Synthesis And Characterization Of Novel Fluoro-Azobenzoate Ester

In this study, three new series of 3-fluoro-4-cyanophenyl esters were synthesized using 2-fluoro-4-hydroxybenzonitile as main raw material, including p-substituted azobenzene carboxylic acid esters (series I), p-substituted 3-methoxy-azobenzene carboxylic acid esters (series II) and p-substituted phenyl-acrylic esters (series III). Their structures were characterized and confirmed with 1H nuclear magnetic resonance (1H NMR) and infrared spectra (IR). The liquid-crystalline behaviors of these compounds were also investigated by differential scanning calorimetry (DSC) and polarizing optical microscopy (POM).It was found in this study that:(1) In series I, 11 new 4-hydroxy-azobenzene acid esters were synthesized, and 9 of them exhibited liquid-crystalline properties. When the alkoxyl group substituted on the benzene ring of azobenzene is small, such as ethoxyl or propoxyl, the compounds did not exhibit liquid-crystalline properties. With the increasing number of carbon atoms in alkoxy groups (i.e. when n>3), these compounds showed mesophases. Moreover, the relationship between the number of carbon atoms in the alkoxyl chain and the transition temperatures, as well as the relationship between the number of carbon atoms (indicated by n) in the alkoxyl chain and the nematic range were studied. It was shown that the clearing points decreased as n increases firstly (when n<8), and the clearing points increased as n increases (when n>8). but the nematic range and the melting points increased in a zigzag mode as n increases.(2) In series II, 10 compounds were synthesized and and 9 of them exhibited liquid-crystalline properties. And the trends of their melting points, clearing points and nematic range formed similar patterns as those in series I.(3) In series III, 3 compounds were synthesized, and they all exhibited liquid-crystalline properties. They possessed lower clearing points and nematic ranges, compared to the relative compounds in series I and II. This phenomenon was due to that the interactions between the aromatic cores of substituted phenyl acrylic acid esters (in the case of series III are weaker than those of substituted azobenzen carboxylic acid esters (in the case of series I and II).