Luminescent liquid crystals derived from quinoxaline and thiophene

Full text: Columnar Liquid Crystals (LCs) with luminescent properties are interesting candidates for applications in the field of organic electronics, particularly on OLEDs [1]. Here in, we present the synthesis and characterization of three new symmetric molecules with LC properties containing quinoxaline and thiophene units. The molecules have the 2,5-dissubstituted-thiophene with peripheral portions of 2,3-bis(3,4-bis(alkoxy)phenyl)quinoxaline-6-carboxylate and 1,3,4- oxadiazole heterocyclic as spacer group. The molecules differentiate by the size of the alkoxy chain linear or branched, varying from twelve and eight carbons. The insertion of two units of 1,3,4-oxadiazole heterocyclic might give luminescence and increases the electron deficiency behavior of the molecules [2]. The quinoxaline group may influence the LC properties as well as the fluorescent properties [3]. The thiophene has a donating electron characteristic in opposition to the electron withdrawing heterocycles (quinoxaline and oxadiazole) giving rise to a push-pull system. The structures of molecules were characterized for NMR ¹H and ¹³C. Thermal properties were studied by POM, DSC and TGA. The compounds exhibited the columnar mesophase assigned by POM that is to be confirmed by XRD analysis underway. Preliminary optical studies were performed in CHCl₃ solution. All compounds showed an absorption band around 370 nm, attributed to the π-π* transitions. The compounds exhibited blue fluorescence with maxima wavelengths around 450 nm. Investigations of possible interactions of the donor/acceptor portions in the molecules leading to excimer formation, as well as the measurements of mesomorphic and optical behavior properties in thin films with a variation of temperature, are currently underway. (author)