Interlayer Electronic Decoupling Unlocks Giant Birefringence in π‐Conjugated Metal‐Organic Frameworks

ABSTRACT

Giant birefringence (Δ n > 1.0) is an essential requirement and a remaining challenge for advanced polarization optics. Organic π‐conjugated molecules possess high intrinsic polarizability favorable for birefringent materials design, although this potential is often hindered by dense, cofacial π–π stacking, which induces strong interlayer electronic coupling and severely limits optical anisotropy. Metal‐organic frameworks (MOFs) offer a solution that utilizes coordination bonds to modulate the packing configuration of ligands. However, a dimensionality dilemma remains: 3D MOFs often possess high symmetry that cancels optical anisotropy, while 2D MOFs typically inherit eclipsed stacking of conjugated ligands, locking birefringence at low levels. Herein, we propose an anion‐induced coordination competition strategy (AICCS) to disrupt dense stacking. By steering inorganic anions ( T _d of SO ₄ ²⁻ and D _3h of NO ₃ ⁻ ) and π‐conjugated 2,3,6,7,10,11‐hexahydroxytriphenylene (HHTP) ligands to competitively coordinate with La ³⁺ , we successfully force in‐plane slip of adjacent HHTP layers in resulted MOFs. This precisely engineered slip‐stacking decouples interlayer electronic states and liberates the latent polarizability of the π‐system. Consequently, we achieved a dramatic birefringence enhancement from the suppressed state in the parent LaHHTP (Δ n = 0.12) to record‐high values of Δ n = 1.1 in LaHHTP‐SO ₄ and Δ n = 1.3 in LaHHTP‐NO ₃ , providing a versatile route to design next‐generation anisotropic optical crystals.