Self-assembly of tetracationic amphiphiles bearing a calix[4]arene core. Correlation between the core structure and the aggregation properties

Abstract

Water-soluble, conformationally-immobilized calix[4]arenes (1 and 2_n) with cone and 1,3-alternate conformations have been synthesized: at the para-position of each phenyl unit 1 has a Me₃N⁺CH₂ group and 2_n has a Me₃N⁺[CH₂]_nOCH₂ group. Examinations with surface tension, fluorescence and dynamic light-scattering established that in water cone-1 aggregates into small micellar particles whereas such molecular aggregates are not detected for 1,3-alternate-1. In 2_n both the cone and 1,3-alternate isomers formed aggregates in water but the cone isomers always gave CAC (critical aggregation concentration) values lower than the 1,3-alternate isomers. These results consistently indicate that the cone 2_n isomers with a cone-shaped hydrophobic surface are more cohesive intermolecularly than the 1,3-alternate 2_n isomers with a cylindrical hydrophobic surface. From the molecular shape one can expect that the cone isomers favourably form a globular micelle whereas the 1,3-alternate isomers favourably form a two-dimensional lamella. This was evidenced by the fact that 1,3-alternate-2_n can form stable vesicular aggregates detectable by an electron microscope whereas cone-2_n cannot form such stable aggregates. These results demonstrate that the aggregation properties of calix[4]arene-containing amphiphiles can be controlled by the conformational structure difference in the calix[4]arene core.