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

Abstract

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 CO 2 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.

Programmed Pore‐Shape Fixing in a Soft Porous Molecular Crystal—Navigating the Phase Interconversion Landscape

“…Although the same effect was reported for a metal–organic framework (MOF) in 2013, [6] the second example involving a MOF was only reported in 2018 [12] . Since then only a handful of additional articles have described similar observations for MOFs [13–18] …”

“…[12] Since then only a handful of additional articles have described similar observations for MOFs. [13][14][15][16][17][18] From the various structure-property studies by the groups of Atwood [10] and Ripmeester [19] it is possible to construct a partial phase interconversion scheme for TBC4 based on the two stimuli of CO 2 gas pressure and heat (Text S2). However, some of the aspects of this scheme are conflicting owing to the lack of conclusive in situ experimental data.…”

Phase switching and shape-memory effect in a molecular material: revisiting the Werner complex [Ni(4-MePy)4(NCS)2]

“…In principle, should FMOMs exhibit structural transformations between polymorphic phases, then they could also function as SMMs (Scheme 1). Indeed, recent studies have demonstrated the SME in a handful of 3D FMOMs, 41–47 as evidenced by their successive sorption isotherms. Despite these encouraging findings, the SME remains an underexplored phenomenon in molecular FMOMs, with only one known example, a platinum( ii )–diimine complex that was studied for its vapochromic shape-memory characteristics.…”

Correlating Phonons and Deformations: A Method for Structural Phase Transformation Analysis in Metal–Organic Frameworks

“…23 In the field of structural phase transitions, metal−organic frameworks (MOFs) have drawn significant attention. The flexibility inherent to some MOFs 24,25 allows them to undergo structural changes when exposed to a variety of external stimuli such as temperature, 26,27 pressure, 28−30 presence of guest molecules, 31,32 or electric field. 33,34 Notably, these structural changes have the potential to alter the key characteristics of MOFs and have been proved advantageous, as they may improve adsorption-related applications such as separation and catalysis, 35−37 or contribute to mechanical energy storage and absorption.…”

“…In the field of structural phase transitions, metal–organic frameworks (MOFs) have drawn significant attention. The flexibility inherent to some MOFs , allows them to undergo structural changes when exposed to a variety of external stimuli such as temperature, , pressure, − presence of guest molecules, , or electric field. , Notably, these structural changes have the potential to alter the key characteristics of MOFs and have been proved advantageous, as they may improve adsorption-related applications such as separation and catalysis, − or contribute to mechanical energy storage and absorption. , Numerous studies have explored the relationship between structural changes and individual phonon modes and their groups. Ryder et al performed a comprehensive experimental and theoretical analysis of vibrational modes in three zeolitic imidazolate frameworks (ZIFs): ZIF-4, ZIF-7, and ZIF-8 .…”