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.