The loss of endgroup effects in long pyridyl-endcapped oligoynes on the way to carbyne.

The versatility of carbon is revealed in its all-carbon forms (allotropes), which feature unique properties (consider the differences between diamond, graphite, graphene and fullerenes). Beyond natural sources, there are many opportunities to expand the realm of carbon chemistry through the study of new carbon forms. In this work, the synthesis of oligo-/polyynes is used to model the elusive carbyne. The chemical stabilization of oligoynes by sterically encumbered endgroups, particularly the 3,5-bis(3,5-di-tert-butylphenyl)pyridyl group, is key to assemble an extended series of stable oligoynes. The final member of this series is the longest monodisperse polyyne isolated and characterized so far, featuring 24 contiguous alkyne units (48 carbons). Spectroscopic and X-ray crystallographic analysis show that endgroups influence the properties of oligoyne derivatives, but this effect diminishes as length increases toward the polyyne/carbyne limit. For instance, with ultraviolet-visible spectroscopy, molecular symmetry clearly documents the evolution of characteristics from oligoynes to polyynes (in which endgroup effects are absent). The combined experimental data are used to refine predictions for the D∞h structure of carbyne.