Unusual solvent-regulated inversion of a metal stereocenter in an enantiopure Eu2L4 helicate: a new strategy for CPL inversion

Abstract

Developing artificial metal helicates with stimuli-responsive helicity inversion is a significant challenge in functional supramolecular chemistry. However, most reported helicity inversions are based on the synchronous transformation of P/M conformation and the metal center's Δ/Λ configuration, requiring the overcoming of substantial energy barriers. Herein, we report the first example of Δ/Λ configurational inversion of metal centers independent of the P/M conformational conversion. The helicate (NMe₄)₂[Eu₂(L^R)₄], maintaining a constant P helical conformation, undergoes a local metal center configurational conversion from ΛΛ-(NMe₄)₂[Eu₂(L^R)₄] → ΔΛ-(NMe₄)₂[Eu₂(L^R)₄] → ΔΔ-(NMe₄)₂[Eu₂(L^R)₄] upon increasing the content of CHCl₃ in CH₃CN. Associated with the conversion, the inversion of circularly polarized luminescence (CPL) activity took place with the luminescence dissymmetry factor (g_lum value) changing from −0.23 to +0.44. The understanding thus gained as to the solvent's impact on local chirality inversion enabled the design of CPL optical switches or sensors based on lanthanide helicates.