Two-dimensional structure of self-assembled alkyl-substituted polyphenylene dendrimers on graphite

Three types of polyphenylene dendrimers were demonstrated to spontaneously form stable, pinhole-free monolayers on graphite with a complex two-dimensional structure (Loi et al, Macromolecules 2001, 34, 366 1). For this paper, we extended our study using four now dendrimers tu draw more general conclusions, Monolayers were prepared by spin-coating and analyzed by atomic force microscopy. The dendrimers consist of a core of twisted, interlocked benzene rings and an external shell of alkyl chains. All structures observed can be divided into three types which can coexist on one sample: (1) granular regions, where individual dendrimers could be identified in a glasslike phase, (2) diffuse, homogeneous regions, and (3) nanorod regions consisting of parallel rows of 5.1-5.9 nm spacing, Dendrimers with alkyl chains of different lengths showed the same spacing between nanorods. This indicates that the alkyl chains arc, oriented parallel to the direction of the nanorods. In addition, some dendrimers form two-dimensional crystals on graphite. The concentration of the solution and the adsorption energy are two important factors determining which structure is formed.