Photophysical Properties, DNA Photocleavage, and Photocytotoxicity of a Series of Dppn Dirhodium(II,II) Complexes

A series of dirhodium(II,II) complexes of the type cis-[Rh-2(mu-O2CCH3)(2)(dppn)(L)](2+), where dppn = benzo[t]dipyrido[3,2-a:2',3'-h]quinoxaline and L = 2,2'-bipyridine (bpy, 1), 1,10-phenanthroline (phen, 2), dipyrido[3,24.2'3'-h]quinoxaline (dpq, 3), dipyrido[3,2-a:2',3'-c]phenazine (dppz, 4), and dppn (5), were synthesized and their photophysical properties investigated to probe their potential usefulness as photodynamic therapy agents. The ability of the complexes to bind and photocleave DNA was also probed, along with their toxicity toward human skin cells in the dark and when irradiated with visible light. Nanosecond time-resolved absorption measurements established that the lowest energy excited state in 1 -5 is dppn-localized (3)pi pi* with lifetimes of 2.4-4.1 mu s in DMSO, with spectral features similar to those of the free dppn ligand (tau = 13.0 mu s in CHCl3). Complexes 1-4 photocleave DNA efficiently via a mechanism that is mostly mediated by reactive oxygen species, including O-1(2) and O-2(-). The DNA photocleavage by 5 is significantly lower than those measured for 1-4 in air, and the absence of 02 in solution or the addition of azide, SOD (superoxide dismutase), or D2O does not affect the efficiency of the photocleavage reaction. The toxicity of 1-5 toward Hs-27 human skin fibroblasts is significantly greater upon irradiation with visible light than in the dark. In contrast to the DNA photocleavage results, 5 exhibits the largest increase in toxicity upon irradiation within the series. These results are explained in terms of the known ability of the complexes to transverse cellular membranes, the toxicity of the complexes in the dark, and their photophysical properties.