Polyyne

A polyyne is an organic compound with a carbon chain containing alternating single and triple carbon–carbon bonds. Its conjugated chain can have distinctive electronic and optical properties.

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Alkyne: An alkyne is a hydrocarbon containing at least one carbon–carbon triple bond. Each triple bond in a polyyne chain is an alkyne unit.
Polyynic carbon chain: A linear sequence of carbon atoms linked by alternating single and triple bonds. This chain is the central structural unit shared by polyynes.
Carbyne: A hypothetical or transient linear carbon allotrope consisting of an extended chain of carbon atoms. Carbyne is the infinite-chain ideal often compared with finite polyynes.
Polyene: An organic compound containing multiple carbon–carbon double bonds. Polyynes alternate single and triple bonds, unlike polyenes' double-bond sequences.
Carbon–carbon bond: A covalent bond joining two carbon atoms, formed by sharing one or more pairs of electrons. Polyyne structure alternates single and triple carbon–carbon bonds.
Peierls transition: A distortion of a one-dimensional equally spaced lattice that lowers its energy by opening an electronic band gap. Bond alternation in an ideal carbon chain is related to Peierls distortion.
Carbon nanotube: A cylindrical nanostructure formed from a rolled sheet of graphene. Polyynes can be encapsulated inside nanotubes, which help protect them.
Cumulene: A compound containing consecutive cumulated double bonds between carbon atoms. Cumulenes have adjacent double bonds rather than polyyne bond alternation.
Carbon hybridization: A model describing how atomic orbitals combine to form bonding orbitals and molecular geometry. The triple-bonded carbons in polyynes are sp-hybridized.
Electronic band structure: The ranges of allowed electron energies in a periodic solid, together with forbidden energy gaps. Long carbon chains can show length-dependent electronic gaps.