Viologen

[2] Other viologens have been commercialized because they can change color reversibly many times through reduction and oxidation.

The second reduction yields a yellow quinoid compounds: The electron transfer is fast because the redox process induces little structural change.

Their tendency to form host–guest complexes is key to the molecular machines recognized by the 2016 Nobel Prize in Chemistry.

Viologens have been modified to optimize their performance in such batteries, e.g. by incorporating them into redox-active polymers.

Further research shows that the diradical exists as a mixture of triplets and singlets, although an ESR signal is absent.

Compound 3 is a very strong reducing agent, with a redox potential of −1.48 V. The widely used herbicide paraquat is a viologen.

The toxicity of the 2,2'-, 4,4'-, or 2,4'-bipyridylium-based viologens is related to their ability to form stable free radicals.

[9][10][11] Viologens have been commercialized as electrochromic systems because of their highly reversible and dramatic change of color upon reduction and oxidation.