Calculations and model reactions show that a simple iterative Diels–Alder strategy is a viable method for the synthesis of most single-chirality carbon nanotubes.
Carbon nanotubes have properties that depend on their chirality — that is, how the tube is constructed if it is considered to be rolled up from a single sheet of a graphite-like structure. Present synthetic methods, however, deliver a mixture of all chiralities that must then be separated. The alternative approach would be to use organic synthesis to selectively build single-chirality carbon nanotubes.
Now, Larry Scott and co-workers from Boston College have shown1 using model reactions and calculations that a very simple synthetic strategy should be applicable to most types of carbon nanotubes. The strategy works because carbon nanotubes are just a specific type of polyaromatic hydrocarbons (PAHs), so once a short section of a nanotube is available, longer tubes can be built by repeated Diels–Alder reactions in the 'bay regions'. These reactions, however, are notoriously difficult because the loss of aromaticity causes a large activation energy to be set-up.
The calculations and model reactions — a Diels–Alder between a reactive acetylene and either perylene or a bisanthene — indicate that the large activation energy becomes less of a problem as the PAH increases in size. The search is now on for methods to synthesize the necessary endcaps — fairly large PAHs that should therefore be reactive — for a variety of different nanotubes.
References
Fort, E. H., Donovan, P. M. & Scott, L. T. Diels–Alder reactivity of polycyclic aromatic hydrocarbon bay regions: implications for metal-free growth of single-chirality carbon nanotubes. J. Am. Chem. Soc. 10.1021/ja907802g (2009).
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Davey, S. Diels–Alder delivers. Nature Chem (2009). https://doi.org/10.1038/nchem.464
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DOI: https://doi.org/10.1038/nchem.464
