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

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-beta-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.