Synthesis and characterization of triangulene

Abstract

Triangulene, the smallest triplet-ground-state polybenzenoid (also known as Clar's hydrocarbon), has been an enigmatic molecule ever since its existence was first hypothesized1. Despite containing an even number of carbons (22, in six fused benzene rings), it is not possible to draw Kekulé-style resonant structures for the whole molecule: any attempt results in two unpaired valence electrons2. Synthesis and characterization of unsubstituted triangulene has not been achieved because of its extreme reactivity1, although the addition of substituents has allowed the stabilization and synthesis of the triangulene core3,4 and verification of the triplet ground state via electron paramagnetic resonance measurements5. Here we show the on-surface generation of unsubstituted triangulene that consists of six fused benzene rings. The tip of a combined scanning tunnelling and atomic force microscope (STM/AFM) was used to dehydrogenate precursor molecules. STM measurements in combination with density functional theory (DFT) calculations confirmed that triangulene keeps its free-molecule properties on the surface, whereas AFM measurements resolved its planar, threefold symmetric molecular structure. The unique topology of such non-Kekulé hydrocarbons results in open-shell π-conjugated graphene fragments6 that give rise to high-spin ground states, potentially useful in organic spintronic devices7,8. Our generation method renders manifold experiments possible to investigate triangulene and related open-shell fragments at the single-molecule level.

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Figure 1: Scheme that illustrates the generation of triangulene 1.
Figure 2: Comparison of different molecular species on NaCl.
Figure 3: Characterization of triangulene on Cu and Xe.

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Acknowledgements

We acknowledge R. Allenspach, D. Peña, J. Repp and I. Tavernelli for valuable comments on the manuscript. The research leading to these results received funding from the European Research Council (ERC) Advanced Grant CEMAS (agreement no. 291194), the ERC Consolidator Grant AMSEL (682144), the European Union project PAMS (610446) and the Initial Training Network QTea (317485).

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N.P., Z.M., L.G. and G.M. performed the STM/AFM experiments; N.P. and N.M. performed the DFT calculations; A.M. and D.J.F. synthesized the precursor molecules; all the authors analysed the data and contributed to the manuscript.

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Correspondence to Niko Pavliček.

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The authors declare no competing financial interests.

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Pavliček, N., Mistry, A., Majzik, Z. et al. Synthesis and characterization of triangulene. Nature Nanotech 12, 308–311 (2017). https://doi.org/10.1038/nnano.2016.305

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