Viologen

The Viologens are diquaternary derivatives of 4,4'-bipyridyl. The name comes from the fact that this class of compounds is easily reduced to the radical mono cation, which is intensely blue coloured. In an experimental electrolysis setup, viologen in solution with sodium sulfate is reduced at the cathode with simultaneous formation of hydrogen gas. oxygen generated at the anode is capable of oxidizing the radical ion back to the viologen.

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Further reduction yields a yellow quinoid compound. Diquaternary derivatives of 2,2'-bipyridyl give a green radical anion.

Viologens are investigated for use in electrochromic systems because of their ability to change color reversibly many times upon reduction and oxidation. Paraquat is a viologen used as a herbicide.

In extended viologens conjugated oligomers such as based on aryl, ethylene and thiophene units are inserted between the pyridine units.^[1] The bipolaron di-oktyl 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 diradical 3b contribute equally to the hybrid structure. The driving force for the contributing 3b is the restoration of aromaticity with the biphenyl unit. From X-ray crystallography it is established that the molecule is coplanar and with slight nitrogen pyramidalization and that the central carbon bonds a 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 remarkably an ESR signal is absent. In this sense the molecule resembles Chichibabin's hydrocarbon discovered in 1907. The blue color in solution and metallic green color as crystals are also in common.

Compound 3 is a very strong reducing agent with a redox potential of - 1.48 V again because aromaticity is restored. The compound is also a liquid crystal with multiple liquid crystal phases in the melt as a result of the molecule's structure with a flat and rigid core and flexible linear alkyl arms.

References

1. ^ W. W. Porter, T. P. Vaid and A. L. Rheingold (2005). "Synthesis and Characterization of a Highly Reducing Neutral "Extended Viologen" and the Isostructural Hydrocarbon 4,4' '-Di-n-octyl-p-quaterphenyl". J. Am. Chem. Soc. 127 (47): 16559-16566. doi:10.1021/ja053084q.