Self-Organization of a Fan-Shaped Dendrimer at the Air-Water Interface

The orientation of hexagonal cylinders self-assembled from fan-shaped dendrimers is strongly affected by surface anchoring. Planar ordering of the cylinders is obtained on a water surface, while the cylinder assemblies align homeotropically on carbon substrates. To understand self-assembling mechanism of the planar ordering, we investigate the surface texture of Langmuir-Blodgett monolayers and multilayers of an asymmetric fan-shaped second generation dendrimer containing a CO₂C₃H₇ core group of the cylinder and dodecyl (C₁₂H₂₅) alkyl tails at the air-water interface. Surface pressure-area (π-A) isotherm and Brewster angle microscopy (BAM) measurements show that the molecule forms a stable monolayer on a water surface with two phase transitions (gaseous phase, expanded phase, condensed phase). The electron diffraction (ED) patterns, transmission electron microscopy (TEM) images, and atomic force microscopy (AFM) images of the condensed monolayers clearly reveal that the molecule forms edge-onoriented monolayers with hexagonal packing on the water surface, suggesting fractional cylindrical configuration at the air-water interface.Uponfurther compression, the condensed monolayer is transformed into a multilayer, and the interfacial structure becomes planar morphology. These results demonstrate the origin of planar alignment of self-assembled dendrimers on water surface.