Dual Photoreactivity of a New Rh₂(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(OCCH₃NH)₂(dppn)(CH₃CN)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 ³ππ* state that is advantageous for the sensitization of singlet oxygen (¹O₂), which complex 2 produced with a quantum yield, Φ ⁴⁶⁰, of 0.22(7) with 460 nm excitation. In addition, one equivalent of CH₃CN is released from 2 upon irradiation with visible light, generating cis-H,H-[Rh₂(OCCH₃NH)₂(dppn)(H₂O)(CH₃CN)]²⁺ in aqueous media with photoinduced ligand exchange quantum yield, Φ_LE⁴⁵⁰, 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.