Photoswitch selects by size

Replicating things from nature is one of the specialities of chemists. Anion receptors are no exception. When you make them yourself, applications such as sensing or transport are obvious candidates. A highly regarded synthetic anion receptor is calix[4]pyrrole, owing to its strong affinity with halide ions. Researchers from Sander Wezenberg’s group at Leiden University have created a variant that can be activated with light: one isomer admits all manner of halogens, whilst the other keeps larger ions outside the proverbial door.

The modification they gave calix[4]pyrrole was a molecular ‘loop’ of dithienylethene. It consists, amongst other things, of three five-membered rings joined together that, under the influence of light, form a six-membered ring between them. The loop variant without the six-membered ring creates an opening where calix[4]pyrrole can use its halogen affinity to bind halide ions such as chloride, bromide, or iodide.

If you shine light on it at a wavelength of 300 nm (ultraviolet), the six-membered ring isomer forms and the opening becomes smaller. Chloride is not particularly affected by this, but the affinity with bromide becomes 160 times smaller — the greatest change in affinity ever recorded in this type of receptor. To break the six-membered ring (and allow bromide and iodide back in), you need to use light with a wavelength of 630 nm (red).

‘One of the possible future applications could be in sensing,’ says Wezenberg. ‘Not necessarily as a sensor itself, but as an extractant in cases where quantitative analysis would be hampered by background noise. You could then selectively extract the analyte, separate it completely from the receptor again using light, and subsequently quantify it.’

In an NWO-M1 project, Wezenberg’s group is investigating the extraction of caesium chloride/bromide ion pairs. ‘Calixpyrrole is selective for caesium salts,’ Wezenberg explains, ‘and that could be interesting for the purification of radioactive wastewater. Because that is rather expensive technology, you can justify the use of a somewhat more complex — and therefore more costly — molecule.’

But the group is not there yet. Wezenberg: ‘For the time being, we are developing the technology to control binding affinity with light at a fundamental level, which is a considerable challenge. However, the difference in affinity for bromide between the light-switchable states as described in this article is the greatest compared to what has been achieved thus far. The calixpyrrole receptors bridged with a molecular switch therefore appear to be highly suited to controlling binding properties.’

Villarón, D. et al. (2023) Chem. Commun. 59, DOI: 10.1039/D3CC02264A