Systematic density functional cluster model calculations reveal that the [2 + 2] cycloadditions of ethylene on the (100) surfaces of group IV semiconductors (silicon, germanium, and diamond) follow diradical mechanisms. Details of the reaction pathways vary subtly with the bonding nature of the surface dimers, including their zwitterionicity, weakness of their π-bond, and readiness to react as a diradical.

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Formation of a bilayer or a multilayer organic thin film on semiconductor surfaces is promising for the manufacture of new-generation microelectronic/nanoelectronic materials. A prerequisite for realization of such a goal is the formation of monolayer organic thin films with reactive functional groups exposed as a template for further chemical manipulations. We report herein a theoretical prediction that attachments of diacetylenes on Si(100)-2 × 1, Ge(100)-2 × 1, and Si(111)-7 × 7 surfaces could produce monolayers of reactive [3]cumulenic or/and enynic surface species. Our density-functional cluster-model calculations revealed the following:  (i) Diacetylene can undergo either stepwise [4 + 2]-like or stepwise [2 + 2]-like cycloadditions onto a XX dimer of X(100)-2 × 1 surfaces (X = Si and Ge) via a common singlet−diradical intermediate. (ii) On Si(100), the [2 + 2]-like pathway is kinetically favored over the [4 + 2]-like one, preferentially giving rise to enynic adspecies. (iii) On Ge(100), both pathways are competitive, resulting concomitantly in [3]cumulenic and enynic adspecies. (iv) On Si(111)-7 × 7, diacetylene can undergo either 1,4 addition or 1,2 addition onto a rest atom−adatom pair with the former process being favored over the latter both thermodynamically and kinetically and leading preferentially to the formation of [3]cumulenic adspecies on the surface. It is noteworthy that the [4 + 2]-like cycloaddition of diacetylene on Ge(100) 2 × 1 is among the few examples of [4 + 2] cycloaddition with a 1,3-diyne acting as a four-electron component affording six-membered cyclic [3]cumulenes.