Engineering Fast Reverse Intersystem Crossing and Radiative Decay in [1,2,4]Triazolo[1,5-a]pyridine-Based TADF Emitters via Donor Strength and Positional Control of Dihedral Angle and Reorganization Energy: A Density Functional Theory Study.

Achieving a balance between the radiative decay rate (kr) and the reverse intersystem-crossing (rISC) rate (krISC) persists as a fundamental hurdle in thermally activated delayed-fluorescence (TADF) emitters because the processes depend oppositely on the overlap of frontier molecular orbitals. We computationally designed and explored 30 [1,2,4]triazolo[1,5-a]pyridine (TP)-based donor-π-acceptor emitters divided into two molecular series, 2,6TP and 2,7TP, according to the site for substituting the donor-appended phenyl linker on the TP acceptor core. The relationships among the donor-acceptor dihedral angle (θDA), singlet-triplet energy gap (ΔEST), spin-orbit coupling (SOC), radiative decay rate (kr), reverse intersystem crossing rates (krISC), reorganization energy (λ), and activation energy (ΔGrISC) governing the excited-state dynamics were systematically analyzed. The ortho-substituted dimethylacridine-based emitters exhibit nearly optimal dihedral angles (∼77°-78°), resulting in low reorganization energies (λ ≈ 0.22-0.30 eV), small ΔEST values (∼0.05 eV), moderate SOC matrix elements (∼0.3-0.4 cm-1), enhanced kr values (>105 s-1), and krISC values exceeding 106 s-1, together with low activation barriers for rISC (ΔGrISC ≈ 0.08-0.10 eV). The results further demonstrate that ΔGrISC is governed strongly by λ in addition to ΔEST, indicating that minimizing ΔEST alone is insufficient to ensure efficient TADF performance. Overall, this study offers a comprehensive design framework for developing high-performance TP-based TADF emitters for potential organic light-emitting diode applications through synergistic optimization of molecular geometry, reorganization energy, and activation energy.