Fluorescence, metal ion binding, and viscosity properties of poly[2- and 4-vinylpyridine N-oxide]s in aqueous solution

Poly(2-vinylpyridine N-oxide) (P2VPNO) and poly(4-vinylpyridine N-oxide) (P4VPNO) were synthesized by the oxidation of the corresponding poly(vinylpyridine)s. They are soluble in water and have a characteristic fluorescence in aqueous solutions. The fluorescence intensities of P2VPNO were over 3 times that of P4VPNO in 10(-2) M aqueous solutions at ambient temperature, which was accounted for by the difference in the microenvironment of the pyridine N-oxide fluorophore. The N-oxide polymers formed strong complexes with Tb3+ in aqueous solution, and the fluorescence intensities of Tb3+ were greatly enhanced upon complex formation. The increase in fluorescence intensity is due both to the replacement of inner-coordinated water molecules of Tb3+ by N-oxide polymers and to a ligand-to-metal energy transfer. P4VPNO is a better complexing agent for Tb3+ than P2VPNO. The reduced viscosity of P4VPNO increased sharply at polymer concentrations smaller than 1.0 g/dL, as in polyelectrolytes. However, in the case of P2VPNO, the concentration dependence of the reduced viscosity was similar to that of nonionic polymers. These differences are due to the position of the N-O group. P2VPNO and P4VPNO have a charge separation across the N+-O- bond with a large dipole moment. P4VPNO forms a partial complex with water which transforms the N-O group into a weak acid. The complex then behaves like a polyelectrolyte, and upon dilution, the reduced viscosity increases due to chain expansion. In the case of P2VPNO, the N-O group is close to the polymer backbone, which sterically decreases the formation of the water complex, and its reduced viscosity then behaves like that of nonionic polymers. Although P2VPNO does not behave like a polyelectrolyte, addition of Tb3+ reduces the solution viscosity due to a complexation of Tb3+ with the ligands.