Bifunctional luminescent rhenium(I) complexes containing an extended planar diimine ligand and a biotin moiety

We report the synthesis, characterization, and photophysical and electrochemical properties of four bifunctional luminescent rhenium(I) complexes that contain an extended planar diimine ligand and a biotin moiety, [Re(N-N)(CO)(3)(L)](PF6) (N-N = dipyrido[3,2-a:2',3'-c]phenazine, dppz; L = 4-((biotinamido)methyl)pyridine (py-CH2NH-biotin; 1), 4-(N-((6-biotinamido)hexanoyl)aminomethyl)pyridine, (py-CH2NHCOC5H10NH-biotin; 2); N-N = benzo[i]dipyrido[3,2-a:2',3'-c]phenazine, dppn; L = py-CH2NH-biotin (3), py-CH2NHCOC5H10NH-biotin (4)). Upon irradiation, complexes 1-4 exhibit intense and long-lived emission in deoxygenated fluid solutions at 298 K and in low-temperature glass. The luminescence is assigned to a triplet intraligand ((IL)-I-3) (pi --> pi*) (diimine) excited state. However, the excited state of the dppn complexes 3 and 4 is likely to possess substantial triplet metal-to-ligand charge-transfer ((MLCT)-M-3) (dpi(Re) --> pi*(dppn)) character. The interactions of these complexes with double-stranded calf thymus DNA have been studied by absorption and emission titrations. The binding of the complexes to avidin has also been investigated by HABA assays and emission titrations.