NMR and kinetic investigations of the multistep photochromism of 3-thienyl-naphthopyrans

The photocolouration under UV irradiation and the thermal relaxation after irradiation of 3-phenyl-3H-naphtho[2,1-b]pyrans linked to one, two, or three thiophene units in 3 position of the pyran moiety have been investigated by NMR. The characterization of each of the four transoid photomerocyanines, as well as the kinetic parameters of photochemical and thermal reactions were obtained and discussed.