When enriching closed Z-1 in MeOH solution by UV light irradiation, the corresponding UV/Vis spectral changes are very similar to those of conventional ring-closed diarylethenes with a broad absorbance band emerging in the visible range between 400 nm -500 nm (Figure ). The theoretically obtained UV/Vis spectra of open 1 and closed Z-1 in MeOH are shown in Figure . A good match between experimental and theoretical absorption spectra were obtained when four explicit MeOH solvent molecules in proximity to the thiolate moiety were included in the calculations, which shows a significant influence of the surrounding medium (see Supporting Information and Supporting Figure Elucidation of the molecular structures as well as double bond configurations of protonated closed Z-1 and protonated closed E-1 was achieved again by a comprehensive NMR analysis (see Supporting Information for details). The key experiments for elucidating the double bond configurations are described in the following. A 1 H-13 C HSQMBC experiment directly allowed measuring coupling constants for the enethiol fragments and assigning E and Z configurations. Annealing the Z-1 / E-1 isomer mixture in MeOD-d4/TFA-d1 to 50 °C shows that the protonated closed 1 forms are now greatly stabilized and do not convert thermally to open 1 for several hours. Thus, a repeat of the irradiation experiment in MeOD-d4/TFA-d1 (10 equiv.) at 25 °C was conducted (Figure ). At the higher temperature the same sequential behavior of first Z-1 and then E-1 accumulation was observed under irradiation. A final E-1 : Z-1 ratio of 48% : 52 % is established in the photostationary state (pss) and again a quantitative combined yield was achieved in the photoreaction (Figure ). Adding 20 equiv. Et3N to the sample resulted in fast recovery of open 1 in up to 82% (Figure , Supporting Figure ). The significantly greater thermal stability of the protonated closed Z-1/E-1 isomers at lower pH can straightforwardly be explained by protonation of the thiolate. The resulting thiol possesses a drastically reduced nucleophilicity and thus diminished capability for attacking carbon C8' (Figure ), closing the thiophene ring, and opening up the pyrido[1,2-a]pyrimidin-5-ium ring. The kinetics of thermal isomerizations of protonated closed Z-1 and E-1 were scrutinized by 1 H NMR spectroscopy at 50 °C in the dark (Figure ). The experimentally determined variation in isomer content over the course of the experiment was modeled using the COPASI tool. It was found that thermal isomerization from protonated closed Z-1 to open 1 proceeds fastest with an associated Gibbs energy of activation of DG ‡ = 25.6 kcal mol -1 . The corresponding Gibbs energy of activation for isomerization from protonated closed Z-1 to protonated closed E-1 was determined to 26.2 kcal mol -1 , and for protonated closed E-1 to protonated closed Z-1 to 26.7 kcal mol -1 (Supporting Figure ). ) was added directly afterwards to convert the system from dissipative into thermally stable in a sequential fashion (Supporting Video 3, Supporting Figure ).