Evidence and mechanism of efficient thermally activated delayed fluorescence promoted by delocalized excited states

The design of organic compounds with nearly no gap between the first excited singlet (S₁) and triplet (T₁) states has been demonstrated to result in an efficient spin-flip transition from the T₁ to S₁ state, that is, reverse intersystem crossing (RISC), and facilitate light emission as thermally activated delayed fluorescence (TADF). However, many TADF molecules have shown that a relatively appreciable energy difference between the S₁ and T₁ states (~0.2 eV) could also result in a high RISC rate. We revealed from a comprehensive study of optical properties of TADF molecules that the formation of delocalized states is the key to efficient RISC and identified a chemical template for these materials. In addition, simple structural confinement further enhances RISC by suppressing structural relaxation in the triplet states. Our findings aid in designing advanced organic molecules with a high rate of RISC and, thus, achieving the maximum theoretical electroluminescence efficiency in organic light-emitting diodes.