The thermoreversible photochromic reaction spiro-indoline-oxazine reversible arrow photomerocyanine was photosensitized by triplet energy transfer 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 photosensitized 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 concentration on the reaction yield. The results obtained for six photochromes (spiro-indoline-oxazines differently substituted in the indoline and/or oxazine moiety) show that energy transfer occurs at a diffusion-controlled rate and that the limiting quantum yield of the sensitized reaction, approximately unity, is greater than the yield obtained by direct excitation of the photochrome. An upper limit to the triplet energies of the spiro-oxazines was established. Direct evidence for the tripler state was only obtained for a nitro-spiro-oxazine from low temperature phosphorescence emission of both the closed and open forms.
