Computational insights on the isomerization of photochromic oxazines

We investigated the isomerization of the simplest member of a family of photochromic oxazines with the aid of density functional theory, using three different functionals. Specifically, we simulated the thermal interconversion of the two enantiomers, associated with this compound, and established that the opening of the oxazine ring dictates the rate of the overall degenerate process. The M062X functional provides the best match to experimental data, whereas B3LYP calculations fail to model accurately the ground-state potential-energy surface of this system. In addition, we also modeled the absorption spectra of this compound and its photogenerated isomer with time-dependent calculations. The resulting data support the original assignment of the experimental spectra and confirm that the oxazine ring opens upon excitation. The MPW1PW91 functional provides the best match to experimental data, whereas M062X calculations fail to model accurately the spectroscopic parameters of this particular system. Furthermore, the MPW1PW91 calculations demonstrate that the photoinduced opening of the oxazine ring occurs along the potential-energy surface of the first triplet excited state. Indeed, the photoinduced isomerization appears to involve: (1) the initial excitation of one isomer to the second singlet excited state, (2) its thermal relaxation to the first triplet excited state, (3) its ring opening to produce the other isomer, and (4) the thermal relaxation of the product to the ground state. Thus, our calculations provide valuable information on the elementary steps governing the isomerization of this particular photochromic compound in the ground state and upon excitation. These useful mechanistic insights can guide the design of novel members of this family of photoresponsive compounds with specific properties.