Viologens

Viologens are a family of redox-active organic compounds based on the 4,4'-bipyridinium core. They are typically formed as quaternary salts in which both nitrogens of the bipyridine units are alkylated, yielding divalent viologen cations that are paired with counterions such as chloride. The best-known member is methyl viologen, commonly called paraquat, along with other derivatives such as ethyl, propyl, and benzyl viologens. Substituents and counterions are used to tune solubility, redox potential, and stability.

Redox chemistry and colors are central to viologens. The viologen dication (MV2+) can be reduced in one step to the radical cation (MV+•), a species that is typically intensely colored (blue in many solutions). A further one-electron reduction yields the neutral viologen (MV0), which is usually colorless. The redox couples MV2+/MV+• and MV+•/MV0 are well defined and highly reversible, and the exact potentials depend on substituents and solvent.

Applications include use as redox mediators and electron acceptors in electrochemical and photochemical systems, as well as in electrochromic devices where color changes accompany redox cycling. Viologens have been explored in redox-flow batteries as organic electrolytes and in various catalytic and polymerization processes. Some derivatives, notably paraquat, are highly toxic and face strict regulatory controls in many countries, highlighting the need for careful handling and environmental consideration.