Rosa M. Tejedor

Photoinduced supramolecular chirality in side-chain liquid crystalline azopolymers

A series of liquid crystalline homopolymers that have photoisomerizable cyanoazobenzene groups in the side chain has been synthesized and characterized. Thin films of these polymers have been processed in order to study the absorption spectra and circular dichroism responses after illumination with 488 nm circularly polarized light. The tendency of the azobenzene units to aggregate and the modification of the electronic spectra by illumination were studied by UV-vis spectroscopy. Illumination with circularly polarized light of the azopolymers gave rise to a photoinduced circular dichroism response, which suggests supramolecular structures with a chiral orientation of the azobenzene chromophores in H-aggregates. The photoinduced supramolecular order depends on the polymer structure, i.e., on the Tg and the length of the flexible spacer, and the nature of the liquid crystal phase. Identical absorption spectra and reverse circular dichroism spectra were obtained after irradiation with circularly polarized light of opposite handedness and this is due to the photoinduction of enantiomeric supramolecular structures. The thermal stability above Tg of the photoinduced supramolecular chirality is determined by the polymer structure.

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

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.

Chiral photochemical induction in liquid crystals

Experiments carried out to induce chirality into liquid crystals by means of photochemical procedures are reviewed. After basic considerations on the techniques used to detect supramolecular chirality in such organizations, the present Feature Article is focused on chiral photoinduction experiments performed on calamitic, columnar and bent-shaped liquid crystals. The most intense research corresponds to calamitic systems, either photochromic polymers or systems doped with a photoactive chiral trigger. Only recently have investigations on columnar or bent-shaped liquid crystals begun to receive attention.

Synthesis, characterization and photoreactivity of liquid crystalline cinnamates

Abstract A series of liquid crystalline cinnamate derivatives has been synthesized and characterized. The reactivity of photoreactive samples containing these compounds and a low percentage of a triplet sensitizer has been investigated at different temperatures and phases. No topochemical reaction occurs by irradiation on the solid phase. However, by irradiation at mesophase the presence of photoproducts have been detected by FTIR and UV-Vis. However, when temperature increases above 200xa0°C, thermal reactivity predominates. The photoproducts were studied by NMR. A relatively low percentage of E – Z isomerization was measured together with [2+2] photocycloaddition. The resulting isomeric cyclobutane rings depend on the molecular ordering of the fluid phase. Thus smectic mesomorphic ordering mainly favors a β-truxinic acid derivative but isotropic phase mainly favors a δ-truxinic acid derivative.

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. Stereomutation of the chiral supramolecular organization is detected when polymeric films of P8S and P8R experience a different thermal history.

Photoinduction and Control of the Supramolecular Chirality in Liquid Crystalline Azomaterials

Liquid crystals (LC) containing azobenzene moieties are materials of interest in different optical technologies. Applications of azomaterials derived from the capability of reversible isomerization between the E- and Z-configurations. Among the different applications reported so far, photoinduction or chirality deserves to be studied in details as can provide, e.g., new chiroptical switches and molecular rotors based on achiral compounds. Here, we review our recent work on the formation, manipulation and control of the chiral supramolecular organization of LC azomaterials using circular polarized light as the chiral stimulus.