Molecular carbon allotropes: cyclo[n]carbons

This thesis details the first structural characterisation of a cyclo[n]carbon. Carbon allotropes built from rings of two-coordinate atoms, known as cyclo[n]carbons, have fascinated chemists for many years, but until now they could not be isolated or structurally characterised, due to their high reactivity. This thesis describes two different routes to cyclo[18]carbon by atomic manipulation of a cyclocarbon oxide C24O6 and a bromocyclocarbon C18Br6 precursor on bilayer NaCl on Cu(111) at 5 K. It then discusses initial attempts towards the on-surface synthesis of antiaromatic cyclo[n]carbons. Chapter 1 reviews the history of synthetic carbon allotropes, in particular the cyclo[n]carbons, including all theoretical and experimental investigations into these molecular carbon allotropes. An introduction to the field of scanning probe microscopy is then detailed. Chapter 2 discusses the on-surface generation of cyclo[18]carbon by elimination of carbon monoxide from a cyclocarbon oxide molecule, C24O6. Characterisation of cyclo[18]carbon by high-resolution atomic force microscopy reveals a polyynic structure with defined positions of alternating triple and single bonds. The high reactivity of cyclocarbon and the cyclocarbon oxides allows covalent coupling by atom manipulation, which has significant promise for the synthesis of other carbon allotropes and carbon-rich materials from the coalescence of cyclocarbon molecules. Chapter 3 details the synthesis of cyclo[18]carbon by dehalogenation of a bromocyclocarbon precursor, C18Br6, in 64% yield. This method generates cyclo[18]carbon in a higher yield than from the cyclocarbon oxide, C24O6. The experimental images of C18 are compared with simulated images for four theoretical model geometries, including possible bond-angle alternation: D18h cumulene, D9h polyyne, D9h cumulene and C9h polyyne. Cumulenic structures with (D9h) and without (D18h) bond-angle alternation can be excluded. Polyynic structures with (C9h) and without (D9h) bond-angle alternation both show a good agreement with the experiment and are challenging to differentiate. Chapter 4 explores the synthesis and properties of a series of cyclocarbon oxides as precursors to antiaromatic cyclo[n]carbons. The synthesis of cyclocarbon oxide precursors to cyclo[24]carbon (C32O8) and cyclo[20]carbon (C28O8) is discussed however the on-surface sublimation of both molecules proves unsuccessful. Initial investigations into the synthesis of a cyclo[16]carbon precursor, C22O6, are also detailed. Finally, a number of possible future avenues for this project are described.