Triangulene Explained

Triangulene (also known as Clar's hydrocarbon) is the smallest triplet-ground-state polybenzenoid. It exists as a biradical with the chemical formula .[1] It was first hypothesized by Czech chemist Erich Clar in 1953.[2] 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.[3] Other attempts by Japanese researchers have been successful only in making substituted triangulene derivatives.[4]

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).[3]

[''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 predicts[5] that [''n'']triangulenes have (n − 1) unpaired electrons, which are associated to (n − 1) non-bonding states. When electron–electron interactions are included, theory predicts[5] [6] [7] that the ground state total spin quantum number S of [''n'']triangulenes is S = . 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 largest[8] 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 rationalized[5] in terms of a theorem[9] 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, 4,[10] 5[11] and 7[12] (the hitherto largest triangulene homologue) have been reported. In addition, the on-surface synthesis of [3]triangulene dimers[13] 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,[14] where signatures of both spin fractionalization and Haldane gap were observed.

Notes and References

  1. Web site: triangulene C22H12 ChemSpider. www.chemspider.com. 2017-02-19.
  2. Ball . Philip . Elusive triangulene created by moving atoms one at a time . Nature . February 2017 . 542 . 7641 . 284–285 . 10.1038/nature.2017.21462 . 28202993 . 2017Natur.542..284B . 4398214 . free .
  3. Pavliček . Niko . Mistry . Anish . Majzik . Zsolt . Moll . Nikolaj . Meyer . Gerhard . Fox . David J. . Gross . Leo . Synthesis and characterization of triangulene . Nature Nanotechnology . April 2017 . 12 . 4 . 308–311 . 10.1038/nnano.2016.305 . 28192389 . 2017NatNa..12..308P .
  4. Morita. Yasushi. Suzuki. Shuichi. Sato. Kazunobu. Takui. Takeji. Synthetic organic spin chemistry for structurally well-defined open-shell graphene fragments. Nature Chemistry. 3. 3. 197–204. 10.1038/nchem.985. 21336324. 2011. 2011NatCh...3..197M.
  5. Fernández-Rossier . J. . Palacios . J. J. . Magnetism in Graphene Nanoislands . Physical Review Letters . 23 October 2007 . 99 . 17 . 177204 . 10.1103/PhysRevLett.99.177204 . 17995364 . 0707.2964 . 2007PhRvL..99q7204F . 10045/25254 . 9697828 . free .
  6. Wang . Wei L. . Meng . Sheng . Kaxiras . Efthimios . Graphene NanoFlakes with Large Spin . Nano Letters . 1 January 2008 . 8 . 1 . 241–245 . 10.1021/nl072548a . 18052302 . 2008NanoL...8..241W .
  7. Güçlü . A. D. . Potasz . P. . Voznyy . O. . Korkusinski . M. . Hawrylak . P. . Magnetism and Correlations in Fractionally Filled Degenerate Shells of Graphene Quantum Dots . Physical Review Letters . 10 December 2009 . 103 . 24 . 246805 . 10.1103/PhysRevLett.103.246805 . 20366221 . 0907.5431 . 2009PhRvL.103x6805G . 18754119 .
  8. Ortiz . Ricardo . Boto . Roberto A. . García-Martínez . Noel . Sancho-García . Juan C. . Melle-Franco . Manuel . Fernández-Rossier . Joaquı́n . Exchange Rules for Diradical π-Conjugated Hydrocarbons . Nano Letters . 11 September 2019 . 19 . 9 . 5991–5997 . 10.1021/acs.nanolett.9b01773 . 31365266 . 1906.08544 . 2019NanoL..19.5991O . 195218794 .
  9. Lieb . Elliott H. . Two theorems on the Hubbard model . Physical Review Letters . 6 March 1989 . 62 . 10 . 1201–1204 . 10.1103/PhysRevLett.62.1201 . 10039602 . 1989PhRvL..62.1201L .
  10. Mishra . Shantanu . Beyer . Doreen . Eimre . Kristjan . Liu . Junzhi . Berger . Reinhard . Gröning . Oliver . Pignedoli . Carlo A. . Müllen . Klaus . Fasel . Roman . Feng . Xinliang . Ruffieux . Pascal . Synthesis and Characterization of π-Extended Triangulene . Journal of the American Chemical Society . 10 July 2019 . 141 . 27 . 10621–10625 . 10.1021/jacs.9b05319 . 31241927 . 195696890 .
  11. 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 . Atomically precise bottom-up synthesis of π-extended [5]triangulene . Science Advances . July 2019 . 5 . 7 . eaav7717 . 10.1126/sciadv.aav7717 . 31360763 . 6660211 . 2019SciA....5.7717S . free .
  12. Mishra . Shantanu . Xu . Kun . Eimre . Kristjan . Komber . Hartmut . Ma . Ji . Pignedoli . Carlo A. . Fasel . Roman . Feng . Xinliang . Ruffieux . Pascal . Synthesis and characterization of [7]triangulene . Nanoscale . 2021 . 13 . 3 . 1624–1628 . 10.1039/d0nr08181g . 33443270 . 231605335 .
  13. Mishra . Shantanu . Beyer . Doreen . Eimre . Kristjan . Ortiz . Ricardo . Fernández-Rossier . Joaquín . Berger . Reinhard . Gröning . Oliver . Pignedoli . Carlo A. . Fasel . Roman . Feng . Xinliang . Ruffieux . Pascal . Collective All-Carbon Magnetism in Triangulene Dimers . Angewandte Chemie International Edition . 13 July 2020 . 59 . 29 . 12041–12047 . 10.1002/anie.202002687 . 32301570 . 7383983 . 2003.00753 .
  14. 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 . Observation of fractional edge excitations in nanographene spin chains . Nature . 13 October 2021 . 598 . 7880 . 287–292 . 10.1038/s41586-021-03842-3. 34645998 . 2105.09102 . 2021Natur.598..287M . 234777902 .

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