A new family of photochromic compounds based on the photoinduced opening and thermal closing of [1,3]oxazine rings

We have developed a new family of photochromic compounds with fast switching speeds and outstanding fatigue resistances. Their molecular skeleton combines a 3H-indole heterocycle and a phenoxy fragment to form a [1,3]oxazine core, which can be assembled in two synthetic steps starting from commercial reagents. Following this general synthetic strategy, we have prepared ten compounds differing in the substituent (R¹) on the chiral center of their [1,3]oxazine ring, the group (R²) in the para position, relative to the oxygen atom, of their phenoxy fragment and/or the substituent (R³) in the para position, relative to the nitrogen atom, on the 3H-indole heterocycle. The [1,3]oxazine ring of four of these compounds opens in less than 6 ns, upon laser excitation, with quantum yields ranging from 0.08 to 0.28. The photoinduced ring-opening process generates zwitterionic isomers, incorporating a 3H-indolium cation and a phenolate anion. These two fragments can be designed independently to absorb in the visible region by regulating the nature of the substituents R¹ and R². The photogenerated isomers revert thermally to the original species with first-order kinetics and lifetimes ranging from 25 to 140 ns. In fact, a complete switching cycle can be completed on a nanosecond timescale with these photochromic compounds. Furthermore, their photoinduced isomerization is not accompanied by degradation and they tolerate thousand of switching cycles in air even in the presence of molecular oxygen. Thus, our novel photochromic compounds can, in principle, evolve into innovative photonic materials with unprecedented performance.