Unlocking the Multi-Modal Emission Mechanism of Solid–Liquid Responsive ESIPT and AIE in a Phenol-Derived Molecule: A QM/MM Study

To bridge the gap between aggregation-caused quenching (ACQ) and aggregation-induced emission (AIE), exploring the excited-state intramolecular proton transfer (ESIPT) process and AIE behavior of molecules that exhibit efficient fluorescence emission in both dilute solution and aggregated states is of great significance for the development of next-generation luminescent materials. In this study, we systematically investigated the photophysical behavior of the 2,6-di(benzo[<i>d</i>]thiazol-2-yl)-4-<i>tert</i>-butylphenol (DTP) molecule in various dilute solution environments and in the aggregated state using density functional theory (DFT) and quantum mechanics/molecular mechanics (QM/MM) methods. We confirmed the presence of three distinct emission mechanisms in three different solvents. In highly polar dimethyl sulfoxide (DMSO), the ESIPT process is suppressed by deprotonation, leading to anionic emission. In acetonitrile, only the ESIPT process occurs. In methanol, both deprotonation and ESIPT processes take place simultaneously. AIE phenomena are observed in both concentrated Acetonitrile (ACN)/water mixtures and the solid state. Based on analyses of electron-hole distributions, reorganization energies, and radiative decay rates, we rationalize the origin of the AIE behavior. This study provides valuable insights into the design and development of efficient luminescent materials that combine the advantageous features of both ESIPT and AIE mechanisms.