Photoswitching in azo dyes bearing thienylpyrrole and benzothiazole heterocyclic systems

Visible light promotes the conversion of the E-isomer of benzothiazol-2-yl and benzothiazol-6-yl diazenes to the thermal unstable Z-isomer that reverts in few seconds to the initial form. The kinetics of the thermal Z-E process is strongly influenced by the linkage position of the NN function to the benzothiazole heterocycle. Thienylpyrrole azo dyes functionalized with benzothiazol-6-yl groups are particularly interesting since they show an excellent compromise between the switching speed (rate constants: 0.068-0.12 s⁻¹) and the amplitude of the absorbance variation (37-42%).