Can Luminescent Ru(II) Polypyridyl Dyes Measure pH Directly?

Abstract

Two molecularly engineered Ru(II) complexes for direct pH optosensing in environmental or physiological media based on luminescence lifetime measurements-namely, Na(2)[Ru(bpds)(2)(F(15)ap)] and Na(2)[Ru(pbbs)(2)(pyim)] (where bpds = 2,2'-bipyridine-4,4'-disulfonate, F(15)ap = 5-perfluorooctanamide-1,10-phenanthroline, pbbs = 1,10-phenanthroline-4,7-(diyl)bis(benzenesulfonate), and pyim = 2-(2'-pyridyl)imidazole)-have been prepared. The suitability of these two luminophores as general-purpose pH indicators has been assessed to determine the general features of Ru(II) dyes required for such application. Their photochemical properties were investigated at different pH values in various buffer solutions using absorption spectroscopy, as well as steady-state and time-resolved luminescence. Both dyes display a parallel absorption and emission behavior as a function of pH (2-10), namely, higher luminescence in acidic solutions together with a 8-10 nm bathochromic shift in their (blue) absorption and 6-39 nm bathochromic shift in their (red) luminescence maxima in basic media, respectively. Similar ground-state acidity values (pK(a)) of 6.5 +/- 0.2 for the amide group of the F(15)ap complex and 6.9 +/- 0.2 for the imidazole NH moiety of the pyim complex have been measured. However, dramatic differences in their luminescence lifetimes as a function of pH were found. The HA and A(-) forms of *[Ru(bpds)(2)(F(15)ap)](2-) conveniently display lifetimes of 372 and 263 ns, respectively, regardless of the solution acidity and buffer nature. Their relative contributions to the overall decay (0%-100%) are dependent on the solution pH indicating excited-state proton exchange rates well below the decay rates of the acidic and basic forms. However, *[Ru(pbbs)(2)(pyim)](2-) deactivation kinetics show a pH-independent component of 80 ns at high pH and an acidity-sensitive one that varies from 610 ns (at pH 2) to 170 ns (at pH 10). Both components are also dependent on the buffer nature and concentration, also indicating the lack of an acid-base equilibrium in its excited-state but an irreversible proton transfer by the buffer species. Density functional theory calculations have demonstrated the difficult accessibility of the base to the acidic perfluoroamide proton of the F(15)ap complex, severely slowing the excited-state proton transfer kinetics of the luminescent dye. Therefore, we conclude that the design of Ru(II) polypyridyl lifetime-based pH indicators not affected by the buffer nature and concentration requires the absence of proton exchange during the radiative deactivation of both the acidic and basic species, which then would remain in their ground-state relative ratio. This feature may be achieved by (a) mild excited-state acidities and (b) structural features that shield the exchangeable proton from the buffer access.