A sensitive phosphorescent thiol chemosensor based on an iridium(III) complex with alpha,beta-unsaturated ketone functionalized 2,2 '-bipyridyl ligand

An iridium(III)-containing phosphorescent chemosensor Ir(ppy)(2)(L)(PF6) (1, ppy = 2-phenylpyridine) containing a 2,2'-bipyridyl ligand (L) functionalized with an alpha,beta-unsaturated ketone for selective detection of thiol was synthesized and characterized by spectroscopic and photophysical measurements. The structure of complex 1 was determined by X-ray crystallography. In order to get an insight into 1,4-addition reactions of thiol to complex 1, the adduct 2 from reaction of 1 with benzenethiol was successfully prepared and characterized. Complex 1 shows a lowest energy absorption at ca. 450 nm, primarily ascribable to an intraligand charge transfer (ILCT) transition from the HOMO (pi) resident on the fragment -C(O)C6H4N(C2H5)(2) to the LUMO (pi*) localized on the 2,2'-bipyridyl moiety in the functionalized 2,2'-bipyridyl ligand as suggested from DFT computational studies. Complex 1 is weakly emissive at ca. 587 nm at ambient temperature, arising likely from the (ILCT)-I-3 excited state. Upon addition of thiol to a semi-aqueous solution of complex 1, the lowest energy absorption is obviously blue-shifted and the emission is remarkably enhanced due probably to a conversion from the primary ILCT state to the predominant [pi(ppy)->pi*(L)] LLCT and the [5d(Ir)->pi*(L)] MLCT state caused by the formation of the 1-thiol adduct. The sensing properties of 1 to thiol were also investigated by ESI-MS spectrometry and H-1 NMR spectroscopy.