Light-Induced Ring-Closing Dynamics of a Hydrogen-Bonded Adduct of Benzo[1,3]oxazine in Protic Solvents

Abstract

Benzo[1,3]oxazine, an organic optical switching compound, is known to be in an equilibrium between its closed form (OX) and its open zwitterionic form (IN). Here we report a light-induced ring-closing mechanism of a hydrogen-bonded adduct (p-IN, partially protonated open isomer of benzo[1,3]oxazine) based on the observations of femtosecond and nanosecond transient absorption spectra in protic solvents. Femtosecond transient measurements upon visible excitation reveal the appearance of two states having different dynamical signatures. One corresponds to a conventional intramolecular charge transfer excited state. The other one is a concerted electron–proton transfer product (d-IN, embedded solvent molecule released from p-IN). Without steric hindrance, the main molecular structure tends to be planar upon excitation, and two intermediates, IN and OX, are involved in the sequential thermal transformation before the return to the ground state of p-IN. Specifically, in alcoholic solvents, d-IN converts to the original p-IN compound within 1 ms via the dominant pathway d-IN → IN → OX → p-IN and the side pathway d-IN → IN → p-IN, which is found to be feasible in energy; in contrast, in aqueous solution with increasing strength of intermolecular hydrogen bonding, the rate of the thermal transformation is enhanced by 1 order of magnitude.