Molecules featuring a conjugated sequence of sp-hybridized carbon atoms can take the form of either oligoynes or [n]cumulenes. The two classes of molecules differ structurally based on the terminal carbons of the conjugated framework, namely sp-carbons for oligoynes and sp2-carbons for cumulenes. In shorter derivatives, the so-called endgroups often play a defining role in the structural-property characteristics of oligoynes and [n]cumulenes. At longer lengths, however, the influence of endgroups diminishes as the effective conjugation length is reached, and properties reach a consistent value that is independent of length. In the case of oligoynes and [n]cumulenes, this point would provide a polymer (e.g., a polyyne) with characteristics of carbyne, the linear allotrope of carbon derived from sp-hybridized carbon. The molecular length necessary for oligo-/polyynes and [n]cumulenes to model carbyne remains, however, unknown. To study polyynes and [n]cumulenes, the development of synthetic methods that overcome and/or circumvent the kinetic instability of longer molecules must be addressed. We have devoted significant effort toward the synthesis of polyynes and [n]cumulenes with unprecedented lengths, yet the properties of carbyne remain elusive (at least when not encased within a nanotube). Nevertheless, our path toward carbyne has revealed many intriguing properties for oligoynes, polyynes, and [n]cumulenes. For example, it is often posited that steric shielding is crucial toward stabilization of oligoynes and [n]cumulenes by preventing intermolecular cyclization reactions. Our studies show, however, that steric bulk is equally important toward preventing other decomposition pathways in [n]cumulenes and polyynes. We have also demonstrated that molecular symmetry plays a significant role in the electronic absorptions as one progresses from oligoynes to polyynes, while symmetry in [n]cumulenes can be used to explore electrohelicity. Finally, appropriately substituted oligoynes or [n]cumulenes can function as molecular wires as a result of their linear, conjugated structures. This presentation will explore selected synthetic, electronic, and structural properties of sp-hybridized molecules we have discovered on our journey toward carbyne.

Modelling carbyne with sp-hybridized carbon oligomers and polymers

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ACS Fall 2022

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