Effects of Methyl Substitution on Radiative and Solvent Quenching Rate Constants of [Ru(phen)2dppz]2+ in Polyol Solvents and Bound to DNA

Methyl substituents on the distant benzene ring of the dppz ligand in the "light switch" complex [Ru(phen)2dppz]2+ have profound effects on the photophysics of the complexes in water as well as in the polyol solvents ethylene glycol, glycerol, and 1,2- and 1,3-propanediol. Whereas 11,12-dimethyl substitution decreases the rate of quenching by diminishing hydrogen bonding by solvent, the 10-methyl substituent in addition also decreases both the radiative and the nonradiative rate constant for decay to the ground state of the non-hydrogen-bonded excited state species. For both the 10-methyl and the 11,12-dimethyl derivatives, the effect of methyl substitution on the equilibrium of solvent hydrogen bonding to the excited state is due to changes in the entropy terms, rather than in the enthalpy, indicating that the effect is a steric perturbation of the solvent cage around the molecule. When intercalated into DNA, the effects of methyl substitution is smaller than those in polyol solvent or water, suggesting that the water molecules that quench the excited state by hydrogen bonding to the phenazine aza nitrogens mainly access them from the same groove as in which the Ru(II) ion resides. Since the Δ-enantiomer of [Ru(phen)210-methyl-dppz]2+ has an absolute quantum yield of up to 0.23 when bound to DNA, a value 7000 times higher than in pure water solution, it is promising as a new luminescent DNA probe.