Synthesis and Thermodynamic Study of Photoresponsive Azobenzene-Based Pseudorotaxanes

Supramolecular chemistry provides design principles for artificial systems that move in a controlled way in response to external stimuli. Light driven molecular pumps, which can transport macrocyclic through an energy ratchet powered solely by light are an example. They are typically pseudorotaxane in which a non-symmetric axle (azobenzene, ammonium station and pseudostopper) interacts with a crown ether ring, E/Z isomerization modulates the thermodynamic landscape that enables the pumping. Four asymmetric axles were designed and synthesized to investigate how structure influences pumping efficiency, varying the bulky pseudostopper (phenyl vs 2,4-dimethylphenyl) and the linkage position (meta vs para) between the azobenzene and the ammonium station. Since ion pairing at the ammonium station can strongly influence host-guest binding strength, four salts of each axle were synthesized: PF6-, ClO4-, OTs- and SCN- to evaluate the effect of the counterion. Following synthesis, all the axles were characterized by multidimensional NMR and UV-Visible spectroscopy. All derivatives show typical azobenzene behaviour, remaining unaffected by the nature of the counterion while, the absorption maximum (λmax) was influenced by the connectivity: meta linked derivatives were slighty blue shifted when compared to their para derivatives analogous owning to inductive effects. The stability of the pseudorotaxanes formed with dibenzo-24-crown-8 a crown (DB24C8) was evaluated via single point 1H NMR experiments. PF6- lead to the most stable pseudorotaxane due to its weakly coordinating nature; whereas OTs- showed the lowest association constants thanks to the competitive ion pairing with the ammonium station. Then, unsubstituted meta showed higher association constants than their para analogous. These results show how structural tuning can control the behaviour of light-driven molecular pumps. Kinetic studies of threading and dethreading will be the next step towards functional applications.