Síntese de cristais líquidos funcionalizados contendo o heterociclo 1,3,4-oxadiazol

Abstract

Neste trabalho são descritas a síntese, a caracterização e os estudos das propriedades ópticas, térmicas, elétricas e organizacionais de novos materiais líquido-cristalinos funcionalizados e derivados do heterociclo 1,3,4-oxadiazol 2,5-dissubstituído. Quatro projetos foram desenvolvidos, os quais diferem na funcionalização e forma dos cristais líquidos (CLs), totalizando 59 produtos finais. 1) CLs iônicos: um estudo sistemático permitiu demonstrar que variações estruturais afetam simultaneamente e de forma variada as propriedades térmicas e fotofísicas estudadas; 2) CLs discóticos: os compostos apresentaram mesofase Colhex e fluorescência na região do azul (370-400 nm), com rendimentos quânticos dependentes da estrutura; 3) CLs curvados contendo o grupo 2-octinoiloxi almejando a obtenção de mesofases nemáticas biaxiais: a substituição do anel benzênico pela ligação C?C gerou um abaixamento do ponto de fusão dos materiais e os produtos apresentaram a predominância da mesofase N com a presença de clusters cibotáticos do tipo SmC; 4) CLs curvados contendo grupos segregantes almejando mesofases esméticas polares: os compostos apresentaram um interessante polimorfismo, exibindo uma predominância de mesofases Colob e SmC. Medidas eletroópticas confirmaram um ordenamento ferro e/ou antiferroelétrico para alguns materiais.<br>

Abstract : In this doctoral thesis are described the synthesis, characterization and study of the optical, thermal, electrical and organizational properties of new functionalized liquid crystalline materials of 2,5-disubstituted 1,3,4-oxadiazole derivatives. The synthesized compounds had their structure and purity confirmed by IR spectroscopy, 1H, 13C and 29Si NMR, melting point and elemental analysis (CHN) techniques. Subsequently, the thermal and liquid crystalline properties were investigated by POM, DSC, TGA, XRD and electro-optical measurements. In parallel, the photophysical properties of some compounds were studied in solution and in film by UV-vis absorption and emission spectroscopy. In this thesis, four different projects have been developed, which differ in the functionalization type and in the shape of liquid crystals: 1) 29 ionic and non-ionic calamitic molecules with several structural modifications were synthesized. This allowed a detailed systematic study of the effect of structural modifications on the thermal, liquid-crystalline and photophysical properties of these materials. It was shown that every parameter studied affected simultaneously and in a different way these properties; 2) 12 flat molecules with structural variations were synthesized in order to obtain fluorescent and columnar mesophase forming materials. Generally the compounds showed mesophase Colhex and fluorescence in the blue region (370-400 nm) with quantum yields dependent on the structure. In a general way, the compounds showed a Colhex mesophase and blue fluorescence (370-400 nm), with quantum yield dependent on the structure; 3) 10 bent-core molecules with reduced rigid center and smaller aliphatic chains were synthesized in order to obtain biaxial nematic mesophases. Furthermore, the 2-octinoiloxi group was used to study the effect of the substitution of a benzene ring by one C=C triple bond. The molecules showed predominance of a nematic mesomorphism with the presence of SmC cybotactic clusters. The results also showed that the substitution of a benzene ring by a C=C bond resulted in the lowering of the materials melting point, which also caused a shortening of the mesomorphic behavior range; 4) 8 bent-core molecules with slightly elongated rigid center, larger aliphatic chains and containing segregating groups were prepared aiming polar smectic mesophases. The compounds exhibited an interesting polymorphism, showing a predominance of Colob and SmC mesophases. Electro-optic measurements confirmed ferro and/or antiferroeletric order for some materials, which depend on the molecular structure and temperature analyzed.