Oligoynes and polyynes

In the sequence of carbon allotropes, diamond (sp 3 carbon) and graphite (sp 2 carbon) are followed by carbyne (sp carbon). In spite of intense research interest and effort, however, a verifiable example of carbyne remains elusive. In the absence of synthetic carbyne, polyynes have been targeted and synthesized as model systems to help predict the properties of carbyne. By the early 2000s, the efforts of several research groups had provided intriguing clues regarding carbyne, but polyynes longer than 10–14 acetylene units remained out of reach. Over the last 15 years, however, polyynes of previously unimaginable lengths have been synthesized and studied in both solution and the solid state. These advances have been achieved through synthetic developments, including rotaxination, in combination with a more thorough understanding of polyyne reactivity. With more complete series of polyynes in hand, several of the anticipated properties of carbyne have been confidently predicted based on the molecular model systems.