Halogen‐Bond Coupled Halogenated‐π‐Conjugation Enables Giant Birefringence in Hydrogen‐Bonded Organic Frameworks

Birefringent crystals are essential optical materials for modulating and detecting the polarization state of light. However, achieving large optical anisotropy remains a significant challenge, mainly because it is difficult to rationally align anisotropic fundamental building blocks (FBBs) to maximize their collective contribution. Here, we propose a halogen-bond (XB) coupled halogenated-π-conjugation strategy. Using hydrogen-bonded pyridinedicarboxylic acid as a template, we synthesized XOF-1, Hybrid-XOF-1, and XB-HOF-1 through decarboxylation-halogenation reactions. All three compounds are built from 0D halogen-bonded FBBs and extended by halogen and/or hydrogen bonds. Notably, all three compounds exhibit large birefringence (0.69-0.97), with XB-HOF-1 reaching Δn = 0.97 @ 546 nm, over 80 times that of the commercial benchmark MgF₂ and the highest value reported for any pyridine-containing system. Theoretical calculations and structural analyses reveal that this exceptional performance originates from the synergistic combination of maximized in-plane anisotropy, induced by collinear arrangement of N···I-Cl halogen bonds along one crystallographic axis, and minimized out-of-plane polarizability, induced by coplanar alignment of halogenated π-conjugated planes perpendicular to another axis. Our work establishes an effective strategy for pushing the boundaries of optical anisotropy and offers a new blueprint for the rational design of high-performance birefringent materials.