sv:Viologen Viologen, 4,4'-bipyridylium salt, compound of the form R₂C₁₀H₈N₂²⁺2X^- , where R can be a substituent for a nitrogen atom and <math>X^-<math> the anion needed to keep charge neatrality. The formula is very often shortened to just <math>V^{\cdot}<math>, where <math>\cdot<math> determinates its redox-state.

The reduction has an intermediate radical product <math>V^+<math> which is known to be very iron blue and where its name comes from ("violene"). The final neatural reduction <math>V^0<math> is yellow.

A special case of anion is the methyl, which makes the colouring process more reversible, since the compount is gathered in less size on the electrode surface and there can re-oxidation be perform more easily.

Viologen is intensively investigated and is used as a dye in electrochromic system because its ability to colour upon reduction and bleached upon oxidation.