The back-switch at 300 nm irradiation did not allow to fully recover the initial E-isomer (Table ). Indeed, only between 50% and 85% of 4c is recovered. For 4d, 85% and 83% of the Eisomer is obtained in acetonitrile-d3 and DMSO-d6, respectively. Interestingly, the back-switch is more effective in ethanol as a total conversion of 4d to the initial E/Z ratio of 95/5 was observed. All those results agree well with the literature. Moreover, a full degradation of 4d in CHCl3 was noticed which can be explained as an effect of the degradation of the solvent at 300 nm irradiation, leading to the formation of HCl. Indeed, mass spectrometry analysis of the compound obtained after irradiation coupled with UV analysis revealed the addition of HCl to the ethylenic double bond. Except for this issue, the photoisomerization was perfectly reversible for all the compounds and in all solvents, over at least 10 cycles under successive irradiation at 375 nm (1.5 W, 10 s) and 300 nm (350 mW, 30 s). Furthermore, the isomerization occurred within several minutes despite the moderate power of the LEDs used as the illumination source (Figure and S8). The rate constant of the direct E→Z photoisomerization (Table ) decreases with increasing the solvent polarity and goes from 38•10 -3 s -1 in chloroform to 6•10 -3 s -1 in ethanol for compound 4c. The same trend is observed for compound 4d whose isomerization constant decreased from 29•10 - 3 s -1 in chloroform to 5•10 -3 s -1 in ethanol. The highest rate constant for the photoinduced back-