Mechanistic insights into thermally activated delayed fluorescence synergistically governed by ESIPT-induced ICT and reorganization energy.

Excited-state intramolecular proton transfer (ESIPT) can serve as an effective pathway for inducing thermally activated delayed fluorescence (TADF), but the underlying mechanism by which it triggers TADF remains to be fully elucidated. Herein, we explore the photophysical mechanisms of ESIPT-mediated TADF behavior in salicylaldehyde Schiff base derivatives. Theoretical calculations reveal that the key to truly triggering TADF lies not in the favorable ESIPT kinetic barrier but in whether ESIPT further enhances the intramolecular charge transfer (ICT). The synergistic enhancement effect of ESIPT and ICT reduces the singlet-triplet energy gap (ΔEST), thereby facilitating rapid intersystem crossing (ISC) and reverse intersystem crossing (RISC). Furthermore, the reorganization energy (λ) is another crucial factor governing TADF. Even in systems with an extremely small ΔEST, a large λ significantly suppresses the ISC/RISC. These results provide an important molecular design strategy for developing high-performance ESIPT-TADF materials.