Metal-Organic Frameworks (MOFs) are gaining attention in the chemical world for a variety of novel uses. In essence, they are structures where metallic ions are held in a three-dimensional grid pattern by organic molecules that link the ions together. The structure is naturally porous and can be thought of as a molecular sponge, with great potential for efficient storage of materials - for example, there has been a great deal of research devoted to using MOFs for high density storage of hydrogen for energy applications.
The properties of an MOF may be tuned by the choice of metal and the linking organic component. The organics are typically aromatic structures with multiple functional groups attached. These functional groups are not only going to determine how the grid structure is formed with the metal, but also what molecules may be attracted to, and held by, the MOF.
A research group at the University of Buffalo has come up with an interesting variation of a MOF where the structure changes properties upon exposure to ultraviolet (UV) light. Their work was published recently in Chemical Communications.
Their MOF structure, called a UBMOF (presumably for the University of Buffalo), alters the structure by attaching light-sensitive molecules known as diarylethenes to the organic linking component. Diarylethene contains a ring of atoms that is open under normal conditions. However, upon exposure to UV light, the ring of atoms closes down, effectively changing the pore size of the MOF. This property changes the MOF from something that passively attracts and absorbs chemicals to an active ingredient that can absorb specific chemicals and trap them.
The research team also discovered that the crystal changes color on exposure to UV light, becoming a red color instead of its normal colorless state. This suggests that the electronic properties are being altered as well as the pore size. It's possible that the UV light reaction could change the attractive forces within the structure, so that the MOF attracts one species under normal light and another species under UV light.
The next challenge for the research team is to understand the mechanism in greater detail in order to make it reversible. While the UBMOF's properties can be altered in interesting ways, at this point it is only a one-way street. The diarylethene structures can be reopened if they are outside of the MOF structure, but they remain closed when placed within the MOF. It's possible that by using different metals or further altering the organic linking component of the MOF, the ring reaction may become reversible without destroying the unique properties of the diarylethene.
It's easy to conceive of many uses for these materials - for example, targeted drug deliveries where a MOF delivers a tailored dose of an enclosed drug directly to a tumor and releases it upon command. Perhaps it can be used to trap and remove very specific chemical contaminants in a chemical process stream. We look forward to the progress of these materials and are eager to see how they can be used in practical applications.