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

Abstract

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.