However, a significant effect is observed on the absorption of the closed 1 isomers, which lose their broad absorption band in the visible region of the spectrum upon addition of TFA. This behavior is in full accordance with protonation of the thiolate and a resulting dramatic decrease of its electron-donating capability. Overall, the push-pull character of the open 1 chromophore is therefore significantly reduced upon protonation, resulting in a strongly hypsochromically shifted absorption spectrum. Both closed isomers Z-1 and E-1 possess rather similar UV/Vis absorption spectra, which are very well reproduced in the theoretical description (Figure and). It should be noted that the initially expected closed-ring photoproduct typical for diarylethenes would not show such dramatic absorption shifts in response to protonation, which further supports our structural assignments. In conclusion, we report on a unique aza-diarylethene photoswitch motive, which undergoes a