Enhanced Luminescence Properties of Lanthanide Metal–Organic Frameworks with Low Symmetry Structure Induced by Chiral Ligand

Luminescent complexes of lanthanide ions and organic ligands have been extensively studied. However, limited attention has been given to the effect of the lanthanide coordination environment on their fluorescence. Chiral ligands, characterized by their low symmetry, provide an effective approach to inducing asymmetry in coordination structures. This study explores how low-symmetry coordination structures induced by chiral ligands, enhance the luminescent properties of lanthanide ions. We synthesized two pairs of chiral Eu^lll metal–organic frameworks (MOFs): R/S–{[Eu₆(pyr)₁₂(H₂O)₁₂] Cl₆ (H₂O)₆} (1R/1S) and R/S–[Eu₁(pyr)₂(NO₃)] (2R/2S), where “pyr” refers to R/S–pyroglutamic acid. These metal-organic frameworks (MOFs) exhibit distinct luminescent behaviors. The 1R/1S MOFs exhibit weak fluorescence intensity, short lifetimes, and low quantum yields (QYs). In contrast, the 2R/2S MOFs achieve exceptionally high-luminescent QYs of up to 72.2%. This enhancement is attributed to the lower-symmetry coordination structures around the Eu^lll ions. Additionally, circularly polarized luminescence (CPL) experiments revealed that 2R/2S MOFs exhibit significant CPL with |g_lum| values reaching up to 0.092. Our study provides a cost–effective synthesis strategy for lanthanide complexes with excellent luminescence properties.