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Triangulene
Chemical compound
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Triangulene (also known as Clar's hydrocarbon) is the smallest triplet-ground-state polybenzenoid. It exists as a biradical with the chemical formula C22H12. It was first hypothesized by Czech chemist Erich Clar in 1953. Its first confirmed synthesis was published in a February 2017 issue of Nature Nanotechnology, in a project led by researchers David Fox and Anish Mistry at the University of Warwick in collaboration with IBM. Other attempts by Japanese researchers have been successful only in making substituted triangulene derivatives.

A six-step synthesis yielded two isomers of dihydrotriangulene which were then deposited on xenon or copper base. The researchers used a combined scanning tunneling and atomic force microscope (STM/AFM) to remove individual hydrogen atoms. The synthesized molecule of triangulene remained stable at high-vacuum low-temperature conditions for four days, giving the scientists plenty of time to characterize it (also using STM/AFM).

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[n]Triangulenes

Triangulene, as defined here, is a member of a wider class of [n]triangulenes, where n is the number of hexagons along an edge of the molecule. Thus, triangulene may also be referred to as [3]triangulene.

Theory

A tight-binding description of the molecular orbitals of [n]triangulenes predicts7 that [n]triangulenes have (n − 1) unpaired electrons, which are associated to (n − 1) non-bonding states. When electron–electron interactions are included, theory predicts8910 that the ground state total spin quantum number S of [n]triangulenes is S = ⁠n − 1/2⁠. Thus, [3]triangulenes are predicted to have an S = 1 ground state. The intramolecular exchange interaction in triangulene, which determines the energy difference between the S = 1 ground state and the S = 0 excited state, is predicted to be the largest11 among all polycyclic aromatic hydrocarbon (PAH) diradicals, due to maximum overlap of the wave function of the unpaired electrons.

The ground state spin of [n]triangulenes can be rationalized12 in terms of a theorem13 by Elliot H. Lieb, which relates, for a bipartite lattice, the ground state spin of the Hubbard model at half filling to the sublattice imbalance.

Experiments

So far, the ultra-high vacuum on-surface syntheses of [n]triangulenes with n = 3,14 4,15 516 and 717 (the hitherto largest triangulene homologue) have been reported. In addition, the on-surface synthesis of [3]triangulene dimers18 has also been reported, where inelastic electron tunneling spectroscopy provides a direct evidence of a strong antiferromagnetic coupling between the triangulenes. In 2021, an international team of researchers reported the fabrication of [3]triangulene-based quantum spin chains on a gold surface,19 where signatures of both spin fractionalization and Haldane gap were observed.

References

  1. IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "biradical". doi:10.1351/goldbook.B00671 /wiki/International_Union_of_Pure_and_Applied_Chemistry ↩

  2. "triangulene | C22H12 | ChemSpider". www.chemspider.com. Retrieved 2017-02-19. http://www.chemspider.com/Chemical-Structure.58825768.html ↩

  3. Ball, Philip (February 2017). "Elusive triangulene created by moving atoms one at a time". Nature. 542 (7641): 284–285. Bibcode:2017Natur.542..284B. doi:10.1038/nature.2017.21462. PMID 28202993. S2CID 4398214. https://doi.org/10.1038%2Fnature.2017.21462 ↩

  4. Pavliček, Niko; Mistry, Anish; Majzik, Zsolt; Moll, Nikolaj; Meyer, Gerhard; Fox, David J.; Gross, Leo (April 2017). "Synthesis and characterization of triangulene" (PDF). Nature Nanotechnology. 12 (4): 308–311. Bibcode:2017NatNa..12..308P. doi:10.1038/nnano.2016.305. PMID 28192389. http://wrap.warwick.ac.uk/86722/1/WRAP_ch-030317-wrap_-cusersdavid_foxdesktop40triangulene_complete_accepted.pdf ↩

  5. Morita, Yasushi; Suzuki, Shuichi; Sato, Kazunobu; Takui, Takeji (2011). "Synthetic organic spin chemistry for structurally well-defined open-shell graphene fragments". Nature Chemistry. 3 (3): 197–204. Bibcode:2011NatCh...3..197M. doi:10.1038/nchem.985. PMID 21336324. /wiki/Bibcode_(identifier) ↩

  6. Pavliček, Niko; Mistry, Anish; Majzik, Zsolt; Moll, Nikolaj; Meyer, Gerhard; Fox, David J.; Gross, Leo (April 2017). "Synthesis and characterization of triangulene" (PDF). Nature Nanotechnology. 12 (4): 308–311. Bibcode:2017NatNa..12..308P. doi:10.1038/nnano.2016.305. PMID 28192389. http://wrap.warwick.ac.uk/86722/1/WRAP_ch-030317-wrap_-cusersdavid_foxdesktop40triangulene_complete_accepted.pdf ↩

  7. Fernández-Rossier, J.; Palacios, J. J. (23 October 2007). "Magnetism in Graphene Nanoislands". Physical Review Letters. 99 (17): 177204. arXiv:0707.2964. Bibcode:2007PhRvL..99q7204F. doi:10.1103/PhysRevLett.99.177204. hdl:10045/25254. PMID 17995364. S2CID 9697828. /wiki/ArXiv_(identifier) ↩

  8. Fernández-Rossier, J.; Palacios, J. J. (23 October 2007). "Magnetism in Graphene Nanoislands". Physical Review Letters. 99 (17): 177204. arXiv:0707.2964. Bibcode:2007PhRvL..99q7204F. doi:10.1103/PhysRevLett.99.177204. hdl:10045/25254. PMID 17995364. S2CID 9697828. /wiki/ArXiv_(identifier) ↩

