The lowest excited state of [Ru(TAP)2(dppz)]2+ (TAP = 1,4,5,8-tetraazaphenanthrene, dppz = dipyrido[3,2-a:2',3'-c]phenazine) 1 is strongly luminescent, even in water, and very oxidizing. Therefore it is able to oxidise not only guanosine-5'-monophosphate (GMP), as demonstrated by laser flash photolysis, but also guanine-containing polynucleotides such as calf thymus DNA and [poly(dG-dC)]2. The luminescence quenching was found to be faster in H2O than in D2O, as is the back reaction, indicating that both processes probably proceed by proton-coupled electron transfer. These properties, that are controlled by the triplet MLCT state in which the charge has been transferred from the Ru to a TAP ligand, contrast with those of the well known [Ru(phen)2(dppz)]2+ 2.

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As a strategy to synthesize new sequence-specific DNA photoreagents, oligodeoxyribonucleotides bearing a photoreactive [Ru II (tap) 2 (dip)] 2 complex (tap 1,4,5,8-tetraazaphenanthrene; dip 4,7-diphenylphenanthroline) tethered to a central nucleotide base have been prepared and characterized. The resulting Ru-labeled oligonucleotides exhibit absorption and emission properties of the tethered metal complex and bind to complementary single-stranded DNA sequences. The thermal denaturation curves are not significantly affected by the chemical attachment of the complex. Steady-state and time-resolved emission data reveal a significant luminescence quenching upon hybridization of the Ru-labeled oligonucleotide with the complementary target strand, if the strand contains guanines. Based on the behavior of the free complex, the quenching process is attributed to a photoinduced electron transfer from the guanines of the target strand. This primary process is related to the formation of photoproduct(s) on the duplex that generate an irreversible photocrosslinking of the two strands. This work constitutes an initial step in the design of sequence-specific photocrosslinking agents.

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