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 PV(2+)2Cl(-)center dot 2H(2)O and PV center dot+PF(6)(-center dot)pyridine reveals the expected differences in bond lengths and also a structural change from two coplanar central rings in PV center dot+ to a twist of 36 degrees between the two central rings in PV2+. The phenyl viologen radical cation exhibits behavior characteristic of many radical cations, including weak pi-dimerization in the solid state and reversible pi-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.