Light-gated binding in double-motorized porphyrin cages

Abstract

Molecular motors change conformation under the influence of light and when attached to host molecules they may find applications as sensors and switchable catalysts. Here we present a porphyrin macrocyclic host functionalized with two motor appendages for future catalytic applications. The compound is formed as a mixture of six stereoisomers (three sets of enantiomers), which have been separated by (chiral) chromatography. 1H NMR and chiral spectroscopy revealed that in one set of diastereomers the two motors interact with the cavity of the host (bound-bound), whereas in a second set one interacts and the other one does not (bound-loose). In the third set both motors do not interact with the host compound (loose-loose). The motorized hosts bind guest molecules in the order: (loose-loose) > (bound-loose) > (bound-bound). They can be switched with light to pseudo-identical diastereomers, leading to orthogonal behavior in the light-gated binding of guest molecules. Whereas the photo-isomerization of the diastereomer set loose-loose significantly lowers the binding affinity for viologen guests, the opposite is true for the diastereomer set bound-bound, i.e. the binding affinity increases. For the diastereomer set bound-loose no influence on guest binding is observed as the effect of photoisomerization on the motors is cancelled out.