Nature of highly efficient thermally activated delayed fluorescence in organic light-emitting diode emitters: Nonadiabatic effect between excited states

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.