On the other hand, the thermal back-switch is extremely slow at both 25 °C, and 40 °C, with no conversion detected within six months. All these results agree with those presented in our previous studies dealing with compounds 4a and 4b. The direct photoisomerization quantum yields have been determined by UV/vis spectroscopy (as described in the experimental part 5.6) and validated for compounds 4a and 4b (Figure ) by independent in-situ NMR measurements (as described in the experimental part 5.7). The results presented in table show that the direct quantum yields depend on the solvent's polarity being approximately 2 to 5 times larger in chloroform than in ethanol. On the contrary, the inverse quantum yields are largely independent of the solvent polarity for an alkoxy donor moiety. However, they are twice as high in ethanol as in chloroform for an amine donor moiety.