Dual‐Role BF 3 Anchoring Enables Giant Birefringence in Short‐Wave UV‐Transparent Fluoroborates

ABSTRACT Birefringent materials that operate in the short‐wave ultraviolet region (cutoff < 300 nm) are important for polarization control in UV photonics. However, combining ultralarge birefringence (Δ n > 0.5) with a wide bandgap (> 4.0 eV) remains challenging because of the trade‐off between optical anisotropy and energy gap. Here, we present a computation‐guided BF 3 ‐anchoring strategy in which grafting BF 3 onto halogenated pyridine yields BF 3 XPy modules that combine a favorable energy gap with large polarizability anisotropy. The BF 3 group simultaneously serves as a wide‐bandgap anchor that prevents low‐energy transitions and a coplanarity‐locking organizer, whose terminal fluorine atom accepts hydrogen and halogen bonds. Three trifluoroborates, Hy·BF 3 HClPy, [HIm] + [BF 3 FPy] − and [HIm] + [BF 3 ClPy] − ·H 2 O, were obtained using hydantoin (Hy) or imidazole (Im) as co‐formers. Among them, Hy·BF 3 HClPy exhibits a Δ n of 0.549 at 546 nm, a bandgap of 4.10 eV, and a UV cutoff of 263 nm, which renders it the first fluoroborate with Δ n > 0.5, which is more than 45 times that of commercial fluorides (MgF 2 , Δ n = 0.012). Structure–property analysis correlates the birefringence trend to Σ( ρ i ·Δ α i ·cos θ i ), and electrostatic potential calculations identify donor–acceptor electrostatic matching as the driving force for coplanar assembly. This work provides an effective route to high‐performance short‐wave UV birefringent crystals.