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Large magnetic exchange coupling in rhombus-shaped nanographenes with zigzag periphery

Abstract

Nanographenes with zigzag edges are predicted to manifest non-trivial π-magnetism resulting from the interplay of concurrent electronic effects, such as hybridization of localized frontier states and Coulomb repulsion between valence electrons. This provides a chemically tunable platform to explore quantum magnetism at the nanoscale and opens avenues towards organic spintronics. The magnetic stability in nanographenes is thus far greatly limited by the weak magnetic exchange coupling, which remains below the room-temperature thermal energy. Here, we report the synthesis of large rhombus-shaped nanographenes with zigzag peripheries on gold and copper surfaces. Single-molecule scanning probe measurements show an emergent magnetic spin singlet ground state with increasing nanographene size. The magnetic exchange coupling in the largest nanographene (C70H22, containing five benzenoid rings along each edge), determined by inelastic electron tunnelling spectroscopy, exceeds 100 meV or 1,160 K, which outclasses most inorganic nanomaterials and survives on a metal electrode.

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Fig. 1: Synthesis of [4]- and [5]-rhombenes.
Fig. 2: On-surface synthesis and STM characterization of [4]- and [5]-rhombenes.
Fig. 3: Electronic and magnetic characterization of [4]- and [5]-rhombenes.
Fig. 4: Theoretical calculations.

Data availability

Additional STM/STS data and theoretical calculations, materials and methods, solution synthetic procedures and characterization data of chemical compounds (X-ray diffraction, NMR spectroscopy and high-resolution mass spectrometry) are available in the Supplementary Information. Crystallographic data for the structures reported in this Article have been deposited at the Cambridge Crystallographic Data Centre, under deposition numbers CCDC 1978171 (3) and 1978172 (4). Copies of the data can be obtained free of charge via https://www.ccdc.cam.ac.uk/structures/. Source data are provided with this paper.

Code availability

The tight-binding calculations were performed using a custom-made Python program available on the GitHub repository (https://github.com/eimrek/tb-mean-field-hubbard).

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Acknowledgements

We thank D. Passerone for fruitful discussions, D. Schollmeyer for single-crystal X-ray analysis and L. Rotach for technical support. This work was supported by the Swiss National Science Foundation (grant nos. 200020-182015 and IZLCZ2-170184), NCCR MARVEL funded by the Swiss National Science Foundation (grant no. 51NF40-182892), the European Union’s Horizon 2020 research and innovation programme (grant no. 785219, Graphene Flagship Core 2), the Office of Naval Research (N00014-18-1-2708), the Max Planck Society, Ministry of Science and Innovation of Spain (grant nos. PID2019-106114GB-I00 and PID2019-109539GB), Generalitat Valenciana and Fondo Social Europeo (grant no. ACIF/2018/175), MINECO-Spain (grant no. MAT2016-78625) and Portuguese FCT (grant no. UTAPEXPL/ NTec/0046/2017). Computational support from the Swiss Supercomputing Center (CSCS) under project ID s904 is gratefully acknowledged.

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Authors

Contributions

R.F., P.R., A.N. and K.M. conceived the project. Q.C. and X.Y. synthesized and characterized the precursor molecules. S.M. performed the STM experiments. S.M. analysed the data with contributions from M.D.G. K.E. and C.A.P. performed the DFT and GW calculations. R.O., J.F.-R., J.C.S.-G., O.G. and S.M. performed the tight-binding and Hubbard calculations. All authors contributed to discussing the results and writing the manuscript.

Corresponding authors

Correspondence to Akimitsu Narita or Roman Fasel.

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The authors declare no competing interests.

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Peer review information Nature Chemistry thanks Christopher Ehlert and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Supplementary information

Supplementary Information

Supplementary Figs. 1–32, notes 1 and 2, containing experimental STM/STS data, theoretical calculations, synthetic procedures and solution characterization data (X-ray diffraction, NMR spectroscopy and high-resolution mass spectrometry), general methods and materials and references.

Supplementary Data 1

Crystal structure of 3, CCDC 1978171.

Supplementary Data 2

Crystal structure of 4, CCDC 1978172.

Supplementary Data 3

Python script to simulate MFH-LDOS maps of the SOMOs and SUMOs of [5]-rhombene with the mean-field Hubbard tight-binding code available at https://github.com/eimrek/tb-mean-field-hubbard. Note that the simulations are performed with nearest-neighbour hopping, with U/t1 = 1.1. The geometry of the system and output images are included.

Supplementary Data 4

Source data for Supplementary Figs. 4a,b and 7.

Source data

Source Data Fig. 4

Source data for Fig. 4a,b,d

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Mishra, S., Yao, X., Chen, Q. et al. Large magnetic exchange coupling in rhombus-shaped nanographenes with zigzag periphery. Nat. Chem. 13, 581–586 (2021). https://doi.org/10.1038/s41557-021-00678-2

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