J. L. Serrano

Copper-containing dendromesogens: the influence of the metal on the mesomorphism

The synthesis and liquid crystalline behaviour of the first and second generations of a dendrimeric structure based on poly(propyleneimine)(DAB-dendr(NH2)x) are reported. 4-(4-n-Alkoxybenzoyloxy)salicylaldehydes are used as mesogenic moieties attached at the peripheral amino groups of the dendrimers giving rise to dendromesogens with four and eight mesogenic branches. From these dendromesogens, considered as organic ligands, were prepared six metal-containing dendrimers which incorporate two or four copper atoms in their structures. All the dendrimeric ligands and three of the metal-containing dendrimers exhibit liquid crystalline properties which were studied by optical microscopy, DSC, X-ray diffraction and EPR spectroscopy.

Sorption and diffusion of organic solvents through photo-crosslinked SBS block copolymers

Abstract Styrene-butadiene-styrene (SBS) block copolymer exposed to UV radiation in the presence of unimolecular and bimolecular photoinitiators yields a crosslinked polymer. When 2,2-dimethoxy-2-phenylacetophenone is used as photoinitiator the photocrosslinking takes place fundamentally through the addition of the benzoyl and/or dimethoxyphenyl radicals to the double bonds (cis, trans and vinyl) of the SBS block copolymer. The reactivity of the radicals towards the three double bonds is as follows: vinyl > cis > trans. Diffusion and sorption of chloroform and toluene through SBS block copolymer have been studied in the temperature interval of 25–60°C by conventional weight gain experiments. The diffusion results have been analyzed in terms of the simple Fickian model. Diffusion of toluene and chloroform in crosslinked SBS block copolymer was found to deviate slightly from the normal Fickian character which was attributed to the diphasic morphology of the SBS copolymer. The approach to a normal Fickian behaviour as the temperature and crosslinking increase has been explained considering the decrease in the degree of completion of the microphase separation with the increase of both the crosslinking and temperature. Temperature was found to activate the diffusion process, which was supported by higher intrinsic diffusion coefficient values and slopes values at higher temperatures. Entropy and enthalpy of sorption were estimated by using the Van’t Hoff relationship. Crosslinked SBS block copolymer–solvent interaction parameter χ were computed using the Flory–Rehner equation. The χ parameter (0.40 and 0.41 for chloroform and toluene, respectively) indicated strong polymer–solvent interaction. The crosslinking densities of the SBS block copolymer irradiated were relatively low ( M c values go from 23,000 for an irradiation time of 30 min until 5000 for an irradiation time of 120 min).

FLCs with a five-membered ring in the mesogenic core

A study has been undertaken of the structure-activity relationship of eight new chiral compounds having either a pyrazole (series P) or an isoxazole (series I) ring as a central bridge in the mesogenic core. The presence of dimers in the pyrazole compounds accounts for their lower P s values in comparison with the isoxazole analogues. The corresponding four beta-diketone precursors have also been studied and these, as expected given their bent molecular shape, show much worse mesomorphic and ferroelectric behaviour. In order to complete the study, the molecular dipoles of the three types of derivative have been determined using AM1 calculations. Two types of chiral tail have been incorporated into the compounds: alkoxy and alkanoyloxy. The latter tail gives rise to the best mesomorphic and ferroelectric properties. A study of the tail conformations by MM2 calculations provides an explanation of these results. The highest P s value (137 nC cm) has been obtained for the isoxazole derivative with the (2S)-2-...

Supramolecular chirality of columnar mesophases consisting of H-bonded complexes of melamine and polycatenar benzoic acids

The present work describes a series of H-bonded complexes consisting of a melamine derivative and polycatenar acids that show columnar liquid crystalline behaviour. Chirality has been transferred from the molecular blocks (either the acids, A3* and A4*, or the melamine, T2*) to the columnar organization. Particular effort and attention has been paid to structural characterization by NMR techniques [1D NMR spectroscopy and diffusion-ordered spectroscopy (DOSY) experiments] of the complexes in solution. The mesomorphic arrangements of all the complexes have been studied by polarizing optical microscopy (POM), differential scanning microscopy (DSC) and powder X-ray diffraction. The chiral nature of the supramolecular organization in the mesophase has been confirmed by circular dichroism spectroscopy on neat samples over a range of temperatures.

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.

New chiral Schiff's bases with a 1,3,4‐thiadiazole ring in the mesogenic core: synthesis, mesomorphic and ferroelectric properties

Two new homologous series of chiral esters derived from Schiffs bases containing a 1,3,4‐thiadiazole unit (series 6 and 7) were synthesized and their liquid crystalline and ferroelectric properties investigated. All the compounds of series 6 exhibit SmC*–SmA dimorphism and the compounds of series 7 exhibit a SmC* phase. All the compounds of these series are ferroelectric liquid crystals.

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.

Persistence of short range order in the fluid phases of a mesogen copper complex studied by EPR

Abstract The synthesis and thermal characterization of a new copper complex, bis[N-(4-n-pentoxyphenyl)-4-n-decyloxysalicylaldiminate] Cu(II), having mesomorphic properties is given. The EPR spectrum of this material has been measured for its different phases as a function of temperature. The results are compared with those for two other Cu(II) compounds, bis[4-n-decyloxysalicylate] Cu[II] and bis[N-(n-pentyl)-4-n-decyloxysalicylaldiminate] Cu(II) which have a related molecular structure, but are not mesomorphic. The influence on the spectra of exchange interactions in all the phases, as well as the molecular motion in the fluid phases is discussed, and it is concluded that the exchange interactions are not destroyed by the molecular motion. These facts suggest that molecular motion in the high temperature phases takes places whilst keeping a short range order, resulting in a locally correlated motion.

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.

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.