Luminescent iridium(III) arylbenzothiazole complexes: Photophysics, electrochemistry, bioconjugation, and cellular uptake

Three luminescent cyclometalated iridium(III) diimine complexes [Ir(bsn)2(N^N)](PF6) (Hbsn = 2-(1-naphthyl)benzothiazole; N^N = 5-amino-1,10-phenanthroline (phen-NH2) (1), 5-isothiocyanato-1,10-phenanthroline (phen-NCS) (2), and N-butyl-N′-1,10-phenanthrolin-5-yl thiourea (phen-TU-Bt) (3)) have been synthesized and characterized. Complex 1 is a precursor complex of complex 2, and the isothiocyanate group of complex 2 enables it to function as a biological labeling reagent. To investigate its reactivity towards the primary amine group, complex 2 was reacted with a model substrate n-butylamine, resulting in the formation of complex 3. The photophysical and electrochemical properties of these complexes have been investigated. Upon photoexcitation, these complexes displayed intense and long-lived luminescence in fluid solutions at 298 K and in butyronitrile glass at 77 K. In view of the structured emission profiles and the long emission lifetimes, the emission of these complexes has been assigned to a triplet intraligand (3IL) (π → π∗)(bsn) excited state, probably with mixing of some triplet metal-to-ligand charge-transfer (3MLCT) (dπ(Ir) → π∗(N^N)) character. Owing to the specific reactivity of the isothiocyanate group towards primary amines, complex 2 was conjugated to various proteins and polymers. Photoexcitation of these conjugates resulted in intense and long-lived 3IL (π → π∗)(bsn) emission in aqueous buffer at 298 K. Additionally, the uptake of complexes 1–3 by HeLa cells has been investigated by various methods. Furthermore, the cytotoxicity of the complexes and the polymeric conjugates towards the cells has been examined by the 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyltetrazolium bromide assay.