A ditopic receptor for cation binding and facilitated transport through a supported liquid membrane

The binding properties of an artificial receptor towards a series of cations including Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Ba²⁺, Fe²⁺ and Al³⁺ in acetonitrile are described. The receptor comprises a photo-responsive pyrene unit connected via a short spacer to a 2,2′:6′,2″-terpyridine metal ion binding site. Interaction of cations with the receptor was monitored by changes in absorption profile and the association constants calculated for 1:1 and 1:2 cation:ligand binding fall within the range log β = 3-12. The receptor is highly fluorescent and quenching of the emission is observed upon cation binding. The potassium picrate transport properties of the membrane-bound receptor are also described. This receptor when immobilised in a polymer support, which separates two aqueous solutions, has been shown to transport potassium ions in the dark with a flux rate of 1.5 × 10⁸ mol/s m². In contrast, when the membrane-bound receptor is selectively illuminated with light (λ > 400 nm), the flux increases to 2.0 × 10⁸ mol/s m². The transport efficiency depends on the nature of the trap used in the receiver phase.