Thiophenylazobenzene - an alternative photoisomerization controlled by lone pair∙∙∙π interaction.

Azoheteroarene photoswitches have attracted attention due to their unique properties. We present here the stationary photochromism and the ultrafast photoisomerization mechanism of thiophenylazobenzene (TphAB) - an alternative sulfur-based azoheteroarene design. TphAB demonstrates impressive fatigue resistance and photoisomerization efficiency, and shows favorably separated (E)- and (Z)-isomer absorption bands, allowing for highly selective photoconversion. The (Z)-isomer of TphAB adopts an unusual orthogonal geometry where the plane of the thiophene is perfectly perpendicular to that of the benzene. This geometry is stabilized by a rare lone pair∙∙∙π interaction between the S-atom and the benzene. The photoisomerization of TphAB occurs on the sub-ps to ps timescale and is governed by the lone pair∙∙∙π interaction. Therefore, the adoption and disruption of the orthogonal geometry requires significant movement along the inversion reaction coordinates (CNN and NNC angles). Our results establish TphAB as an excellent photoswitch with versatile properties that expand the application possibilities of AB derivatives.