Superquenching in cyanine pendant poly(L-lysine) dyes: Dependence on molecular weight, solvent, and aggregation

A series of poly(L-lysines) ranging in number of repeat units (N-PRU) from 6 to 900 has been synthesized and the photophysics of the series and monomer cyanine dye have been studied in solution. In water or aqueous dimethyl sulfoxide, the oligomers and polymers exhibit high sensitivity to fluorescence quenching by oppositely charged electron acceptors; in this study, 9,10-anthraquinone-2,6-disulfonate was used as a quencher for the cationic fluorescent polyelectrolytes. Quenching constants (K-SV) measured in 50:50 (v/v) dimethyl sulfoxide-water increase monotonically with increase in N-PRU ranging from 630 M-1 for monomer to 1.2 x 10(9) M-1 for dilute solutions of the polymer having N-PRU similar to900. The polymers having N-PRU > 100 exhibit predominantly J-aggregate absorption and fluorescence and enhanced susceptibility to quenching. For the polymers exhibiting strong J-aggregation, the effective exciton domain quenched by a single quencher reaches similar to100 PRU. The results of this study permit a semiquantitative analysis of superquenching of fluorescent polyelectrolytes in solution and the factors that control it.