Viologen
Organic compounds that switch between blue, violet and yellow when reduced or oxidised
No single value: Viologen is a material, and its colour varies. The background shows its family, blue.
Viologens are a group of organic salts based on modified bipyridine molecules. They are known for their ability to change colour reversibly—showing blue, violet, or yellow—when they gain or lose electrons. The name comes from the violet shade seen during reduction. Viologens are used in herbicides, batteries, and as redox indicators.
- Viologens change colour between blue, violet, and yellow when reduced or oxidised
- Paraquat, a methyl viologen, is a widely used herbicide
- Viologens are used in electrochromic devices and as redox indicators
- Their colour change is highly reversible and fast
What colour is Viologen?
Viologens are synthetic organic compounds that can display a range of colours depending on their chemical state. In their dication form, they are colourless or pale, but when reduced by gaining electrons, they become intensely blue or violet. Further reduction leads to a yellow colour. This ability to switch between colours sets viologens apart from many other compounds, making them useful as indicators and in devices that rely on colour changes.
Where does Viologen come from?
The name 'viologen' is linked to the violet colour that appears when these compounds are reduced. Viologens are derived from bipyridine, a molecule made of two connected pyridine rings. The compounds were first noticed for their colour-changing properties in the 1930s, when paraquat (a methyl viologen) was used as a redox indicator.
How is Viologen made?
Viologens are made by alkylating bipyridine, which means attaching alkyl groups (such as methyl or benzyl) to the nitrogen atoms of bipyridine using alkyl halides like methyl chloride or methyl bromide. This process produces water-soluble salts. The resulting compounds can be further modified to adjust their properties for different uses.
Where Viologen is used
Viologens are best known for their use in the herbicide paraquat. They are also used as colour-changing indicators in chemical reactions, and in electrochromic devices that change colour when an electric current is applied. Some experimental batteries use viologens in their electrolytes, taking advantage of their ability to reversibly accept and donate electrons.
Blue around the world
What the blue family means from country to country.
Lapis lazuli has been mined in the mountains of Badakhshan for more than six thousand years. Ground to powder it gave ultramarine, the costliest blue in European painting.
Royalty and powerSacred and holyAweSource: Lapis lazuliThe chadari is a full-length covering for women with a mesh panel over the eyes. In Kabul and the north it is most often a light blue.
Protection and healingSource: BurqaThe blue band of the flag stands for the wish of the Armenian people to live under peaceful skies. The red above it is for the highlands and the struggle to survive.
Politics and protestHopeSource: Flag of ArmeniaThe flag is sky blue, white and sky blue, created by Manuel Belgrano in 1812. National teams are called the Albiceleste, the white and sky blue.
Politics and protestPrideSource: Flag of ArgentinaBoca Juniors play in blue and gold. The club's story is that in 1907 it took the colours of the flag of the next ship into port, which was Swedish.
Festival and celebrationPrideSource: Boca JuniorsTo be true blue is to be honestly and loyally Australian. The phrase is older than the country, but Australians have made it their own.
Politics and protestTrustSource: True Blue (John Williamson song)Shades and relatives of Viologen
There are many types of viologens, depending on the alkyl group attached to the bipyridine core. Paraquat is a methyl viologen, while others use longer alkyl chains or benzyl groups. Diquat is a related compound based on a different arrangement of the bipyridine rings, and it also acts as a herbicide. The colour shown by a viologen depends on its exact structure and redox state: blue for 4,4'-viologens, green for 2,2'-derivatives, and yellow when fully reduced.
Questions people ask about Viologen
What colour are viologens?
Viologens can appear blue, violet, or yellow depending on their chemical state. The blue or violet colour appears when the compound is reduced, while the fully reduced form is yellow.
Where does the name 'viologen' come from?
The name 'viologen' refers to the violet colour that appears when these compounds are reduced. It is derived from their colour-changing property.
What are viologens used for?
Viologens are used in herbicides (notably paraquat), as colour-changing indicators in chemical reactions, and in electrochromic devices and some experimental batteries.
How are viologens made?
Viologens are made by attaching alkyl groups to bipyridine molecules using alkyl halides such as methyl chloride or methyl bromide, producing water-soluble salts.
Good reading and sources
The facts on this page come from the Wikipedia article "Viologen", written by its contributors and used under the CC BY-SA 4.0 licence, and from its Wikidata record (CC0). Rang rewrote the text in its own words, with AI assistance, and compares every page with the article to make sure no passage is copied. It may be reused under the same CC BY-SA 4.0 licence with a link back.
It may contain mistakes. The swatch is an approximation: named colours are not fixed standards, and every screen shows them a little differently. If something is wrong, tell us. Last refreshed 2026-10-03.
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