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 (T_m) 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 T_m 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.