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.