Spectroscopy and Mechanisms of the Photo- and Thermal Reactions of Photochromic Anils

The principal emission from the salicylideneanilines, salicyclidene‐2‐aminoanthracene, and 2‐hydroxynaphthylideneaniline is assigned as a fluorescence originating from a $\mathrm{\pi ,\; \pi *}$ singlet state of a short‐lived intermediate, cis^†. The cis^† arises as the first product of the photochemistry and is a species in which proton transfer has occurred but essentially retains the geometry of the enol tautomer. The cis^† species, in the salicylideneanilines and salicylidene‐2‐aminoanthracene, converts to the final photocolored product. This final product is a nonequilibrium form of the true cis–keto tautomer. The latter is produced from the photocolored species by a thermal reaction. The cis–keto tautomer cannot be produced by a photoreaction from either the enol or the photocolored species. However, for the 2‐hydroxy‐naphthylideneaniline, the cis–keto tautomer is the final photocolored product from the enol. No photocolored products can be produced from the nitrosalicylideneanilines. Viscosity barriers are present that affect both the forward and reverse photochemical reactions for all molecules considered.