Antiaromatic molecule displays record electrical conductance
Organic materials often have a lower production cost than traditional electric conductors like metals and semiconductors. Not all organic systems conduct electricity well, however. A class of organic materials known as antiaromatic compounds -- featuring planar rings of carbon atoms sharing a number of electrons that is a multiple of 4 -- have been predicted to be excellent conductors, but this prediction has been hard to verify since antiaromatic molecules are usually unstable. Now, Shintaro Fujii and Manabu Kiguchi from the Tokyo Institute of Technology and colleagues have performed a systematic study of charge transport in a single, stable antiaromatic molecule. Compared to a structurally related aromatic molecule (where the carbon rings share an extra two electrons), its record electrical conductance is an order of magnitude higher.
left: Antiaromatic norcorrole-based Ni complex, Ni(nor). right: Aromatic Ni porphyrin-based complex, Ni(porph).
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The researchers studied a particular norcorrole-based nickel complex, Ni(nor), which is antiaromatic but stable, and a structurally similar aromatic, porphyrin-based nickel complex, Ni(porph). They measured the conductivities of the two compounds by means of the scanning tunneling microscopy break-junction technique; in such a setup, the current through a single molecule sandwiched between two parts of a broken junction is measured as a function of applied voltage. With a conductance of over 4x10 -4 conductance quanta, Ni(nor) is the most conductive known organometallic complex. Ni(porph) was found to have a value about 25 times less, a result confirming the superior conductivity of antiaromatic molecules. Via theoretical calculations of the molecules' electronic structure and charge transport property, the scientists were able to identify the origin of the antiaromaticity-enhanced conductance: for Ni(nor), the lowest unoccupied molecular orbital lies closer to the Fermi level (the amount of work needed to add one electron to the system) than for Ni(porph).
Fujii and colleagues also succeeded in demonstrating tunability of the single-molecule conductance of Ni(nor). By applying a technique known as electrochemical gating, which enables controlling the molecular energy levels relative to the Fermi level of the source and drain electrodes (of the single-molecule junction) by varying an applied electrochemical potential, the researchers demonstrated a 5-fold modulation of the conductance of Ni(nor).
The findings of Fujii and colleagues show that antiaromatic materials are promising low-cost systems exhibiting high electrical conductance. In the words of the researchers, their study "provides relevant guidelines for the design of molecular materials for highly conducting single-molecule electronics.
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