Coordination-driven self-assembly of a sandwich shaped Eu(iii) complex for induced circularly polarized luminescence

Abstract

Chiral luminescent lanthanide supramolecular complexes have attracted much attention recently due to their excellent circularly polarized luminescence properties. Herein, a sandwich Eu(III) complex, (HNEt₃)[EuL₂], was prepared by the Eu³⁺ ion driven coordination self-assembly of an achiral bis-β-diketonate ligand (L). When chiral guanidine enantiomers, namely, [(R)-HG]⁺ and [(S)-HG]⁺, were used as counter cations to displace [HNEt₃]⁺, the racemic equilibrium of the anionic sandwich assembly was broken by the formation of the compact ion pair between [EuL₂]⁻ and [(R/S)-HG]⁺. The P- and M-helical conformations of P-((S)-HG)[EuL₂] and M-((R)-HG)[EuL₂] were deduced from the exciton couplet pattern of the Cotton effect of the CD spectra, as well as DFT calculation. Circularly polarized luminescence (CPL) measurements showed that P-((S)-HG)[EuL₂] and M-((R)-HG)[EuL₂] emitted preferential left- or right-CPL at 590 and 612 nm, respectively, with modest luminescence dissymmetry factors, |g_lum| = 0.024 at 590 nm and 0.013 at 596 nm, and high luminescence quantum yields, 47%. This work represents the first example of the successful preparation of a sandwich lanthanide complex with a CPL feature.