Charge density effects on the fluorescence behavior of dansyl labels to strong polyelectrolytes

Fluorescent spectra of dansyl-labeled copolymers containing 2-acrylamido-2-methylpropanesulphonic acid (AMPS) and N,N-dimethylacrylamide (DMAA) with 0.2 mol% chromophore were observed in aqueous solution. Two emission bands appeared at 336 and 533 nm for protonated and unprotonated species of the label, respectively. These wavelengths were independent of the mole fraction FAMPS of AMPS ranging from 0.186 to 0.738 but the relative intensity I336/I533 increased proportionally to FAMPS. The locally excited emission (LE), b∗ band of the dansyl label was found at 405 nm when the chromophore concentration was decreased below 1.45×10−5 mol/l. With increasing charge density in the solid copolymer the b∗ band was strengthened and the a∗ band for the twisted intramolecular charge-transfer state was weakened and even disappeared owing to the existence of ion-pair attractions. It was found that for the copolymer in dimethyl sulfoxide (DMSO)/tetrahydrofuran (THF) mixtures the change in charge density did not alter the emission wavelength λem of the a∗ band. The charge density dependence for the threshold of DMSO/THF composition, at which the relative emission intensity RF abruptly decreased, was similar to that inducing the volume phase transition in the corresponding polyelectrolyte gels.