Hydrogen‐Bond‐Network Enables Near‐Coplanar Fluorooxoborates with Large Deep‐Ultraviolet Birefringence

Deep‐ultraviolet birefringent materials require simultaneously large optical anisotropy and wide bandgaps, yet these properties are often difficult to reconcile in fluorooxoborates because the incorporation of tetrahedral [BO 4− x F x ] units tends to distort otherwise anisotropic borate frameworks. Here we introduce a hydrogen‐bond modulation strategy to suppress such structural distortion and promote near‐coplanar alignment of optically active building units. Inspired by the tension‐distributing architecture of spider webs, the planar π‐conjugated cations, [C(NH 2 ) 3 ], were employed as multidirectional hydrogen‐bond donors to construct a highly interconnected hydrogen‐bond network throughout the crystal lattice. Guided by this concept, a new fluorooxoborate, Li[C(NH 2 ) 3 ]B 6 O 8 F 4 , was synthesized. The cooperative hydrogen‐bond network effectively constrains the [B 6 O 9 F 4 ] FBBs into a near‐coplanar arrangement, reducing the dihedral angle between adjacent B 3 O 3 rings to less than 7°. An interplay between terahertz spectroscopy and solid‐state density functional theory calculations reveals substantially enhanced cation–anion vibrational coupling over the hydrogen‐bond‐free analogue α ‐KB 3 O 4 F 2 , providing direct evidence for the collective constraint imposed by the hydrogen‐bond network. The resulting cooperative alignment of the π‐conjugated cations and anionic chains generates pronounced optical anisotropy, yielding a birefringence of 0.15 at 546 nm and a wide bandgap of 6.87 eV. Hydrogen‐bond‐network engineering effectively regulates framework coplanarity and provides a general design principle for high‑performance DUV birefringent materials.