The three-dimensional construction of arrays of functional molecules on an
electrode surface, such as organic
semiconductors and redox-active molecules, is a considerable challenge in the fabrication of sophisticated junctions for molecular devices. In particular, well-defined organic layers with precise molecular gradients are anticipated to function as novel
metal/organic interfaces with specific electrical properties, such as a space charge layer at the
metal/
semiconductor interface. Here, we report a strategy for the construction of a three-dimensional molecular array with an electrical connection to a
metal electrode by exploiting dendritic molecular architecture. Newly designed dendritic molecules consisting of
viologens (1,1'-disubstituted-4,4'-bipyridilium salts) as the framework and mercapto groups as anchor units form unique self-assembled monolayers (
SAMs) on a
gold surface reflecting the molecular design. The dendritic molecules exhibit a conical shape and closely pack to form cone arrays on the substrate, whereas, in
solution, they expand into more flexible conformations. Differences in the introduction position of the anchor units in the dendritic structure result in apical- and basal-type cone arrays in which the spatial concentration of the
viologen units can be precisely configured in the cones. The concentration in apical-type
SAMs increases away from the substrate, whereas the opposite is true in basal-type
SAMs.