Light‐Responsive Oligothiophenes Incorporating Photochromic Torsional Switches

We present a quaterthiophene and sexithiophene that can reversibly change their effective π‐conjugation length through photoexcitation. The reported compounds make use of light‐responsive molecular actuators consisting of an azobenzene attached to a bithiophene unit by both direct and linker‐assisted bonding. Upon exposure to 350 nm light, the azobenzene undergoes trans‐to‐cis isomerization, thus mechanically inducing the oligothiophene to assume a planar conformation (extended π‐conjugation). Exposure to 254 nm wavelength promotes azobenzene cis‐to‐trans isomerization, forcing the thiophenic backbones to twist out of planarity (confined π‐conjugation). Twisted conformations are also reached by cis‐to‐trans thermal relaxation at a rate that increases proportionally with the conjugation length of the oligothiophene moiety. The molecular conformations of quaterthiophene and sexithiophene were characterized by using steady‐state UV‐vis spectroscopy, X‐ray crystallography and quantum‐chemical modeling. Finally, we tested the proposed light‐responsive oligothiophenes in field‐effect transistors to probe the photo‐induced tuning of their electronic properties.