Deciphering the ESIPT mechanism and AIE behavior of HNBT under controlled polarity gradients: A theoretical study

Combining excited-state intramolecular proton transfer (ESIPT) with aggregation-induced emission (AIE) is an effective strategy for designing fluorescent probes with high selectivity and strong anti-interference capability. Herein, we uncover a polarity and aggregation-regulated ESIPT-TICT-AIE interplay in 2-hydroxynaphthylbenzothiazole (HNBT) using density functional theory (DFT) and time-dependent DFT calculations. Solvent-dependent geometrical and electronic analyses reveal that photoexcitation strengthens intramolecular hydrogen bonding and induces pronounced intramolecular charge transfer, as evidenced by frontier molecular orbital (FMO) and density of states (DOS) analyses. Electrostatic potential (ESP) and dipole moment calculations further demonstrate enhanced charge separation and excited-state polarization in polar solvents, providing a driving force for proton transfer. These effects collectively lower the ESIPT barrier, as confirmed by potential energy curves (PECs), infrared spectra, charge variation balance (CVB), and natural bond orbital (NBO) analyses, while simultaneously promoting twisted intramolecular charge transfer (TICT) characteristics. In water/ACN mixtures, increasing water fraction further strengthens hydrogen bonding and reduces the HOMO-LUMO gap, which modulates the excited-state properties of HNBT. However, aggregation suppresses TICT-related nonradiative decay while enhancing radiative transitions, leading to pronounced AIE emission. Spectral simulations confirm exclusive enol* emission with quenched keto* emission due to the ESIPT-TICT competition. Overall, the emission of HNBT is governed by a dynamic balance between ESIPT-driven proton transfer, TICT-mediated nonradiative decay, and aggregation-induced restriction of intramolecular motion. Solvent polarity stabilizes charge separation and facilitates ESIPT, while aggregation shifts the balance toward radiative decay, enabling strong AIE emission. These findings clarify the subtle solvent-regulated excited-state dynamics of HNBT and provide theoretical guidance for designing highly sensitive, environment-responsive AIE-ESIPT fluorescent probes.