P. Hidalgo

New 1,2,4‐ and 1,3,4‐oxadiazole materials: synthesis, and mesomorphic and luminescence properties

The synthesis and liquid crystalline properties of new series of 1,2,4‐ and 1,3,4‐oxadiazole derivatives (2a–f and 5a–f respectively) are reported. These compounds contain only one terminal flexible alkoxy chain, the other terminal substituent is a protecting benzyl group. All compounds of series 2 exhibit an enantiotropic nematic phase. The homologue with the longest chain (2f) displays an enantiotropic dimorphism smectic A– nematic. None of the compounds of series 5 shows mesomorphism and only crystal–isotropic transitions were observed. The liquid crystalline properties were investigated by differential scanning calorimetry, polarizing optical microscopy and X‐ray measurements. Luminescence properties, in chloroform solution, of 2f and the series 5 compounds were evaluated. Good photoluminescence quantum yields for compounds of series 5 were observed. Compound 2f, incorporating the 1,2,4‐oxadiazole ring shows a very strong reduction in emissive properties.

Properties of thermotropic liquid crystals induced by hydrogen bonding between pyridyl‐1,2,4‐oxadiazole derivatives and benzoic acid, 4‐chlorobenzoic acid or 4‐methylbenzoic acid

Supramolecular liquid crystalline complexes have been obtained from binary mixtures of 3‐(4‐pyridyl)‐5‐(4‐n‐alkoxy)phenyl‐1,2,4‐oxadiazoles with benzoic acid, 4‐chlorobenzoic acid or 4‐methylbenzoic acid. Neither the oxadiazole derivatives nor the carboxylic acids are mesomorphic, but the H‐bonded complexes are. Their liquid crystalline properties were investigated by differential scanning calorimetry, polarizing optical microscopy and X‐ray diffraction.

Azo compounds and Schiff 's bases derived from 5-(4-pyridyl)-2-amino-1,3,4-thiadiazole: synthesis, mesomorphic properties and structural study by semi-empirical calculations

The synthesis and liquid crystalline properties of new Schiff s bases (series 3a-f) and azo compounds (4a-f) incorporating pyridine and 1,3,4-thiadiazole rings are reported. The first homologues in the series of Schiffs bases (3a-c) show a monotropic mesophase and the homologues 3d-f display an enantiotropic SmA phase. The only azo compounds to exhibit liquid crystalline properties are the homologues 4e, f which show a monotropic nematic phase. These series are compared with Schiffs bases and azo compounds analogues (series 5 and 6), previously reported by us and an AM1 study of the structure/mesomorphic property relationship is described.

Novel chiral liquid crystals based on amides and azo compounds derived from 2‐amino‐1,3,4‐thiadiazoles: synthesis and mesomorphic properties

Novel chiral amides (Ia–Ie, II) and azo compounds (III, IV) with a 1,3,4‐thiadiazole unit in the rigid core were synthesized and their liquid crystalline properties investigated by polarizing optical microscopy and differential scanning calorimetry. The amides Ia–Ie contain a chiral alkoxy chain derived from (R)‐2‐octanol and an achiral chain varying from 6 to 10 carbon atoms at the end of the rigid core. In amide II one of the terminal group is a chiral alkoxy chain derived from (S)‐isoleucine and the other terminal substituent is an achiral n‐decyloxy chain. Azo compounds III and IV contain an achiral n‐decyloxy chain and a chiral alkoxy chain derived from (R)‐2‐octanol and (S)‐isoleucine, respectively, at the end of the rigid core. The first homologue in the series of amides (Ia) exhibits enantiotropic smectic X (SmX)–chiral nematic (N*) dimorphism and the homologues Ib–Ie display enantiotropic SmX–chiral smectic C (SmC*)–N* mesomorphism. Amide II displays an enantiotropic smectic A phase. The azo compounds III and IV do not show smectic order and only an enantiotropic N* phase was observed. Thus, the mesomorphic behaviour depends on the nature of the central linkage and on the nature of the chiral alkoxy chain.

Columnar liquid crystalline benzenetrisamides with pendant 1,3,4-oxadiazole groups

Novel liquid crystal materials based on 1,3,5-benzenetrisamide derivatives with three pendant 2-phenyl-5-(mono-, di-, and/or tri-n-alkoxyphenyl)-1,3,4-oxadiazole arms (Ia–c, IIa–c) were prepared. The mesomorphic properties of these compounds were characterised and studied by differential scanning calorimetry, polarising optical microscopy and X-ray diffraction. The formation of a columnar mesophase was found to be dependent on the number of alkoxy side chains. The compounds Ib and IIb, with a total of six alkoxy chains, and compounds Ic and IIc, with nine alkoxy chains, exhibited an enantiotropic hexagonal columnar (Colh ) phase with high isotropisation temperatures; however, compounds Ia and IIa with a total of three alkoxy chains formed a crystalline phase. Compounds IIb and c were room temperature liquid crystals.

