Isolation and Characterization of Phenyl Viologen as a Radical Cation and Neutral Molecule

The chemical synthesis, isolation, and characterization of phenyl viologen (PV) as a dication, radical cation, and neutral species are described. Single-crystal X-ray diffraction of PV2+2Cl-·2H2O and PV•+PF6-·pyridine reveals the expected differences in bond lengths and also a structural change from two coplanar central rings in PV•+ to a twist of 36° between the two central rings in PV2+. The phenyl viologen radical cation exhibits behavior characteristic of many radical cations, including weak π-dimerization in the solid state and reversible π-dimerization in solution. Electrical conductivity measurements of neutral phenyl viologen, the first such measurements of a neutral viologen, reveal that it is a significantly better conductor than the radical cation. Differences in geometric relaxation during charge transfer offer a possible explanation for the higher conductivity of the neutral viologen.