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 long molecular axis. The mesogenic units consist of an azobenzene core which has an electron donating oxy unit at the 4-position and an electron withdrawing carboxy unit at the 4'-position. This mesogenic unit has an intrinsically high dipole moment. The effects of the orientation of the dipoles of the mesogens on the thermotropic and lyotropic aggregation behavior of these compounds have been investigated. The thermotropic properties can be rationalized by assuming that the antiparallel orientation of the mesogenic units is the most favorable one. Compound 1, which has both dipoles pointing away from the headgroup, gives almost fully extended bilayers with parallel mesogens upon sonication in water. Compounds 2, with alternating dipoles, and 3, which has both dipoles pointing toward the headgroup, give interdigitated bilayers. The compounds also form monolayers at the water-air interface. 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 surface potential is small. Instead, the observed differences are caused by the position and orientation of the carbonyl groups. The orientation of the dipole moments of the mesogenic units has only a minor effect on the lyotropic aggregation behavior of these compounds