Photochromism of spirooxazines I. Investigation of the primary processes in the ring‐opening reaction by picosecond time‐resolved absorption and emission spectroscopy

Abstract The heterolytic bond cleavage between the spiro‐carbon and the neighbouring oxygen is commonly accepted as the primary photochemical step after exciting indolino‐spirooxazines with UV‐photons. The nonplanar intermediate “X” generated in this way is said to relax very fast to a distribution of open forms, which are in structure similar to merocyanines. In this paper, the existence of an intermediate “X” is proven for various derivatives of indolino‐spironaphthoxazines. The build‐up time of these “X”‐forms is generally shorter than 2 ps, and the following reaction to the planar forms takes 2 to 12 ps depending on the nature of the solvent as well as the kind of substituent added to the parent molecule. In contrast to nitro‐substituted compounds the triplet route is found to be negligible in the investigated derivatives.