Luminescent ruthenium(II) amidodipyridoquinoxaline biotin complexes that display higher avidin-induced emission enhancement

A series of luminescent ruthenium(II) amidodipyridoquinoxaline biotin (dpq-B) complexes [Ru(N–N)2(N–N′)](PF6)2 (N–N = 2,2′-bipyridine (bpy), 1,10-phenanthroline (phen), 4,7-diphenyl-1,10-phenanthroline (Ph2-phen); N–N′ = 2-((2-biotinamido)ethyl)amidodipyrido[3,2-f:2′,3′-h]quinoxaline (dpq-C2-B), 2-((6-biotinamido)hexyl)amidodipyrido[3,2-f:2′,3′-h]quinoxaline (dpq-C6-B)) has been designed as new luminescent probes for avidin. The electrochemical and photophysical properties of these complexes have been investigated. Upon irradiation, all the complexes exhibited metal-to-ligand charge-transfer (3MLCT) (dπ(Ru) → π∗(diimine)) emission in fluid solutions at 298 K and in low-temperature glass. In aqueous buffer, the emission was extremely weak, probably a consequence of hydrogen-bonding interactions between the amide moiety of the dpq-B ligands and the water molecules. The avidin-binding properties of all the complexes have been studied by 4′-hydroxyazobenzene-2-carboxylic acid (HABA) assays, luminescence titrations, kinetics experiments and confocal microscopy using avidin-conjugated microspheres.