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 anth racene (9EA) covalently bound to polymethacrylic acid (PMA) in pH 11 w ater and (2) vinyldiphenylanthracene (DPA) bound to polystyrene sulfon ate (PSS) in neutral water, where in each case the chromophores compri se less than 1 mol % of the polymer. The quencher used was Tl+ (from T lNO3) with additional ionic strength provided by KNO3. Quenching exper iments were performed as a function of quencher concentration and ioni c 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-V olmer plots for the steady-state and time-resolved data agree, which i mplies that quenching is almost entirely diffusive. At low ionic stren gths, the rates of fluorescence quenching in these polyelectrolyte sol utions exceed the diffusion-controlled rate expected for homogeneously distributed reactants by approximately 2 orders of magnitude. A drama tic reduction in the reaction rate is observed for only slight increas es in the ionic strength, and at high salt concentrations the rate asy mptotically approaches this diffusion-controlled limit. The Stern-Volm er plots exhibit negative curvature corresponding to that observed if a fraction of the fluorophores are inaccessible to quenchers. This ina ccessibility is interpreted in the context of a diffusion/reaction the ory. A simple model for the quenching dynamics using a Smoluchowski di ffusion 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.