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

Here we present a molecular architecture that can reversibly change the geometric conformation of its pi-system backbone via irradiation with two different wavelengths. The proposed 'molecular actuator' consists of a photoswitchable azobenzene orthogonally connected to a pi-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 pi-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 pi-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 pi-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.