STUDY OF THE FATIGUE PROCESS AND THE YELLOWING OF POLYMERIC FILMS CONTAINING SPIROOXAZINE PHOTOCHROMIC COMPOUNDS

Photochromic polyurethane films containing 5-substituted 1,3-dihydro-3,3-dimethyl-1-alkylspiro[2H-indole-2,3'-[3H]pyrido[3,2-f][1,4]benzoxazine] and 1,3-dihydro-1,3,3-trimethylspiro[2H-indole-2,3'-[3H]-naphth-[2,1-b][1,4]oxazine] were degraded under xenon light exposure. The loss of the photochromic response was monitored by spectrophotometry as a function of the irradiation time; meanwhile, the photoproduct formation and the amount of photochromic material still available were analyzed by chromatography after liquid-solid extraction. At the same time, the fade rates were monitored after flash-excitation photolysis or after the photostationary state during degradation. It was clearly demonstrated that the loss of photochromism was due exclusively to a photooxidation process of the initial dye, and not to an acceleration of the bleaching or a potential screen effect caused by the photoproducts.