pH-dependent excited-state dynamics of [Ru(bpy)(3)](2+)-modified amino acids: Effects of an amide linkage and remote functional groups

The pi-I-dependent photophysical properties of a series of polypyridyl ruth enium-substituted amino acids were investigated by steady-state and time-re solved luminescence spectroscopy. [H3N-DAPA(Rub(2)m)-OH](PF6)(3) (1), [H3N- DABA(Rub(2)m)-OH](PF6)(3) (2), [H3N-Orn(Rub(2)m)-OH](PF6)(3) (3), and [H3N- Lys(Rub(2)m)-OH](PF6)(3) (4) were obtained by formation of an amide link be tween the omega-NH2 group of the respective commercially available amino ac id 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 interacti ons between the ruthenium chromophore and the amino acid moieties, the ener getics 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 ex cited-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 t he 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 nona diabatic pathways in the excited-state decay of the ruthenium moiety. Our r esults are discussed with respect to the rapidly growing interest in ruthen ium-substituted amino acids as spectroscopic and mechanistic tools in biolo gical systems.