Poly(azoaromatic viologen): its synthesis and reversibly photocontrollable supramolecular assembly in a polymer/low-molar-mass system

Poly(azoaromatic viologen) (PAV) having both photoisomerizable azobenzene chromophore and complexable bipyridinium unit in the backbone of the polymer chain was synthesized in an attempt to construct reversibly photo-induced, supramolecular assembling, polymer/low-molar-mass systems. It was found that PAV could form 1/1 intermolecular complex with Eosin Y (EY) dye and the interactions between PAV and EY changed reversibly by light-irradiation. The conductivity of the solution decreased after complexation. PAV solution irradiated with ultraviolet light (λmax=365 nm) showed an increase in conductivity. PAV exhibited a slightly larger trend to bind with EY under ultraviolet irradiation than that in the dark. PAV changed its conformation under irradiation with ultraviolet light from trans form to cis form, and recovered from cis form to trans form when it was kept in the dark again. The complex association constant (Ka) between the cis-PAV2n+ and EY2− was larger than that between the trans-PAV2n+ and EY2−. PAV displayed a photo-stimulated “on–off” binding with EY. The reversibly photo-induced change of properties of the complex solution has been explained by isomerization-diffusion mechanism.