Supramolecular Chirality in New Chiral Liquid Crystal Azopolymers Obtained By ATRP

The synthesis of chiral liquid crystalline polymers of well-controlled structure (linear and three-arms star shaped) with distinct average chain lengths and low polydispersity was achieved by Atom Transfer Radical Polymerization (ATRP) of a new optically active monomer (S)-4-[6-(2-methacryloyloxypropanoyloxy)hexyloxy)]-4’-ethoxyazobenzene [(S)-ML6A], containing the L-lactic residue of one absolute configuration in the side-chain. All the obtained polymeric samples, characterized by DSC, XRD and POM, exhibit a smectic A1/2 (fully interdigitated) liquid-crystalline phase and high cleaning points, with transition temperatures dependent on the average polymerization degree and the macromolecular structure. The chirality originated at molecular level by the asymmetric functionality of L-lactic acid residue provides the polymers, in the smectic phase, of highly homogeneous conformations with a prevailing chirality related to the presence of H-aggregates having conformational dissymmetry of one prevailing screw sense. By irradiating with circularly polarised light (CPL), it is possible to photomodulate the chiroptical properties of these intrinsically chiral polymeric thin films. Upon irradiation with left-handed CPL (l-CPL), the circular dichroism (CD) spectra of the films show enhancement of ellipticity and a net inversion of sign. The effect is reversible and the mirror image of the CD spectrum can be restored by pumping with right-handed CPL radiation (r-CPL). As compared with similar achiral polymeric systems, the results put into evidence the ability by l-CPL to invert the supramolecular chirality of the materials and demonstrate the essential role of azoaromatic aggregates.