Theoretical Study of the Photochemistry of a Reversible Three-State Bis-Thiaxanthylidene Molecular Switch

The ground- and the lowest singlet excited-state potential energy surfaces of the bis-thiaxanthylidene (3) molecular switch are investigated using a density functional method specifically designed to treat molecular systems typified by strong non-dynamic electron correlation. The results of the theoretical calculations suggest that the unique ability of substituted bis-thiaxanthylidenes to switch between three states of luminescencenon-fluorescent state, blue fluorescent state, and red fluorescent statecan be explained by specific features on the excited state potential energy surface: the potential barrier around the FranckCondon point of the anti-folded conformer and the existence of conical intersection in the vicinity of the syn-folded conformer. It is suggested that the twisted conformer, if made more stable via chemical modification, should fluoresce in the near-infrared region (gimel approximate to 740760 nm), thus offering a possibility for a four-state switching of luminescence in a single-component molecular system.