Solvent and counterion cooperatively induced inversion of the stereocenter Δ/Λ and CPL in a mononuclear Eu(iii) complex

Abstract

The development of lanthanide complexes with stimuli-responsive dynamic chiral inversion has significant potential for applications in chiroptical switches and chiral sensing. However, the variable coordination numbers and coordination geometries of Ln(III) ions pose substantial challenges in controlling the chiral inversion of lanthanide complexes. Herein, we present the first example of solvent and counterion cooperatively induced inversion of the Eu(III) stereocenter Δ/Λ in mononuclear complexes. In Cs[Eu(L^L)₄], where Cs⁺ serves as the counterion, the addition of chloroform to an acetonitrile solution of the complex resulted in a reversal of Eu(III) center configuration from Δ to Λ, accompanied with an inversion of the circularly polarized luminescence (CPL) signal (g_lum value shifting from +0.15 to −0.13). However, when (NMe₄)⁺ was used as the counterion, (NMe₄)[Eu(L^L)₄] did not exhibit this inversion behavior under the same conditions. Notably, the addition of Cs⁺ ions to a solution of (NMe₄)[Eu(L^L)₄] restored the inversion feature. This understanding of the impact of Cs⁺ ions and solvent on Δ/Λ inversion contributes to the development of CPL switches and sensors based on chiral lanthanide supramolecules.