Insights into High-Level Reverse Intersystem Crossing in a Multiresonance Thermally Activated Delayed Fluorescent Compound

Multiresonance thermally activated delayed fluorescence (MR-TADF) materials have recently garnered substantial interest owing to their narrow emission bands, near-unity photoluminescence quantum yields, and high external quantum efficiencies of fabricated light-emitting diodes. A key question, however, is whether higher-lying triplet states (Tm) can facilitate hot-exciton harvesting through reverse intersystem crossing (hRISC), a process that is believed to result in delayed fluorescence and an increase in triplet exciton utilization in MR-TADF. Here, we critically assess the MR-TADF emitter 2PTZBN by combining ultrafast transient absorption spectroscopy, streak camera measurements, and density functional theory calculations. Despite theoretical indications of appreciable spin–orbit coupling between higher-lying electronic states, our experimental data reveal that the hRISC channel is upstaged by strong internal conversion (IC) from Tm to T1. Quantitative analysis places the upper limit of hRISC efficiency at ∼3%, indicating the dominant role of nonradiative losses in these MR-TADF molecules. These findings challenge the assumption that MR-TADF systems inherently realize efficient hot-exciton harvesting and point to the need for different structural modifications to suppress IC and enhance hRISC for MR-TADF molecules.