Photochromic Torsional Switch (PTS): a light- driven actuator for the dynamic tuning of p- conjugation extension

Here we present a molecular architecture that can reversibly change the geometric conformation of its p-system backbone via irradiation with two different wavelengths. The proposed ‘molecular actuator’ consists of a photoswitchable azobenzene orthogonally connected to a p-conjugated bithiophene by both direct and aliphatic linker-assisted bonding. Upon exposure to 350 nm light, the trans azobenzene moiety isomerizes to its cis form, causing the bithiophene to assume a semiplanar anti conformation (extended p-conjugation). Exposure to 254 nm light promotes the isomerization of the azobenzene unit back to its initial extended trans conformation, thus forcing the bithiophene fragment to twist out of coplanarity (restricted p-conjugation). The molecular conformation of the bithiophene was characterized using steady-state UV-vis and nuclear magnetic resonance spectroscopy, as well as ab initio computations. The proposed molecular design could be envisaged as a p-conjugation modulator, which has potential to be incorporated into extended linear p-systems, i.e. via the terminal a-thiophene positions, and used to tune their optical and electronic properties.