Enhancing Magneto-Optical Activity via Coordination Distortion in Chiral Er ₄ M ₈ Clusters

Chiral lanthanide molecular materials exhibiting robust magneto-optical (MO) responses are attractive for applications in optical sensing, optical readout, and magnetic-field-controlled circular-polarization optics. However, achieving strong MO responses remains challenging due to the formally parity-forbidden nature and weak oscillator strengths of f – f transitions. Herein, we present three pairs of chiral R / S -Er 4 M 8 clusters (M = Co 2+ for Er 4 Co 8; Cu 2+ for Er 4 Cu 8; Zn 2+ for Er 4 Zn 8 ) that display pronounced circular dichroism (CD) and magnetic circular dichroism (MCD) signals spanning the UV–vis–NIR spectral region. Despite possessing nearly identical cluster frameworks, these clusters display markedly distinct MO behaviors. Interestingly, the g MCD of Er 4 Zn 8 exceeds those of Er 4 Co 8 and Er 4 Cu 8 . Notably, Er 4 Zn 8 exhibits a record-value of 0.044 T –1 for the 4 I 15/2 → 4 I 13/2 transition (1400–1600 nm), exceeding its Co and Cu analogues by factors of 5.6 and 2.6, respectively. Structural analyses indicate that this enhancement correlates with pronounced local coordination asymmetry at the Er 3+ sites, which promotes odd-parity crystal-field mixing and electric-dipole borrowing within the 4 f manifold, thereby strengthening the Zeeman-driven MCD response. These findings suggest site-specific structural distortion as a powerful design principle for enhancing and amplifying f – f transition-driven magneto-optical responses.