An Expandable Hydrogen-Bonded Organic Framework Characterized by Three-Dimensional Electron Diffraction

A molecular
crystal of a 2-D hydrogen-bonded organic framework
(HOF) undergoes an unusual structural transformation after solvent
removal from the crystal pores during activation. The conformationally
flexible host molecule, ABTPA, adapts its molecular conformation
during activation to initiate a framework expansion. The microcrystalline
activated phase was characterized by three-dimensional electron diffraction
(3D ED), which revealed that ABTPA uses out-of-plane
anthracene units as adaptive structural anchors. These units change
orientation to generate an expanded, lower density framework material
in the activated structure. The porous HOF, ABTPA-2,
has robust dynamic porosity (SABET = 1183 m2 g–1) and exhibits negative area thermal expansion.
We use crystal structure prediction (CSP) to understand the underlying
energetics behind the structural transformation and discuss the challenges
facing CSP for such flexible molecules.