Coordination-driven self-assembly of a sandwich shaped Eu() 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, (HNEt 3 )[Eu L 2 ], was prepared by the Eu 3+ 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 [HNEt 3 ] + , the racemic equilibrium of the anionic sandwich assembly was broken by the formation of the compact ion pair between [Eu L 2 ] − and [( R / S )-HG] + . The P - and M -helical conformations of P -(( S )-HG)[Eu L 2 ] and M -(( R )-HG)[Eu L 2 ] 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)[Eu L 2 ] and M -(( R )-HG)[Eu L 2 ] 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. A chiral guanidine cation as a counterion of a double decker sandwich europium( iii ) complex, [Eu L 2 ] − , breaks the racemization equilibrium of assemblies and induces circularly polarized emission from Eu 3+ ions.