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)](PF 6 ) (N-N = dipyrido[3,2- a:2‘,3‘- c ]phenazine, dppz; L = 4-((biotinamido)methyl)pyridine (py-CH 2 NH-biotin; 1 ), 4-( N -((6-biotinamido)hexanoyl)aminomethyl)pyridine, (py-CH 2 NHCOC 5 H 10 NH-biotin; 2 ); N-N = benzo[ i ]dipyrido[3,2- a:2‘,3‘- c ]phenazine, dppn; L = py-CH 2 NH-biotin ( 3 ), py-CH 2 NHCOC 5 H 10 NH-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 ( 3 IL) (π → π*) (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 ( 3 MLCT) (dπ(Re) → π*(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.