Construction and Band Gap-Regulation of Ordered Macro-Microporous Single Crystals of an Amine-Linked Covalent Organic...

Heterogeneity engineering provides an effective route to manipulate the chemical and physical properties of covalent organic frameworks (COFs) but is still under development for their single-crystal form. Here, we report the strategy based on a combination of the template-assisted modulated synthesis with a one-pot crystallization-reduction method to directly construct ordered macro-microporous single crystals of an amine-linked three-dimensional (3D) COF (OM-COF-300-SR). In this strategy, the colloidal crystal-templating synthesis not only assists the formation of ordered macropores but also greatly facilitates the in situ conversion of linkages (from imine to amine) in the COF-300 single crystals. The as-synthesized OM-COF-300-SR₁₂₀ exhibits a reversible symmetry change from a tetragonal I4₁/a to monoclinic I2/c space group after activation, which was not observed previously. On the other hand, this strategy allows for a flexible control over the degree of amination (from 0 to 100%, as determined by X-ray photoelectron spectroscopy (XPS) analysis) in COF-300 crystals to regulate their band gap (from 2.57 to 2.81 eV) for the optimization of photocatalytic activity. The high degree of amination and the embedded ordered macropores render OM-COF-300-SR₁₂₀ with superior photocatalytic activity (with a reaction rate constant of 0.9572 min^-1) to its nonmacroporous counterpart (NM-COF-300-SR₁₂₀, 0.2303 min^-1) for the degradation of rhodamine B. In addition, the significant contribution of ordered macropores to confront mass transfer resistance in COF single crystals was also confirmed by the much higher catalytic activity of Au/OM-COF-300-SR₁₂₀ (with an activity parameter of 7.96 × 10³ s^-1 mol^-1) as compared with Au/NM-COF-300-SR₁₂₀ (1.43 × 10³ s^-1 mol^-1) in the model reduction reaction of 4-nitrophenol by NaBH₄.