Close columnar packing of triangulenium ions in Langmuir films

Three new tris(dialkylamino)trioxatriangulenium (ATOTA+) salts rendered amphiphilic by attachment of two (5a x PF6 and 5b x PF6) or four (5c x PF6) n-decyl chains have been synthesized, and their Langmuir films have been studied by grazing incidence X-ray diffraction (GIXD). Compounds 5a x PF6 and 5b x PF6 both self-assemble into 2D-crystalline Langmuir monolayers, in which the planar triangular shaped carbenium ions form columnar aggregates segregated from the PF6- ions. The column width is found to be close to the width of the triangulenium moiety itself (approximately 17 angstroms), while the repeat distance along the columnar aggregates is only 3.45 angstroms, implicating a near cofacial columnar structure with only a small tilt of the planar carbenium ions relative to the columnar axis. A detailed Bragg rod analysis confirmed an 8-9 degrees tilt and inferred a large anisotropy in the smearing/thermal displacement along the pi-pi stacking and lamellar packing directions. Specular X-ray reflectivity (SXR) was used to confirm the model derived from the GIXD data and elucidate the average position of the disordered PF6- ions, showing that the majority of the anions are accommodated in the ATOTA+ layer rather than in the water subphase.