Positional and Charged Effects of Heterocyclic N Atoms on Mesogenic Properties of Stilbazoles and Analogous N-Oxides

The synthesis and the phase behavior of stilbazoles and corresponding N-oxides containing different positions of heterocyclic N atoms, i.e. trans-2'-, 3'-, 4'- stilbazoles and their N-oxides, are reported. Their phase transition temperatures and the mesogenic behavior were investigated through the DSC, polarizing optical microscope and X-ray diffraction (XRD) measurements. Increasing dipole moments by oxidation of stilbazoles to analogous N-oxides induces the S A phase with an interdigitated bilayer packing. Both 2'-stilbazoles and analogous N-oxides do not possess mesogenic phases, which may he due that π electron polarizabilities are reduced by the molecular twist originated from dipoles along the ortho-direction. Besides, the positonal effects of the N-oxides on mesogenic properties are more distinct than those of analogous stilbazoles, and new mesogenic phases have been generated by tuning the position of the N-oxide function in the charged liquid crystals. Consequently. unique mesogenic properties may he introduced through adjusting the dipolar direction and strength of the heterocyclic atoms in the molecules.