Supramolecular Chirality and Reversible Chiroptical Switching in New Chiral Liquid‐Crystal Azopolymers

Abstract

The synthesis of chiral liquid-crystalline polymers of well-controlled structure (linear and three-armed star-shaped) with distinct average chain lengths and low polydispersity was achieved by atom transfer radical polymerisation (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, characterised by differential scanning calorimetry (DSC), X-ray diffraction (XRD) and polarised optical microscopy (POM), exhibit a smectic A(1/2) (fully interdigitated) liquid-crystalline phase and high cleaning points, with transition temperatures dependent on the average polymerisation degree and the macromolecular structure. The chirality originated at the molecular level by the asymmetric functionality of the 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). The results show the ability of l-CPL to invert the supramolecular chirality of the materials and demonstrate the essential role of azoaromatic aggregates.

Consequences of Chirality in Directing the Pathway of Cholesteric Helix Inversion of π‐Conjugated Polymers by Light

“…Azobenzenes attached as side chains to covalent polymers have shown chiroptical switching. [ 27–31 ] Whereas efficient photoswitching has often been observed in these materials, they lack the cooperative trigger to induce changes at a mesoscopic length scale due to: i) the low local density of photoactive molecules, ii) the often random organization and flexibility of the polymer main chain, and iii) the presence of flexible spacers between the mesogens (azobenzene) and the main chain. Azobenzenes in direct conjugation with an electronically active polymer backbones can lead to significant structural reorganization, [ 32–35 ] which offers the possibility of tuning the electronic properties of the polymer by employing the state of polarization of light.…”

Proton conductive ionic liquid crystalline poly(ethyleneimine) polymers functionalized with oxadiazole

“…Similar behavior has been previously reported for some sidechain liquid-crystalline polymers and it is explained by the connement of the polymeric backbones in a thin sublayer, so that the polymeric backbones produce an electron-density maximum comparable to that of the mesogenic cores. 28,29 Proton conductive properties…”

Experimental aspects of solid state circular dichroism

“…[109][110][111] Supramolecular assemblies J-aggregates in PVA/aqueous solution were dried as a film and mounted between optical glass plates 112 : CD spectra were collected rotating the sample around the optical axis and about the vertical axis. More recently Yao et al 113 revisited the same topics in solution phase detecting different conformers according to the apparent CD measurements.…”