Breaking the Birefringence Barrier in B–O Crystals via B–C π-scaffolding
Jiajia Li, Miao‐Bin Xu, Mingchang Wang, Zhi Lin, Yuan Lin, Yunxia Hu, Jia-Min Lian, Jin Chen, Ke‐Zhao Du
Abstract
Optically anisotropic crystals exhibiting large birefringence (Δ n ) are crucial for manipulating polarized light in optical technologies. While traditional borates offer structural diversity and inherent anisotropy, achieving large Δ n (>0.3) combined with short-wave UV transparency remains challenging, and surpassing the Δ n = 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 (C 2 O 4 ) or nitrate (NO 3 ) anions yielded four new birefringent crystals: HPyXB(C 2 O 4 )(OH) (X = Cl, Br) and [HPyXB(OH) 2 ]·(NO 3 ) (X = Cl, Br). Crucially, the crystal packing differs significantly between the two series. In HPyXB(C 2 O 4 )(OH), the constituent π-units (PyX and C 2 O 4 ) 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 ]·(NO 3 ) enforce a highly synergistic, near-coplanar alignment of the [HPyXB(OH) 2 ] + moiety and the NO 3 anion. This optimized structural arrangement leads to exceptionally large birefringence for the [HPyXB(OH) 2 ]·(NO 3 ) series, with measured Δ n values reaching 0.533 for X = Cl (calculated 0.554) at 546 nm. These values drastically exceed those of the HPyXB(C 2 O 4 )(OH) counterparts (Δ n = 0.182–0.194) and are more than four times larger than that of the commercial benchmark α-BaB 2 O 4 (Δ n = 0.12@546 nm). Significantly, [HPyXB(OH) 2 ]·(NO 3 ) 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.