New Luminescent Cyclometalated Iridium(III) Diimine Complexes as Biological Labeling Reagents

We report the synthesis, characterization, and photophysical and electrochemical properties of thirty luminescent
cyclometalated iridium(III) diimine complexes [Ir(N-C)2(N-N)](PF6) (HN-C = 2-phenylpyridine, Hppy; 2-(4-methylphenyl)pyridine, Hmppy; 3-methyl-1-phenylpyrazole, Hmppz; 7,8-benzoquinoline, Hbzq; 2-phenylquinoline, Hpq; N-N =
4-amino-2,2‘-bipyridine, bpy-NH2; 4-isothiocyanato-2,2‘-bipyridine, bpy-ITC; 4-iodoacetamido-2,2‘-bipyridine, bpy-IAA; 5-amino-1,10-phenanthroline, phen-NH2; 5-isothiocyanato-1,10-phenanthroline, phen-ITC; 5-iodoacetamido-1,10-phenanthroline, phen-IAA). The X-ray crystal structure of [Ir(mppz)2(bpy-NH2)](PF6) has also been investigated.
Upon irradiation, all the complexes display intense and long-lived luminescence under ambient conditions and in
77-K glass. On the basis of the photophysical and electrochemical data, the emission of most of these complexes
is assigned to an excited state of predominantly triplet metal-to-ligand charge-transfer (3MLCT) (dπ(Ir) → π*(N-N))
character. In some cases, triplet intraligand (3IL) (π → π*)(N-N or N-C-) excited states have also been identified.
In view of the specific reactivity of the isothiocyanate and iodoacetamide moieties toward the primary amine and
sulfhydryl groups, respectively, we have labeled various biological molecules with a selection of these luminescent
iridium(III) complexes. The photophysical properties of the luminescent conjugates have been investigated. In addition,
a heterogeneous assay for digoxin has also been designed on the basis of the recognition of biotinylated anti-digoxin by avidin labeled with one of the luminescent iridium(III) complexes.