Two-Dimensional Ordering of Viologen Polymers Fixed on Charged Surface of Bilayer Membranes: A Peculiar Odd-Even Effect on Redox Potential and Absorption Spectrum

Polyion complex films were prepared from viologen ionene polymers and anionic bilayer membranes. Electrochemical reduction and electrochromic behavior of the complex films were examined. A peculiar odd-even effect of alkyl chain connection among viologen groups was found in the absorption spectra, in the redox potential, and in the diffusion coefficients (D_app) of the charge transport process. Blue shift in the absorption spectrum indicated dimer radical formation in the odd-numbered polymer. While in the even-numbered polymer, dimerization was suppressed. Cyclic voltammetry showed that the odd polymers were more easily reduced than the even polymers. The D_app value of the odd polymer was smaller than that of the even polymer. That suggests the dimer radical formed in the odd polymer acts as a trapping site of the hopping electron. The odd-even effect was assumed to be explained in the fixed conformation of the viologen polymers ordered on the regularly charged surface of the bilayer membranes.