Selective Phosphorescence Chemosensor for Homocysteine Based on an Iridium(III) Complex

A new homocysteine-selective sensor based on the iridium(III) complex Ir(pba)2(acac) (Hpba = 4-(2-pyridyl)benzaldehyde; acac = acetylacetone) was synthesized, and its' photophysical properties were studied. Upon the addition of homocysteine (Hcy) to a semi-aqueous solution of Ir(pba)2(acac), a color change from orange to yellow and a luminescent variation from deep red to green were evident to the naked eye. The blue-shift of the absorption spectrum and enhancement of the phosphorescence emission upon the addition of Hcy can be attributed to the formation of a thiazinane group by selective reaction of the aldehyde group of Ir(pba)2(acac) with Hcy, which was confirmed by 1H NMR studies. Importantly, Ir(pba)2(acac) shows uniquely luminescent recognition of Hcy over other amino acids (including cysteine) and thiol-related peptides (reduced glutathione), in agreement with the higher luminescent quantum yield of the adduct of Ir(pba)2(acac) with Hcy (0.038) compared with that of the adduct with Cys (~0.002). Both surface charge analysis and the electrochemical measurement indicated that a photoinduced electron-transfer process for Ir(pba)2(acac)-Cys might be responsible for the high specificity of Ir(pba)2(acac) toward Hcy over Cys.