Photoluminescence of Water-Soluble Conjugated Polymers: Origin of Enhanced Quenching by Charge Transfer

Abstract

Quenching of the photoluminescence (PL) emission from conjugated macromolecules by charge transfer to cationic electron acceptors is studied by changing the ion concentration in buffered aqueous solutions, by changing the concentration of acceptors, and by varying the temperature. A weakly bound complex is formed (in water) between polyanionic conjugated macromolecules and cationic electron acceptors with Coulomb binding energy ∼150 meV. The mean distance between the polymer(-) and quencher(+) was estimated as 10 Å, consistent with the effective charge-transfer distance. At high quencher concentrations, quenching by acceptors within the sphere-of-action becomes important; a modified Stern−Volmer equation is used to estimate the radius (∼400 Å). High-temperature studies of concentrated quencher solutions independently indicate a crossover from static quenching to sphere-of-action quenching with a radius ∼400 Å. Since the radius is comparable to the radius of gyration of the macromolecule, the high molecular weight of the conjugated polymer enhances the quenching.