Dipyridophenazine Complexes of Os(II) as Red-Emitting DNA Probes:  Synthesis, Characterization, and Photophysical Properties

Polypyridyl complexes of Os(II) bearing one dipyridophenazine (dppz) derivative and two ancillary ligands derived from bipyridine (bpy) or phenanthroline (phen) exhibit emission maxima at ∼740 nm and average excited-state lifetimes in the 10 ns range upon binding to DNA by preferential intercalation of the dppz ligand. A family of [Os(L^1)(L^2)(L^3)]^(2+) and [Os(L^1)_2(L^2)]^(2+) complexes with simple modifications in the ancillary phen or bpy ligands (L^1 and L^3) as well as the intercalating dppz ligand (L^2) was prepared. By cyclic voltammetry, electron-donating substituents on the ancillary ligands lowered the Os(3+/2+) reduction potential but did not affect the reduction potential of the dppz ligand. A methyl substituent at the 7-, 8-, or 6-position of the dppz ligand shifted the phenazine reduction toward the negative but did not affect the Os(3+/2+) potential. Absorption titrations indicated intercalative binding to DNA with high affinity (K_B ∼10^6 M^(-1)) for the family of complexes, although at high ratios (50:1) of base pairs to metal, complexes with ancillary 4,7-dimethylphenanthroline or 4,4‘-dimethylbipyridine ligands exhibit less hypochromism (26−27%) in the π−π* transition on the dppz ligand compared to complexes with 5,6-dimethylphenanthroline (30−37%) or the parent phen (31−35%). By steady-state and time-resolved emission spectroscopy, complexes bound to DNA by intercalation with substituents on the 4,7- or 4,4‘-positions of the ancillary phen or bpy displayed lower quantum yields for emission (Φ_(em)) compared to complexes with the parent phen, while complexes with methyl substituents on the dppz ligand had the greatest Φ_(em). Studies with poly d(AT), poly d(GC), and mixed-sequence DNA revealed that the emission yields are also sequence-dependent. Comparative luminescence studies in CH_2Cl_2 demonstrated that these effects arise from a combination of (i) the inherent sensitivity of the excited state to ligand structure and (ii) perturbations in DNA binding geometry introduced by substituents on the ancillary and intercalating ligands. Our results clarify the relationships between ligand architecture and emission yield and lifetime in the presence and absence of DNA and illustrate the utility of dppz complexes of Os(II) as luminescent probes for DNA