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.

The spectroelectrochemical properties of a novel light switch for DNA, Ru(phen) 2 (PHEHAT) 2+ (phen ) 1,10phenanthroline; PHEHAT ) 1,10-phenanthrolino [5,6-b]1,4,5,8,9,12-hexaazatriphenylene), are examined and compared to those of Ru(phen) 2 (DPPZ) 2+ (DPPZ ) dipyrido[3,2-a;2′,3′-c]phenazine) and Ru(phen) 2 (HAT) 2+ (HAT ) 1,4,5,8,9,12-hexaazatriphenylene). The excited Ru(phen) 2 (PHEHAT) 2+ luminesces in organic solvents but not in water. It is shown that the orbitals involved in the absorption and luminescence spectroscopy are not the same as those in the electrochemistry. In aqueous solution, this complex luminesces upon intercalation of the PHEHAT ligand into the stacking of the DNA bases. Two modes of distribution of the complex on DNA can be evidenced from the titration curves of the complex with DNA. Laser flash photolysis experiments show that the excited state is able to abstract an electron from GMP (guanosine-5′-monophosphate) with a rather low efficiency, leading to the reduced complex and oxidized GMP. However, this process is not accompanied by the formation of photoproduct with GMP and cannot be detected with DNA on the time scale of the experiments.

Quantum-chemical methods are applied to study the nature of the excited states relevant in the photophysical processes (absorption and emission) of a series of polyazaaromatic-ligand-based ruthenium-(II) complexes. The electronic and optical properties of the free polyazaaromatic ligands and their corresponding ruthenium(II) complexes are determined on the basis of correlated Hartree-Fock semiempirical approaches. While the emission of complexes containing small-size ligands, such as 1,10phenanthroline or 2,2′-bipyridine, arises from a manifold of metal-to-ligand charge-transfer triplet states ( 3 MLCTs), an additional ligand-centered triplet state ( 3 L) is identified in the triplet manifold of complexes containing a π-extended ligand such as dipyrido[3,2-a:2′,3′-c]phenazine, tetrapyrido[3,2-a:2′,3′-c:3′′,2′′-h: 2′′′,3′′′-j]phenazine, and 1,10-phenanthrolino [5,6-b]-1,4,5,8,9,12-hexaazatriphenylene. Recent experimental data are interpreted in light of these theoretical results; namely, the origin for the abnormal solvent-and temperature-dependent emission measured in π-extended Ru complexes is revisited.