Unique TADF Behavior Around Zero Gap Between Singlet and Triplet Excited States

In recent years, thermally activated delayed fluorescence (TADF) has garnered significant attention due to its straightforward application in creating high-efficiency organic light-emitting diodes (OLEDs). To develop high-performance TADF materials, researchers have focused on designing novel molecules with a small energy gap between the lowest excited singlet and triplet states (dE_ST). Detailed analysis indicates a substantial contribution from higher-lying excited states in spin-flipping processes. Most recently, a heptazine derivative that violated Hund’s rule by exhibiting a negative dE_ST was reported, sparking considerable interest among photophysics researchers. In this study, we discovered an unusual thermal behavior in a donor-acceptor type TADF molecule, TMCz-BO, which shows nominal negative dE_ST but can be explained without a negative dE_ST through comprehensive kinetic analysis across various temperatures and solvents. Although activation energy has traditionally been considered temperature-independent, we emphasize that it should be viewed as a dynamic parameter influenced by environmental temperature, especially in cases of small energy gaps. Inadequate analysis could confuse and obscure a true understanding of the TADF mechanism. We intend to systematically organize the peculiar ST gap behavior around the zero gap and present an ideal molecular design.