Nominal appearance of inverted singlet and triplet excited state behaviour induced by the presence of a higher lying triplet excited state

In recent years, thermally activated delayed fluorescence (TADF) has attracted intense attention owing to its straightforward application to high-efficiency organic light-emitting diodes. Further, to develop high-performance TADF materials, many researchers have designed novel molecules that have a small energy gap between the lowest excited singlet and triplet states (∆E_ST), and detailed analysis suggest a significant contribution of higher lying excited states for spin flipping processes. In this study, we studied the spin-flipping process of the donor–acceptor TADF molecule with 1,3,6,8-tetramethyl-9H-carbazole as the donor unit and 5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracene as the acceptor unit (TMCz-BO) and observed peculiar thermally activated behaviour similar to that of the negative gap molecule HzTFEX2 (a heptazine derivative). By comprehensive kinetic analysis across various temperatures in various solvents, the peculiar thermal behaviour of TMCz-BO can be explained with a normal four-state configuration without a negative small energy gap between the S1 and T1 states. While the activation energy has previously been treated as being temperature independent, we stress that it should be a dynamic parameter affected by the environmental temperature, especially in the case of a small energy gap of ~10 meV. Insufficient analysed results would cause confusion and inhibit the visualization of the true picture of the TADF mechanism. We anticipate that the comprehensive analysis in this study will provide information for the molecular design of advanced TADF emitters in organic light-emitting diodes.