Dipyridophenazine complexes of Os(II) as red-emitting DNA probes: Synthesis, characterization, and photophysical properties

Polypyridyl complexes of Os(II) bearing one dipyridophenazine (dppz) deriva tive and two ancillary ligands derived from bipyridine (bpy) or phenanthrol ine (phen) exhibit emission maxima at similar to 740 nm and average excited -state lifetimes in the 10 ns range upon binding to DNA by preferential int ercalation 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 th e ancillary ligands lowered the Os(3+/2+) reduction potential but did not a ffect the reduction potential of the dppz ligand. A methyl substituent at t he 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 similar to 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-dimethylphenant hroline or 4,4'-dimethylbipyridine ligands exhibit less hypochromism (26-27 %) in the pi-pi* transition on the dppz ligand compared to complexes with 5 ,6-dimethylphenanthroline (30-37%) or the parent phen (31-35%). By steady-s tate and time-resolved emission spectroscopy, complexes bound to DNA by int ercalation with substituents on the 4,7- or 4,4'-positions of the ancillary phen or bpy displayed lower quantum yields for emission (Phi(em)) compared to complexes with the parent phen, while complexes with methyl substituent s on the dppz ligand had the greatest Phi(em). Studies with poly d(AT), pol y d(GC), and mixed-sequence DNA revealed that the emission yields are also sequence-dependent. Comparative luminescence studies in CH2Cl2 demonstrated that these effects arise from a combination of (i) the inherent sensitivit y of the excited state to ligand structure and (ii) perturbations in DNA bi nding geometry introduced by substituents on the ancillary and intercalatin g ligands. Our results clarify the relationships between ligand architectur e and emission yield and lifetime in the presence and absence of DNA and il lustrate the utility of dppz complexes of Os(II) as luminescent probes for DNA.