EFFECT OF SUBSTITUENTS ON THE PHOTOCHROMISM OF A SPIRO[INDOLINE-NAPHTHOXAZINE] UNDER LASER EXCITATION

Spirooxazines have been extensively investigated in the last few years because of their remarkable photostability. Laser flash photolysis wa s used to study the mechanism of the photocoloration reaction of a spi ro [indoline-naphthoxazine] substituted either with an electron-withdr awing group, the 8'-nitro derivative (compound I), or with electron-do nating groups, the 6'-morpholino and 6'-piperidino derivatives (compou nds II and III), in toluene solution at 297 K; the unsubstituted compo und (IV) was also studied for comparison. The photomerocyanine is form ed 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 hom ologous nitroindolinospiropyrans. In the case of electron-donating com pounds (II and III) the photomerocyanine is formed exclusively via an excited singlet pathway; however, the quantum yields of photocoloratio n are much higher for compounds II and III (0.5) than for the unsubsti tuted one (0.2), but lower than for the nitro derivative.