Indeed, while the UV spectrum shows a decrease/increase of the characteristic E absorption band, the NMR intensity of the protons of the E styryl moiety is also decreasing upon excitation at 375 nm. While the styryl protons may be easily identified since the vicinal coupling constants are always larger for Z that for E isomer, they appear within a multiplet and therefore the accurate quantification of the E/Z ratio, via the peak integrals is rather cumbersome. Hence, we instead quantified the protons belonging to the two methoxy groups appearing at d of 4.00 and 3.95 ppm for Z-isomer and at 3.70 and 3.65 for E-isomer that are perfectly resolved and not overlapping (Figures ). As indicated in Table , upon irradiation at l max , the isomer ratio changes from 98/2 to 25/75 in CDCl 3 , which gives an initial conversion yield a 0 Z of 0.73. In MeCN, the conversion yield is even higher (0.81), while in DMSO and EtOH a 0 Z is decreasing to 0.45 and 0.6, respectively. Upon irradiation at 300 nm, the reverse conversion between PSS1 and PSS2 was less efficient reaching 0.31 in MeCN, 0.35 in DMSO and 0.25 in EtOH, while the highest value, 0.45, was obtained in CHCl 3 . Importantly, both the direct and reverse photoisomerization occurred within several minutes, in every solvent tested, by irradiation at 375 or 300 nm, respectively, despite the moderate power of the irradiating source (Figure , 5B). The isomerization process was also perfectly reversible for at least 5 cycles under consecutive irradiation at 375 then 300 nm (Figure ), highlighting a high resistance to fatigue. From the kinetic profiles of the direct and reverse switch, it is also possible to determine the isomerization rate constants k (Table ). The rate constant of the direct E ® Z isomerization decreases with the polarity of the solvent, from 27 x 10 -3 s -1 in CHCl 3 to 10 x 10 -3 s -1 in EtOH, excepted for the more viscous DMSO, for which ' (→* +,-amounts to 6 x 10 -3 s -1 , although the rate constant of the reverse reaction was the highest for this solvent (' (→* +.. =224 x 10 -3 s -1 ). On the other hand, the thermal back-switch is extremely slow at both 25°C and 40°C, no conversion being detected within two weeks. This negligible thermal switch is of paramount importance for a high temporal control of the switch, which should be triggered only by irradiation to achieve selectivity.