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Abstract— The luminescence quenching of ruthenium‐tris‐l,4,5,8‐tetraazaphenanthrene [Ru(tap)32+] by nucleotides approaches the diffusion rate only with guanosine‐5′‐monophosphate (GMP), the most reducing nucleotide, and leads to an electron transfer with the production of the monoreduced complex and the oxidized base. The resulting deprotonated GMP(‐H)radical recombines with the monoreduced complex according to a bimolecular equimolar process. The pH dependence of the decay of the transient reduced complex, in the presence of an oxidant (oxygen or benzoquinone) indicates the formation of Ru(tap)2(tapH)2+i.e. the reduced protonated species, subsequent to the electron transfer, with a pKa of 7.6 as confirmed from pulse radiolysis experiments. As the non‐protonated reduced complex, Ru(tap)2(tap2)+ has a higher reducing power than the protonated one, oxygen is able to reoxidize only the non‐protonated species, whereas benzoquinone reoxidizes both species but with different rate constants. The flash photolysis of Ru(tap),2+ in the presence of DNA and the effect of Mg2+ ions and GMP as supplementary additives also show the existence of a photo‐induced electron transfer with the nucleic acid, which can be correlated to the photosensitized cleavage of DNA by this complex.

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