Three approaches for the synthesis of dendrimers with poly(p-phenylene)-PPP derived cores are described. The respective limitations of these approaches are assessed in view of the goal of this research:  to make available structurally perfect, high molecular weight polymers with a dense sequence of sterically demanding dendrons.

The degree of polymerization (DP n ) of poly(p-phenylene) (PPP) 3 with appendant third-generation, Fréchet-type dendrons was determined as 110 after a chemical modification. This DP n is extraordinarily high and shows that the mechanistically complex Suzuki polycondensation can thus be employed even in the case of sterically highly loaded (dendronized) AA-type monomers, which paves the way to obtain extremely rigid nanorods. A molecular model of 3 was obtained by molecular dynamics simulations which show that the diameter of the rodlike dendrimer fluctuates between approximately 2 and 4 nm in vacuo. Scanning force microscopy (SFM) on 3 adsorbed on graphite indicates the formation of multilayer films made of densely packed nanorods. SFM with molecular resolution reveals highly ordered domains in which the molecules are packed parallel to each other, separated by a lateral periodicity of 4.8 ± 0.5 nm. Lateral and vertical spacings indicate that the (110) plane of the densely packed nanorods is exposed at the film surface. SFM further indicates that different domains exhibit three molecular orientations, reflecting the 3-fold symmetry of the graphite substrate. Chain ends can be resolved at grain boundaries. Time-dependent SFM experiments show reorientation of small domains on a time scale of minutes.

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