Light-driven molecular pumps: entanglement of thermodynamic and kinetic effects in the photocontrolled threading-unthreading of pseudorotaxanes.

Abstract

Light-powered molecular pumps represent an intriguing class of artificial nanomachines capable of using the energy of photons to perform directional transport. Pseudorotaxanes composed of macrocyclic crown ethers that encircle axles based on azobenzene photoswitches and secondary ammonium recognition sites have emerged as promising architectures, as light can modulate both the kinetics and thermodynamics of complex formation, thereby enabling directionally biased motion by an energy ratchet mechanism. In this study, we examine the effect of photoisomerization on the threading-unthreading dynamics of a symmetrical axle bearing decoupled azobenzene and dibenzylammonium units. The results are compared with those obtained on a previously reported more compact axle in which the two units share a phenyl ring. We found that, while Z-azobenzene significantly slows down the (un)threading kinetics with respect to the E isomer, it does not destabilize the pseudorotaxane. Hence, such a decoupling challenges a core design requirement for photoinduced molecular pumps - namely, the light-induced modulation of both energy barriers and binding affinities. Our results underscore the critical role of electronic and spatial proximity between the photoisomerizable unit and the ring recognition site in achieving coupled kinetic and thermodynamic control. These insights provide refined design principles for the development of efficient light-driven molecular pumps based on modular supramolecular motifs.