Predicting intersystem crossing efficiencies of organic molecules for efficient thermally activated delayed fluorescence

Efficient intersystem crossing (ISC) and reverse ISC (RISC) are of central importance in thermally activated delayed fluorescence (TADF) materials to harvest both singlet and triplet excitons for electroluminance with theoretically 100% internal quantum efficiency. However, facile and accurate prediction of the ISC and RISC rates of TADF molecules in donor-acceptor architectures is challenging. Herein, we investigated five theoretical methods in simulating the ISC and RISC processes based on six typical TADF molecules. Both the singlet-triplet energy splitting (ΔE_ST) and spin-orbit coupling (SOC) were evaluated at the density functional theory (DFT) and time-dependent DFT levels using frontier orbital energy levels and overlap extents, n-orbital composition and electronic density transition analyses, and natural transition orbital (NTO) studies on the ground or excited state structures. Compared to the experimental results, NTO and n-orbital analyses based on the configuration of the lowest triplet excited state were found to be the most applicable approaches in predicting credible ISC and RISC rates in low computational costs, offering facile and reliable approaches to computational screening for high-performance TADF materials. This work could provide important clues for not only the fundamental understandings of facilitating ISC and RISC processes but also efficient computational methods in predicting high-performance TADF materials.