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The discovery and utilization of metal-free organic emitters with thermally activated delayed fluorescence (TADF) is a huge breakthrough toward high-performance and low-cost organic light-emitting diodes. Time-dependent second-order perturbation theory including spin–orbit and nonadiabatic couplings, combined with time-dependent density functional theory, is employed to reveal the nature of highly efficient TADF in pure organic emitters. Our results demonstrate that except energy gaps between the lowest singlet (S1) and triplet (T1) excited states the nonadiabatic effect between low-lying excited states should play a key role in the T1 → S1 upconversion for TADF emitters, especially donor–acceptor–donor (D–A–D) molecules. We not only clarify the reason why D–A–D molecules with large S1–T1 energy gaps show efficient TADF but also explain the experimental observation that D–A–D-type compounds with S1–T1 gaps close to those of their D–A-shape counterparts display more efficient T1 → S1 upconversion.
Chen et al. (Tue,) studied this question.