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Achieving small singlet–triplet energy gaps in polycyclic heteroaromatic emitters

Nature Materials volume 24, pages 1576–1583 (2025)Cite this article

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Abstract

In polycyclic heteroaromatic (PHA) emitters, which possess great potential for application in ultrahigh-definition displays, the origin of a small singlet–triplet energy gap (ΔES1T1) and its relationship with the molecular structure still remain poorly established. Here we derive an effective expression for ΔES1T1, in which ΔES1T1 positively depends on 2KHL (where KHL is the exchange energy between the highest occupied molecular orbital and the lowest unoccupied molecular orbital (LUMO)) and on the energy gap between LUMO and LUMO + 1 (ΔELUMO–LUMO+1). This expression for ΔES1T1 is validated over a series of 100 reported PHA emitters. It allows us to easily identify various molecular design approaches for managing ΔES1T1 by synergistically regulating 2KHL and ΔELUMO–LUMO+1. The proof-of-concept PHA molecules were synthesized and characterized to further confirm the validity of this expression for ΔES1T1. Overall, our work provides a physical picture to not only modulate ΔES1T1 in emerging PHA emitters but also design and screen such materials with small ΔES1T1.

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Fig. 1: Physical picture for reducing ΔES1T1.
Fig. 2: Relationships among ΔES1T1, 2KHL and ΔELUMO–LUMO+1 for experimentally reported MR-type molecules.
Fig. 3: Molecular design principle for MR-type molecules with small ΔES1T1.
Fig. 4: Photophysical and electroluminescence properties of IV-DABNA.

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All relevant data are included in the Article and its Supplementary Information.

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Acknowledgements

X.-K.C. acknowledges support through grants from the National Key Research & Development Program of China (nos. 2024YFB3612900 and 2022YFB4200600), the National Natural Science Foundation of China (no. 52473190), the Natural Science Foundation of Jiangsu Province (no. BK20240042), and the Science and Technology Project of Suzhou (no. ZXL2024394). K.W. acknowledges support through grants from the National Key Research & Development Program of China (no. 2020YFA0714601), the National Natural Science Foundation of China (nos. 52422309 and 52373193), and the Science and Technology Project of Suzhou (no. ZXL2022490). X.-H.Z. acknowledges support through a grant from the National Natural Science Foundation of China (no. 52130304). C.A. acknowledges support through a grant from MEXT/JSPS KAKENHI (no. 23K20039). X.-C.F. acknowledges support through grants from the National Natural Science Foundation of China (no. 52403240), the Natural Science Foundation of Jiangsu Province (no. BK20230507) and the Jiangsu Province Excellent Postdoctoral Program (no. 2023ZB515). X.T. acknowledges support through a grant from JSPS KAKENHI (no. 22K20536). R.W. acknowledges support through a grant from the Jiangsu Province Excellent Postdoctoral Program (no. 2024ZB898). R.W., X.X., X.-C.F., T.-F.C., H.W., K.W., X.-K.C. and X.-H.Z. acknowledge support from the Suzhou Key Laboratory of Functional Nano & Soft Materials, Suzhou Key Laboratory of Advanced Photonic Materials (grant SZS2023010), Collaborative Innovation Center of Suzhou Nano Science & Technology and the 111 Project. We highly appreciate stimulating discussions with Z. Qu at Jilin University. J.-L.B. acknowledges support through a grant from the Office of Naval Research under award no. N00014-24-1-2114.

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Author notes

  1. These authors contributed equally: Rajat Walia, Xin Xiong, Xiao-Chun Fan.

Authors and Affiliations

  1. Institute of Functional Nano and Soft Materials (FUNSOM), Joint International Research Laboratory of Carbon-Based Functional Materials and Devices, Soochow University, Suzhou, People’s Republic of China

    Rajat Walia, Xin Xiong, Xiao-Chun Fan, Ting-Feng Chen, Hui Wang, Kai Wang, Xian-Kai Chen & Xiao-Hong Zhang

  2. Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou, People’s Republic of China

    Rajat Walia, Xin Xiong & Xiao-Chun Fan

  3. State Key Laboratory of Bioinspired interfacial Materials Science, Soochow University, Suzhou, People’s Republic of China

    Kai Wang & Xian-Kai Chen

  4. School of Materials Science and Engineering, Suzhou University of Science and Technology, Suzhou, People’s Republic of China

    Yi-Zhong Shi

  5. Center for Organic Photonics and Electronics Research (OPERA), Kyushu University, Fukuoka, Japan

    Xun Tang & Chihaya Adachi

  6. Department of Chemistry and Biochemistry, The University of Arizona, Tucson, AZ, USA

    Jean-Luc Bredas

  7. Jiangsu Key Laboratory of Advanced Negative Carbon Technologies, Soochow University, Suzhou, People’s Republic of China

    Xiao-Hong Zhang

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Contributions

K.W., C.A., X.-K.C. and X.-H.Z. conceived the project. X.-K.C. proposed the physical picture of small singlet–triplet energy gaps in PHA emitters. R.W. carried out all the theoretical calculations under the supervision of X.-K.C. X.X., X.C.-F., T.-F.C. and H.W. carried out the experiments with the help of Y.-Z.S. under the supervision of K.W. X.-K.C., K.W., X.-H.Z., R.W., C.A. and J.-L.B. wrote the manuscript. All authors discussed the results and commented on the final manuscript.

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Correspondence to Kai Wang, Chihaya Adachi, Xian-Kai Chen or Xiao-Hong Zhang.

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Nature Materials thanks You-Xuan Zheng and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Supplementary Notes 1–6, Figs. 1–16, Tables 1–6, Equations (1)–(15), Methods and references.

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Walia, R., Xiong, X., Fan, XC. et al. Achieving small singlet–triplet energy gaps in polycyclic heteroaromatic emitters. Nat. Mater. 24, 1576–1583 (2025). https://doi.org/10.1038/s41563-025-02309-4

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