Ultrafast switching of photochromic oxazine: A theoretical explanation

We have shown that ultrafast switching of photochromic oxazine (PCOX) can occur via a singlet mechanism. Previously, only a triplet model of PCOX photoconversion was proposed by other authors. According to our TDDFT/DFT calculations, PCOX undergoes a photoinduced S0 -> S3 electron transition. As a result, the electron density between the C2 and O1 atoms decreases and with the participation of thermal motion, the bond between them is broken. In the resulting excited open form of PCOX, an intramolecular stacking excimer is formed. During its relaxation to the ground state, the stacking is preserved, preventing the rotation of the aromatic systems around the alkyl linker connecting them and thus facilitating a very rapid recovery of the C2-O1 bond. Therefore, the photochemical pathway of PCOX can be represented as the following scheme: Closed(S0) -> Closed(S3) -> Open stacked(S3) -> Open stacked(S0) -> Closed(S0).