Tuning Relaxation Pathways And Aggregate-State Fluorescence Properties Of 2-(2-Hydroxyphenyl)Benzothiazole Derivatives By Switching Position Of Electron-Withdrawing Substituent

Excited-state intramolecular charge transfer (ESICT) and excited-state intramolecular proton transfer (ESIPT) can occur either concertedly or sequentially, or even independently, thus harnessing the steady-state fluorescence features of a molecule. In this study, two 4-nitrostyryl-modified 2-(2-hydroxyphenyl)benzothiazole (HBT) isomers (o-NO2-HBT and m-NO2-HBT) are rationally designed, in which the 4-nitrostyryl group is located ortho and meso, respectively, to the phenolic hydroxyl. Unlike the para-isomer (p-NO2-HBT), which exhibits dual emission [solvatochromicenol-emission (EICT emission) and non-solvatochromic keto-emission (K-emission) based on a sequential ESICT-ESIPT process and aggregation-induced emission enhancement (AIEE) property, o-NO2-HBT shows only non-solvatochromic K-emission and anti-aggregation-caused quenching (ACQ), while m-NO2-HBT demonstrates EICT emission and ACQ. Theoretical calculations reveal that the substituent position and the resultant dipole moment induce differences in the relative energies of EICT* and K* and in the energy barriers for the forward and reverse ESIPT processes. Furthermore, based on the elucidated emission mechanisms, a red-emitting ESIPT-active AIEgen (o-CN-CF3-HBT), with a solid-state fluorescence efficacy of 41.0%, is strategically designed and successfully applied in lipid droplet imaging. Therefore, the results of this study provide important insights into designing high-performance AIEgens for future applications in various cuttingedge research areas.