Abstract
Reverse intersystem crossing (RISC), originally considered forbidden in purely organic materials, has recently become possible by minimizing the energy gap between the lowest excited singlet state (S1) and lowest triplet state (T1) in thermally activated delayed fluorescence systems. However, direct spin-inversion from T1 to S1 is still inefficient when both states are of the same charge transfer (CT) nature (that is, 3CT and 1CT, respectively). Intervention of locally excited triplet states (3LE) between 3CT and 1CT is expected to trigger fast spin-flipping. Here, we report the systematic design of ideal thermally activated delayed fluorescence molecules with near-degenerate 1CT, 3CT and 3LE states by controlling the distance between the donor and acceptor segments in a molecule with tilted intersegment angles. This system realizes very fast RISC with a rate constant (kRISC) of 1.2 × 107 s−1, resulting in organic light-emitting diodes with excellent performance, particularly at high brightness. An organic molecule, TpAT-tFFO, which is designed to support rapid reverse intersystem crossing allows the fabrication of efficient organic light-emitting diodes.
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