Aloir Antonio Merlo

Novel selenoesters fluorescent liquid crystalline exhibiting a rich phase polymorphism

A simple and efficient procedure for the synthesis of a new class of selenoesters 4a and 4b was developed. Polarized-light optical microscopy (POM), differential scanning calorimetry (DSC), and X-ray diffraction showed that the selenoester 4a with a shorter alkyl chain displayed a wide nematic range (ΔT = 110.7 °C), while 4b with a longer alkyl chain possesses a rich phase polymorphism: Cr → SmI → SmC → N → I mesophase sequence during heating cycle and I → N → SmC → SmI → SmX → Cr transition on cooling. For 4b, a large temperature range (135.7 °C) from crystal to isotropic phase can be observed. In addition, UV-Vis, steady-state fluorescence emission and excitation spectra in solution were also applied in order to characterize their photophysical behaviour. Compounds 4a and 4b are fluorescent in the blue region and present a Stokes shift at around 80 nm and 39 nm in dichloromethane and dioxane, respectively.

Selenides and diselenides containing oxadiazoles: a new class of functionalised materials

A simple and efficient procedure for the synthesis of a new class of organoselenium liquid crystal compounds was developed. The coupling of aryl bromides with elemental selenium was catalysed using copper oxide nanopowder in the presence of potassium hydroxide employing dimethyl sulfoxide as the solvent. This is the first report of the synthesis and characterisation of liquid crystal-based diselenides. Their mesophases were characterised by polarising optical microscopy and differential scanning calorimetry. Compounds 1, 2 and 4b exhibited the smectic A phase. In addition, these compounds showed weak blue fluorescence in solution (λmax. em. 350–405 nm) and a Stokes shift of around 90 nm.

Disubstituted 3,5‐isoxazolines. An easy route to polymer liquid crystal materials

The synthesis and phase behaviour are described of new side‐chain liquid crystal polyacrylates (9a–9d) with a 3,5‐isoxazoline ring in their mesogenic core. The new homopolymers were synthesised by free radical polymerisation. Phase transition temperatures exhibited dependence on degree of polymerisation and on terminal groups. The resulting materials exhibited typical smectic A mesomorphism.

A new synthetic approach based on (−)-menthone for chiral liquid crystals

Starting from (−)-menthone, a new chiral building block useful for liquid crystal preparation was synthesized. This chiral moiety was attached to selected phenols under mild conditions by esterification. Rigid cores of tolanebenzoates and phenylbenzoates were prepared using the palladium cross-coupling reaction or by traditional liquid crystal synthesis methods. This convergent approach ended with a second esterification or palladium cross-coupling reaction to furnish new liquid crystal materials with smectic A, smectic C* and N* phases, as well as blue phases (BP). Thermal behavior, and the effect of chiral moiety branches and molecular packing in the smectic phases, have been investigated using differential scanning calorimetry, optical microscopy and X-ray diffraction.

Synthesis of liquid-crystalline 3,5-diarylisoxazolines

The synthesis and mesomorphic properties of new liquid-crystalline 3,5-disubstituted isoxazolines are presented and discussed. These isoxazoline derivatives have been synthesised by reacting oximes with the appropriate 4-substituted styrene dipolarophiles in the presence of N-chlorosuccinimide and pyridine. The isoxazolines 3a–d and 7a–g prepared by this methodology are used as scaffolds for further molecular derivation through a molecular elongation strategy. The selected scaffolds 3a–b and 7f–g were transformed into liquid crystals (LCs) by the addition of an arylacetylene group at the C3 or C5 carbon atoms on the isoxazoline ring by a Sonogashira reaction. The relevant LC compounds 14a–c, 15, 16, 17 and 18a–c were synthesised in fair to good yields. The final compounds display both nematic and smectic A liquid-crystalline phases.

Chiral liquid-crystalline polyacrylates from (S)-(-)-2-methyl-1-butanol. Synthesis, mesomorphic properties and light scattering

A sintese e o comportamento mesomorfico de dois poliacrilatos quirais liquido cristalinos com cadeia lateral derivados do ( S)-(-)-2-metil-1-butanol sao descritos. Estes novos poliacrilatos quirais mostram dependencia das temperaturas de transicao de fase com a massa molar e o comprimento do espacador. O poliacrilato contendo quatro unidades metilenicas no espacador mostra fase nematica quiral, enquanto que o poliacrilato com espacador contendo onze unidades metilenicas mostra fase esmetica. Alem disto, a fase nematica quiral foi encontrada para massa molar menor e fase esmetica para massa molar maior. Experimentos de espalhamento de luz foram realizados em solucoes diluidas do poliacrilato contendo onze unidades metilenicas em tetrahidrofurano e diclorometano. Valores de massa molar ponderal media, raio de giro e o segundo coeficiente virial foram determinados atraves de espalhamento de luz estatico, enquanto que o coeficiente de difusao e o raio hidrodinâmico foram obtidos por espectroscopia de correlacao de fotons. Uma comparacao entre as tecnicas indica que o comportamento da cadeia e tipico de cadeia linear polidispersa em bom solvente. The synthesis and mesomorphic behavior of two chiral side chain liquid crystalline polyacrylates from (S)-(-)-2-methyl-1-butanol are described. These new polyacrylates show a dependence of the phase transition temperatures on both, molecular weight and spacer length. The polyacrylate with four methylene units in the spacer exhibits a chiral nematic phase whereas the polyacrylate with a spacer containing eleven methylene units presents a smectic phase. In addition, the chiral nematic phase appears for low molecular weight and smectic phase for high molecular weight polyacrylates. Light scattering experiments were performed in dilute solutions of the polyacrylate containing eleven methylene units in tetrahydrofuran and dichloromethane. Values for weight averaged molecular weight, radius of gyration and second virial coefficient were determined by static light scattering whereas the diffusion coefficient and the hydrodynamic radius of the chains were obtained by photon correlation spectroscopy. A comparison between both techniques indicates that the chain behavior can be taken as typical for a polydisperse linear chain in a good solvent.

