Critical role of intermediate electronic states for spin-flip processes in charge-transfer-type organic molecules with multiple donors and acceptors

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Abstract

Spin-flip in purely organic molecular systems is often described as a forbidden process; however, it is commonly observed and utilized to harvest triplet excitons in a wide variety of organic material-based applications. Although the initial and final electronic states of spin-flip between the lowest singlet and lowest triplet excited state are self-evident, the exact process and the role of intermediate states through which spin-flip occurs are still far from being comprehensively determined. Here, via experimental photo-physical investigations in solution combined with first-principles quantum-mechanical calculations, we show that efficient spin-flip in multiple donor–acceptor charge-transfer-type organic molecular systems involves the critical role of an intermediate triplet excited state that corresponds to a partial molecular structure of the system. Our proposed mechanism unifies the understanding of the intersystem crossing mechanism in a wide variety of charge-transfer-type molecular systems, opening the way to greater control over spin-flip rates.

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Fig. 1: Temperature dependence of spin-flip processes in CzCN derivatives in solution.
Fig. 2: Spin-flip process through an intermediate higher-lying triplet excited state of 4CzIPN.
Fig. 3: Theoretical description of the excited states of 4CzIPN.
Fig. 4: Illustration of the intermediate state for the spin-flip process in other donor–acceptor-type molecules.

Data availability

The data that support the findings of this study are available from the corresponding authors upon reasonable request.

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Acknowledgements

This work was supported in part by the Japan Science and Technology Agency, ERATO, Adachi Molecular Exciton Engineering Project, under JST ERATO grant no. JPMJER1305, and the Japan Society for the Promotion of Science KAKENHI (grant nos. JP17J04907, JP18H02047 and JP18H03902). Work at the Georgia Institute of Technology is supported by the Department of Energy (no. DE-EE0008205). X.-K.C. and J.-L.B. are also grateful to Kyulux for generous support of their TADF activities.

Author information

The project was conceived and designed by H. Nakanotani and H. Noda. H. Noda, M.M. and N.N. synthesized the organic compounds used in this study. H. Noda, Y.K. and M.M. prepared the samples and measured their properties. T.H. performed the TAS measurements. X.-K.C. carried out the quantum-chemical calculations under the supervision of J.-L.B. H. Nakanotani, H. Noda and X.-K.C. analysed all data. C.A. supervised the project. All authors contributed to writing the paper and critically commented on the project. H. Noda and X.-K.C. contributed equally to this work.

Correspondence to Hajime Nakanotani or Jean-Luc Brédas or Chihaya Adachi.

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Supplementary Information

Supplementary Methods, Supplementary Table 1, Supplementary Figs. 1–14 and Supplementary refs. 1–4.

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