Solution Photoreactivity of Phenanthrenequinone Diimine Complexes of Rhodium and Correlations with DNA Photocleavage and Photooxidation

The transient absorption spectra of Rh(III) complexes containing one or two phi ligands (phi = 9,10-phenanthrenequinone diimine) and various ancillary ligands were measured at pH 5.0 utilizing visible and UV excitation. The spectra were all consistent with a primarily phi ligand-centered (LC) nπ* transition. The spectral profile obtained with visible excitation of the complexes is slightly different and significantly weaker at pH 8.0, where now Rh(phi)2(phen)3+, Rh(phi)2(bpy)3+, and Rh(phen)2(phi)3+ are deprotonated. Irradiation of these complexes in basic media with 308 nm laser excitation leads to irreversible ligand-loss photochemistry. The LC excited states of Rh(phi)2(phen)3+, Rh(phi)2(bpy)3+, and Rh(phen)2(phi)3+ are reductively quenched by the purine DNA bases, A, dA, dAMP, AMP, G, dG, dGMP, and GMP, with rate constants ranging from 1.4 × 109 M-1 s-1 to 4.7 × 109 M-1 s-1 at pH 5.0, but no quenching was observed for dC or dT. Absorbances assigned to the reduced Rh(II) complex and dG· were observed in the transient absorption spectrum. There are some parallels between the observed photochemistry in solution and the DNA photocleavage results, in particular the presence of oxidative damage to the DNA bases.