Crystalline self-assembly into monolayers of folded oligomers at the air-water interface

Insertion of the 1,3-bis(ethynylene)benzene unit as a rigid spacer into a linear alkyl chain, thus separating the two resulting stems by 9 A. induces chain folding at the air-water interface. These folded molecules self-assemble into crystalline monolayers at this interface, with the plane of the folding unit almost perpendicular to the water surface, as determined by synchrotron grazing-incidence X-ray diffraction. Three distinct molecular shapes, of the types U, inverted U, and M, were obtained in the two-dimensional crystalline state, depending upon the number of spacer units, and the number and position of the hydrophilic groups in the molecule. The molecules form ribbons with a higher crystal coherence in the direction of stacking between the molecular ribbons, and a lower coherence along the ribbon direction. A similar molecule, but with a spacer unit that imposes a 5 A separation between alkyl chains, yields the conventional herringbone arrangement.