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

Novel anionic [RuL2L'](2-) complexes. where L stands for (1,10-phenanthroli ne-4,7-diyl)bis(benzenesulfonate) (pbbs: 3a) or (2.2'-bipyridine)-4,4'-disu lfonate (bpds: 3b), and L' is N-(1,10-phenanthrolin-5-yl)tetradecanamide (p ta: 2a) or N-(1,10-phenanthrolin-5-yl)acetamide (paa: 2b). were synthesized , and their interaction with the prototypical surfactants sodium dodecylsul fate (SDS), cetyl trimethyl ammonium bromide (CTAB), and Triton X-100 (TX-1 00) was investigated by electronic absorption, luminescence spectroscopy, e mission-lifetime determinations. and O-2-quenching measurements. [Ru(pbbs)( 2)(pta)](2)- (5a) displayed cooperative self-aggregation in aqueous medium at concentrations above 1.3 mum the observed association was enhanced in th e presence of either beta -cyclodextrin or NaCl. This amphiphilic Ru-II com pound showed the strongest interaction with all the detergents tested: nucl eation of surfactant molecules around the luminescent probe was observed be low 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 Sa bound to TX-100 micelles, the other complexes showed smalle r variations. The O-2-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-life time experiments in H2O/D2O allowed the deter mination 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 ration alized in terms of the Ru-II complex structure. the electrostatic/hydrophob ic interactions, and the probe environment.