Asymmetric induction in quadruple-stranded europium(iii) helicates and circularly polarized luminescence

Abstract

Chiral supramolecular lanthanide-helicates are regarded as promising chiroptical materials due to their combination of ground and excited state chirality and special luminescence properties from Ln³⁺, named circularly polarized luminescence (CPL). However, the sophisticated and costly asymmetric syntheses decelerate their research progress. Herein, a chiral induction strategy is employed to break the racemic equilibrium of lanthanide helicate by the formation of a compact ion pair. The synthesized chiral guanidine cation ((R/S)-HG) helps to effectively transfer chirality to anionic quadruple-stranded helicate by electrostatic, H bonds, and multiple C–H⋯F and N–H⋯F interactions. The diastereoselective control was confirmed by X-ray crystallography and CD and CPL spectroscopy, where (S)-HG prefers to bind a P (ΔΔ) helical enantiomer, in contrast a M helicate for (R)-HG. Additionally, the inconsistency of CD and CPL spectra in assessing the perturbation of a racemic equilibrium discloses their complementary advantages on monitoring molecular chirality. In the case of diastereomeric enrichment equilibrium, three pairs of helicates show high luminescence quantum yields of 42%–54%, and large |g_lum| with the values of 0.137–0.266. This work provides an effective strategy to synthesize excellent CPL materials based on racemic lanthanide helicate.