The rates for up-conversion intersystem crossing (UISC) from the T₁ state to the S₁ state are calculated for a series of organic emitters with an emphasis on thermally activated delayed fluorescence (TADF) materials. Both the spin-orbit coupling and the energy difference between the S₁ and T₁ states (ΔE_ST) are evaluated, at the density functional theory (DFT) and time-dependent DFT levels. The calculated UISC rates and ΔE_ST values are found to be in good agreement with available experimental data. Our results underline that small ΔE_ST values and sizable spin-orbit coupling matrix elements have to be simultaneously realized in order to facilitate UISC and ultimately TADF. Importantly, the spatial separation of the highest occupied and lowest unoccupied molecular orbitals of the emitter, a widely accepted strategy for the design of TADF molecules, does not necessarily lead to a sufficient reduction in ΔE_ST; in fact, either a significant charge-transfer (CT) contribution to the T₁ state or a minimal energy difference between the local-excitation and charge-transfer triplet states is required to achieve a small ΔE_ST. Also, having S₁ and T₁ states of a different nature is found to strongly enhance spin-orbit coupling, which is consistent with the El-Sayed rule for ISC rates. Overall, our results indicate that having either similar energies for the local-excitation and charge-transfer triplet states or the right balance between a substantial CT contribution to T₁ and somewhat different natures of the S₁ and T₁ states, paves the way toward UISC enhancement and thus TADF efficiency improvement.

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