Cristais Líquidos Ferroelétricos - CLF. Uma abordagem sintética

Synthetic methods used for the synthesis of the Ferroelectric Liquid Crystals (FLCs) will be considered with emphasis on the chiral synthesis methodologies and mesomorphic electrooptical properties.

Synthesis and characterization of some novel tetrazole liquid crystals

The synthesis and characterization of new 4-[(2-alkyl)-2H-tetrazol-5-yl]phenyl 4-alkyloxybenzoates (5a–i) and 5-{4-[(4-substituted)benzyloxy]phenyl}-2-alkyl-2H-tetrazoles (7a–e) are reported. The reported tetrazoles contain two alkyl chains (six to ten carbon atoms) except for 5h and 7e which have a chiral citronellyl group linked at the N-2 nitrogen atom of the tetrazole ring and 5i having a bromine atom at the para position of benzoate. The regiochemistry in the alkylation step was established unequivocally by single-crystal X-ray analysis for 5a and 5f. The regioselectivity in the alkylation step was also confirmed by analysis of 13C NMR chemical shifts of the C5 carbon atom in the tetrazole ring and the methylene carbon atom vicinal to the nitrogen atom. The mesogenic behavior of the liquid crystals was characterized by polarizing optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD) techniques. The tetrazolyl benzoates 5a–h show a nematic mesophase. The lengthening of alkyl chains favors the appearance of the smectic mesophase. For instance, 5g displayed nematic and smectic C mesophases, while 5h which has the chiral citronellyl group displayed nematic and smectic A mesophases. Tetrazoles 7a–e did not show mesomorphic behavior mainly due to replacement of the carbonyl by a methylene group.

Synthesis and characterization of new chiral liquid crystalline polyacrylates from L-isoleucine

The synthesis of chiral side chain liquid crystalline polyacrylates with a two-stereogenic centre from L-α-aminoacid is described. The chiral tail is 2-chloroalcohol obtained from L-isoleucine and the spacer group has either four or eleven methylene units. The mesogenic moiety is derived from phenyl benzoate. The stereochemistry of the key intermediate (2S,3S)-(+)-4- [1-(2-chloro-3-methyl)pentyloxy]phenyl benzoate (6) obtained by a Mitsunobu reaction was established by single crystal X-ray analysis. This result indicates that the nucleophilic displacement of chiral diazonium salts proceeds with overall retention of configuration. The liquid crystalline behaviour of polyacrylates P13 and P14 was investigated by DSC, optical microscopy, small angle X-ray scattering and depolarized light scattering. The polyacrylate P13, with eleven methylene units in the spacer, exhibits a chiral smectic A phase whereas the polyacrylate P14, with a spacer containing four methylene units, displays a chiral nematic phase.

Structure-property relationships of smectic liquid crystalline polyacrylates as revealed by SAXS

A influencia da estrutura quimica dos grupos mesogenicos e do tamanho dos grupos espacadores, no comportamento de fase de uma serie de cristais liquidos polimericos de cadeia lateral (SCLCP), foram estudados utilizando-se espalhamento de raios-X a Baixo Ângulo (SAXS) e Microscopia Otica de Luz Polarizada (POM). Analises do arranjo das mesofases em amostras nao orientadas e orientadas por acao do campo magnetico sao descritas. O papel do tamanho do espacador lateral no empacotamento local e na largura da camada esmetica determinados nas mesofases SmA e SmC e elucidado. Os ângulos θ formados entre os grupos mesogenicos e a normal as camadas nas mesofases SmC foram determinados. Um estudo a respeito do grau de ordem em funcao da temperatura, para os polimeros esmeticos foi possivel atraves de medidas de SAXS. Uma ordenacao particular em um dos SCLCPs estudados e relacionada com a coexistencia de duas fases distintas. The influence of the chemical structure of the mesogenic groups and the length of the spacer groups on the phase behavior in a series of side-chain liquid crystalline polyacrylates (SCLCP) have been studied using Small Angle X-ray Scattering (SAXS) and Polarized Optical Microscopy (POM). Analyses of the mesophase arrangement in unaligned and aligned samples by magnetic field are reported. The role of the spacer length on the local packing and on the thickness of the layers encountered in the SmA and SmC mesophases is elucidated. The tilt angles θ of the mesogenic cores related to the normal of the layers in the SmC mesophases are measured. A study about the degree of order as a function of temperature for the smectic polymers was possible using SAXS measurements. A particular arrangement in one of the studied SCLCPs is related to the coexistence of two different phases.