Milagros Piñol

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.

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.

Photo-Induced Alignment of LC Polymers by Photoorientation and Thermotropic Self-Organization

Abstract Optical anisotropy is generated in films of photochromic LCPs upon irradiation with linearly polarized light. Annealing of these films above the glass transition temperature results in a significant amplification of the photo-induced anisotropy. The amplification takes place even in the case of copolymers with only 10% of azobenzene side groups and if only 7% of the saturation value of anisotropy are induced. Photoorientation and photoinduced alignment are restricted by thermal pretreatment and ageing at room temperature.

Polarized photoluminescence and order parameters of “in situ” photopolymerized liquid crystal films

Oriented liquid crystal polymer films with different degrees of cross-linking have been obtained by “in situ” photopolymerization of different mesogenic samples at various temperatures. These samples are mixtures of a naphthalene based monoacrylate and a crosslinking agent (diacrylate). Photoluminescence has been obtained by adding a cyanoterphenyl chromophore. The order parameters of the polymer films have been derived from dichroic measurements. An increase of the dichroism has been found when the diacrylate content and/or the polymerization temperature decrease. The dichroism is also bigger if the chromophore is in a “guest-host” configuration than if it is covalently attached to the polymer backbone. The dichroic ratios do not change under annealing up to 200 °C but some photodegradation of the luminescence is observed in the naphthalene containing films.

Alternative synthetic strategies for Schiff base derived liquid crystal polymers: A comparative study

Semiflexible liquid crystalline polymers based on Schiff bases as mesogenic cores have been synthesized through different synthetic strategies. The mesogenic cores are derived from 1,5-naphthalenediamine linked to a polymethylenic spacer by means of ether or ester groups. The synthetic strategies differ in the disconnection of the molecules to design the synthesis of the repeating units. Polymers were synthesized either as polyazomethines, where imine bonds are formed in the polymer synthesis, or as polyethers or polyesters, where the ether or ester groups are formed at the polymer-forming stage. The influence of the synthetic method and the linking groups between mesogenic core and flexible spacer on the thermal and mesogenic properties has been analyzed by DSC, thermogravimetry, and optical microscopy. The possibility of obtaining hydroxy-functionalized polymers by different synthetic strategies has been studied as well as the influence of these groups on the thermal and mesogenic properties.

Emissive Anisotropic Polymeric Materials derived from Liquid Crystalline Fluorenes

The synthesis and characterization of photoluminescent crosslinking agents containing a fluorene core are described. They exhibit an efficient fluorescence and have a promesogenic molecular shape that depending on the substitution results in the development of thermotropic mesophases. A difunctionalized fluorene derivative has been incorporated into a liquid crystal host in order to produce polarized light emitting films. These films have been prepared by the in-situ photopolymerization technique.

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.

Polarised luminescent films containing fluorene cross-linkers obtained by in situ photo-polymerisation

The paper describes the synthesis of several direactive compounds with luminescent properties based on the fluorene moiety. These difunctionalised compounds have been incorporated as cross-linking agents on liquid crystalline reactive mixtures to be used for the preparation of anisotropic films by the in situ polymerisation technique. The generation of polarised luminescence on the oriented films has been investigated.