Highly Selective Aluminum(III) Ion Sensing with Luminescent Iridium(III) Complexes Bearing a Distorted 2,2′-Bipyridine-3,3′-diol Moiety Utilizing a Rigidified Seven-Membered Chelate Ring

To create an ion sensor utilizing a rigidified seven-membered chelate ring, we developed two Ir(III) complexes with 2,2′-bipyridine-3,3′-diol (bpy(OH)₂, bpydL) ligands as reaction centers, namely Ir1 ([Ir(ppy)₂{bpy(O^-)(OH)}], ppy = 2-phenylpyridine) and Ir2 ([Ir(bzq)₂{bpy(O^-)(OH)}], bzq = benzo[h]quinoline), and evaluated their reactivities toward metal ions by spectrophotometry. When they are reacted with Al³⁺, these complexes exhibit dramatic enhancements in emission intensity (775-fold for Ir1 and 51.0-fold for Ir2) and distinct orange to green changes in emission color. The reactions of Ir1 and Ir2 with Al³⁺ were found to barely be affected by nearly all common metal ions. We conclude that these high selectivities arise from the high affinities of the (O,O) atoms in bpydL for hard metal ions and the increased strain of the seven-membered chelate ring due to the coordination of bpydL to the Ir(III) center in each complex, which excludes large metal ions out of the chelate ring.