Cation-π Interactions and Rattling Motion through Two-Dimensional Carbon Networks: Graphene vs Graphynes

We study the cation-pi interactions of alkali and alkaline earth cations with various forms of graphyne and graphdiyne and compare the results with those of graphene. The objective of the work is to explore the role of sp hybridized carbons present in various graphynes in determining the stabilities of the cation-pi complexes. The two-dimensional network materials, graphene and graphynes are represented by a series of model compounds and their superstructures. Systematic investigations using density functional theory with the dispersion-including M06-2X functional and a triple-zeta basis set reveal that all the cations have a stronger binding with graphynes than graphene. The binding strengths of various ions across the model systems follow the order: Be2+ > Mg2+ > Ca2+ > Li+ > Na+ > K+. Electrostatic potential and molecular orbital analyses are used to explain relative binding strengths at various active sites of model systems. Further, we investigated the passage of the cations through the pores of various graphynes by estimating the energy barriers and rates of diffusion. gamma-GY and rhombic GY are found to exhibit selectivity for the passage of certain cations. This opens up the possibility of these novel carbon materials to act as excellent platforms for ion filters.