Spin pairing (`dimerisation) of the viologen radical cation: kinetics and equilibria

Viologen radical cations dimerise (spin pair) in aqueous solution, causing the colour of the viologen dye to alternate between blue (monomer) and dimer (red). Equilibrium constants K of viologen dimerisation have been determined using UV¯vis spectroscopy for alkyl viologen radical-cation species in halide solutions. Equilibrium constants of dimerisation were obtained by deconvolution of optical spectra. Values of deltaG°dim have a crude linear dependence on n, the length of the alkyl substituent chain, for n=2 to 8. Rate constants of spin-pairing kdim (i.e. dimerisation of the radical cation) have been determined. Bjerrum plots indicate that the rate-limiting reaction during dimerisation involves anions: it is postulated that the dimerisation reaction is two-step, with dimer formation occurring after the formation of an association pair comprising a radical cation and a counter anion. Recent comproportionation results are used to analyse the structure of the dimer and the role of anions during dimerisation. The possibility that dimer comprises anions placed between the planes of the two bipyridilium radicals is also discussed.