Synthesis and characterization of photoactive columnar liquid crystals containing azobenzene and quinoxaline moieties

Abstract

Abstract In this study, we report the synthesis and characterization of thermal and optical properties of photoactive molecules containing both quinoxaline and azobenzene moieties. Two series of molecules in which the quinoxaline is connected to the azobenzene without a flexible alkyl chain were accessed via esterification reactions between the 4-(hydroxy)-4′-substituted-azobenzenes (substituents on azobenzene ring: -ODecyl, -NO2, -Cl, -OMe and -O(2-Et-Hexyl)) and 2,3-bis(4-methoxyphenyl)quinoxaline-6-carboxylic acid or 2,3-bis(3,4-bis(dodecyloxy)phenyl)quinoxaline-6-carboxylic acid. The other two series of molecules containing an alkyl chain spacer of six carbon atoms between both aromatic systems were obtained via O-alkylation reactions of the same quinoxaline acids mentioned above with 4-(6-bromohexyloxy)-4′-substituted-azobenzenes. The absence of long alkyl chains on the quinoxaline side led to glassy materials and some of them showed a calamitic type mesophase, mostly monotropic. On the other hand, a considerable increase of side-chain density in compounds containing the long alkyl chains on the quinoxaline ring gave materials with hexagonal columnar phases (Colh). In these materials, the incorporation of small polar substituents such as nitro or chlorine on the azobenzene system increased the stability of the Colh. The presence of alkyl chain spacer was responsible for lowering the melting points and room temperature Colh phases were observed for some of these compounds. The molecular arrangement of molecules on the Colh is different from the analogous without the alkyl spacer between the aromatic systems. The absorption and fluorescence properties, as well as the photoisomerization of the azo group in solution were investigated. Colh phase was able to be destroyed under UV light and this process was found to be reversible.