Microscopic chiral pockets in a tris(chelated) iridium(III) complex as sites for dynamic enantioselective quenching
An attempt to develop a chiral luminescent probe with enantioselectivity is described. A tris(chelated) iridium(III) complex with bulky ligands, [Ir(bzq)2(phen)]+ (bzqH = benzo[h]quinoline; phen = 1,10-phenanthroline), was synthesized and optically resolved into Δ- and Λ-enantiomers. The complex (denoted as Ir(III)) was emissive in non-polar organic solvents, while in aprotic and protic polar solvents it was less emissive. When Ir(III) was adsorbed by the colloidal particles of synthetic saponite (denoted as SAP), it recovered its emitting capability even in a solvent containing water. Emission from Ir(III) adsorbed by SAP was quenched by adding [Ru(acac)3] (acac = acetylacetonato, denoted as Ru(III)). The Stern–Volmer constant (KSV) exhibited remarkable stereoselectivity; KSV = 15 900 ± 900 M−1 for Δ-Ir(III)/Δ-[Ru(acac)3] (or Λ-Ir(IIII)/Λ-[Ru(acac)3]) and 7000 ± 500 M−1 for Δ-Ir(III)/Λ-[Ru(acac)3] (or Λ-Ir(III)/Δ-[Ru(acac)3]). The dependence of the emission lifetime on the concentration of Ru(III) indicated that quenching proceeded dynamically. On the basis of the molecular model, it was postulated that a microscopic pocket surrounded by the bulky ligands at the top of adsorbed Ir(III) acted as a site discriminating the chirality of an approaching molecule.