Photochromic behaviour of Berry Red studied in solution and polymer films

In this work, the photokinetic study of Reversacol Berry Red (2,2-bis(4-methoxyphenyl)-5,6-dimethyl-2H-naphtho[1,2-b]pyran), a photochromic compound from James Robinson Ltd., has been carried out in solution and in polymeric films (Paraloid B-72 and Polyvinyl butyral) in order to quantitatively characterize its photochromic properties by means of objective parameters in different environments.In solution, Berry Red shows reversible and intense photocolouration at room temperature and significant solvatochromic effect in the spectrum of the coloured form. Embedded into polymeric films, Berry Red maintains its thermally reversible photochromic behaviour. However, the photocolouration quantum yield is appreciably reduced and the kinetic parameters of bleaching are significantly altered. The matrix has the effect of lowering both the activation energy (up to a half of the value in solution) and the pre-exponential factor of thermal bleaching (by about seven orders of magnitude). These compensating effects make the bleaching rates in films close to that in solution. The negative values of activation entropy determined in polymer films are related to the rearrangement of the molecule into the activated complex structure.