Triangulene

Names
Preferred IUPAC name Dibenzo[cd,mn]pyrene-4,8-diyl
Other names [3]Triangulene
Identifiers
3D model (JSmol)	Interactive image
ChemSpider	58825768
PubChem CID	102378663
CompTox Dashboard (EPA)	DTXSID701337130
InChI InChI=1S/C22H12/c1-4-13-10-15-6-2-8-17-12-18-9-3-7-16-11-14(5-1)19(13)22(20(15)17)21(16)18/h1-12H Key: YUXIWEBPPQSVAK-UHFFFAOYSA-N
SMILES c1cc2cc3cccc4c3c-5c2c(c1)[CH]c6c5c(ccc6)[CH]4
Properties
Chemical formula	C22H12
Molar mass	276.338 g·mol−1
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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Triangulene (also known as Clar's hydrocarbon) is the smallest triplet-ground-state polybenzenoid.^[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]

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]

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.

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.

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.