Synthesis and mesomorphic properties of oxadiazole esters derived from (R)‐2‐octanol, (S)‐2‐n‐octyloxypropanol and (2S,3S)‐2‐chloro‐3‐methylpentanol

A novel series of chiral liquid crystalline materials based on 1,3,4‐ and 1,2,4‐oxadiazole derivatives were synthesised. These compounds contain a chiral chain derived from (R)‐2‐octanol (Ia–Ic), (S)‐ethyl lactate (IIa–IIc) or (S)‐isoleucine (IIIa–IIIc). Their liquid crystalline properties were studied by polarizing optical microscopy and differential scanning calorimetry. With the exception of compounds IIa and IIb, all of the new compounds exhibit an enantiotropic chiral nematic (cholesteric) phase. A monotropic chiral smectic C phase (ferroelectric) was also found in compounds Ia, IIa and IIb, whereas in compound IIIa an enantiotropic chiral smectic C phase was observed. Compounds Ic, IIc, IIIb and IIIc are purely chiral nematic (cholesteric) in character. In addition, a monotropic blue phase was observed in compounds Ib and IIIa.

New supramolecular liquid crystals induced by hydrogen bonding between pyridyl‐1,2,4‐oxadiazole derivatives and 2,5‐thiophene dicarboxylic acid

Supramolecular liquid crystalline complexes were obtained from binary mixtures of 3‐(4‐pyridyl)‐5‐(4‐n‐alkoxy)phenyl‐1,2,4‐oxadiazoles and 2,5‐thiophene dicarboxylic acid. Although the oxadiazole derivatives and the dicarboxylic acid are non‐mesomorphic, the H‐bonded complexes exhibit mesomorphism. Their liquid crystalline properties were investigated by differential scanning calorimetry and polarizing optical microscopy. The complexes exhibit enantiotropic nematic phases. A structural study involving AM1 semi‐empirical calculations is also described.

Novel chiral calamitic liquid crystalline oxadiazoles as ferroelectric materials

Novel liquid crystalline materials based on chiral calamitic 1,2,4‐ and 1,3,4‐oxadiazole derivatives were synthesized and their thermotropic mesomorphism investigated by polarizing optical microscopy and differential scanning calorimetry. The structures of their smectic phases were investigated by X‐ray diffraction. The existence of ferroelectric properties in the smectic C phase was also studied.

Symmetric esters derived from 1,3,4-oxadiazole: synthesis, mesomorphic properties and structural study by semi-empirical calculations

Three homologous series of symmetric esters derived from 2-alkylthio-5-(p -hydroxy)phenyl-1,3,4-oxadiazole (series 3, 4 and 5) were synthesized and their liquid crystalline properties investigated by optical microscopy and differential scanning calorimetry. Depending on the chain length, nematic and smectic C phases were observed in the series 3 ; smectic A and smectic C phases in series 4. None of the homologues of series 5 shows mesomorphic properties. These three series are compared with other previously reported oxadiazoles. A structural study of AM1 semi-empirical calculations is also described.

Novel amides and Schiff's bases derived from 1,3,4-oxadiazole derivatives: Synthesis and mesomorphic behaviour

The synthesis and liquid crystalline properties of novel achiral amides (Ia–g, IIa–g and IVa,b), achiral Schiffs bases (IIIa–g and Va–g), chiral amides (VI, VII) and chiral Schiffs bases (VIII–XI) incorporating a 1,3,4-oxadiazole ring are reported. All amides of the series I and II display an enantiotropic smectic A phase. The amide IVa,b did not show mesomorphic properties. Amides of the series Ia–g and IIa–g contain a flexible n-tetradecylthio chain, the other terminal substituent is an n-alkoxy chain and n-alkyl chain, respectively (n = 4–10) and the 1,3,4-oxadiazole is in the terminal rigid core. Amides Ia–g have broader mesomorphic range and higher thermal stability than the corresponding amides IIa–g. Amides IVa,d contain the 1,3,4-oxadiazole ring in the centre of the rigid core and two flexible alkoxy chains as flexible terminal substituents. Thus, the mesomorphic properties are favoured if 1,3,4-oxadiazole is shifted to a terminal position of the rigid core. Schiffs bases IIIa–g display an enantiotropic dimorphism smectic C–smectic A. Schiffs bases IIIa–g have a broader mesomorphic range than the analogous amides Ia–g. Schiffs bases Va–g exhibit a dimorphism smectic A–nematic, and in contrast to this the analogous amide IVa,b did not show mesomorphism. The chiral amides VI and VII and chiral Schiffs bases X and XI did not show mesomorphic properties and only the chiral Schiffs bases VIII and IX display a chiral smectic C phase in a short mesomorphic range. A density functional theory theoretical study at the B3LYP/6–311++G(d,p) level was performed in order to analyse the structural features that must be related with the mesomorphic behaviour of the reported compounds.