Theoretical Characterizations of TADF Materials: Roles of ΔG and the Singlet–Triplet Excited States Interconversion

The thermally activated delayed fluorescence (TADF) phenomenon has attracted increasing attention because it can harvest 100% of the electro-pumped carriers to form singlet bound excited state for fluorescence. It is generally believed that the small energy gap between S₁ and T₁ (ΔE_ST) is essential for TADF to facilitate the reverse intersystem crossing (rISC). However, for a few donor–acceptor (D–A) organic compounds with small ΔE_ST, the TADF phenomenon is absent, indicating that ΔE_ST might not be a good molecular descriptor. Here, using our self-developed thermal vibration correlation function (TVCF) formalism in combination with quantum chemistry calculations, we revisit the key factors that dominate the TADF property for 11 D–A systems with small ΔE_ST. Based on our theoretical results in comparison to experiments, we conclude that the activation energy ΔG is a good molecular descriptor to characterize the TADF performance because a significantly better linear relationship is observed between ΔG and the rISC rate constant (k_rISC) compared to that between ΔE_ST and k_rISC. These findings provide deeper understanding of the TADF mechanism, shedding light on the molecular design of high-performance TADF materials.