Shape-Memory Effect Triggered by π–π Interactions in a Flexible Terpyridine Metal–Organic Framework

Shape-memory polymers
and alloys are adaptable materials capable
of reversing from a deformed, metastable phase to an energetically
favored original phase in response to external stimuli. In the context
of metal–organic frameworks, the term shape-memory is defined
as the property of a switchable framework to stabilize the reopened
pore phase after the first switching transition. Herein we describe
a novel flexible terpyridine MOF which, upon desolvation, transforms
into a nonporous structure that reopens into a shape-memory phase
when exposed to CO2 at 195 K. Based on comprehensive in situ experimental studies (SC-XRD and PXRD) and DFT energetic
considerations combined with literature reports, we recommend dividing
shape-memory MOFs into two categories, viz responsive and nonresponsive,
depending on the transformability of the gas-free reopened pore phase
into the collapsed phase. Furthermore, considering the methodological
gap in discovering and understanding shape-memory porous materials,
we emphasize the importance of multicycle physisorption experiments
for dynamic open framework materials, including metal–organic
and covalent organic frameworks.