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

The π-conjugated [B₃O₆] 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 [B₃O₆] 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 B₃O₃(OH)₃ and [C(NH₂)₃]. During the hydrothermal process, B(OH)₃ undergoes condensation to form planar B₃O₃(OH)₃ units, while preserving the C_{3 h} symmetry of the precursors. The resulting crystal, C(NH₂)₃B₃O₃(OH)₃Cl, crystallizes in the hexagonal P6₃/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(NH₂)₃B₃O₃(OH)₃Cl 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.