ethynylene

Ethynylene is the divalent fragment derived from acetylene by removing both terminal hydrogens, giving the linear -C≡C- unit that can link two organic groups. In practice, ethynylene refers to the fragment R−C≡C−R′, where two substituents occupy the ends of a carbon–carbon triple bond. The unit is strictly linear at the C≡C axis and the two carbons are sp-hybridized; the triple bond provides a short, rigid linkage that can contribute to conjugation in π systems.

Ethynylene groups are widely used as linking units in conjugated materials, notably polyphenylene ethynylene polymers, which consist of phenyl rings connected by ethynylene bridges. These materials are studied for optoelectronic applications such as light-emitting diodes and molecular wires. In synthesis, ethynylene linkages are typically introduced by forming C≡C bonds through cross-coupling reactions, including Sonogashira coupling between aryl or vinyl halides and terminal alkynes, or related methods that build the acetylenic linkage.

Reactivity: terminal ethynyl groups (−C≡C−H) participate in various cycloadditions and coupling processes; the internal ethynylene unit (−C≡C−) serves as a rigid spacer that can influence molecular planarity and electronic communication between adjacent units.

See also: ethynyl, acetylene, polyphenylene ethynylene, Sonogashira coupling.