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Synthesis

Antiaromaticity flips single-molecule switch

Study shows comparatively unstable antiaromatic molecules can work well in molecular electronic devices

by Mitch Jacoby
November 6, 2017 | APPEARED IN VOLUME 95, ISSUE 44

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Pinned between gold electrodes (yellow), TBTP serves as a single-molecule switch as its core undergoes reversible oxidation between a nonconducting neutral state (red) and a conducting antiaromatic dication (green).
Pinned between gold electrodes (yellow), TBTP serves as a single-molecule switch as its core undergoes reversible oxidation between a nonconducting neutral state (red) and a conducting antiaromatic dication (green).

Ultrasmall electronic devices require ultrasmall components. That’s why scientists have been working for the past few decades on electronic circuitry in which function is controlled by just a single molecule. Those molecules nearly always sport conjugated aromatic backbones because aromaticity in those molecules usually goes hand-in-hand with enhanced conductivity. But a team led by Latha Venkataraman and Luis M. Campos of Columbia University and Jeffrey B. Neaton of Lawrence Berkeley National Laboratory has shown that aromatic isn’t the only way to go: The team has demonstrated a highly conducting, reversible, single-molecule switch based on antiaromaticity (Sci. Adv. 2017, DOI: 10.1126/sciadv.aao2615). In classical organic chemistry, Hückel’s rule states that conjugated cyclic molecules with 4n+2 π electrons exhibit enhanced stability because they are aromatic. Related molecules with 4n π electrons are comparatively unstable and described as antiaromatic, a concept pioneered by Columbia’s Ronald Breslow, who died Oct. 25. Intrigued by the possibility that a so-called unstable antiaromatic molecule might function as a switch, the team used a scanning tunneling microscope to pin a nonconducting thiophenylidene derivative (TBTP, shown) between gold electrodes. They showed that a quick electrooxidation step reversibly removes two electrons, converting the neutral molecule to a highly conducting antiaromatic dication that works stably and reproducibly as a switch.

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