Introducing a new family of biotinylated Ir(III)-pyridyltriazole lumophores: Synthesis, photophysics, and preliminary study of avidin-binding properties

Six new biotinylated reagents derived from monocationic pyridyl-1,2,3-triazole-based Ir(III) complexes of the general formula [(C<<^>>N)(2)Ir(N<<^>>N-spacer-X-CO-biotin)]+, where HC<<^>>N is 2-phenylpyridine, N<<^>>N is the neutral chelating (2-pyridyl)-1,2,3-triazole ligand, the term spacer refers to alkyl (-C11H22) or aromatic (p-phenyl- or 4,4'-biphenyl-) chains, and X is NH or O, have been prepared and characterized. Upon photoexcitation, all six complexes in fluid CH2Cl2 and aqueous solutions at room temperature displayed intensewith quantum yields as high as 0.60 (complex 4b)and quite long-lived blue-green luminescence, corresponding in each case to a vibronically structured emission profile peaking at ca. 480 and ca. 508 nm. These features, in combination with the reduced rigidochromic shift that was observed from the same samples frozen at 77 K, suggested the (LC)-L-3-type excited states as the prevalent contributors to the emission. The interactions of these new biotinylated complexes with avidin have been studied by 4'-hydroxyazobenzene-2-carboxylic acid (HABA) assays and emission titrations. In general, the amplification of the emission intensity of the Ir(III) complexes occurring upon their interaction with avidin was observed. It is also worth mentioning how a similar or better response was displayed by [(C<<^>>N)(2)Ir(N<<^>>N-spacer-O-CO-biotin)](+)-type complexes, in which biotin is appended to the Ir(III) lumophore through an ester moiety.