Diquat is an isomer of viologens, being derived from 2,2-bipyridine (instead of the 4,4-isomer). It also is a potent herbicide that functions by disrupting electron-transfer.
Extended viologens have been developed based on conjugated oligomers such as based on aryl, ethylene, and thiophene units are inserted between the pyridine units. The bipolaron di-octyl bis(4-pyridyl)biphenyl viologen 2 in scheme 2 can be reduced by sodium amalgam in DMF to the neutral viologen 3.
The resonance structures of the quinoid 3a and the biradical 3b contribute equally to the hybrid structure. The driving force for the contributing 3b is the restoration of aromaticity with the biphenyl unit.  It has been established using X-ray crystallography that the molecule is, in effect, coplanar with slight nitrogen pyramidalization, and that the central carbon bonds are longer (144 pm) than what would be expected for a double bond (136 pm). Further research shows that the diradical exists as a mixture of triplets and singlets, although an ESR signal is absent. In this sense, the molecule resembles Tschischibabin's hydrocarbon, discovered during 1907. It also shares with this molecule a blue color in solution, and a metallic-green color as crystals.
Compound 3 is a very strong reducing agent, with a redox potential of &minus;1.48 V.