Promoting Reverse Intersystem Crossing in Thermally Activated Delayed Fluorescence via Heavy-Atom Effect

Thermally activated delayed fluorescence (TADF) molecules are promising for realizing durable organic light-emitting diodes in all color regions. Fast reverse intersystem crossing (RISC) is a way of improving the device lifetime of TADF-based organic light-emitting diodes. To date, RISC rate constants (kRISC) of 10^8 s−1 have been reported for metal-free TADF molecules. Here, we report the heavy-atom effect on TADF and a molecular design for further promoting RISC. First, the RISC mechanism of a sulfur-containing TADF molecule (with kRISC of 10^8 s−1) was comprehensively investigated via density functional theory. The role of the heavy-atom effect on the rapid RISC process was clarified. Our calculations also predicted that much larger kRISC (>10^10 s−1) will be obtained for selenium- and tellurium-containing TADF molecules. However, a polonium-containing molecule promotes phosphorescence without exhibiting TADF, indicating that too strong heavy-atom effect is unfavorable for achieving both rapid RISC and efficient TADF.