Interlayer Shifting in Two-Dimensional Covalent Organic Frameworks

Layer-stacking
structures are very common in two-dimensional covalent
organic frameworks (2D COFs). While their structures are normally
determined under solvent-free conditions, the structures of solvated
2D COFs are largely unexplored. We report herein the in situ determination
of solvated 2D COF structures, which exhibit an obvious difference
as compared to that of the same COF under dried state. Powder X-ray
diffraction (PXRD) data analyses, computational modeling, and Pawley
refinement indicate that the solvated 2D COFs experience considerable
interlayer shifting, resulting in new structures similar to the staggered
AB stacking, namely, quasi-AB-stacking structures, instead of the
AA-stacking structures that are usually observed in the dried COFs.
We attribute this interlayer shifting to the interactions between
COFs and solvent molecules, which may weaken the attraction strength
between adjacent COF layers. Density functional theory (DFT) calculations
confirm that the quasi-AB stacking is energetically preferred over
the AA stacking in solvated COFs. All four highly crystalline 2D COFs
examined in the present study exhibit considerable interlayer shifting
upon solvation, implying the universality of the solvent-induced interlayer
stacking rearrangement in 2D COFs. These findings prompt re-examination
of the 2D COF structures in solvated state and suggest new opportunities
for the applications of COF materials under wet conditions.