Return of the closed Z-1 species to the thermodynamically most stable open 1 via closing the thiophene ring and opening the C=N bond occurs spontaneously in methanol at 0 °C in the dark with a corresponding Gibbs energy of activation of 20.4 kcal mol -1 (Figure ). This behavior establishes fully reversible photoswitching of aza-diarylethene 1 in high yields for both switching states.