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.