pH dependent excited state properties of ruthenium modified amino acids

The pH dependent emission properties of a series of [Ru(bpy)(3)](2+) substi tuted amino acids were investigated by steady-state and time-resolved lumin escence spectroscopy. Both emission intensities and lifetimes were found to depend on the protonation state of the amino acid moiety. Protonation of t he amine function results in increasing non radiative decay rates which are reverse proportional to the number of methylene groups separating the chro mophore from the amino acid moiety. For comparison, two derivatives were in vestigated in which the amino acid moiety is replaced by an amine and a tet raalkylammonium function, respectively. The data suggest that the intramole cular presence of an ammonium ion modulates the electronic properties of th e amide link which connects the ruthenium chromophore to the rest of the mo lecule. This in turn results in a more localized MLCT excited state and in increased non radiative decay rates. Our study demonstrates how amide group s can mediate electronic interactions between functional groups which are n ot or weakly coupled otherwise. The relevance of such effects for the appli cation of spectroscopic labels and optical sensing devices is discussed.