Dearomatization of Benzenoid Arenes Triggered by Triplet Excited State Intramolecular Proton Transfer.

The detailed mechanism of photoinduced dearomatization of benzenoid arenes is investigated using both the high-level ab initio method and density functional theory. The results suggest that the optically allowed singlet excited state (S 2 ) can quickly decay to the lowest triplet excited state (T 1 ) through a barrierless internal conversion and intersystem crossing. Importantly, we find a triplet excited state intramolecular proton transfer (T-ESIPT) pathway to produce a diradical triplet intermediate ( 3 M O-H ), which can trigger the subsequent [4 + 2] dearomatization reaction. Furthermore, the diastereoselectivity of the reaction was illustrated by the rotation of the O - H group of 3 M O-H , which could be effectively modulated by the solvent effect (arising from the strength of the intermolecular hydrogen bond) and the substituted effect (arising from the strength of the electron-donation group). This photochemical mechanism can explain well the experimental observations, and the novel T-ESIPT process can open a new door in studying the photoinduced proton transfer reactions.