TADF Regulation by Tuning Keto‐Energy Levels, ISC/RISC Equilibrium in Coordination Polymers with Excited State Intramolecular Proton Transfer (ESIPT)

Abstract Thermally activated delayed fluorescence (TADF) and long persistent luminescence (LPL) are emerging photophysical hotspots, which involve the singlet/triplet excited states and two‐way energy transfer processes between them via intersystem crossing (ISC) or reverse intersystem crossing (RISC). Herein, a new tactic is reported for the regulation of keto‐energy levels and ISC/RISC equilibrium via the introduction of electron‐withdrawing fluorine in a series of excited state intramolecular proton transfer (ESIPT) ligands and corresponding Cd(II) coordination polymers. Among them, the fluorine p‐substituted to the hydroxy group along the nodal plane on the phenol site improves the thermodynamic stability of the keto‐isomers of the ligand, thereby lowering the keto‐energy level and promoting the ESIPT process effectively. Uniquely, the lower‐energy keto excited state makes the ISC/RISC equilibrium shift to the direction of RISC progress, and thus arousing efficient TADF in the constructed coordination polymer. A comprehensive transient photophysical and theoretical study verifies the greatly promoted RISC progress, rather than suppressed ISC is aroused in the p‐substituted system, suggesting a clear‐cut strategy in the design of coordination polymers with good TADF performance.