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

Abstract

Giant birefringence (Δn > 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 SO₄ ^2- and D_3h of NO₃ ^-) and π-conjugated 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP) ligands to competitively coordinate with La³⁺, 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 (Δn = 0.12) to record-high values of Δn = 1.1 in LaHHTP-SO₄ and Δn = 1.3 in LaHHTP-NO₃, providing a versatile route to design next-generation anisotropic optical crystals.