Luminescence quenching by DNA-bound viologens: Effect of reactant identity on efficiency and dynamics of electron transfer in DNA

Photoinduced electron transfer from two intercalating photoactive donors, Ru(phen)₂dppz²⁺ and ethidium, to intercalating viologen acceptors of the N,N′-dialkyl-6-(2′-pyridiniumyl)phenanthridinium family has been investigated through steady-state and time-resolved luminescence quenching measurements. Efficient quenching of the emission from these donors bound to DNA is observed at low concentrations of acceptor (1-10 eq.), and in time-resolved emission experiments it is determined that electron transfer occurs on the nanosecond time scale. Furthermore, transient absorption measurements confirm that the quenching is the result of a charge-transfer process; upon photoreaction of intercalated Ru(phen)₂dppz²⁺ with a viologen acceptor, an intermediate with spectral properties resembling the expected charge-separated pair is observed. The quenching yields and kinetics obtained with this quencher are in marked contrast to those observed with these same donors paired with Rh(phi)₂bpy³⁺ as an acceptor. The differing efficiencies of electron transfer for these donor/acceptor pairs bound to DNA as compared to others previously described are discussed qualitatively in terms of the structural and electronic properties of the different reactants.