Cyclic and linear acetylenic molecular scaffolding

During the past decade, the construction and investigation of expanded acet ylenic pi-chromophores has become a central area of chemical research. It h as been fueled by the availability of new synthetic methods, in particular Pd(0)-catalyzed cross-coupling reactions, the discovery of the antitumor ac tivity of a series of natural compounds possessing reactive enediyne pi-chr omophores, and the need for new nanoscale molecular and polymeric materials that exhibit unusual electronic and optical functions and properties. In t his review, synthetic approaches to the cyclo[n]carbons (cyclo-C-n), n-memb ered monocyclic rings of sp-hybridized C-atoms with unique electronic struc tures resulting from two perpendicular systems of conjugated rr-orbitals - one in-plane and one out-of-plane - are presented. In the following section s, the syntheses and properties of perethynylated molecules, constituting a versatile "molecular construction kit" for acetylenic molecular scaffoldin g, are discussed. Examples of such compounds are perethynylated annulenes, radialenes, olefins and cumulenes, or transition metal complexes. The artic le concludes by outlining advances ion novel acetylenic polymers, such as p oly(triacetylene), the third linearly conjugated polymer with a non-aromati c, all-carbon backbone.