Topochemical Synthesis of Single-Crystalline Hydrogen-Bonded Cross-Linked Organic Frameworks and Their Guest-Induced Elastic Expansion

Covalently
linked single-crystalline porous organic materials are
highly desired for structure–property analysis; however, periodically
polymerizing organic entities into high dimensional networks is challenging.
Here, we report a series of topologically divergent single-crystalline
hydrogen-bonded cross-linked organic frameworks (HCOFs)
with visible guest-induced elastic expansions, which mutually integrate
high structural order and high flexibility into one framework. These
HCOFs are synthesized by photo-cross-linking molecular
crystals with alkyldithiols of different chain lengths. Their detailed
structural information was revealed by single-crystal X-ray analysis
and experimental investigations of HCOFs and their corresponding
single-crystalline analogues. Upon guest adsorption, HCOF-2 crystals composed of a 3D self-entangled polymer network undergo
anisotropic expansion to more than twice their original size, while
the 2D-bilayer HCOF-3 crystals exhibit visible, layered
sorption bands and form delaminated sheets along the plane of its
2D layers. The dynamic expansion of HCOF networks creates
guest-induced porosity with over 473% greater volume than their permanent
voids, as calculated from their record-breaking aqueous iodine adsorption
capacities. Temperature-gated DMSO sorption investigations illustrated
that the flexible nature of cross-linkers in HCOFs provides
positive entropy from the coexistence of multiple conformations to
allow for elastic expansion and contraction of the frameworks.