Breaking the Birefringence Barrier in B–O Crystals via B–C π-scaffolding

Optically anisotropic crystals exhibiting large birefringence (Δn) are crucial for manipulating polarized light in optical technologies. While traditional borates offer structural diversity and inherent anisotropy, achieving large Δn (>0.3) combined with short-wave UV transparency remains challenging, and surpassing the Δn = 0.5 threshold has been unrealized in this material class. Herein, we employ a B-C π-scaffolding strategy to introduce 2-halogen-pyridine-5-boronic acid, PyXB(OH)₂, as novel π-conjugated B-O functional modules. Combining these with either oxalate (C₂O₄) or nitrate (NO₃) anions yielded four new birefringent crystals: HPyXB(C₂O₄)(OH) (X = Cl, Br) and [HPyXB(OH)₂]·(NO₃) (X = Cl, Br). Crucially, the crystal packing differs significantly between the two series. In HPyXB(C₂O₄)(OH), the constituent π-units (PyX and C₂O₄) adopt a noncoplanar arrangement with a large dihedral angle. In contrast, specific intermolecular interactions (dominated by hydrogen bonds and electrostatic interactions) within [HPyXB(OH)₂]·(NO₃) enforce a highly synergistic, near-coplanar alignment of the [HPyXB(OH)₂]⁺ moiety and the NO₃ anion. This optimized structural arrangement leads to exceptionally large birefringence for the [HPyXB(OH)₂]·(NO₃) series, with measured Δn values reaching 0.533 for X = Cl (calculated 0.554) at 546 nm. These values drastically exceed those of the HPyXB(C₂O₄)(OH) counterparts (Δn = 0.182-0.194) and are more than four times larger than that of the commercial benchmark α-BaB₂O₄ (Δn = 0.12@546 nm). Significantly, [HPyXB(OH)₂]·(NO₃) represents the first B-O containing crystal to surpass the 0.5 birefringence threshold, validating B-C π-scaffolding as a powerful strategy for designing ultrahigh-performance birefringent materials.