Resonance Raman investigation of Ru(phen)(2)(dppz)(2+) and related complexes in water and in the presence of DNA

Resonance Raman (rR) spectroscopy was utilized to gain insight into the electronic structure of dppz (dppz = dipyrido[3,2-alpha:2',3'-c]phenazine) complexes of Ru(II) and Os(II). The time-resolved resonance Raman (TR3) spectrum of Ru(phen)(2)(dppz)(2+) (phen = 1,10-phenanthroline) collected under 355 nm (fwhm approximate to 10 ns) excitation is consistent with the population of a ligand-centered (LC) (3) pi pi* state of the dppz ligand. Since emission from the Ru(II) --> (dppz) metal-to-ligand charge transfer (MLCT) excited state is observed, both states must possess similar energies. Lowering of the MLCT state energy in Ru(phen)(2)(F-2-dppz)(2+) and Os(phen)(2)(dppz)(2+) with respect to that of the (3) pi pi* State results in the disappearance of the LC (3) pi pi* excited-state peaks in the rR spectrum, indicative of its fast deactivation to the lower MLCT state in these complexes. addition of DNA to the samples containing Ru(phen)(2)(dppz)(2+) and Ru(phen)(2)(F-2-dppz)(2+) leads to a decrease in intensity of the peaks associated with the intercalating phenazine part of the dppz ligand. Further increase in the DNA:Ru ratio causes broadening of the spectrum. This broadening has been interpreted in terms of strong pi pi interaction between the intercalated dppz ligand and the DNA bases.