Reverse Intersystem Crossing Rates in All-organic and Heavy-metal containing Thermally Activated Delayed Fluorescent molecules A Quantum Structure Approach

Full understanding of the mechanism of Thermally Activated Delayed Fluorescence (TADF) process, we studied optical kinetic properties of a series of carbazole based all-organic molecules, and gold-centered carbene-metal-amides, applying Density Functional Theory. We have calculated spin orbit coupling matrix elements (SOCME), reverse intersystem crossing rates (k_RISC) and energy difference between singlet and triplet states (∆E_ST). The calculated k_RISC values are in good agreement with available experimental data. Our findings emphasize that to enable reverse intersystem crossing and eventually TADF, large SOCME and minimal ∆E_ST values must be obtained simultaneously. We also find that the obtained k_RISC values are higher where ∆E_ST values are closer to reorganization energy. The k_RISC of carbene-metal-amides is more than five orders of magnitude higher than that of carbazole based molecules. A small change in the absorption peak of optical absorption spectra with and without spin orbit coupling (SOC) is observed for carbazole-based emitters whereas the carbene-metal-amides show a significant red shift in optical absorption spectra. This result indicates the role of spin-orbit coupling induced by heavy metal.