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

Abstract

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, <b>bpydL</b>) ligands as reaction centers, namely <b>Ir1</b> ([Ir(ppy)₂{bpy(O^-)(OH)}], ppy = 2-phenylpyridine) and <b>Ir2</b> ([Ir(bzq)₂{bpy(O^-)(OH)}], bzq = benzo[<i>h</i>]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 <b>Ir1</b> and 51.0-fold for <b>Ir2</b>) and distinct orange to green changes in emission color. The reactions of <b>Ir1</b> and <b>Ir2</b> 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 <b>bpydL</b> for hard metal ions and the increased strain of the seven-membered chelate ring due to the coordination of <b>bpydL</b> to the Ir(III) center in each complex, which excludes large metal ions out of the chelate ring.