Luis Oriol

Phototunable lasing in dye-doped cholesteric liquid crystals

In this letter, we report the results of phototunable lasing in dye-doped cholesteric liquid crystals (DD–CLC). Photoexcitation of DD–CLC films gives rise to laser emission in the violet-UV range. Control of the structure of the chiral dopant driven by UV phototransformation is exploited in order to obtain a permanent variation of the cholesteric pitch. Laser emission wavelength tuning, by means of photoinduced shifting of the selective reflection band of the cholesteric liquid crystals is established. A tuning interval of about 35 nm, in the wavelength range of 385–415 nm, is observed.

Widely tunable ultraviolet-visible liquid crystal laser

Quasicontinuous tuning of a dye doped cholesteric liquid crystal (CLC) mirrorless laser in the ultraviolet-visible wavelength range is demonstrated using a single original device based on CLC as resonator and several resonant dyes. The thought is to combine the CLC pitch gradient and the distribution of different dyes. In the same cell, six dyes are combined in order to nearly cover the whole wavelength range from ultraviolet (370 nm) to red (680 nm). Some of the used dyes work as emitter, while others work in the Forster regime to decouple the excitation and emission processes. The relevant aspect of the device is that a simple translation of the cell respect to the same pump beam enables fine tuning of the laser wavelength in almost all the visible range, up to the ultraviolet.

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.

Synthesis and Characterization of Maltose-Based Amphiphiles as Supramolecular Hydrogelators

Low molecular mass amphiphilic glycolipids have been prepared by linking a maltose polar head and a hydrophobic linear chain either by amidation or copper(I)-catalyzed azide-alkyne [3 + 2] cycloaddition. The liquid crystalline properties of these amphiphilic materials have been characterized. The influence of the chemical structure of these glycolipids on the gelation properties in water has also been studied. Glycolipids obtained by the click coupling of the two components give rise to stable hydrogels at room temperature. The fibrillar structure of supramolecular hydrogels obtained by the self-assembly of these gelators have been characterized by electron microscopy. Fibers showed some torsion, which could be related with a chiral supramolecular arrangement of amphiphiles, as confirmed by circular dichroism (CD). The sol-gel transition temperature was also determined by differential scanning calorimetry (DSC) and NMR.

Supermolecular Liquid Crystals with a Six‐Armed Cyclotriphosphazene Core: From Columnar to Cubic Phases

New liquid crystals having a non-conventional structure have been synthesised from a six-armed cyclotriphosphazene core, [N(3) P(3) (OC(6) H(4) OH-4)(6) ], which was condensed with polycatenar acids. Reactions were monitored by (31) P{(1) H} and (1) H NMR spectroscopy and the chemical structure of the resulting materials was confirmed by different spectroscopic techniques and mass spectrometry (MALDI-TOF). Results were in accordance with monodisperse, fully functionalised cyclotriphosphazenes. Thermal and mesomorphic properties were studied by optical microscopy, differential scanning calorimetry and X-ray diffraction. All of the synthesised phosphazenes, substituted with benzyl ether chains, show a high thermal stability and exhibit mesomorphic properties, which depend on the number and type of alkyl terminal chains located at the periphery of the mesogens. Mesomorphic properties range from Col(h) for cph-A1 and cph-A2 to a cubic phase detected for cph-A3, which has the larger number of alkyl chains. Furthermore, helical order was detected on X-ray data of cph-A2, which has chiral branched chains. Circular dichroism spectra of annealed films at mesophase temperature show a signal attributed to the chiral helical arrangement of the mesogenic chromophores.

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.

Light induced molecular release from vesicles based on amphiphilic linear-dendritic block copolymers

We report on the synthesis and characterization of new amphiphilic linear-dendritic block copolymers (LDBCs) as well as their self-assembly in water to form polymeric vesicles. We have also investigated the encapsulation of both hydrophilic and hydrophobic fluorescent probes by these stimuli-responsive polymeric vesicles and the use of light as an external stimulus to trigger the release of the probes. The LDBCs are composed of a 2000 g mol−1 poly(ethylene glycol) (PEG) linear segment linked to a fourth generation 2,2-di(hydroxymethyl)propionic acid (bis-MPA) based dendron with 4-isobutyloxyazobenzene units at the periphery. By tuning the chemical structure of the azobenzene moieties, a specific LDBC has been identified that forms stable vesicles in water as it is clearly revealed by TEM and Cryo-TEM images. Encapsulation of both hydrophobic and hydrophilic molecules by these vesicular self-assemblies has been investigated by fluorescence analysis as well as the release of the loaded probes triggered by low intensity UV illumination, which is related to trans-to-cis isomerisation of the azobenzene moieties. Thus, the potential of these photo-responsive vesicles as light-responsive nanocarriers is demonstrated.

Responsive linear-dendritic block copolymers.

The combination of dendritic and linear polymeric structures in the same macromolecule opens up new possibilities for the design of block copolymers and for applications of functional polymers that have self-assembly properties. There are three main strategies for the synthesis of linear-dendritic block copolymers (LDBCs) and, in particular, the emergence of click chemistry has made the coupling of preformed blocks one of the most efficient ways of obtaining libraries of LDBCs. In these materials, the periphery of the dendron can be precisely functionalised to obtain functional LDBCs with self-assembly properties of interest in different technological areas. The incorporation of stimuli-responsive moieties gives rise to smart materials that are generally processed as self-assemblies of amphiphilic LDBCs with a morphology that can be controlled by an external stimulus. Particular emphasis is placed on light-responsive LDBCs. Furthermore, a brief review of the biomedical or materials science applications of LDBCs is presented.

Synthesis and characterization of reactive liquid crystals and polymers based on terphenyl derivatives

Abstract A series of liquid crystalline methacrylates based on terphenyl derivatives bearing a donor–π-acceptor structure has been synthesized. The mesomorphic, optical absorption, photoluminescent and non-linear optical properties of the monomers have been investigated. Polymerization of these monomers was first carried out in solution but the dicyanovinyl group seems to inhibit the reaction despite the fact that the DSC curves of the monomer provide evidence for the occurrence of thermal polymerization. An IR study of this thermal polymerization confirms that only methacrylate groups are involved in the polymerization process. The mesomorphic and thermal properties of the resulting side-chain polymers have also been investigated.

Hydroxy-functionalized liquid-crystalline polyazomethines I. Synthesis, characterization and structure-mesogenic behaviour relationship

Abstract A new series of polyazomethines based on hydroxy-functionalized mesogenic units has been synthesized and characterized. Different structural variations have been checked in order to obtain mesogenic polyazomethines with low melting temperatures and a broad nematic range. Completely conjugated mesogenic units with a lateral substituent or alteration of coaxiality are shown to be the most suitable structural variations. No mesogenic behaviour was observed for polyazomethines with a flexible mesogenic core due to unfavourable conformational equilibrium. Polyazomethines have complex thermal behaviour and show an increase in the degree of polymerization on annealing.