Interaction of sulfonated ruthenium(ii) polypyridine complexes with surfactants probed by luminescence spectroscopy

Article abstract of DOI:10.1002/1522-2675(20010919)84:9<2708::AID-HLCA2708>3.0.CO;2-M

Novel anionic [RuL₂L′]^2- complexes, where L stands for (1,10-phenanthroline-4,7-diyl)bis(benzenesulfonate) (pbbs; 3a) or (2,2′-bipyridine)-4,4′-disulfonate (bpds; 3b), and L′ is N-(1,10-phenanthrolin-5-yl)tetradecanamide (pta; 2a) or N-(1,10-phenanthrolin-5-yl)acetamide (paa; 2b), were synthesized, and their interaction with the prototypical surfactants sodium dodecylsulfate (SDS), cetyl trimethyl ammonium bromide (CTAB), and Triton X-100 (TX-100) was investigated by electronic absorption, luminescence spectroscopy, emission-lifetime determinations, and O₂-quenching measurements. [Ru(bpds)₂(pta)]^2- (5a) displayed cooperative self-aggregation in aqueous medium at concentrations above 1.3 μM; the observed association was enhanced in the presence of either β-cyclodextrin or NaCl. This amphiphilic Ru¹¹ compound showed the strongest interaction with all the detergents tested: nucleation of surfactant molecules around the luminescent probe was observed below their respective critical micellar concentrations. As much as a 12-fold increase of the emission intensity and a 3-fold rise in the lifetime were measured for 5a bound to TX-100 micelles; the other complexes showed smaller variations. The O₂-quenching rate constants decreased up to 1/8 of their original value in H₂O (e.g., for [Ru(bpds)₂(pta)]^2- (6a) bound to CTAB micelles). Luminescence-lifetime experiments in H₂O/D₂O allowed the determination of the metal-complex fraction exposed to solvent after binding to surfactant micelles. For instance, such exposure was as low as 25% for pta complexes · CTAB aggregates. The different behaviors observed were rationalized in terms of the Ru¹¹ complex structure, the electrostatic/hydrophobic interactions, and the probe environment.