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%).

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► Thienylpyrrole azo dyes functionalized with benzothiazolyl groups showed photochromic properties.
► The kinetics of the Z–E back reaction is influenced by the linkage position of the azo function to the benzothiazole heterocycle.
► The linkage of the azo function to the 6 position of the benzothiazole led to slower isomerisation kinetics.