Monodirectional photocycle drives proton translocation

Photoisomerization of retinal is pivotal to ion translocation across the bacterial membrane and has served as inspiration for the development of artificial molecular switches and machines. Light-driven synthetic systems in which a macrocyclic component transits along a nonsymmetric axle in a specific direction have been reported, however, unidirectional and repetitive translocation of small substrates, including ions, has not been achieved. Herein, we describe unique protonation-controlled isomerization behavior for hemi-indigo dyes bearing N-heterocycles, owing to intramolecular hydrogen bond formation. Light-induced isomerization from Z to E isomer is unlocked when protonated, while reverse E→Z photoisomerization occurs in the neutral state. As a consequence, associated protons are displaced in one direction with respect to the photoswitchable scaffold. We foresee that selective inhibition of photoswitching paths by substrate binding can be applied as general method toward directional transport and pumping of that substrate, which will prove critical in developing artificial systems in which concentration gradients can be effectively generated using (solar) light energy.