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

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 (NMe 4 ) 2 [Eu 2 (R/S- L 1–4 ) 4 ] (ΔΔ/ΛΛ- 1 – 4 ) with different structural rigidity, as well as in crown ether-modified (NMe 4 ) 2 [Eu 2 (R/S- L 5 ) 4 ] (ΔΔ/ΛΛ- 5 ) that exhibits adjustable coordination geometry symmetry. Notably, encapsulating bulkier [NEt 4 ] + instead of a [NMe 4 ] + 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 NMe 4 -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.