Probing the presence of hydrophobic sites in anionic polyelectrolytes with tris(polypyridine)chromium(III) complexes

The emission properties of tris(polypyridine)chromium(III) complexes, [Cr(LL)3]3+ (LL=2,2′-bipyridine (bpy), 1,10-phenanthroline (phen), 4,7-dimethylphenanthroline (dim), and 4,7-diphenylphenanthroline (dip)), in air-saturated aqueous solutions, are dramatically modified in the presence of anionic polyelectrolytes. Investigated polyions included poly(acrylic acid) (PAA), poly(methacrylic acid) (PMA), and sodium poly(styrenesulfonate) (PSS). All four chromium complexes displayed tunable emission properties in the polyelectrolyte microenvironment, in a degree that depended on the hydrophobic character of both the macromolecule and the polyamine. The largest effect was observed for 2.5×10−5 M aqueous solutions of [Cr(dip)3]3+ in 4×10−3 M PMA at approximately pH 5 and t=15°C, where IPMA/Iw=25.66 (IPMA=emission intensity in aqueous solutions containing PMA; Iw=emission intensity in water). Decay lifetimes revealed that [Cr(dip)3]3+ units are distributed in hydrophobic and hydrophilic sites of the PMA, giving rise to short and long-lived intermediates. Distribution of [Cr(dip)3]3+ units in PAA and PSS is homogeneous throughout the polyelectrolyte, and the excited-state intermediates decay following a first order exponential rate law. All other investigated chromium polypyridines showed a homogeneous distribution in solutions containing PMA, PAA or PSS.