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.
The solution
Use the MEX-1 Beamlineto check if the Ln oxidation states in the MOFs actually change.
MEX-1 X-ray absorption spectroscopy allowed us to compare the oxidation states of the Lns in the MOFs before and after UV-induced colour change. Interestingly--though less exciting of a result--is that we saw no Ln oxidation state change in any of the MOFs, indicating that a different mechanism is responsible for the colour change.
The impact
Our result is important because it provides insight into the photoswitching mechanism, the decipherment of which provides the researchers with the insight they need to tailor their MOF to change colour when binding and releasing guest molecules, not just in the presence of UV. Optimising MOFs in this way is key to integrating them into green or wider technologies everyday people can use and understand, for example “at home” CO2 capture, i.e. sensing by colour change.
This case study, written by Dr Pria Ramkissoon was the runner up of ANSTO Australian Synchrotron's Case Study Competition Round 3 2025.
References:
L. Jing, E. Deplazes, J. K. Clegg and X. Wu, A charge-neutral organic cage selectively binds strongly hydrated sulfate anions in water, Nature Chemistry 2024, 16, 335-342. https://www.nature.com/articles/s41557-024-01457-5
R. J. Goodwin and N. G. White, Clever cryptand cage coordinates contaminants, Nature Chemistry 2024, 16, 299-300. https://www.nature.com/articles/s41557-024-01459-3