The light intensity received by the sample was measured before and after the kinetic measurements on compounds 4a and 4b according to the procedure described in ref . We The minimum concentration of the starting photo-isomers for the validity of equation 10 was determined based on a series of kinetic measurements on compound 4a for increasing photoisomer concentration ranging from 5 to 80 mM. By this way a concentration of 60 mM was found sufficient to perform quantum yield measurements on our photoswitches.