Bichromophoric photochromes based on the opening and closing of a single oxazine ring

We have designed bichromophoric photochromes based on the photoinduced opening and thermal closing of a [1,3]oxazine ring. In particular, we have synthesized six compounds incorporating fused 3H-indole and 4-nitrophenoxy fragments and pendant biphenyl, styryl, biphenylvinyl, or stilbenylvinyl groups. The laser excitation of two of these six molecules cleaves a [C-O] bond and opens their [1,3]oxazine ring in less than 6 ns with quantum yields of 0.08 and 0.28, respectively. This photoinduced process generates simultaneously a 4-nitrophenolate anion and a 3H-indolium cation. Both chromophores absorb in the same region of the electromagnetic spectrum. As a result, an intense band appears at ca. 440 nm upon the photoinduced opening of the [1,3]oxazine ring. In both instances, the photogenerated species switches back to the original isomer with first-order kinetics and lifetimes of 38 and 140 ns, respectively. Both compounds have excellent fatigue resistances and retain their photochemical behavior within rigid poly(methyl methacrylate) matrices. However, the thermal reisomerization within the polymer matrix is significantly slower and requires several microseconds to occur. The other four compounds do not undergo ring opening upon excitation under otherwise identical experimental conditions. Indeed, either photoinduced electron transfer or intersystem crossing compete successfully with the ring-opening process.