Photomechanical actuation and manipulation of the electronic properties of linear pi-conjugated systems

A photodynamic molecular architecture has been synthesized by covalent fixation of a photoisomerizable azobenzene group at two fixed points of a conformationally flexible pi-conjugated quaterthiophene chain. The crystallographic structure shows that the two systems lie in parallel planes with a short interplane distance. Theoretical modelization and experimental analysis by 1H NMR and cyclic voltammetry unequivocally show that trans to cis photoisomerization of the azobenzene group induces dimensional and conformational changes in the underlying pi-conjugated system. These geometrical changes produce, in turn, an increase of the HOMO level and a narrowing of the HOMO-LUMO gap, thus providing a first example of photomechanical control of the electronic properties of the pi-conjugated system.