Photoluminescence of water-soluble conjugated polymers: Origin of enhancedquenching by charge transfer

Quenching of the photoluminescence (PL) emission from conjugated macromolec ules by charge transfer to cationic electron accepters is studied by changi ng the ion concentration in buffered aqueous solutions, by changing the con centration of accepters, and by varying the temperature. A weakly bound com plex is formed tin water) between polyanionic conjugated macromolecules and cationic electron accepters with Coulomb binding energy similar to 150 meV . The mean distance between the polymer((-)) and quencher((+)) was estimate d as 10 Angstrom, consistent with the effective charge-transfer distance. A t high quencher concentrations, quenching by accepters within the sphere-of -action becomes important; a modified Stern-Volmer equation is used to esti mate the radius (similar to 400 Angstrom). High-temperature studies of conc entrated quencher solutions independently indicate a crossover from static quenching to sphere-of-action quenching with a radius similar to 400 Angstr om. Since the radius is comparable to the radius of gyration of the macromo lecule, the high molecular weight of the conjugated polymer enhances the qu enching.