Here, we demonstrate unique protonation-controlled isomerization behavior for N-heterocyclic hemi-indigos 1-6 (Fig. ). Almost quantitative conversion from Z to E isomer and vice versa is induced by addition of acid and base, respectively. This conversion is strongly accelerated by visible-light irradiation. More remarkably, the forward Z→E photoisomerization takes place only when protonated, while the reverse E→Z photoisomerization occurs in the neutral state as is shown for all six derivatives. Such behavior is to our best knowledge unprecedented for molecular photoswitches, as usually a dynamic photoequilibrium is established under continuous irradiation (using a wavelength at which both isomers absorb). Since in the presence of acid, the neutral and protonated species are in equilibrium, they are interchanged photochemically in a specific order under irradiation with a single wavelength of light as is depicted in Fig. . We came to the realization that by maintaining a constant flux through the cycle, associated protons are being displaced in one direction (with respect to the hemi-indigo scaffold), where the system operates by an information ratchet mechanism.