Photo-induced reaction of dye in polymer media—I. Unannealed polymer matrices

Abstract

The amber Aberchrome 540 dye [( E)-α-(2,5-dimethyl-3-furylethylidene) (isopropylidene) succinic anhydride] in its ring-opened form ( A) when irradiated with u.v. light ( λ > 300 nm) undergoes conrotatory ring-closure to give a red dye ( B) [7,7a-dihydro-2,4,7,7,7a-pentamethylbenzo [b] furan-5,6-dicarboxylic anhydride]. The red form B may be reversed to form A in the photodecolouration reaction using visible light. The kinetics of the photodecolouration of form B in a number of unannealed solid polymer films have been investigated over the range 303–343 K by following the changes in λ max at 493 nm with a spectrophotometer. Analyses of the results have shown that the photodecolouration in various polymer matrices follows a simple first-order reaction. This allows for the extraction of the apparent first-order rate constants and activation energies. The polymer matrices employed are polystyrene, poly( p-t-butylstyrene), poly(methyl methacrylate), poly(n-butyl methacrylate) and poly(vinyl acetate). The rate of the photo-induced reaction of the dye has been found to be very sensitive to changes in the micro-environment of the dye in the polymer matrix such as the polarity, free volume and the molecular motions. This finding is consistent with the results of the dielectric relaxation studies of some of the polymers. A comparison is made between this reaction and the thermally induced isomerization of other dyes in polymer films, e.g. spiropyran derivatives.