An Aldol Reaction-Based Iridium(III) Chemosensor for the Visualization of Proline in Living Cells

A long-lived aldol reaction-based iridium(III) chemosensor [Ir(ppy)₂(5-CHOphen)]PF₆ (1, where ppy = 2-phenylpyridine and 5-CHOphen = 1,10-phenanthroline-5-carbaldehyde) for proline detection has been synthesized. The iridium(III) complex 1, incorporating an aldehyde group in N^N donor ligand, can take part in aldol reaction with acetone mediated by proline. The transformation of the sp²-hybridized carbonyl group into a sp³-hybridized alcohol group influences the metal-to-ligand charge-transfer (MLCT) state of the iridium(III) complex, resulting in a change in luminescence in response to proline. The interaction of the iridium(III) complex 1 with proline was investigated by ¹H NMR, HRMS and emission titration experiments. Upon the addition of proline to a solution of iridium(III) complex 1, a maximum 8-fold luminescence enhancement was observed. The luminescence signal of iridium(III) complex 1 could be recognized in strongly fluorescent media using time-resolved emission spectroscopy (TRES). The detection of proline in living cells was also demonstrated.