Mechanistic Insights into Ion Transport and Selectivity in Membrane embedded Ligand-Appended Pillar[5]arene and Pillar[6]arene Channels

Ligand-appended pillararene (LAP) channels offer a versatile platform for ion-selective membranes. LAP5 and LAP6, composed of five and six aromatic units, respectively, exhibit distinct ion transport behaviors due to differences in ring size. We investigated three LAP5 and three LAP6 channels functionalized with diglycolamine (DGA), carbamoylmethyl phosphine oxide (CMPO), and propionamide phosphonic acid (PropPhos), chosen for their known affinities toward lanthanides. The permeability and selectivity of each channel were evaluated for Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, La³⁺, Eu³⁺, and Yb³⁺ using potential of mean force (PMF) calculations, which were converted into permeability coefficients to quantify ion selectivity. Our results show that permeability differences between LAP5 and LAP6 strongly depend on ring edge size which is related to ligand chemistry. For CMPO and DGA channels, increasing the ring size from LAP5 to LAP6 leads to higher permeabilities for all ions, as the edge region expands while PropPhos exhibits the opposite trend, with a reduced edge size in LAP6 leading to lower permeabilities. Ion selectivity further correlates with the ratio of ion coordination to channel versus water molecules, reflecting competition between hydration and ligand coordination. Monovalent ions, which are readily dehydrated, show selectivity primarily governed by the ligand's hydrophobic/hydrophilic environment. In contrast, divalent ions and trivalent lanthanide ions maintain strong hydration shells, and their selectivity arises from the balance between ion–water and ion–channel interactions.