Up-Conversion Intersystem Crossing Rates in Organic Emitters for Thermally Activated Delayed Fluorescence: Impact of the Nature of Singlet vs Triplet Excited States

The rates for up-conversion intersystem crossing (UISC) from the T 1 state to the S 1 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 S 1 and T 1 states (Δ E ST ) are evaluated, at the density functional theory (DFT) and time-dependent DFT levels. The calculated UISC rates and Δ E ST values are found to be in good agreement with available experimental data. Our results underline that small Δ E 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 Δ E ST; in fact, either a significant charge-transfer (CT) contribution to the T 1 state or a minimal energy difference between the local-excitation and charge-transfer triplet states is required to achieve a small Δ E ST . Also, having S 1 and T 1 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 T 1 and somewhat different natures of the S 1 and T 1 states, paves the way toward UISC enhancement and thus TADF efficiency improvement.