QUENCHING KINETICS OF ANTHRACENE COVALENTLY BOUND TO A POLYELECTROLYTE .1. EFFECTS OF IONIC-STRENGTH

Steady-state and time-resolved fluorescence quenching experiments have been performed for the following polyelectrolytes: (1) 9-ethanol anthracene (9EA) covalently bound to polymethacrylic acid (PMA) in pH 11 water and (2) vinyldiphenylanthracene (DPA) bound to polystyrene sulfonate (PSS) in neutral water, where in each case the chromophores comprise less than 1 mol % of the polymer. The quencher used was Tl+ (from TlNO3) with additional ionic strength provided by KNO3. Quenching experiments were performed as a function of quencher concentration and ionic strength. The quencher concentration ranged from 0 to 3 mM, and the ionic strength ranged from 2 to 100 mM. At each ionic strength Stern-Volmer plots for the steady-state and time-resolved data agree, which implies that quenching is almost entirely diffusive. At low ionic strengths, the rates of fluorescence quenching in these polyelectrolyte solutions exceed the diffusion-controlled rate expected for homogeneously distributed reactants by approximately 2 orders of magnitude. A dramatic reduction in the reaction rate is observed for only slight increases in the ionic strength, and at high salt concentrations the rate asymptotically approaches this diffusion-controlled limit. The Stern-Volmer plots exhibit negative curvature corresponding to that observed if a fraction of the fluorophores are inaccessible to quenchers. This inaccessibility is interpreted in the context of a diffusion/reaction theory. A simple model for the quenching dynamics using a Smoluchowski diffusion equation and a Poisson-Boltzmann potential of mean force for a rod-like polymer is briefly discussed and shown to account for many, but not all, aspects of the observations.