We became interested in transforming hemi-indigo into a pH-sensitive molecular switch for several reasons. Earlier studies had shown that incorporation of a pyrrolic ring can energetically favor the otherwise metastable E isomer owing to formation of an intramolecular hydrogen bond. Furthermore, while hemi-indigo recently regained interest as potential photoswitch, its pyridyl derivative was found to not undergo photoisomerization, which is likely due to excited state intramolecular proton transfer (ESIPT) as has been demonstrated for indigo. Likewise, for the E isomer of some structurally related indole-containing hemithioindigo derivatives, which are stabilized by intramolecular hydrogen bonding, E→Z photoisomerization was inhibited, which was also ascribed to ESIPT. Based on these findings, we hypothesized first that protonation of N-heterocyclic hemi-indigo would induce thermal isomerization over the double bond by disruption and (re-)formation of a stabilizing intramol-ecular hydrogen bond in the Z and E isomer, respectively. Second, and most importantly, hydrogen bond disruption was envisioned to enable photoisomerization, such that one-way and effectively quantitative conversion would be attained in single and opposite directions for the neutral and protonated species.