Long-Range Charge Transfer (LRCT)-Driven Enhancement of SOC and rISC in Donor-Modified MR-TADF Emitters

Multi-Resonance Thermally Activated Delayed Fluorescence (MR-TADF) materials are leading candidates for high-efficiency narrowband Organic Light-Emitting Diodes (OLEDs), yet the electronic factors governing their reverse Intersystem Crossing (rISC) rates remain a subject of debate. In this work, we established a computational framework to investigate the factors governing the S1-T1 crossing in donor-modified MR-TADF emitters. By employing Orbital-Optimized DFT (OO-DFT), we achieved accurate predictions of the singlet-triplet gap (ΔEST) in close agreement with experimental data (RMSE = 0.079 eV). Using a specialized Charge Transfer descriptor CTS1T1, we screened 8 candidates exhibiting Long-Range Charge Transfer (LRCT) character during the S1-T1 transition out of a 20-molecule database. All selected candidates show enhanced SOC upon donor modification, with Spin-Orbit Coupling Matrix Element (SOCME) values of up to 1.1 cm-1. Marcus theory was applied to estimate qualitative spin-conversion trends and identify candidates with favorable crossing regimes. MECP analysis of the selected candidates shows how reorganization energy, seam accessibility, and energy-gap matching affect the S1-T1 crossing. ORCA ESD calculations of the elementary radiative rate kr, ISC rate kISC, and Franck-Condon contribution kISCFC were used to assess vibronic participation in the S1-T1 spin-flip process. These findings provide a theoretical perspective on the S1-T1 spin-flip mechanism and offer practical design insights for developing high-efficiency MR-TADF materials with intrinsically fast spin conversion.