PHOTOSENSITIZATION OF PHOTOCHROMISM OF SPIRO-INDOLINE-OXAZINES BY CAMPHORQUINONE

The thermoreversible photochromic reaction spiro-indoline-oxazine reve rsible arrow photomerocyanine was photosensitized by triplet energy tr ansfer using camphorquinone (E(T) = 215 kJ mol(-1)) as sensitizer and visible light excitation (lambda(exc) = 436 nm). From absorbance/time data, obtained during steady irradiation, the quantum yield of the pho tosensitized reaction was determined. The rate parameter For the donor -acceptor interaction was obtained from quenching measurements of the sensitizer phosphorescence and from the effect of the acceptor concent ration on the reaction yield. The results obtained for six photochrome s (spiro-indoline-oxazines differently substituted in the indoline and /or oxazine moiety) show that energy transfer occurs at a diffusion-co ntrolled rate and that the limiting quantum yield of the sensitized re action, approximately unity, is greater than the yield obtained by dir ect excitation of the photochrome. An upper limit to the triplet energ ies of the spiro-oxazines was established. Direct evidence for the tri pler state was only obtained for a nitro-spiro-oxazine from low temper ature phosphorescence emission of both the closed and open forms.