Effect of substituents on the photochromism of a spiro[indoline-napthoxazine] under laser excitation

Abstract

Spirooxazines have been extensively investigated in the last few years because of their remarkable photostability. Laser flash photolysis was used to study the mechanisms of the photocoloration reaction of a spiro[indoline-naphthoxazine] substituted either with an electron-withdrawing group, the 8′-nitro derivative (compound I), or with electron-donating groups, the 6′-morpholino and 6′-piperidino derivatives (compounds II and III), in toluene solution at 297 K; the unsubstituted compound ( IV) was also studied for comparison. The photomerocyanine is formed in the photocoloration process via a triplet pathway in the case of I with a high quantum yield (0.7), as was previously observed for homologous nitroindolinospiropyrans. In the case of electron-donating compounds ( II and III) the photomerocyanine is formed exclusively via an excited singlet pathway; however, the quantum yields of photocoloration are much higher for compounds II and III (0.5) than for the unsubstituted one (0.2), but lower than for nitro derivative.