Triangulene (also known as Clar's hydrocarbon) is the smallest
triplet-ground-state polybenzenoid. It exists as a
biradical with the chemical formula . It was first hypothesized by
Czech chemist
Erich Clar Erich Clar (23 August 1902 in Hřensko – 27 March 1987 in Estepona) was an Austrian organic chemist who studied polycyclic aromatic hydrocarbon chemistry. He is considered as the father of that field. In 1941, he authored "Aromatische Kohlenwasser ...
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
, mottoeng = Mind moves matter
, established =
, type = Public research university
, endowment = £7.0 million (2021)
, budget = £698.2 million (202 ...
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
Xenon is a chemical element with the symbol Xe and atomic number 54. It is a dense, colorless, odorless noble gas found in Earth's atmosphere in trace amounts. Although generally unreactive, it can undergo a few chemical reactions such as the ...
or
copper
Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish ...
base. The researchers used a combined
scanning tunneling and
atomic force
Atomic may refer to:
* Of or relating to the atom, the smallest particle of a chemical element that retains its chemical properties
* Atomic physics, the study of the atom
* Atomic Age, also known as the "Atomic Era"
* Atomic scale, distances com ...
microscope (STM/AFM) to remove individual
hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...
atoms. The synthesized molecule of triangulene remained stable at high-
vacuum
A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or " void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often di ...
low-
temperature
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer.
Thermometers are calibrated in various temperature scales that historically have relied on ...
conditions for four days, giving the scientists plenty of time to characterize it (also using STM/AFM).
'n''riangulenes
Triangulene, as defined here, is a member of a wider class of
'n''riangulenes, where ''n'' is the number of hexagons along an edge of the molecule. Thus, triangulene may also be referred to as
riangulene.
Theory
A
tight-binding description of the
molecular orbitals
In chemistry, a molecular orbital is a mathematical function describing the location and wave-like behavior of an electron in a molecule. This function can be used to calculate chemical and physical properties such as the probability of find ...
of
'n''riangulenes predicts
that
'n''riangulenes have (''n'' − 1) unpaired electrons, which are associated to (''n'' − 1) non-bonding states. When electron–electron interactions are included, theory predicts
that the ground state total spin quantum number ''S'' of
'n''riangulenes is ''S'' = . Thus,
riangulenes 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 among all
polycyclic aromatic hydrocarbon
A polycyclic aromatic hydrocarbon (PAH) is a class of organic compounds that is composed of multiple aromatic rings. The simplest representative is naphthalene, having two aromatic rings and the three-ring compounds anthracene and phenanthrene. ...
(PAH) diradicals, due to maximum overlap of the wave function of the unpaired electrons.
The ground state spin of
'n''riangulenes can be rationalized
in terms of a theorem by
Elliot H. Lieb, which relates, for a bipartite lattice, the ground state spin of the
Hubbard model
The Hubbard model is an approximate model used to describe the transition between conducting and insulating systems.
It is particularly useful in solid-state physics. The model is named for John Hubbard.
The Hubbard model states that each ...
at half filling to the sublattice imbalance.
Experiments
So far, the ultra-high vacuum on-surface syntheses of
'n''riangulenes with ''n'' = 3,
4, 5 and 7 (the hitherto largest triangulene homologue) have been reported. In addition, the on-surface synthesis of
riangulene dimers 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
riangulene-based quantum spin chains on a gold surface,
where signatures of both spin fractionalization and Haldane gap were observed.
References
{{PAHs
Polycyclic aromatic hydrocarbons
Free radicals