Luminescent Cyclometalated Iridium(III) Arylbenzothiazole Biotin Complexes

A series of luminescent cyclometalated iridium(III) arylbenzothiazole biotin complexes, [Ir(N−C) 2 L](PF 6 ) (N−CH = 2-phenylbenzothiazole, Hbt ( 1 ); 2-((1,1‘-biphenyl)-4-yl)benzothiazole, Hbsb ( 2 ); 2-(2-thienyl)benzothiazole, Hbtth ( 3 ); 2-(1-naphthyl)benzothiazole, Hbsn ( 4 ); L = 4-( N -((6-biotinamido)hexyl)amido)-4‘-methyl-2,2‘-bipyridine), have been synthesized and characterized. The photophysical and electrochemical properties of these complexes have been investigated. Upon excitation, these complexes displayed intense and long-lived luminescence in fluid solutions at 298 K and in low-temperature glass. The emission is derived from an 3 IL (π → π*) (N−C - ) excited state, probably with some mixing of 3 MLCT (dπ(Ir) → π*(N−C - )) character. The avidin-binding properties of these complexes have been investigated by luminescence titrations and competitive assays using native biotin. Upon binding to avidin, these complexes showed significant emission enhancement and lifetime extension. On the basis of these observations, homogeneous competitive assays for biotin and resonance-energy transfer (RET) quenching using modified avidin have been studied.