PRAIRIES · Supramolecular hierarchical self-assembly of organic moleculesonto surfaces towards bottom-up nanodevices: an host-driven action

Final Activity Report Summary - PRAIRIES (Supramolecular hierarchical self-assembly of organic moleculesonto surfaces towards bottom-up nanodevices: an host-driven action)

"There is plenty of room at the bottom". These were the famous words of Richard P. Feynman in 1959 that led to the birth of nanotechnology and nanoscience. Electronic devices based on inorganic semiconductors have been part of our daily lives for the last 60 years. Their miniaturisation has occurred gradually over the years, however, according to Moore's law the contemporary microelectronic industry's "top-down" manufacturing technique will soon reach its limits. Therefore, the recent development and increased knowledge of organic semiconductors has led to a tendency to explore alternative avenues with a focus on the creation of electronic devices based on organic molecules. The invention of techniques such STM (1981) and AFM (1986) have facilitated this research, allowing the imaging and manipulation of surfaces and molecules at the nanometre scale (0.1-100 nm). The next step is therefore the development of methods for the controlled fabrication of molecular assemblies and their integration into usable macroscopic systems. In this respect, the "bottom-up" approach offers considerable advantages over any other methodology (i.e. "top-down") for the construction of nanoscale functional materials and devices. This approach generally exploits the hierarchical self-assembly of functional molecules through multiple non-covalent interactions to prepare long range ordered and defect-free assemblies barely accessible through conventional covalent synthesis. However, an intrinsic drawback of investigating such systems in solution or in a crystal is that molecular components cannot be directly addressed on a nanometric scale. As a consequence, the best engineering methodology involves modifying the surfaces of bulk materials such as metals or semiconductors by deposition of functional organic materials. The modified surfaces are then characterised using scanning probe microscopies (e.g. STM, AFM). To this end, surface-confined, supramolecularly constructed, bi-dimensional (2D) networks, featuring regular porous domains (controllable both in shape and size) are of particular significance in this research domain because their cavities can be used as receptors for the confinement of other remotely controlled functional molecules (e.g. molecular switches, luminescent chromophores). Since these complex nanostructures could ultimately find applications as optoelectronic devices, research efforts in this domain have been gathering momentum in recent years. The crafting of complex specialised tools, represented by architectures at the molecular level, has been always considered a task for Nature. Nature's strategy consists of creating libraries of molecular elements with embedded information, so that a device for creating solar energy, e.g. a photosynthetic reaction centre, is spontaneously arranged or self-assembled from its elements. The RTN - PRAIRIES project has demonstrated that the understanding and formation of artificial molecular-sized architectures is possible. Through a molecular library conceived and characterised by partner nodes, the network has shown the first examples of extended bi-molecular architectures at surfaces. Such architectures, in conjunction with new molecular elements, will eventually make it possible to craft artificial tools at the molecular level, using the arrangement strategies in liquid media employed by Nature. The first artificial tools that can be envisaged are photovoltaics and sensors devices. In fact, the PRAIRIES network has achieved a new strategy for the conception of the new molecular elements needed to achieve the spontaneous arrangement of such complex artificial molecular devices, through the use of sophisticated theoretical supercomputer methods. Such a study, unique in its kind and literature, has given birth to a new area of studies, at the intersection of nanotechnology and supramolecular chemistry, which we have denoted supramolecular engineering.

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