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A radical departure for supramolecular chemistry

Nature Reviews Chemistry volume 5, pages 442–443 (2021)Cite this article

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An experiment that challenged established dogma opened an entirely new avenue for research in the assembly and control of mechanically interlocked compounds.

Viologens, a class of compounds known formally as 1,1′-disubstituted 4,4′-bipyridinium ions, were first synthesized by Weidel and Russo1 in the 1880s by reacting 4,4′-bipyridine with an alkyl halide under reflux. Their informal name was coined in the early 1930s by Michaelis2 who was the first to report the electrochemical behaviour of these ions. At around –400 mV, the dicationic viologen undergoes a one-electron reduction to produce a radical cation, and there is an associated colour change from colourless to blue. Michaelis became interested in the applications of these molecules as redox indicators in biological systems. Hünig3 was also very active in studying the dimerization of viologens, starting around 1950 and has more than 100 publications devoted to the chemistry of bisquaternary 4,4′-bipyridinium salts. In the mid-1960s, Kosower and Cotter4 observed a further colour change — to violet/purple — at high concentrations in aqueous solutions and attributed this change to the formation of a radical–radical pi dimer, or pimer. More recently, many strategies have been employed to increase the stability of the dimer.

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Fig. 1: Good things can happen when you bend the rules.

References

  1. Weidel, H. & Russo, M. Studien über das pyridin. Monatshefte für Chemie 3, 850–885 (1882).

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  3. Hünig, S. & Berneth, H. in Organic Chemistry. Topics in Current Chemistry Vol. 92 (Springer, 1980).

  4. Kosower, E. M. & Cotter, J. L. Stable free radicals. II. The reduction of 1-methyl-4-cyanopyridinium ion to methylviologen cation radical. J. Am. Chem. Soc. 86, 5524–5527 (1964).

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  1. Chemistry Program, NYUAD Water Research Center, New York University Abu Dhabi (NYUAD), United Arab Emirates, Abu Dhabi, UAE

    Ali Trabolsi

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  1. Ali Trabolsi
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Correspondence to Ali Trabolsi.

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Trabolsi, A. A radical departure for supramolecular chemistry. Nat Rev Chem 5, 442–443 (2021). https://doi.org/10.1038/s41570-021-00299-w

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