The zeroth order kinetic constants measurements of compounds 4a and 4b were performed by following the time evolution of the O-CH3 proton signal integrals (at 3.94 and 3.89 ppm for the E-isomer and 3.62 and 3.56 ppm for the Z-isomer) by acquiring one spectrum every 30 s during the first 14 min of illumination (375 nm) and performing a linear fit of the time evolution of the O-CH3 proton signal integrals (Figure ).