pH-Dependent Excited-State Dynamics of [Ru(bpy)(3)](2+)-Modified Amino Acids: Effects of an Amide Linkage and Remote Functional Groups

Abstract

The pH-dependent photophysical properties of a series of polypyridyl ruthenium-substituted amino acids were investigated by steady-state and time-resolved luminescence spectroscopy. [H(3)N-DAPA(Rub(2)m)-OH](PF(6))(3) (1), [H(3)N-DABA(Rub(2)m)-OH](PF(6))(3) (2), [H(3)N-Orn(Rub(2)m)-OH](PF(6))(3) (3), and [H(3)N-Lys(Rub(2)m)-OH](PF(6))(3) (4) were obtained by formation of an amide link between the omega-NH(2) group of the respective commercially available amino acid and [Rub(2)(m-OH)](2+) (b = bipyridine, m-OH = 4'-methyl-2,2'-bipyridine-4-carboxylic acid). Due to the absence of significant electronic interactions between the ruthenium chromophore and the amino acid moieties, the energetics and extinction coefficients of the absorption spectra of 1-4 do not change as a function of pH. The luminescence intensities of these complexes, however, show a marked dependence on pH. At low pH (<2), quenching via excited-state protonation of the amide link leads to short lifetimes. In the pH 2-8 range, the lifetimes depend on the amino acid side chain length of the complex. At high pH (>9), lifetimes are approaching that of [Ru(bpy)(3)](2+), suggesting that the amino acid moiety has a negligible effect on nonadiabatic pathways in the excited-state decay of the ruthenium moiety. Our results are discussed with respect to the rapidly growing interest in ruthenium-substituted amino acids as spectroscopic and mechanistic tools in biological systems.