FLUORESCENT DYES AS MODEL HYDROPHOBIC MODIFIERS OF POLYELECTROLYTES -A STUDY OF POLY(ACRYLIC ACID)S LABELED WITH PYRENYL AND NAPHTHYL GROUPS

A commercial poly(acrylic acid) (PAA, M-nu = 150 000) has been labelle d with low levels (0.4 to 3.1 mol%) of either pyrene, naphthalene or b oth chromophores introduced randomly along the chain. The photophysica l properties of the polymers have been investigated by steady-state an d time-resolved fluorescence spectroscopy. The pH-behaviour of the pol ymers has been examined by non-radiative energy transfer (NRET) measur ements and by the ratio (I-E/I-M) of excimer to monomer intensities. T he pH-induced expansion of the pyrene-labelled PAA is accompanied by a sharp decrease in the ratio I-E/I-M, signalling the disruption of pyr ene excimer forming sites. In the case of the doubly labelled PAA, the extent of NRET, which monitors the increase in the naphthalene to iso lated pyrene separation distance, occurs over a wide pH range (3.0 to 4.6). In contrast, the decrease in I-E/I-M, which reports on the fract ion of pyrene groups located in excimer forming sites, takes place sha rply at pH 4.5 which is also the polymer pK(a) derived from potentiome tric titrations. (C) 1998 Elsevier Science Ltd. All rights reserved.