Promoting Reverse Intersystem Crossing in Thermally Activated Delayed Fluorescence via the Heavy-Atom Effect
Katsuyuki Shizu, Yongxia Ren, Hironori Kaji
Abstract
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 ( k RISC ) 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, we reproduced all the relevant rate constants of a sulfur-containing TADF molecule (with k RISC of 10 8 s –1 ) 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 k RISC (>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 a too strong heavy-atom effect is unfavorable for achieving both rapid RISC and efficient TADF.