Light-Driven Supramolecular Pumping by Changing Shape Complementarity.

Protein pumps in the cell membrane are capable of actively moving solutes uphill to build concentration gradients, a process pivotal to biological energy conversion. These pumps dissipate light or chemical energy to generate and maintain a nonequilibrium steady state. Achieving continuous unidirectional displacement of substrates or (sub)components in artificial molecular systems is a tremendous challenge. In this work, we realize the repeated transit of a molecular axle through a photoswitchable macrocyclic ring. The axle contains different termini, which have different shape complementarities with the interconverting ring isomers. This approach allows one to invert the relative heights of the kinetic barriers for ring slippage over the axle termini and to change the stability of the pseudorotaxane complex (formed between the ring and the axle). Under steady-state irradiation conditions, this modulation of kinetic barriers results in an effective and continuous unidirectional translation of the axle with respect to the ring by an energy ratcheting mechanism. Further, photoconversion is enhanced by the bound axle, which adds to the directionality of the system by an information ratcheting effect. This work brings opportunities toward substrate pumping across membranes to generate and maintain electrochemical gradients using light.