From High Symmetry to High Symmetry: A Self‐Assembly Strategy for Synthesizing Large Birefringent Borates

ABSTRACT The π‐conjugated [B 3 O 6 ] groups have been one of the essential fundamental building blocks for designing large birefringent crystals used in the deep‐ultraviolet optics. However, whether it is possible to realize the perfect coplanar alignment of the [B 3 O 6 ] groups for maximizing optical anisotropy remains an outstanding problem. We propose a symmetry‐inherited self‐assembly strategy to realize ideal two‐dimensional alignment of birefringent planar building blocks B 3 O 3 (OH) 3 and [C(NH 2 ) 3 ]. During the hydrothermal process, B(OH) 3 undergoes condensation to form planar B 3 O 3 (OH) 3 units, while preserving the C 3 h symmetry of the precursors. The resulting crystal, C(NH 2 ) 3 B 3 O 3 (OH) 3 Cl, crystallizes in the hexagonal P 6 3 / m space group and exhibits the C 6 h symmetry of the molecular building blocks. The alignments of all the atoms strictly conform to the restriction imposed by the mirror symmetry, forming an ideal two‐dimensional layered configuration. This symmetry‐preserved coplanar arrangement enforces the decoupling of in‐plane and out‐of‐plane optical responses, thereby maximizing macroscopic anisotropy. As a result, C(NH 2 ) 3 B 3 O 3 (OH) 3 Cl possess a large birefringence of 0.184 at 546 nm together with a wide bandgap of 6.8 eV. This “from high symmetry to high symmetry” strategy converts preferably microscopic planar anisotropy into macroscopic uniaxial optical anisotropy through self‐assembly, offering a clear design principle for constructing highly anisotropic DUV optical crystals.