Raúl O. Garay

Symmetric bent-shaped liquid crystal dimers showing transitions between optically uniaxial and biaxial smectic phases

We report the synthesis of a new homologous series of symmetric bent-shaped liquid crystal dimers bearing oxybiphenylcarboxylate mesogens linked by a pentamethylene spacer. The dimers were characterised by optical, thermal, X-ray diffraction and computational techniques. Shorter dimers showed an I-N-SmA-SmC phase sequence with strong first-order phase transitions. For the longer dimers, the unconventional uniaxial SmA phase present in shorter members collapsed. A 9:1 binary mixture of a bent dimer with a non-mesogenic dimer exhibited upon the SmA-SmC phase transition a maximum layer contraction of 1%.

Synthesis of a Thienothiophene Conjugated Polymer

A new conducting polymer was prepared by chemical and electrochemical polymerization of 3,6-dimethylthieno[3,2-b]thiophene. The galvanostatic deposition afforded uniform, adherent and dark blue films of PDMTT. Electrochemical characterization by cyclic voltammetry showed that it can be repeatedly driven between the doped and undoped species with a coulombic efficiency of nearly 100%.

Improved synthesis of bis(chloromethyl)arene monomers

We report here on the experimental conditions for radical chlorination that provide improved yields of 2,5-bis(chloromethyl) pyridine and pyrazine, as well as on theoretical calculations which were performed in order to gain some insight regarding the factors that affect reactivity in these systems. The difficulties encountered previously in the methyl functionalization of N-heteroarenes are not due to radical selectivity that produces low yield of the dichlorides or to lower reactivity of this type of compounds, but to the post-halogenation reactions that occur on the desired products. Therefore, a careful handling during and after the reaction allowed us to find conditions for product optimization of α, α′ -dichlorinated monomers.

Synthesis of conjugated polymers. Polymerizability studies of bis-sulfonium salts

Although the bis-sulfonium salt precursor route is one of the most versatile procedures available for the synthesis of conjugated polymers, little effort has been made to explore the scope of this polymerization reaction. In this study, a series of bis-sulfonium salts were synthesized and in order to assess their polymerizability, isotopic exchange at the benzylic positions as well as their reactivity was studied by 13C-NMR spectroscopy. Meanwhile the generation of the reactive intermediate was analyzed by UV spectroscopy. In addition, the reaction heats of the equilibria leading to the intermediate polymerizable species from the corresponding precursors were calculated by computational methods. The relationship between the chemical structure of the bis-sulfonium salts, their ability to polymerize, and the theoretical results was then analyzed in an effort to establish a reactivity pattern that could help to predict the type of bis-sulfonium salt that can be polymerized by this synthetic route and to contribute to the rational design of new conjugated polymers.

4'-[2-(2-Ethoxyethoxy)ethoxy]biphenyl-4-carboxylic acid: correlation between its crystalline and smectic phases.

The crystal structure of the dimeric title compound, C(19)H(22)O(5), is dominated by a head-to-head hydrogen-bonding interaction between centrosymmetrically related carboxyl groups in each monomer. The result is a dimeric axis of unusual length (ca 34 A), but still shorter than what could be expected for a fully extended chain, owing to two turning points in the oligoethoxy ends. This allows for an explanation of the structure of the smectic mesophase exhibited by this compound and at the same time fully validates former geometric estimations based on PM3 calculations.

Mesomorphic Properties of Diesters with p-Substituted-Arylazomethine Pendant Groups

The substituent steric and electronic effects on mesophase stability and glass-forming properties of a series of 1,4-bis-(4-n-alkyloxybenzoyloxy)-2-N-(p-Y-aryl)azomethinebenzenes were evaluated. The diesters showed a typical though mild dependence of the monotropic nematic–isotropic transition temperatures on the substituent size that was attributed to the conformational flexibility of the pendant azomethine group. In contrast, the dependence of the entropy change associated with the nematic–isotropic transition showed an atypical behavior ascribed to the extreme reduction of the mesogen axial ratio. The introduction of the bulky arylazomethine group on a lateral position originated significant crystallization supercooling. The appearance of glassy mesophases was detected in some of the diesters; the OEt and CF3 groups were the more effective substituents to promote the formation of nematic glasses.

Synthesis and Characterization of Bent-rod Liquid Crystals

Two series of diesters with bend-rod shapes were synthesized and their thermal properties characterized by POM and DSC. The central group conformational mobility and polarity as well as the length of the mesogenic groups were varied in order to correlate these parameters with mesophase stability. Results indicate that series II diesters are enantiotropic and that their mesophase sensitivity to central group structural changes is limited.

Synthesis of Poly(m-pyridylene-1,2-diphenylvinylene)

The synthesis by dehalogenating polycondensation and characterization of a new soluble conjugated polymer, poly(m-pyridylene-1,2-diphenylvinylene), DP-PPyV, is reported here. It shows good mechanical properties and a λmax = 330 nm. The maximum intensity peak of MALDI-TOF corresponds to 1.800 Da.

Polymerization Mechanism of α,α’-bis(Tetrahydrothiophenio)-β-xylene Dichloride

A experimental study was performed regarding the influence of the base nature and solvent on the reactive intermediate concentration in the base-promoted bissulfonium salts polymerization. Such polymerization reaction is part of a synthetic procedure used to prepare conjugated polymers. In addition, a theoretical study suggest that a one-electron transfer could be involved in the initiation step.

Methoxide-induced fragmentation of 2,2,3-trihalogeno- and 2,2-dihalogeno-3-methoxy-1,3-diphenylpropanones

The reaction of 2,2,3-trihalogeno-1,3-diphenylpropanones with sodium methoxide in methanol occurs with exclusive formation of fragmentation products. Kinetic and stereochemical evidence is interpreted in terms of a concerted intramolecular process promoted by initial attack of methoxide anion at the carbonyl carbon. In the case of 2,2-dihalogeno-3-methoxy-1,3-diphenylpropanones the reaction gives fragmentation, rearrangement, and elimination products. The former products are believed to be formed through competing reactions involving an intermediate formed on addition of methoxide to the substrate. For the fragmentation pathway the results are consistent with a carbanionic-intermediate mechanism.