxazine (SO) was studied in polymer films of poly(methyl-), poly(ethyl-), and poly(isobutyl methacrylate) and compared with similar data of a benzospiropyran (SP). The decays in the dark depart from monoexponential behavior. The data were analyzed by a Gaussian distribution model and by a matrix relaxation model. The rate constant at the center of the Gaussian distribution is equal in all polymers and equal to the unimolecular rate constant reported in nonpolar liquids. This may be related to the inability of the polymer to solvate the transition state. The polymer matrix affects the width of the activation energy distribution; this distribution is broader in more rigid media. The relaxation model is able to provide a mean value for the relaxation time of the matrix, which is relevant for the decay. The Arrhenius activation energy of this relaxation time is similar in each polymer for the MC forms of SO and of SP and is similar to the activation energy of the β-relaxation of the polymers. This indicates that local movements are responsible for the relaxation, which also explains the fact that all kinetic parameters show an Arrhenius-type behavior in temperature ranges including the glass transition temperature. The thermochromism of SO → MC transformation was studied in poly(methyl-) and in poly(ethyl methacrylate). The equilibrium constants depart from the normal behavior in fluid media. This departure is interpreted by assuming a Gaussian distribution of energy difference between the two forms of SO. The widths of the distributions obtained from kinetic and equilibrium data are very similar.