Guilherme D. Vilela

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.

Implications of flexible spacer rotational processes on the liquid crystal behavior of 4,5-dihydroisoxazole benzoate dimers

The synthesis of some novel non-symmetric liquid crystal dimers, {3-[4-(octyloxyphenyl)]-4,5-dihydroisoxazol-5-yl}alkyl 4-(decyloxy)benzoates (5a–d) and 4-{3-[4-(octyloxyphenyl)]-4,5-dihydroisoxazol-5-yl}alkyl 4-{[6-(octyloxy)naphthalen-2-yl]ethynyl}benzoate (9a–d), are reported. The liquid-crystalline properties, theoretical calculations based on the conformational aspects of the flexible alkyl spacer and X-ray experiments are discussed. The syntheses of the key intermediates, 2-{3-[4-(octyloxy)phenyl]-4,5-dihydroisoxazol-5-yl}alkanol (3a–d), presenting the flexible alkyl spacer were achieved through [3+2] cycloaddition reactions between nitrile oxides, which were generated in situ by oxidation of the respective aromatic oximes, and dipolarophile alkenols (CH2CH(CH2)nOH, n = 1, 2, 3, and 4). The benzoates 5a–d were synthesized through esterification of 3a–d and p-n-decyloxybenzoic acid (4). The esters 9a–d were synthesized through derivatization of isoxazolines 3a–d into 4-{3-[4-(octyloxyphenyl)]-4,5-dihydroisoxazol-5-yl}alkyl 4-bromobenzoate (7a–d) followed by a Sonogashira reaction with 2-ethynyl-6-octyloxynaphthalene (8). 5a and 5b showed a monotropic smectic C phase. 9a/c displayed a enantiotropic nematic (N) mesophase, whereas 9b/d showed a monotropic nematic mesophase. No mesophase was observed for 7a–d. An odd–even effect was observed for 5a–d and 9a–d associated with the crystal to isotropic phase transition and crystal to nematic phase, respectively, as the length of the spacer was increased from 1 to 4 carbon atoms. The transitional properties were higher for odd-numbered members (n = 1 and 3) for all of the series studied. The X-ray data of compounds 5a and 5b are in agreement with polarizing optical microscopy observations with the assignment of an SmC mesophase. Density functional theory calculations using the B3LYP hybrid functional with the level 6-311G(d,p) basis set were performed for molecules 5a–d to correlate the conformation of the flexible spacer and the transitional properties. The conformational analysis showed that the most stable conformation for 5a–d is one where all of the carbon atoms of the flexible spacer are orientated at 180° (antiperiplanar orientation) except for 5a because the spacer is too short. The odd-numbered members have a more bent shape and are less elongated molecules than the even-numbered members. Thus, mesomorphic behavior is dictated by the conformational constraint imposed by the flexible spacer on the mesogenic groups.

3,5-diarylisoxazoles: A New Entry to Soft Crystal Phase

This work describes the synthesis and characterization of a new liquid-crystalline compounds based on isoxazoles. Classical synthetic methodologies were employed in the preparation of this compounds, and the [3+2] 1,3-dipolar cycloaddition was the key step to prepare isoxazolines as precursors to isoxazoles. The structural and thermal characterization was performed using 1H and 13C NMR techniques, polarized-light optical microscopy and differential scanning calorimetry. Compounds 7a-e displayed the SmA mesophase. Soft crystal phase (CrE) was observed for compounds 7a and 7b containing chlorine and bromine atoms. For 7e containing a nitro group crystal phases below SmA mesophase were also observed.

3-(4-Bromophenyl)-5-(4-hydroxyphenyl)isoxazole: a versatile, new molecular building block for side-chain liquid crystal oligomers and polymers

The synthesis and liquid crystalline behaviour of new comb-shaped methacrylate oligomers, derived from a new key intermediate in the synthesis of heterocyclic liquid crystals, i.e., 3-(4-bromophenyl)-5-(4-hydroxyphenyl)isoxazole, are reported. These new thermotropic liquid crystals exhibit a smectic A mesophase (SmA) above a soft crystal phase (CrE). The new methacrylate oligomers were synthesized by free radical polymerization of the corresponding monomers. Additionally, one main-chain liquid crystal polymer (MCLCPs) was prepared using a Mizoroki–Heck polymerization.Graphical abstract3-(4-Bromophenyl)-5-(4-hydroxyphenyl)isoxazole (2) is a new key intermediate in the synthesis of polymer and non polymer ordered liquid crystal phase.

Crystal structure and absolute configuration of (3aR,3′aR,7aS,7′aS)-2,2,2′,2′-tetra­methyl-3a,6,7,7a,3′a,6′,7′,7′a-octa­hydro-4,4′-bi[1,3-benzodioxol­yl], obtained from a Pd-catalyzed homocoupling reaction

The crystal structure of a homocoupled compound with absolute configuration 3aR,3′aR,7aS,7′aS was determined. The molecule contains two similar moieties composed of two fused rings. Its supramolecular structure is controlled mainly by C—H⋯O interactions.

Büchwald-Hartwig Reaction Applied to Synthesis of New Luminescent Liquid Crystal Triarylamines Derived from Isoxazoles

The present work describes the synthesis and characterization of novel series of triarylamines isoxazoles (TAA) addressed to the organic photovoltaic materials. Diarylisoxazoles were synthesized by sequential [3+2] 1,3-dipolar cycloaddition reaction between arylnitrile oxides and selected arylalkenes followed by MnO2-oxidation. Isoxazoles were coupled to diarylamines by Büchwald-Hartwig reaction to afford desired compounds 6a–k. Some TAA display liquid-crystalline behaviour and UV-Vis absorption and fluorescence emission were analysed for all samples of TAA 6a–k.

The 2:1 cycloadducts from [3+2] 1,3-dipolar cycloaddition of nitrile oxide and vinylacetic acid. Synthesis and liquid crystal behaviour

Four liquid crystals (LC) 3,7a-bis(4-alkyloxyphenyl)-7,7a-dihydro-6H-isoxazolo[2,3-d][1,2,4]oxadiazol-6-yl)acetic acid (7a–d) were synthesised and the mesomorphic behaviour reported. The LCs were characterised as 2:1 bisadducts, which were obtained from a double [3 + 2] 1,3-dipolar cycloaddition. In the first step, the cycloaddition of 4-alkyloxyphenylnitrile oxide (4a–d) and vinylacetic acid (5) gave the initial unobserved 1:1 cycloadducts 2-[3-(4-alkyloxyphenyl)-4,5-dihydroisoxazol-5-yl]acetic acid (6a–d). In the second step, the addition of a second equivalent of 4 to 6 yielded the 2:1 bisadducts 7a–d without any traces of 6. All compounds 7a–d were unstable during the transition from the mesophase to the isotropic state upon first heating as evidenced by the large peaks in the differential scanning calorimetry traces. Due to the chemical instability of the compounds upon heating, the transition temperature related to the smectic C to smectic A transitions was acquired by means of an image processing method. X-Ray diffraction experiments were also used to analyse the liquid-crystalline phases. A theoretical calculation was performed using density functional theory (DFT) methods at the PBE1PBE/6-311+G(2d,p) level (with solvent effect) in order to get information about the energetic profile of the 2:1 cycloaddition. DFT studies revealed that the cycloaddition process is controlled by the HOMO(dipolarophile) – LUMO(1,3-dipole), and that the double [3 + 2] 1,3-dipolar cycloaddition reaction is quite possible.