  9. Wang, Wei L.; Meng, Sheng; Kaxiras, Efthimios (1 January 2008). "Graphene NanoFlakes with Large Spin". Nano Letters. 8 (1): 241–245. Bibcode:2008NanoL...8..241W. doi:10.1021/nl072548a. PMID 18052302. /wiki/Bibcode_(identifier) ↩

  10. Güçlü, A. D.; Potasz, P.; Voznyy, O.; Korkusinski, M.; Hawrylak, P. (10 December 2009). "Magnetism and Correlations in Fractionally Filled Degenerate Shells of Graphene Quantum Dots". Physical Review Letters. 103 (24): 246805. arXiv:0907.5431. Bibcode:2009PhRvL.103x6805G. doi:10.1103/PhysRevLett.103.246805. PMID 20366221. S2CID 18754119. /wiki/ArXiv_(identifier) ↩

  11. Ortiz, Ricardo; Boto, Roberto A.; García-Martínez, Noel; Sancho-García, Juan C.; Melle-Franco, Manuel; Fernández-Rossier, Joaquı́n (11 September 2019). "Exchange Rules for Diradical π-Conjugated Hydrocarbons". Nano Letters. 19 (9): 5991–5997. arXiv:1906.08544. Bibcode:2019NanoL..19.5991O. doi:10.1021/acs.nanolett.9b01773. PMID 31365266. S2CID 195218794. /wiki/ArXiv_(identifier) ↩

  12. Fernández-Rossier, J.; Palacios, J. J. (23 October 2007). "Magnetism in Graphene Nanoislands". Physical Review Letters. 99 (17): 177204. arXiv:0707.2964. Bibcode:2007PhRvL..99q7204F. doi:10.1103/PhysRevLett.99.177204. hdl:10045/25254. PMID 17995364. S2CID 9697828. /wiki/ArXiv_(identifier) ↩

  13. Lieb, Elliott H. (6 March 1989). "Two theorems on the Hubbard model". Physical Review Letters. 62 (10): 1201–1204. Bibcode:1989PhRvL..62.1201L. doi:10.1103/PhysRevLett.62.1201. PMID 10039602. /wiki/Bibcode_(identifier) ↩

  14. Pavliček, Niko; Mistry, Anish; Majzik, Zsolt; Moll, Nikolaj; Meyer, Gerhard; Fox, David J.; Gross, Leo (April 2017). "Synthesis and characterization of triangulene" (PDF). Nature Nanotechnology. 12 (4): 308–311. Bibcode:2017NatNa..12..308P. doi:10.1038/nnano.2016.305. PMID 28192389. http://wrap.warwick.ac.uk/86722/1/WRAP_ch-030317-wrap_-cusersdavid_foxdesktop40triangulene_complete_accepted.pdf ↩

  15. Mishra, Shantanu; Beyer, Doreen; Eimre, Kristjan; Liu, Junzhi; Berger, Reinhard; Gröning, Oliver; Pignedoli, Carlo A.; Müllen, Klaus; Fasel, Roman; Feng, Xinliang; Ruffieux, Pascal (10 July 2019). "Synthesis and Characterization of π-Extended Triangulene" (PDF). Journal of the American Chemical Society. 141 (27): 10621–10625. doi:10.1021/jacs.9b05319. PMID 31241927. S2CID 195696890. https://boris.unibe.ch/141648/2/Postprint.pdf ↩

  16. Su, Jie; Telychko, Mykola; Hu, Pan; Macam, Gennevieve; Mutombo, Pingo; Zhang, Hejian; Bao, Yang; Cheng, Fang; Huang, Zhi-Quan; Qiu, Zhizhan; Tan, Sherman J. R.; Lin, Hsin; Jelínek, Pavel; Chuang, Feng-Chuan; Wu, Jishan; Lu, Jiong (July 2019). "Atomically precise bottom-up synthesis of π-extended [5]triangulene". Science Advances. 5 (7): eaav7717. Bibcode:2019SciA....5.7717S. doi:10.1126/sciadv.aav7717. PMC 6660211. PMID 31360763. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6660211 ↩

  17. Mishra, Shantanu; Xu, Kun; Eimre, Kristjan; Komber, Hartmut; Ma, Ji; Pignedoli, Carlo A.; Fasel, Roman; Feng, Xinliang; Ruffieux, Pascal (2021). "Synthesis and characterization of [7]triangulene". Nanoscale. 13 (3): 1624–1628. doi:10.1039/d0nr08181g. PMID 33443270. S2CID 231605335. /wiki/Doi_(identifier) ↩

  18. Mishra, Shantanu; Beyer, Doreen; Eimre, Kristjan; Ortiz, Ricardo; Fernández-Rossier, Joaquín; Berger, Reinhard; Gröning, Oliver; Pignedoli, Carlo A.; Fasel, Roman; Feng, Xinliang; Ruffieux, Pascal (13 July 2020). "Collective All-Carbon Magnetism in Triangulene Dimers". Angewandte Chemie International Edition. 59 (29): 12041–12047. arXiv:2003.00753. doi:10.1002/anie.202002687. PMC 7383983. PMID 32301570. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7383983 ↩

  19. Mishra, Shantanu; Catarina, Gonçalo; Wu, Fupeng; Ortiz, Ricardo; Jacob, David; Eimre, Kristjan; Ma, Ji; Pignedoli, Carlo A.; Feng, Xinliang; Ruffieux, Pascal; Fernández-Rossier, Joaquín; Fasel, Roman (13 October 2021). "Observation of fractional edge excitations in nanographene spin chains". Nature. 598 (7880): 287–292. arXiv:2105.09102. Bibcode:2021Natur.598..287M. doi:10.1038/s41586-021-03842-3. PMID 34645998. S2CID 234777902. /wiki/ArXiv_(identifier) ↩