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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The data that support the findings of this study are available from the corresponding authors upon reasonable request.
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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.
The authors declare no competing interests.
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