The development of circularly polarized luminescence (CPL) emitters with a luminescence dissymmetry factor (|glum|) approaching the theoretical limit of 2 remains a central challenge in chiral photonics. Herein, considerable glum values (1.48-1.65) were achieved in helicates (NMe4)2Eu2(R/S-L1-4)4 (ΔΔ/ΛΛ-1-4) with different structural rigidity, as well as in crown ether-modified (NMe4)2Eu2(R/S-L5)4 (ΔΔ/ΛΛ-5) that exhibits adjustable coordination geometry symmetry. Notably, encapsulating bulkier NEt4+ instead of a NMe4+ counterion within the inner cavity of ΔΔ/ΛΛ-2 further elevates |glum| to an unprecedented 1.71, the highest value for any chiral molecular emitter. While the NMe4-ΔΔ/ΛΛ-3 achieves an ultrahigh CPL brightness (BCPL = 3625 M-1 cm-1), resulting from its large glum value (1.63) and high quantum yield (ΦPL = 32%). Structural and spectroscopic analyses demonstrate that the high CPL activity originates from a conformationally rigidified square antiprismatic (SAP) geometry around the Eu(III) center. This structure and property relationship is vividly demonstrated by the crown-ether-functionalized ΔΔ-5, where gradual binding of Cs+ ions triggers a pronounced glum fluctuation (1.48 → 0.62 → 1.41) through a perturbation and subsequent restoration of the SAP environment. Furthermore, the multicrown-ether binding sites in ΔΔ-5 enable cooperative guest binding, facilitating the first naked-eye CPL recognition of the antispasmodic drug tizanidine. This work establishes rigidifying SAP configuration as a general design principle for maximizing the CPL activity of lanthanide complexes and opening avenues for advanced CPL applications.
Huang et al. (2026) studied this question.