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 cutting-edge research areas.