The influence of dipolar interactions between mesogenic units on the aggregation behavior of a series of isomeric double-chained ammonium amphotropes

A series of novel double-chained ammonium amphotropes has been synthesized which differ in the dipole orientation of their mesogenic units along the l ong molecular axis. The mesogenic units consist of an azobenzene core which has an electron donating, oxy unit at the 4-position and an electron withd rawing carboxy unit at the 4'-position. This mesogenic unit has an intrinsi cally high dipole moment. The effects of the orientation of the dipoles of the mesogens on the:thermotropic and lyotropic aggregation behavior of thes e compounds have been investigated. The thermotropic properties can be rati onalized by assuming that the antiparallel orientation of the mesogenic uni ts is the most favorable one. Compound 1, which has both dipoles pointing a way from the headgroup, gives almost Fully extended bilayers with parallel mesogens upon sonication in water. Compounds 2, with alternating dipoles, a nd 3, which has both dipoles pointing toward the headgroup, give interdigit ated bilayers. The compounds also form monolayers at the water-air interfac e. Compounds 1 and 2 initially lie flat at the water-air interface whereas the mesogens of compound 3 have no interaction with the water surface. The contribution of the dipole of the mesogens to the monolayer electrical surf ace potential is small. Instead, the observed differences are caused by the position and orientation of the carbonyl groups. The orientation of the di pole moments of the mesogenic units has only a minor effect on the lyotropi c aggregation behavior-of these compounds.