Precise elucidations of stacking manners of hydrogen-bonded two-dimensional organic frameworks composed of X-shaped π-conjugated systems
Two-dimensional covalent organic frameworks (2D-COFs) composed of π-conjugated components are promising functional materials. In general, however, 2D-COFs exhibit low crystallinity, preventing their precise structural characterization. On the other hand, non-covalent organic frameworks (nCOFs) are often obtained as single crystals due to highly reversible bond formation, and therefore, are possible to complement the structural evaluation of 2D-COFs. In this study, three X-shaped tetracarboxylic acid derivatives with benzene, tetrathiafulvalene (TTF), and pyrazinoquinoxaline cores (X-Ph, X-TTF and X-PyQ, respectively) were revealed to form isostructural, hydrogen-bonded rhombic network (RhomNet) sheets, which subsequently stacked without interpenetration to give low density frameworks with one-dimensional (1D) inclusion channels. RhomNet structures and their stacking manners were fully revealed based on single crystal X-ray analysis. Consequently, we revealed that the RhomNet sheets of X-Ph, X-TTF and X-PyQ stack in different ways depending on the conformation of the peripheral phenylene groups, molecular symmetry and interlayer interactions, in spite of the same network topology in the RhomNet sheets. The precise characterization of the present RhomNet crystals can provide a new structural insight into porous 2D-COFs.