Dual Photoreactivity of a New Rh 2 (II,II) Complex for Biological Applications.

A new Rh2 (II,II) complex containing one dppn (benzo[ i ]dipyrido[3,2- a :2,3- c ]phenazine) ligand with an extended π-system, cis -H,H-[Rh2 (OCCH3 NH)2 (dppn)(CH3 CN)2 ]2+ ( 2 ), was synthesized and characterized. The dppn ligand, which serves as a DNA base pair intercalator, chelates to a single Rh center and is positioned trans to the amidato N atoms of the bridging acetamide ligand. This ligand also possesses a low-lying dppn-centered 3 ππ* state that is advantageous for the sensitization of singlet oxygen (1 O2 ), which complex 2 produced with a quantum yield, Φ 1 O2 460 , of 0.22(7) with 460 nm excitation. In addition, one equivalent of CH3 CN is released from 2 upon irradiation with visible light, generating cis -H,H-[Rh2 (OCCH3 NH)2 (dppn)(H2 O)(CH3 CN)]2+ in aqueous media with photoinduced ligand exchange quantum yield, ΦLE 450 , of 0.0033(1). Thermal denaturation and relative viscosity studies are consistent with a π-stacking interaction of 2 with double-stranded DNA together with covalent binding to the duplex upon irradiation with visible light. Therefore, 2 exhibits dual photoreactivity towards DNA, making it potentially useful for photochemotherapy with enhanced activity relative to compounds able to achieve only one mode of cell death upon irradiation.