Some metal-organic frameworks (MOF) display large structural changes as a response to external stimuli, and such modifications of their structure can, in turn, lead to drastic changes in their physical and chemical properties. Such stimuli-responsive MOFs are generally referred to as a flexible metal-organic frameworks. They can also be called dynamic metal-organic framework, stimuli-responsive MOFs, multi-functional MOFs, or soft porous crystals. Flexibility of the MIL-53 metal-organic framework

Flexibility of the MIL-53 metal-organic framework

Formally, a metal-organic framework is a coordination network with organic

ligand In coordination chemistry, a ligand is an ion or molecule with a functional group that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's el ...

s containing potential voids. A coordination network is a coordination compound extending, through repeating coordination entities, in

one dimension A one-dimensional space (1D space) is a mathematical space in which location can be specified with a single coordinate. An example is the number line, each point of which is described by a single real number. Any straight line or smooth curve ...

, but with cross-links between two or more individual chains, loops, or spiro-links, or a coordination compound extending through repeating coordination entities in

two 2 (two) is a number, numeral and digit. It is the natural number following 1 and preceding 3. It is the smallest and the only even prime number. Because it forms the basis of a duality, it has religious and spiritual significance in many ...

three dimensions In geometry, a three-dimensional space (3D space, 3-space or, rarely, tri-dimensional space) is a mathematical space in which three values (''coordinates'') are required to determine the position of a point. Most commonly, it is the three-di ...

. A

coordination polymer Coordination may refer to: * Coordination (linguistics), a compound grammatical construction * Coordination complex, consisting of a central atom or ion and a surrounding array of bound molecules or ions ** A chemical reaction to form a coordinat ...

is a coordination compound with repeating coordination entities extending in one, two, or three dimensions. Generally, this kind of material has a well-defined structure, but sometimes some external stimuli can affect its structure, resulting in a different structure without breaking the overall network. A variety of external stimuli like

heat In thermodynamics, heat is energy in transfer between a thermodynamic system and its surroundings by such mechanisms as thermal conduction, electromagnetic radiation, and friction, which are microscopic in nature, involving sub-atomic, ato ...

, light,

solvent A solvent (from the Latin language, Latin ''wikt:solvo#Latin, solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a Solution (chemistry), solution. A solvent is usually a liquid but can also be a solid, a gas ...

, an

electric field An electric field (sometimes called E-field) is a field (physics), physical field that surrounds electrically charged particles such as electrons. In classical electromagnetism, the electric field of a single charge (or group of charges) descri ...

magnetic field A magnetic field (sometimes called B-field) is a physical field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular ...

, etc. can act upon a metal-organic framework, can act to change its internal structure, and can facilitate the transformation process. This structural transformation generally occurs by bond breaking/making, change of coordination number of the

metal ion A metal () is a material that, when polished or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. These properties are all associated with having electrons available at the Fermi level, as against non ...

, change of coordination mode of ligand, ligand length squeezing, solvent exchange, solvent removal, etc. One often discussed example of flexible metal-organic framework is the family of MIL-53 materials, featuring one-dimensional diamond-shaped pores that can expand or contract upon stimulation, such as adsorption of guest molecules (solvent, water, gases, etc.), changes in temperature, and mechanical pressure.

References

See also