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

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. A new tactic is applied for the regulation of keto-energy levels and intersystem crossing or reverse intersystem crossing (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. image