The new tris-heteroleptic complex [Ru(bpy)(dppn)(CH3CN)2](2+) (3, bpy = 2,2'-bipyridine, dppn = benzo[i]dipyrido[3,2-a;2',3'-c]phenazine) was synthesized and characterized in an effort to generate a molecule capable of both singlet oxygen ((1)O2) production and ligand exchange upon irradiation. Such dual reactivity has the potential to be useful for increasing the efficacy of photochemotherapy drugs by acting via two different mechanisms simultaneously. The photochemical properties and photoinduced cytotoxicity of 3 were compared to those of [Ru(bpy)2(dppn)](2+) (1) and [Ru(bpy)2(CH3CN)2](2+) (2), since 1 sensitizes the production of (1)O2 and 2 undergoes ligand exchange of the monodentate CH3CN ligands with solvent when irradiated. The quantum yield of (1)O2 production was measured to be 0.72(2) for 3 in methanol, which is slightly lower than that of 1, Φ = 0.88(2), in the same solvent (λirr = 460 nm). Complex 3 also undergoes photoinduced ligand exchange when irradiated in H2O (λirr = 400 nm), but with a low quantum efficiency (<1%). These results are explained by the presence of the low-lying ligand-centered (3)ππ* excited state of 3 localized on the dppn ligand, thus decreasing the relative population of the higher energy (3)dd state; the latter is associated with ligand dissociation. Cytotoxicity data with HeLa cells reveal that complex 3 exhibits a greater photocytotoxicity index, 1110, than does either 1 and 2, indicating that the dual-action complex is more photoactive toward cells in spite of its low ligand exchange quantum yield.
The preparation, characterization, and photophysical properties of a series of ruthenium(II) complexes possessing the cyclometallating deprotonated 2-phenyl pyridine ligand, phpy − , together with dppn (benzo[i]dipyrido[3,2-a:2′,3′c]phenazine), a ligand with an extended π-system, are reported. Related complexes have been used as efficient dyes in dyesensitized solar cells (DSSCs), and the Ru→dppn metal-to-ligand charge transfer (MLCT) absorption of the new complexes [Ru(phpy)(bpy)(dppn)] + (4) and [Ru(phpy)(dppn) 2 ] + (5) is red-shifted relative to the Ru→bpy MLCT peak in [Ru(phpy)(bpy) 2 ] + (3). These new compounds are compared to conventional complexes where phpy − is replaced by 2,2′bipyridine (bpy), including [Ru(bpy) 3 ] 2+ , [Ru(bpy) 2 (dppn)] 2+ (1), and [Ru(bpy)(dppn) 2 ] 2+(2). Unlike 1 and 2, with long-lived dppn-centered 3 ππ* excited states (τ ∼ 20 μs), the corresponding cyclometallated complexes 4 and 5 exhibit weakly emissive Ru→dppn 3 MLCT states with transient absorption lifetimes of 25 and 45 ps, respectively, which are significantly shorter than that of 3, ∼9 ns. Although it is desirable to shift the absorption of ruthenium dyes used in DSSCs to lower energies, it is evident from this work, that for cyclometallated phpy − complexes, lowering the energy of the 3 MLCT state below that of 3 results in significant shortening of the excited state lifetime. The fast excited state decay, together with the lower energy of the 1 MLCT state, may result in lower charge injection efficiencies from these types of complexes.
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