Rational Design of Azothiophenes—Substitution Effects on the Switching Properties

A series of substituted azothiophenes was prepared and investigated toward their isomerization behavior. Compared to azobenzene (AB), the presented compounds showed red‐shifted absorption and almost quantitative photoisomerization to their (Z) states. Furthermore, it was found that electron‐withdrawing substitution on the phenyl moiety increases, while electron‐donating substitution decreases the thermal half‐lives of the (Z)‐isomers due to higher or lower stabilization by a lone pair–π interaction. Additionally, computational analysis of the isomerization revealed that a pure singlet state transition state is unlikely in azothiophenes. A pathway via intersystem crossing to a triplet energy surface of lower energy than the singlet surface provided a better fit with experimental data of the (Z)→(E) isomerization. The insights gained in this study provide the necessary guidelines to design effective thiophenylazo‐photoswitches for applications in photopharmacology, material sciences, or solar energy harvesting applications.