Fast and Stable Photochromic Oxazines

We have designed and synthesized two photochromic compounds incorporating fused indoline and
benzooxazine fragments. Variable-temperature 1H NMR spectroscopy demonstrates that their
central [1,3]oxazine ring opens thermally with free energy barriers ranging from 14 to 19 kcal
mol-1. The ring-opened species reverts rapidly to the original isomer and can only be detected after
chemical trapping. Specifically, the nucleophilic attack of a hydroxide anion to the indolium cation
of the ring-opened species prevents re-isomerization. Laser excitation of both compounds induces
the opening of the [1,3]oxazine ring in less than 6 ns with quantum yields up to 0.1. The
photoinduced ring opening generates a 4-nitrophenolate chromophore, which absorbs strongly at
440 nm. The photogenerated species reverts to the original form with a lifetime of 22 ns for both
compounds. Thus, these transformations can be exploited to interconvert the two isomers of each
species with nanosecond switching speeds. Furthermore, thousands of switching cycles can be
repeated consecutively without any sign of degradation, even in the presence of molecular oxygen.
These processes can be reproduced efficiently in poly(methyl methacrylate) matrixes. Under these
conditions, the thermal re-isomerization occurs with biexponential kinetics in submillisecond time
scales. In principle, the fast isomerization kinetics and excellent fatigue resistance of both compounds
offer the opportunity to modulate rapidly and efficiently a variety of molecular and macroscopic
properties. Thus, our molecular design can evolve into the realization of a new family of photochromic
compounds and materials with promising photoresponsive character.