Conformational Editing Drives Covalent Hybridization for Nonlinear Optical Crystals

The covalent integration of planar organic molecules with inorganic tetrahedra into crystalline solids is fundamentally constrained by steric repulsion. Open-chain organic moieties predominantly adopt thermodynamically stable trans-conformations, which spatially hinder close approach and covalent bond formation, often leading to centrosymmetric architectures. Here, we first report a solvent-driven conformational editing strategy that overcomes this long-standing synthetic challenge. By modulating HBF₄ concentration, we reprogram biuret from a sterically hindered trans-conformer into a spatially accessible cis-conformer. This in situ conformational switching, validated by interacting region indicator analysis and Raman spectroscopy, providing a general kinetic pathway for previously inaccessible bond formations. The resulting non-centrosymmetric hybrid crystal, [C₂O₂N₃H₅BF₂][BF₄], features [C₂O₂N₃H₅BF₂] units that represent the first synthesis of a covalently linked biuret-[BO₂F₂] hybrid. This compound achieves an outstanding combination of key properties for short-wavelength ultraviolet nonlinear optical applications. Through this conformational editing approach, we further synthesized 14 additional compounds, establishing biuret as a versatile platform for hybrid assembly. Our work demonstrates solvent-mediated conformational control as a general synthetic paradigm for overcoming steric hindrance and accessing previously inaccessible organic-inorganic architectures with tailored optical functionality.