Triangulene (also known as Clar's hydrocarbon) is the smallest triplet-ground-statepolybenzenoid.[1] It exists as a biradical with the chemical formula C 22H 12.[2] It was first hypothesized by Czech chemist Erich Clar in 1953.[3] 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.[4] Other attempts by Japanese researchers have been successful only in making substituted triangulene derivatives.[5]
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references
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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).[4]
[n]Triangulenesedit
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.
Theoryedit
A tight-binding description of the molecular orbitals of [n]triangulenes predicts[6] 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[6][7][8] 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 largest[9] 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[6] in terms of a theorem[10] 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.
Experimentsedit
So far, the ultra-high vacuum on-surface syntheses of [n]triangulenes with n = 3,[4] 4,[11] 5[12] and 7[13] (the hitherto largest triangulene homologue) have been reported. In addition, the on-surface synthesis of [3]triangulene dimers[14] 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,[15] where signatures of both spin fractionalization and Haldane gap were observed.
^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.
^ abcPavliček, 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.