Cyclobutane Scaffold in Bolaamphiphiles: Effect of Diastereoisomerism and Regiochemistry on Their Surface Activity Aggregate Structure.

Cationic bolaamphiphiles have been synthesized starting from meso cis- or chiral trans-1,2-difunctionalized cyclobutane derivatives. They include cis/trans pairs of diastereoisomers, of N- or C-centered bisamides. The goal of this work was to investigate the influence of stereochemistry and regiochemistry on their abilities as surfactants and self-assembly. Very large differences in surface coverage (2-fold), critical micellar concentration (cmc, up to 2 orders of magnitude), and aggregate structure (from lamellae to fibers) for the four molecules are remarkable due to regio- and stereochemistry differences. Computational calculations were carried out to rationalize the experimental findings and a new methodology has been developed to calculate the structure of these bolaamphiphiles at the surface. Although the four surfactants adopt a wicket-like conformation, for N-centered trans, the distance between polar heads is much larger than that for the other three molecules, as suggested by calculations. We have shown that the interplay between the regiochemistry and stereoisomerism, enhanced by rigidity of the cyclobutane ring, affects different physicochemical properties quite differently. That is, the cmc value is mainly governed by stereochemistry, with regiochemistry only modulating this value. On the other hand, regiochemistry definitely governs the morphology of the supramolecular aggregates (i.e., long fibers versus plates or spherical assemblies), with stereochemistry finely modulating their structural parameters. All these results must help in the rational design of new bolaamphiphiles with predictable properties and useful potential applications.