The Carbon Factory: A laboratory for the chemistry of carbon-rich systems

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In adopting the concept of Moore's Law , the semiconductor industry has underwritten 40 years of societal growth through the provision of immense computing power. The International Technology Roadmap for Semiconductors informs researchers of short and long-term research targets necessary to maintain this growth. The 2007 edition of the roadmap predicts geometric scaling of components will reach the limits of the solid state in 10 - 15 years, and highlights the need to develop hybrid solid-state / molecule devices. Although forecasts of the demise of the top down approach are not new, it is remarkable to see the semiconductor industry endorse hybrid device architectures (i.e. those in which molecular structures are integrated within a CMOS device) as targets within the foreseeable future. To meet these challenges, a firm understanding of how an electron can be manipulated within and transferred around a molecular structure will be required, and how such systems can be immobilised onto semiconductor surfaces will be required.Much of our understanding of intramolecular electron transfer originates from studies of linear compounds M(a)-B-M(b) in which two sites, M(a) and M(b) identical in every respect except charge state, are linked by some bridging structure, B. The fundamental principles of intramolecular electron transfer in these mixed valence compounds have been systematically mapped over the last 40 years, with the importance of the interactions between the sites M(a) and M(b) and the bridge B being clearly identified. Conjugated carbon-rich bridges have been identified as being particularly useful in the construction of mixed-valence systems with strong coupling interactions between the M(a) and M(b) sites.The facile electron transfer processes in linear, carbon-bridged mixed-valence compounds have led to significant interest in such systems as molecular-scale wires in electronic device applications. However, it could be reasonably argued that a molecular-sized wire is not sufficient, and higher-level function must be incorporated into the molecular component if true advantages over conventional solid state devices are to be realised. The concepts of charge-transfer in 2-D and 3-D molecular systems have attracted attention from the point of view of designing molecular materials for transistor-like molecular electronic components and molecular switches in which the state of one site can influence the nature of the interactions between the others. Multi-site mixed-valence complexes have been proposed as elements for the construction of quantum cellular automata (QCA)-based logic gates and memory cells. Despite the conceptual simplicity of branched (e.g. X and Y shaped) mixed-valence systems, it has proven difficult to prepare a branched ligand core that permits strong electronic coupling between multiple (more than two) remote sites, and relatively little is known about the mechanisms of charge transfer in these systems. In contrast to linear M(a)-B-M(b) systems, mixed valence systems featuring more than two redox sites, termed here multi-site for convenience, are not well-represented, and generally limited to linear arrays M(a)-B-M(b)-B-M(c). This proposal sets out to develop the synthetic chemistry associated with branched carbon-based bridging ligands and multi-site mixed-valence compounds, studies of electron-transfer within these 2-D conjugated compounds, and mechanisms through which these complexes can be attached to semi-conductor surfaces. A number of aside projects are also described, which initiate new lines of investigation in the chemistry of the longer cumulenes, the role of carbon-rich organometallics in the synthesis of fullerenes and carbon nanotubes, discotic liquid crystals comprised of redox-active mesogens and the role of vibrational coupling on electron-transfer reactions.