These results confirm that Z→E isomerization is enabled in the protonated form, while the reverse isomerization process takes place in the neutral form. To our best knowledge, such a feature has not been observed in photoswitchable systems before. It is also fundamentally different from earlier reported examples of acid-gated photoresponsivity, which allowed both switching directions in one of the states. In our case, only a single (and opposite for neutral and protonated forms) direction of photoswitching is suppressed, affording quantitative conversion by default. It leads to a unique monodirectional cycle of interconversion between species under continuous illumination once a protonation equilibrium is established.