Bistable mesomorphism and supramolecular stereomutation in chiral liquid crystal azopolymers

A series of side-chain liquid crystal polymers based on an azobenzene mesogenic unit bearing a terminal chiral chain has been synthesized. The thermal and mesomorphic properties of the homo- and copolymers have been investigated by polarizing optical microscopy, DSC and X-ray diffraction. Polymers having a chiral methylheptyloxy terminal chain (P8S and P8R) exhibit surprising mesomorphic behavior, which depends on the thermal history of the sample. The influence of the molecular chirality and thermal history on the supramolecular organization of the chromophores have also been studied. Supramolecular chiral aggregation of the azobenzenes seems to be responsible for the observed chiroptical properties of these materials, both in solution and solid state (polymeric films), with the handedness controlled by the molecular chirality. Stereormutation of the chiral supramolecular organization is detected when polymeric films of P8S and P8R experience a different thermal history.