Proton Transfer Quenching of the MLCT Excited State of Ru(phen)2dppz2+ in Homogeneous Solution and Bound to DNA

The bimolecular quenching of the metal-to-ligand charge transfer (MLCT) excited state of Ru(phen)z(dppz)2+ (phen = 1,lO-phenanthroline, dppz = dipyrido[3,2-a:2’,3’-clphenazine) by proton transfer has been investigated in homogeneous acetonitrile solutions and in the presence of calf thymus DNA. In acetonitrile the monoexponential decay of the MLCT excited state emission of R~(phen)2(dppz)~+ is dynamically quenched by proton donors with pKa = 4.7-15.7. The emission lifetimes and quenching when the complex is bound to DNA have been measured for racemic mixtures, as well as Aand A-R~(phen)2(dppz)~+, and the values compared. In the presence of DNA the biexponential decay of the emission is quenched dynamically and three times slower than in acetonitrile when the quencher, in this case hydroquinone, is hydrophilic. Static quenching is observed in the presence of DNA when a hydrophobic proton donor, o-chlorophenol, is utilized. The static quenching with o-chlorophenol is shown to arise solely from the quenching of the long-lived component. These observations are explained in terms of two different modes of binding between the complex and DNA, as well as the different affinities for the aqueous medium of the quenchers.