Vertically Expanded Crystalline Porous Covalent Organic Frameworks

Covalent organic frameworks (COFs) can be developed for
molecular
confinement and separation. However, their proximate π stacks
limit the interlayer distance to be only 3–6 Å, which
is too small for guests to enter. As a result, COFs block access to
the x–y space and limit guest
entry/exit strictly to only the pores along the z direction. Therefore, the extended faces of each layer are hidden
between layers, precluding any interactions with guest molecules.
Here, we report a strategy for opening interlayer spaces of COFs to
attain newly accessible nanospaces between layers. This becomes possible
using coordination bonds to replace the conventional π–π
stacks between layers. We demonstrate this concept by synthesizing
two-dimensional covalent cobalt(II) porphyrin layers through topology-guided
polymerization, which were piled up by bidentate axial pillars through
coordination bonds with cobalt(II) porphyrin along the z direction, assembling vertically expanded COFs via a one-pot reaction.
The resultant frameworks separate the layers with axial pillars and
create discrete apertures between layers defined by the molecular
length of the pillars. Consequently, the originally inaccessible interlayers
are open for guest access, while the polygonal π planes are
exposed to trigger various supramolecular interactions. Vapor sorption,
breakthrough experiments, and computational studies mutually revealed
that the vertically expanded frameworks with optimal interlayer slits
induce additional interactions to discriminate benzene and cyclohexane
and separate their mixtures efficiently under ambient conditions.