Interlayer\nShifting in Two-Dimensional Covalent Organic\nFrameworks

Abstract

Layer-stacking\nstructures are very common in two-dimensional covalent\norganic frameworks (2D COFs). While their structures are normally\ndetermined under solvent-free conditions, the structures of solvated\n2D COFs are largely unexplored. We report herein the in situ determination\nof solvated 2D COF structures, which exhibit an obvious difference\nas compared to that of the same COF under dried state. Powder X-ray\ndiffraction (PXRD) data analyses, computational modeling, and Pawley\nrefinement indicate that the solvated 2D COFs experience considerable\ninterlayer shifting, resulting in new structures similar to the staggered\nAB stacking, namely, quasi-AB-stacking structures, instead of the\nAA-stacking structures that are usually observed in the dried COFs.\nWe attribute this interlayer shifting to the interactions between\nCOFs and solvent molecules, which may weaken the attraction strength\nbetween adjacent COF layers. Density functional theory (DFT) calculations\nconfirm that the quasi-AB stacking is energetically preferred over\nthe AA stacking in solvated COFs. All four highly crystalline 2D COFs\nexamined in the present study exhibit considerable interlayer shifting\nupon solvation, implying the universality of the solvent-induced interlayer\nstacking rearrangement in 2D COFs. These findings prompt re-examination\nof the 2D COF structures in solvated state and suggest new opportunities\nfor the applications of COF materials under wet conditions.