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-oxi des containing different positions of heterocyclic N atoms, i.e. trans-2'-, 3'-, 4'- stilbazoles and their N-oxides, are reported. Their phase transit ion temperatures and the mesogenic behavior were investigated through the D SC, polarizing optical microscope and X-ray diffraction (XRD) measurements. Increasing dipole moments by oxidation of stilbazoles to analogous N-oxide s induces the S-A phase with an interdigitated bilayer packing. Both 2'-sti lbazoles and analogous N-oxides do not possess mesogenic phases, which may be due that pi electron polarizabilities an reduced by the molecular twist originated from dipoles along the ortho-direction. Besides, the positonal e ffects 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 adjust ing the dipolar direction and strength of the heterocyclic atoms in the mol ecules.