Novel luminescent cyclometalated iridium(III) diimine complexes that contain a biotin moiety

We report the synthesis and photophysical and electrochemical properties of a series of cyclometalated iridium(III) diimine complexes equipped with a biotin moiety [Ir(N-C)(2)(bpy-en-biotin)](PF6) (HN-C = 2-phenylpyridine, Hppy (1); 2-(4-methylphenyl)pyridine, Hmppy (2); 1-phenylpyrazole, Hppz (3); 3-methyl-1-phenylpyrazole, Hmppz (4); 7,8-benzoquinoline, Hbzq (5); 2-phenylquinoline, Hpq (6); bpy-en-biotin = 4-(N-((2-biotinamido)ethyl)aminomethyl)-4'-methyl-2,2'-bipyridine). Upon photoexcitation, complexes 1-6 display intense and long-lived emission in fluid solutions at 298 K and in low-temperature glass. The emission is assigned to a triplet metal-to-ligand charge-transfer ((MLCT)-M-3) (dpiT(Ir) --> pi*(bpy-en-biotin)) excited state. However, the excited state of complex 6 is likely to possess substantial triplet intraligand ((IL)-I-3) (pi --> pi*) (pq(-)) character. We have studied the binding of these iridium(III) biotin complexes to avidin by 4'-hydroxyazobenzene-2-carboxylic acid (HABA) assays, luminescence titrations, and competitive assays using native biotin. Homogeneous competitive assays for biotin have also been designed.