Credit: © 2008 RSC

Porous materials can be used as templates for chemical reactions. Producing such materials by self-assembly should enable easy tuning of the pore shapes, and thus control over how strongly guest molecules bind and how they react. However, predicting how changes in the structure will affect reactivity of the guest molecules remains difficult. Now, Linda Shimizu and co-workers from the University of South Carolina have shown that subtle changes in the building blocks — which do not significantly change the framework — can also have a dramatic effect on the reactivity.

Shimizu and co-workers had previously shown that porous materials were formed when bis-urea macrocycles — made by linking together two dibenzylureas — self-assembled into tubes held together by intermolecular hydrogen bonds and π-stacking interactions. They prepared a system where an ether link was used, and showed that absorbed enones were dimerized when irradiated with UV light. Changing the linking group from an ether to a ketone had very little effect on the dimensions of the pores formed, but with this new structure the dimerization did not occur. Instead, the material caused the cis–trans isomerization of methylstryrene under irradiation.

This change in reactivity can be explained by considering the UV characteristics of the two structures. The macrocycle with a ketone link competitively absorbs light at the wavelength necessary to cause the dimerization of enones and thus no reaction occurs. Effective cis–trans isomerization only occurs in the presence of photosensitizers such as benzophenone. In the case of the ketone linker, a benzophenone-like structure is present in the structure of the macrocycle, and this can effectively transfer energy to the guest molecule.