Appearing in Nature Nanotechnology, IBM scientists in collaboration with chemists at the University of Warwick have synthesized and characterized a tricky molecule called triangulene, also known as Clar’s hydrocarbon, which was first hypothesized in 1953.
Anish Mistry, University of Warwick continues, “Chemists have always thought that triangulene would be too unstable to isolate. Building on our previous olympicene collaboration, we have added an extra ring to the molecule, and an extra level of complexity to the science, but have managed to make a previously impossible molecule with potentially really interesting properties.”
This molecule may have applications for quantum computers.
First author on the paper, IBM researcher, Niko Pavliček comments, “In this work, we used our atomic manipulation technique from the aryne and Bergman papers to generate triangulene, which had never been synthesized before. It’s a challenging molecule because it’s highly reactive, but it’s also particularly interesting because of its magnetic properties.”
In their latest research the sharp tip of the combined STM/AFM was used to remove two hydrogen atoms from the precursor molecule. The STM takes its measurement by quantum mechanical tunneling of electrons between a tip brought very close to a sample surface and applying a voltage between them. At appropriately high voltage, the ‘tunneling electrons’ can induce the removal of the specific bonds within the precursor molecule. The product molecule can then be characterized by its molecular orbitals when imaging at milder voltages.
These measurements, combined with density functional theory calculations, confirmed that triangulene keeps it free molecules’ properties on the surface.
The team also used the AFM, with a tip terminated with a single carbon monoxide molecule, to resolve or image the planar molecule with its six fused benzene rings, which appear in a symmetric triangle, for the first time. The results produced some pleasant surprises.
Gross explains, “Radicals feature unpaired electrons, and we previously were investigating sigma-radicals. In these, the unpaired electrons are assigned to certain atoms and we found that these always formed bonds with copper. But we were surprised that no bond formed for triangulene on copper. We think that is because triangulene is a pi-radical, which means it’s unpaired electrons are delocalized.”
It is exactly these unpaired electrons, which make the molecule interesting. In classical physics, a charged particle moving in space possesses angular momentum and produces a magnetic field around it. In quantum mechanics, every particle – moving in space or not – possesses an additional intrinsic angular momentum, which is called their ‘spin’. In most conventional hydrocarbons, electrons are always paired and the effect of their spins cancels. But in molecules like triangulene, the spin of the unpaired electrons leads to magnetism on the molecular scale.
The authors believe that beyond just the science there are also several interesting applications for this work.
Pavliček explains, “Triangulene-like segments incorporated into graphene nanoribbons have been suggested as an elegant way to design organic spintronic devices.”
Graphene nanoribbons are being researched for applications in nanocomposites materials, which are very strong and light. The field of spintronics is being studied by groups around the world, including at IBM, for information storage and processing.
Pavliček continues, “We could also demonstrate that its magnetism survives on xenon or sodium chloride surfaces. However, we cannot get a detailed picture of its magnetic state and possible excitations with our microscope (which lacks a magnetic field), so there is plenty to explore and discover for other groups.”
Some of this research is being conducted under a new collaborative consortium IBM has launched called the IBM Research Frontiers Institute. Within this framework members of the Institute jointly develop and share ground-breaking technologies and explore their business implications.
The research was also partially funded by the European Commission under the H2020 PAMS and ITN QTea projects and ERC grants CEMAS and AMSEL.
us papers, IBM scientists use a unique combined scanning tunneling microscope (STM) and atomic force microscope (AFM), both invented by former IBM scientists in the 1980s and recognized with the Nobel and Kavli Prizes, respectively.
Triangulene, the smallest triplet-ground-state polybenzenoid (also known as Clar’s hydrocarbon), has been an enigmatic molecule ever since its existence was first hypothesized1. Despite containing an even number of carbons (22, in six fused benzene rings), it is not possible to draw Kekulé-style resonant structures for the whole molecule: any attempt results in two unpaired valence electrons. Synthesis and characterization of unsubstituted triangulene has not been achieved because of its extreme reactivity, although the addition of substituents has allowed the stabilization and synthesis of the triangulene core and verification of the triplet ground state via electron paramagnetic resonance measurements. Here we show the on-surface generation of unsubstituted triangulene that consists of six fused benzene rings. The tip of a combined scanning tunnelling and atomic force microscope (STM/AFM) was used to dehydrogenate precursor molecules. STM measurements in combination with density functional theory (DFT) calculations confirmed that triangulene keeps its free-molecule properties on the surface, whereas AFM measurements resolved its planar, threefold symmetric molecular structure. The unique topology of such non-Kekulé hydrocarbons results in open-shell π-conjugated graphene fragments6 that give rise to high-spin ground states, potentially useful in organic spintronic devices. Our generation method renders manifold experiments possible to investigate triangulene and related open-shell fragments at the single-molecule level.
SOURCES- IBM, Warwick University, Nature Communications
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