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

Insertion of the 115-bis(ethynylene)benzene unit as a rigid spacer into a l inear alkyl chain, thus separating the two resulting stems by 9 Angstrom, i nduces chain folding at the air-water interface. These folded molecules sel f-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 molecula r shapes, of the types U, inverted U, and M, were obtained in the two-dimen sional crystalline state, depending upon the number of spacer units, and th e number and position of the hydrophilic groups in the molecule. The molecu les form ribbons with a higher crystal coherence in the direction of stacki ng between the molecular ribbons, and a lower coherence along the ribbon di rection. A similar molecule, but with a spacer unit that imposes a 5 Angstr om separation between alkyl chains, yields the conventional herringbone arr angement.