Theoretical study on the modulation of ESIPT and AIE synergistic mechanisms in α-CAS fluorescent probe by binary solvents with contrasting polarity

Integrating Excited-State Intramolecular Proton Transfer (ESIPT) and Aggregation-Induced Emission (AIE) mechanism is central to achieving high sensitivity in luminescent materials. Based on Chen et al.'s experimental study, this work theoretically investigates the ESIPT and AIE properties of the α-CAS molecule in two mixed solvent systems (water/tetrahydrofuran (THF) and n-hexane/THF), revealing the mechanism by which the solvent environment regulates different AIE emission behavior. Analyses of potential energy curves (PECs), infrared spectra (IR), and radiative decay rates (Kr) confirmed the occurrence of ESIPT and pronounced AIE characteristics. Furthermore, electron–hole distribution analysis revealed a twisted intramolecular charge transfer (TICT) process, indicating that the luminescence of α-CAS is governed by the combined effects of multiple photophysical mechanisms. Notably, in the water/THF system, increasing the water content gradually enhances the polarity of the binary solution, thereby promoting the ESIPT and TICT processes. This leads to pronounced AIE behavior characterized by dominant enol* state fluorescence emission. In contrast, in the n-hexane/THF system, a rising n-hexane proportion gradually reduces solution polarity, suppressing ESIPT and TICT processes and resulting in AIE emission primarily driven by the keto* state. These results provide critical insights into the solvent environment's regulatory role in modulating the ESIPT, TICT, and AIE behaviors of α-CAS, establishing a robust theoretical framework for the design of multi-mode responsive fluorescent probes and highlighting their significant potential for diverse applications.