Preparation of dendritic multisulfides and their assembly on air/water interfaces and gold surfaces

Poly(propylene imine) (PPI) dendrimers of generations 1-5 have been derivatized with dialkyl sulfide chains. On water, these dendrimers form stable LB films with densely packed alkyl chains pointing to the air and cores exposed to the aqueous solution. Self-assembly of the same dendrimers from solution onto a gold surface leads to a flattened orientation of the molecules on the surface, as shown by high contact angle hystereses, high electrochemical capacitances, and low charge transfer resistances. X-ray photoelectron spectroscopy data indicate that the formation of the flattened structure can be attributed to adsorption to the gold surface of not only part of the sulfide moieties in the alkyl chains, but also part of the tertiary amines in the core. The electrochemically determined surface coverages of ferrocene-modified dendritic wedges (with similar molecular structures as those of the dialkyl sulfide PPI dendrimers) confirm the proposed flattened, surface-spread dendritic structure. In contrast, LB films of the sulfide PPI dendrimers that were transferred onto gold appear to largely remain their original, densely packed state, as for these films smaller contact angle hystereses and higher charge transfer resistances are measured than those observed for the monolayers prepared by self-assembly from solution. Additionally, the electrochemical capacitances of the transferred LB films point to a layer thickness comparable to the thickness of a densely packed didecylsulfide monolayer. The results in this study demonstrate that different surface architectures can be prepared of the same compounds by solely varying the preparation procedure.