Mesogenic heterocycles derived from quinoxaline Schiff Bases

Three new series of heterocyclic quinoxaline Schiff Bases 1–2 were prepared, characterized and their mesomorphic properties were investigated. These compounds 1 and 2 are in fact geometric isomers in which an imine moiety (e.g., [sbnd]C[dbnd]N) is inversely incorporated into quinoxaline, leading to an opposite local dipole. Two single crystallographic structures 1 (m=8, n=8) and 2a (m=12, n=8) were determined by X-ray crystallographic analysis in order to understand the effect of mesomorphic properties. Weak H-bonds, CH–π and π–π interactions were found in both crystals, which were attributed to the formation of mesomorphic behavior. Variable temperature FT-IR experiments were performed to confirm the induced H-bonds. All series of compounds 1–2 exhibited N/SmC or SmC phases, which were identified by optical microscope and confirmed by powder X-ray diffraction experiments. Compounds 2a have a slightly wider range of mesophase temperatures than that of compounds 1 and 2b.