The challenge
Metal–organic frameworks (MOFs) are essentially molecular sponges that capture and release smaller molecules, such as CO2, making them a hot topic in the molecular separation and storage space. The issue is, that by eye, it is impossible to tell if a MOF has captured something. So how can you tell if they’re ‘working’? Short answer is that you can’t, unless you have analytical equipment, or a MOF that changes colour when it captures or releases guest molecules.
Hua and co-workers from the University of Melbourne have developed just that: reversible colour changing MOFs, specifically lanthanoid (Ln)-containing MOFs that change colour when exposed to UV radiation, i.e photoswitches. The chemistry behind this change was thought to be due to the Ln redox activity as the colour switching speed correlated with the accessibility of certain oxidation states in the Ln. For example, the Eu(III) MOF switches colour fastest—consistent with the accessible Eu(II) state—while the Ce(III) analogue is the slowest, likely because Ce(II) is hard to access. Determining whether this correlation was causation became the key science question as the answer is the key to optimising the MOFs colour change to various applications.
