Design and Synthesis of Single-Crystalline Covalent Organic Frameworks: The Use of Modulator for Reversible Bond Control

Covalent organic frameworks (COFs), notable for their well-organized crystalline structures, varied framework topologies, and large surface areas, have been actively studied for diverse applications. Their distinct properties are related primarily to their crystalline frameworks; therefore, obtaining accurate structural information is important. Notable advances in the structural analysis of COFs using powder X-ray diffraction and Pawley or Rietveld refinement have been made; however, these methods sometimes encounter limitations. Single-crystal X-ray diffraction (SCXRD) has been suggested as an alternative technique for obtaining accurate structural information on COFs. However, harvesting COFs of sufficiently high purity and quality, with crystals of sufficient size for SCXRD analysis, is difficult. Significant research on growing single-crystalline COFs (SC-COFs) with large crystals using modulators for reversible bond control has recently enabled SCXRD studies. In this review, we introduce various strategies, including molecular design and synthesis methods, for growing SC-COFs, and highlight several notable examples that leverage precise structural information for specific applications.