Controlled Axial Growth and Morphological Evolution of Three-Dimensional Covalent Organic Frameworks Unveiled by Single-Crystal X-ray Diffraction.

Abstract

Precisely tailoring the macroscopic morphology of covalent organic frameworks (COFs) fundamentally drives their physicochemical properties. However, the robust and highly directional nature of covalent bonds makes such control at the single-crystal level a formidable challenge. Resolving this bottleneck, we establish a synergistic Brønsted and Lewis dual-acid catalytic strategy to dictate the controlled axial growth and morphological evolution of large (≥50 μm) three-dimensional(3D) COFs single crystals (the XNU-375-X; X = 1-6, a, p, EtOH) featuring a dia topology. Modulating the concentration of dysprosium trifluoromethanesulfonate (Dy(OTf)3), acting as the Lewis acid, drastically suppresses the twinning rate. Consequently, this targeted regulation drives a continuous morphological transition from octahedral to tetragonal bipyramidal geometries. Single-crystal X-ray diffraction (SCXRD) explicitly confirms the microscopic structural consistency throughout this macroscopic evolution. Crucially, during guest solvent removal and exchange, these crystallographic analyses directly capture a rare structural flexibility and dynamic "breathing" effect, evidenced by a massive 46% volume variation. Density functional theory (DFT) calculations elucidate the underlying growth kinetics. Conditional on the specific exposed facets ({100} versus {001}), Dy3+ exhibits differential adsorption behaviors that effectively passivate lateral free amine sites. To the extent that these sites govern horizontal proliferation, this selective binding simultaneously promotes ordered c-axis stacking and intrinsic self-correction. Ultimately, this work bridges the gap in the precision morphological tailoring of 3D COFs single crystals, providing a robust platform for the anisotropic growth and targeted synthesis of complex porous architectures.