In summary, unprecedented photo-and thermal isomerization behavior was discovered for Nheterocyclic hemi-indigos. Disruption and (re-)formation of intramolecular hydrogen bonding interactions were found to be of major importance. It must be recognized that beside hydrazone, these compounds represent the second family of switches that undergo pH-activated double bond isomerization. More striking though, is the observed protonation-controlled oneway photoisomerization, giving rise to an exceptional monodirectional interconversion cycle.