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The layer stacking of two-dimensional (2D) vinylene-linked covalent organic frameworks can create different topologies and some unexpected properties but is still less explored rather than well-tuned. Here, trimethyl triazacoronene was newly developed as a tritopic monomer. Three pyridine nitrogen atoms evenly embedded at its molecular periphery endow it with not only a noncentrosymmetrically dense-sparse alternating distribution of electron cloud but also a strong electron-deficient character. It enables smooth Knoevenagel condensation with 1,4-phenyl- or 4,4'-diphenyl dialdehyde in the presence of benzoic acid to form two 2D COFs. One of them comprising phenyl nodes shows the less steric interactions between vinylene linkages with the aromatic units in each layer, where all building blocks tend to show an in-plane arrangement under the π-conjugated effect, and the layers could be periodically offset and stacked in a staggered AB mode upon the through-space polar interactions between the interlayer triazacoronene vertexes. For the other COF, its biphenyl nodes favorably adopt a twisted conformation, which could exert an interlocked effect for constraining the slip offset of the layers and result in an eclipsed AA assembly upon the interlayer π-π stacking interaction. These assemblies were confirmed by well-defined powder X-ray diffraction patterns and low-dose imaging in transmission electron microscopy. Moreover, the photophysical properties and photocatalytic activities of as-prepared COFs were fully investigated, which are highly in correlation with their in-plane and stacking structures. These findings provided an efficient strategy for fine-modulating the interlayer alignments of 2D materials with improved properties/functions.
Chi et al. (Fri,) studied this question.