Novel Luminescent Cyclometalated Iridium(III) Diimine Complexes That Contain a Biotin Moiety

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Abstract

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 (3MLCT) (dπ(Ir) → π*(bpy-en-biotin)) excited state. However, the excited state of complex 6 is likely to possess substantial triplet intraligand (3IL) (π → π*) (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.

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