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.
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The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.
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This work was supported by JSPS KAKENHI grant numbers 17H01231 (to H.K.) and 17J09631 (to Y.Wada). Computation time was provided by the Super Computer System, Institute for Chemical Research, Kyoto University. NMR measurements were supported by the international Joint Usage/Research Centre (iJURC) at the Institute for Chemical Research, Kyoto University, Japan. We thank A. Jackson for editing a draft of this manuscript.
The authors declare no competing interests.
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Supplementary text (material synthesis and characterization, theoretical calculations, thermal and photophysical properties), scheme 1, 1H and 13C NMR data, Figs. 1–7 and Tables 1–7.
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Wada, Y., Nakagawa, H., Matsumoto, S. et al. Organic light emitters exhibiting very fast reverse intersystem crossing. Nat. Photonics 14, 643–649 (2020). https://doi.org/10.1038/s41566-020-0667-0
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