Toward carbyne: Synthesis and stability of really long polyynes

Molecules composed of sp-hybridized carbon chains (polyynes) are the simplest of the known conjugated organic oligomers. In comparison to their counterparts such as polyacetylene and polydiacetylene, however, the formation of polyynes has traditionally posed a difficult synthetic challenge. In particular, there is no reliable method to form end-capped polyethynylene, and monodisperse polyynes have therefore been assembled. As a result, structure–property relationships for shorter polyynes have been relatively well established in recent years, while extension of these trends toward longer polyynes has remained a difficult task. Using the Fritsch–Buttenberg–Wiechell (FBW) rearrangement, the formation of diynes through decaynes has become possible and has provided a unique chance to explore the physical characteristics of conjugated polyyne chains. This paper highlights recent advances in the synthesis of extended polyynes, as well as interesting aspects of their NMR, Raman, and UV/vis spectroscopic analyses. These synthetic achievements offer the opportunity to predict some of the properties of the carbon allotrope carbyne. In particular, a set of X-ray crystallographic analyses of t-Bu end-capped polyynes (tBu[n]) shows a definitive experimental trend in reduced bond-length alternation (BLA).