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
Czech may refer to:
* Anything from or related to the Czech Republic, a country in Europe
** Czech language
** Czechs, the people of the area
** Czech culture
** Czech cuisine
* One of three mythical brothers, Lech, Czech, and Rus
*Czech (surnam ...
chemist
Erich Clar
Erich Clar (23 August 1902 – 27 March 1987) was a German organic chemistry, organic chemist, born in Hřensko (a village directly at the border of Germany and the Czech Republic; Czech: Hřensko, German: Herrnskretschen), who studied polycyclic a ...
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
The University of Warwick ( ; abbreviated as ''Warw.'' in post-nominal letters) is a public research university on the outskirts of Coventry between the West Midlands and Warwickshire, England. The university was founded in 1965 as part of ...
in collaboration with
IBM
International Business Machines Corporation (using the trademark IBM), nicknamed Big Blue, is an American Multinational corporation, multinational technology company headquartered in Armonk, New York, and present in over 175 countries. It is ...
.
Other attempts by
Japan
Japan is an island country in East Asia. Located in the Pacific Ocean off the northeast coast of the Asia, Asian mainland, it is bordered on the west by the Sea of Japan and extends from the Sea of Okhotsk in the north to the East China Sea ...
ese 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; it has 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; it has symbol Cu (from Latin ) 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-orang ...
base. The researchers used a combined
scanning tunneling and
atomic force microscope (STM/AFM) to remove individual
hydrogen
Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...
atoms. The synthesized molecule of triangulene remained stable at high-
vacuum
A vacuum (: vacuums or vacua) is space devoid of matter. The word is derived from the Latin adjective (neuter ) meaning "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressur ...
low-
temperature
Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...
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 finding ...
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 any member of a class of organic compounds that is composed of multiple fused aromatic rings. Most are produced by the incomplete combustion of organic matter— by engine exhaust fumes, tobacco, incine ...
(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 Approximation, approximate model used to describe the transition between Conductor (material), conducting and Electrical insulation, insulating systems.
It is particularly useful in solid-state physics. The model is named ...
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