An iridium(iii)-containing phosphorescent chemosensor Ir(ppy)2(L)(PF6) (1, ppy = 2-phenylpyridine) containing a 2,2â²-bipyridyl ligand (L) functionalized with an α,β-unsaturated ketone for selective detection of thiol was synthesized and characterized by spectroscopic and photophysical measurements. The structure of complex 1was determined by X-ray crystallography. In order to get an insight into 1,4-addition reactions of thiol to complex 1, the adduct 2from reaction of 1with benzenethiol was successfully prepared and characterized. Complex 1shows a lowest energy absorption at ca.450 nm, primarily ascribable to an intraligand charge transfer (ILCT) transition from the HOMO (Ï) resident on the fragment -C(O)C6H4N(C2H5)2to the LUMO (Ï*) localized on the 2,2â²-bipyridyl moiety in the functionalized 2,2â²-bipyridyl ligand as suggested from DFT computational studies. Complex 1is weakly emissive at ca.587 nm at ambient temperature, arising likely from the 3ILCT 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 [Ï(ppy)âÏ*(L)] LLCT and the [5d(Ir)âÏ*(L)] MLCT state caused by the formation of the 1-thiol adduct. The sensing properties of 1to thiol were also investigated by ESI-MS spectrometry and 1H NMR spectroscopy. [ABSTRACT FROM AUTHOR]