Me. Morrison et al., QUENCHING KINETICS OF ANTHRACENE COVALENTLY BOUND TO A POLYELECTROLYTE .1. EFFECTS OF IONIC-STRENGTH, Journal of physical chemistry, 98(21), 1994, pp. 5534-5540
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.