Supramolecular Aggregates with Controlled Size and Shape on Solid Surfaces
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Molecule-based nanodevices with advanced functions have been attracting much attention, because the size of microelectronic circuit components will soon reach the scale of atoms or molecules. The idea that one, or a few molecules, could perform the basic functions of electronics was first introduced in 1978. Aviram and Ratner suggested that a molecular diode should be obtained by a donor–spacer–acceptor structure, when it is placed between two electrodes. Such molecular-scale electronic devices differ radically from bulk-scale devices, because one or a few molecules work as a circuit component. In addition, it should be noted that the structure and function of molecular building blocks are rationally controlled by chemical synthesis, and that each molecular component can be aggregated into a larger structure by self-assembly.
To realize molecular-scale devices, the development of new approaches should be required to construct desired molecular nanostructures on a suitable substrate. Whereas large functional molecules have been directly obtained by chemical synthesis, supramolecular assembly is a realistic way to create nanoscale molecular structures. The supramolecular approach, starting from molecular building blocks, can lead to controlled aggregates, which is achieved by selective and directional intermolecular interactions. When noncovalent attractive interactions such as hydrogen bonding are introduced into functional molecules, the selective aggregation results in the controlled formation of molecular nanostructures, which has been widely studied. However, these structures have been obtained exclusively in crystals or dissolved structures. For the use of the molecular nanodevices, the supramolecule should be supported on a suitable surface. The surface-supported supramolecules should be located at suitable positions.
On solid surfaces, scanning probe microscopy, particularly scanning tunneling microscopy (STM), has proven to be a powerful tool for the real-space investigation of adsorbed functional molecules, allowing to directly determine the arrangement, configuration, and conformation of adsorbed molecules with submolecular resolution. Using the technique of STM, several groups have reported on self-assembled structures of adsorbed molecules on metal surfaces. To realize molecular nanodevices, further control of their size and shape should become an important step. Here we demonstrate the controlled supramolecular aggregation of porphyrin derivatives formed on a gold surface. First, we describe the structure of porphyrin and substrate surface used. After the adsorption properties of the porphyrin on the gold surface are described in detail, the supramolecular aggregation of cyano-substituted porphyrins is reported.