Theoretical Study on Photoisomerization Effect with a Reversible Nonlinear Optical Switch for Dithiazolylarylene

DFT and TDDFT methods have been performed to investigate the photoisomerization effect for dithiazolylarylene on solution. The weak S center dot center dot center dot N interaction and CH center dot center dot center dot N hydrogen bond restrain the rotation of the side-chain thiazolyl ring in open-isomer la, the higher stability of which prefers to show a high quantum yield of photoisomerization. The calculated UV-Vis spectrum at around 320 nm for open-isomer la is bathochromically shifted to 647 nm for closed-isomer 1b, in excellent agreement with the experimental photochromic phenomenon. The electron transition in ECD (electron circular dichroism) spectra for closed-isomer 1b with two chiral carbon atoms is dominated by ICT (intramolecular charge transition) and LE (local excitation) corresponding to one positive (440 nm) and one negative Cotton effect (650 nm), respectively, where the two chiral carbon atoms play a slight role in these transitions. The PES in the S-1 and S-0 states, respectively, indicates that the cyclization reaction from open-isomer la to closed-isomer 1b is allowed in the photoexcited state with high-conversion quantum efficiency, while it is forbidden in the thermodynamic process. In addition, the second-order nonlinear optical response for closed-isomer 1b is nearly six times larger than that for open-isomer la. It is also confirmed that the photoirradiation evokes the photoisomerization character to show dramatic difference in the second-order NLO response, which can be applied to designing photochromic materials and reversible NLO switches