Hydrogen-Bonded Displacive-Type Ferroelastic Phase Transition in a New Entangled Supramolecular Compound

The framework entanglements show
structural transitions by the
removal and incorporation of guest molecules, but rarely generate
phase transitions by themselves. In this study, we report a new entangled
hydrogen-bonded supramolecular compound, [(n-C4H9)2NH2]2­H2C4O4·​H4C4O4 (1, H4C4O4 = fumaric acid), which undergoes a reversible ferroelastic
phase transition with the Aizu notation of 2/mF1̅.
Differential scanning calorimetry and specific heat measurements confirm
its typical second-order phase transition at around 228.8 K (Tc), while the results of the deuterated analogue
(2) are different with those of 1, indicating
that proton dynamic motions in hydrogen bonds contribute to the phase
transition. Variable-temperature single-crystal X-ray diffraction
analyses reveal that the cooperative displacements of hydrogen bonds
induce the structural phase transition, which arise from the twisting
motions of the fumaric acid molecules. Simultaneously, two types of
independent hydrogen bonding layers in the entanglement are altered
in response to the transformation of hydrogen bonds aggregates at
the low temperature phase, causing the symmetry breaking. These findings
will open up a new avenue for the design of ferroic materials with
an entangled framework.