AIE mechanism of 2-(2-hydroxyphenyl) benzothiazole derivatives: CASPT2 and spin-flip study

Wang et al. synthesized phenylacetylene modified HBT derivative (HBT- s -Ph) and diphenylacetylene modified HBT derivative (HBT- d -Ph) with aggregation-induced emission (AIE) phenomenon. In this work, we have theoretically compared the photochemical and photophysical processes of HBT- s -Ph and HBT- d -Ph in the gas phase, toluene and solid phase using several theoretical methods. The ground-state, excited-state equilibrium structures, the potential energy curves and the excited-state properties were comparatively calculated for a better understanding of the AIE mechanism. The excited-state intramolecular proton transfer (ESIPT) process within HBT derivatives is very fast in both gas phase, toluene, and solid phase due to the small energy barrier, inducing the formation of Keto tautomer which is responsible for emission peaks observed in experiment in both gas phase, toluene and solid phase. But, the reaction paths are very different from there. In toluene, the non-radiative path via HBT intramolecular rotation along the C–C bond is favorable to occur, leading to low Φ f of HBT- s -Ph and HBT- d -Ph in toluene. In contrast, the non-radiative path of HBT intramolecular rotation is strongly suppressed in solid phase due to the high energy barriers, which is responsible for high Φ f of HBT- s -Ph and HBT- d -Ph in solid phase. The substitution effect on AIE and the large difference of Φ f in HBT- s -Ph and HBT- d -Ph in solid phase have been revealed. The presents results contribute to a better understanding of the different photophysical properties of HBT derivatives in toluene and solid phase, and unveil the AIE mechanism in HBT derivatives, which provides deep insight into molecular design of organic photoelectric materials with high Φ f . • The AIE phenomenon in HBT- s -Ph and HBT- d -Ph is probably induced by the RACI. • The higher Φ f in HBT- d -Ph is found to be related with low exciton coupling, high HR factors in solvent and low possibility of intersystem crossing between singlet state and triplet states.