triangulene
{{Chembox
|ImageFile=Triangulene.svg
|ImageSize=200px
|ImageAlt=Triangulene
|PIN=Dibenzo[cd,mn]pyrene-4,8-diyl
|OtherNames=[3]Triangulene
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{{Chembox Identifiers
| CASNo =
| ChemSpiderID = 58825768
| PubChem = 102378663
| SMILES = c1cc2cc3cccc4c3c-5c2c(c1)[CH]c6c5c(ccc6)[CH]4
|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
|InChIKey=YUXIWEBPPQSVAK-UHFFFAOYSA-N
}}
|Section2=
{{Chembox Properties
| C=22|H=12
| Appearance =
| Density =
| MeltingPt =
| BoilingPt =
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{{Chembox Hazards
| MainHazards =
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{{wiktionary}}
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Triangulene (also known as Clar's hydrocarbon) is the smallest triplet-ground-state polybenzenoid.{{GoldBookRef |title=biradical |file=B00671 }} It exists as a biradical with the chemical formula {{chem|C|22|H|12}}.{{Cite web|url=http://www.chemspider.com/Chemical-Structure.58825768.html|title=triangulene {{!}} C22H12 {{!}} ChemSpider|website=www.chemspider.com|access-date=2017-02-19}} It was first hypothesized by Czech chemist Erich Clar in 1953.{{cite journal |last1=Ball |first1=Philip |title=Elusive triangulene created by moving atoms one at a time |journal=Nature |date=February 2017 |volume=542 |issue=7641 |pages=284–285 |doi=10.1038/nature.2017.21462 |pmid=28202993 |bibcode=2017Natur.542..284B |s2cid=4398214 |doi-access=free }} 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.{{cite journal |last1=Pavliček |first1=Niko |last2=Mistry |first2=Anish |last3=Majzik |first3=Zsolt |last4=Moll |first4=Nikolaj |last5=Meyer |first5=Gerhard |last6=Fox |first6=David J. |last7=Gross |first7=Leo |title=Synthesis and characterization of triangulene |journal=Nature Nanotechnology |date=April 2017 |volume=12 |issue=4 |pages=308–311 |doi=10.1038/nnano.2016.305 |pmid=28192389 |bibcode=2017NatNa..12..308P |url=http://wrap.warwick.ac.uk/86722/1/WRAP_ch-030317-wrap_-cusersdavid_foxdesktop40triangulene_complete_accepted.pdf }} Other attempts by Japanese researchers have been successful only in making substituted triangulene derivatives.{{Cite journal|last1=Morita|first1=Yasushi|last2=Suzuki|first2=Shuichi|last3=Sato|first3=Kazunobu|last4=Takui|first4=Takeji|title=Synthetic organic spin chemistry for structurally well-defined open-shell graphene fragments|journal=Nature Chemistry|volume=3|issue=3|pages=197–204|doi=10.1038/nchem.985|pmid=21336324|year=2011|bibcode=2011NatCh...3..197M}}
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).
[''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{{cite journal |last1=Fernández-Rossier |first1=J. |last2=Palacios |first2=J. J. |title=Magnetism in Graphene Nanoislands |journal=Physical Review Letters |date=23 October 2007 |volume=99 |issue=17 |pages=177204 |doi=10.1103/PhysRevLett.99.177204 |pmid=17995364 |arxiv=0707.2964 |bibcode=2007PhRvL..99q7204F |hdl=10045/25254 |s2cid=9697828 |hdl-access=free }} 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{{cite journal |last1=Wang |first1=Wei L. |last2=Meng |first2=Sheng |last3=Kaxiras |first3=Efthimios |title=Graphene NanoFlakes with Large Spin |journal=Nano Letters |date=1 January 2008 |volume=8 |issue=1 |pages=241–245 |doi=10.1021/nl072548a |pmid=18052302 |bibcode=2008NanoL...8..241W }}{{cite journal |last1=Güçlü |first1=A. D. |last2=Potasz |first2=P. |last3=Voznyy |first3=O. |last4=Korkusinski |first4=M. |last5=Hawrylak |first5=P. |title=Magnetism and Correlations in Fractionally Filled Degenerate Shells of Graphene Quantum Dots |journal=Physical Review Letters |date=10 December 2009 |volume=103 |issue=24 |pages=246805 |doi=10.1103/PhysRevLett.103.246805 |pmid=20366221 |arxiv=0907.5431 |bibcode=2009PhRvL.103x6805G |s2cid=18754119 }} that the ground state total spin quantum number S of [n]triangulenes is S = {{sfrac|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{{cite journal |last1=Ortiz |first1=Ricardo |last2=Boto |first2=Roberto A. |last3=García-Martínez |first3=Noel |last4=Sancho-García |first4=Juan C. |last5=Melle-Franco |first5=Manuel |last6=Fernández-Rossier |first6=Joaquı́n |title=Exchange Rules for Diradical π-Conjugated Hydrocarbons |journal=Nano Letters |date=11 September 2019 |volume=19 |issue=9 |pages=5991–5997 |doi=10.1021/acs.nanolett.9b01773 |pmid=31365266 |arxiv=1906.08544 |bibcode=2019NanoL..19.5991O |s2cid=195218794 }} 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 in terms of a theorem{{cite journal |last1=Lieb |first1=Elliott H. |title=Two theorems on the Hubbard model |journal=Physical Review Letters |date=6 March 1989 |volume=62 |issue=10 |pages=1201–1204 |doi=10.1103/PhysRevLett.62.1201 |pmid=10039602 |bibcode=1989PhRvL..62.1201L }} 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,{{cite journal |last1=Mishra |first1=Shantanu |last2=Beyer |first2=Doreen |last3=Eimre |first3=Kristjan |last4=Liu |first4=Junzhi |last5=Berger |first5=Reinhard |last6=Gröning |first6=Oliver |last7=Pignedoli |first7=Carlo A. |last8=Müllen |first8=Klaus |last9=Fasel |first9=Roman |last10=Feng |first10=Xinliang |last11=Ruffieux |first11=Pascal |title=Synthesis and Characterization of π-Extended Triangulene |journal=Journal of the American Chemical Society |date=10 July 2019 |volume=141 |issue=27 |pages=10621–10625 |doi=10.1021/jacs.9b05319 |pmid=31241927 |s2cid=195696890 |url=https://boris.unibe.ch/141648/2/Postprint.pdf }} 5{{cite journal |last1=Su |first1=Jie |last2=Telychko |first2=Mykola |last3=Hu |first3=Pan |last4=Macam |first4=Gennevieve |last5=Mutombo |first5=Pingo |last6=Zhang |first6=Hejian |last7=Bao |first7=Yang |last8=Cheng |first8=Fang |last9=Huang |first9=Zhi-Quan |last10=Qiu |first10=Zhizhan |last11=Tan |first11=Sherman J. R. |last12=Lin |first12=Hsin |last13=Jelínek |first13=Pavel |last14=Chuang |first14=Feng-Chuan |last15=Wu |first15=Jishan |last16=Lu |first16=Jiong |title=Atomically precise bottom-up synthesis of π-extended [5]triangulene |journal=Science Advances |date=July 2019 |volume=5 |issue=7 |pages=eaav7717 |doi=10.1126/sciadv.aav7717 |pmid=31360763 |pmc=6660211 |bibcode=2019SciA....5.7717S |doi-access=free }} and 7{{cite journal |last1=Mishra |first1=Shantanu |last2=Xu |first2=Kun |last3=Eimre |first3=Kristjan |last4=Komber |first4=Hartmut |last5=Ma |first5=Ji |last6=Pignedoli |first6=Carlo A. |last7=Fasel |first7=Roman |last8=Feng |first8=Xinliang |last9=Ruffieux |first9=Pascal |title=Synthesis and characterization of [7]triangulene |journal=Nanoscale |date=2021 |volume=13 |issue=3 |pages=1624–1628 |doi=10.1039/d0nr08181g |pmid=33443270 |s2cid=231605335 }} (the hitherto largest triangulene homologue) have been reported. In addition, the on-surface synthesis of [3]triangulene dimers{{cite journal |last1=Mishra |first1=Shantanu |last2=Beyer |first2=Doreen |last3=Eimre |first3=Kristjan |last4=Ortiz |first4=Ricardo |last5=Fernández-Rossier |first5=Joaquín |last6=Berger |first6=Reinhard |last7=Gröning |first7=Oliver |last8=Pignedoli |first8=Carlo A. |last9=Fasel |first9=Roman |last10=Feng |first10=Xinliang |last11=Ruffieux |first11=Pascal |title=Collective All-Carbon Magnetism in Triangulene Dimers |journal=Angewandte Chemie International Edition |date=13 July 2020 |volume=59 |issue=29 |pages=12041–12047 |doi=10.1002/anie.202002687 |pmid=32301570 |pmc=7383983 |arxiv=2003.00753 }} 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,{{cite journal |last1=Mishra |first1=Shantanu |last2=Catarina |first2=Gonçalo |last3=Wu |first3=Fupeng |last4=Ortiz |first4=Ricardo |last5=Jacob |first5=David |last6=Eimre |first6=Kristjan |last7=Ma |first7=Ji |last8=Pignedoli |first8=Carlo A. |last9=Feng |first9=Xinliang |last10=Ruffieux |first10=Pascal |last11=Fernández-Rossier |first11=Joaquín |last12=Fasel |first12=Roman |title=Observation of fractional edge excitations in nanographene spin chains |journal=Nature |date=13 October 2021 |volume=598 |issue=7880 |pages=287–292 |doi=10.1038/s41586-021-03842-3|pmid=34645998 |arxiv=2105.09102 |bibcode=2021Natur.598..287M |s2cid=234777902 }} where signatures of both spin fractionalization and Haldane gap were observed.