Rigid Eu(III) Helicates Exhibit Remarkable Circularly Polarized Luminescence Activity and Naked-Eye Recognition...

The development of circularly polarized luminescence (CPL) emitters with a luminescence dissymmetry factor (|g_lum|) approaching the theoretical limit of 2 remains a central challenge in chiral photonics. Herein, considerable g_lum values (1.48-1.65) were achieved in helicates (NMe₄)₂[Eu₂(R/S-L^1-4)₄] (ΔΔ/ΛΛ-1-4) with different structural rigidity, as well as in crown ether-modified (NMe₄)₂[Eu₂(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 |g_lum| to an unprecedented 1.71, the highest value for any chiral molecular emitter. While the NMe₄-ΔΔ/ΛΛ-3 achieves an ultrahigh CPL brightness (B_CPL = 3625 M^-1 cm^-1), resulting from its large g_lum value (1.63) and high quantum yield (Φ_PL = 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 g_lum 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.