Crystalline solids dominate the field of metal-organic frameworks (MOFs), with access to the liquid and glass states of matter usually prohibited by relatively low temperatures of thermal decomposition. In this work, we give due consideration to framework chemistry and topology to expand the phenomenon of the melting of 3D MOFs, linking crystal chemistry to framework melting temperature and kinetic fragility of the glass-forming liquids. Here we show that melting temperatures can be lowered by altering the chemistry of the crystalline MOF state, which provides a route to facilitate the melting of other MOFs. The glasses formed upon vitrification are chemically and structurally distinct from the three other existing categories of melt-quenched glasses (inorganic nonmetallic, organic, and metallic), and retain the basic metal-ligand connectivity of crystalline MOFs, which connects their mechanical properties to their starting chemical composition. The transfer of functionality from crystal to glass points toward new routes to tunable, functional hybrid glasses.

We have discovered and clarified the ultrahigh glass-forming ability of the metal-organic frameworks—ZIF-62 [Zn(Im2−xbImx)].

Desolvated zeolitic imidazolate framework ZIF-4(Zn) undergoes a discontinuous porous to dense phase transition on cooling through 140 K, with a 23 % contraction in unit cell volume. The structure of the non-porous, low temperature phase was determined from synchrotron X-ray powder diffraction data and its density was found to be slightly less than that of the densest ZIF phase, ZIF-zni. The mechanism of the phase transition involves a cooperative rotation of imidazolate linkers resulting in isotropic framework contraction and pore space minimization. DFT calculations established the energy of the new structure relative to those of the room temperature phase and ZIF-zni, while DSC measurements indicate the entropic stabilization of the porous room temperature phase at temperatures above 140 K.

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While glasses formed by quenching the molten states of inorganic non-metallic, organic, and metallic species are known, those containing both inorganic and organic moieties are far less prevalent. Network materials consisting of inorganic nodes linked by organic ligands do however exist in the crystalline or amorphous domain. This large family of open framework compounds, called metal-organic frameworks (MOFs) or coordination polymers, has been investigated intensively in the past two decades for a variety of applications, almost all of which stem from their high internal surface areas and chemical versatility. Recently, a selection of MOFs has been demonstrated to undergo melting and vitrification upon cooling. Here, these recent discoveries and the connections between the fields of MOF chemistry and glass science are summarized. Possible advantages and applications for MOF glasses produced by utilizing the tunable chemistry of the crystalline state are also highlighted.