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

Three luminescent cyclometalated iridium(III) diimine complexes [Ir(bsn)(2)(N boolean AND N)](PF6) (Hbsn = 2-(1-naphthyl) benzothiazole; N boolean AND 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 ((IL)-I-3) (pi -> pi*)(bsn) excited state, probably with mixing of some triplet metal-to-ligand charge-transfer ((MLCT)-M-3) (d pi(Ir) -> pi*(N boolean AND 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 (IL)-I-3 (pi -> pi*)(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. (C) 2011 Elsevier B.V. All rights reserved.