The incident absorbed light intensity Ia(λirr) was estimated to be 6 µeinstein•L -1 •s -1 under the irradiation conditions of the experiments. Finally, the photoisomerization quantum yields ΦEZ of the compounds 4a-d in the four solvents could be determined by solving the equation 4. The backward photoisomerization quantum yields ΦZE were determined at PSS, where the concentration of each isomer is stationary. This fact combined with the negligible impact of the thermal contribution allows to use the previous equations approximating the concentration of each isomer at PSS by the ratio of the product of the photoisomerization quantum yield and the molar extinction coefficient (Eq. 9). In order to verify and validate the UV-Vis approach described in 5.6 for the measurement of ΦEZ the E→Z photoisomerization quantum yields , we used an independent approach based on NMR measurements of compounds 4a and 4d in DMSO-d6 under in-situ light irradiation.