A review of functional linear carbon chains (oligoynes, polyynes, cumulenes) and their applications as molecular wires in molecular electronics and optoelectronics

Linear, sp-hybridised, 1-dimensional (1D) all-carbon wires are conceptually the simplest pi-conjugated organic molecules. Their fundamental experimental and theoretical properties and their materials chemistry applications are under-developed compared to many other pi-conjugated systems. The emphasis of this review is on the emerging applications of functional oligoynes and polyynes in molecular electronics and optoelectronics. The molecules considered herein contain at least two contiguous acetylene units (that is, a conjugated diacetylene or butadiynyl fragment). Relevant examples of cumulenes will also be included. Oligoynes/polyynes/cumulenes represent unique carbon nanostructures, among the so-called "synthetic carbon allotropes". The molecules are structurally versatile because the extent of conjugation and the electronic and photophysical properties can be tailored by judicious selection of end-groups, the number of alkyne units, and the option to incorporate metal atoms into the backbone. Their applications will be considered as molecular wires in electrode|(single)-molecule|electrode architectures, and as active components for non-linear optics, electrochromism, redox-activity, organic light emitting devices, photoinduced electron transfer systems, liquid crystals, bioimaging and biosensing, with emphasis on the role played by the special longitudinal properties of the carbon-wire units. Four to six contiguous triple bonds is generally the length limit of oligoynes for practical applications, due to the synthetic challenges and inherent instability of longer homologues under ambient conditions. The review will also outline appealing future directions for next-generation linear, sp-hybridised, all-carbon molecular wires with new and enhanced properties.