Fast photochromic sterically hindered benzo[1,3]oxazines

A series of new substituted benzo[1,3]oxazines presenting bulky substituents on the chiral oxazine centre were prepared from isopropyl ketones or substituted cyclohexanones. Laser irradiation of these uncoloured compounds in solution promotes the cleavage of the C–O bond and the opening of the [1,3]oxazine ring generating a zwitterionic species, incorporating a 3H-indolium cation and a 4-nitrophenolate anion, that absorbs strongly at 440 nm. The photogenerated coloured open isomers are thermally unstable and revert to the initial closed form with first order kinetics and lifetimes ranging from 13 to 68 ns. These photochromic switches are extraordinarily stable displaying no significant degradation upon repetition of various irradiation/dark cycles.