Theoretical Characterizations of TADF Materials: Roles of Δ 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 S and T (Δ) is essential for TADF to facilitate the reverse intersystem crossing (rISC). However, for a few donor-acceptor (D-A) organic compounds with small Δ, the TADF phenomenon is absent, indicating that Δ 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 Δ. Based on our theoretical results in comparison to experiments, we conclude that the activation energy Δ is a good molecular descriptor to characterize the TADF performance because a significantly better linear relationship is observed between Δ and the rISC rate constant () compared to that between Δ and . These findings provide deeper understanding of the TADF mechanism, shedding light on the molecular design of high-performance TADF materials.