Rigid-rod anion–π slides for multiion hopping across lipid bilayers

Abstract

Shape-persistent oligo-p-phenylene-N,N-naphthalenediimide (O-NDI) rods are introduced as anion–π slides for chloride-selective multiion hopping across lipid bilayers. Results from end-group engineering and covalent capture as O-NDI hairpins suggested that self-assembly into transmembrane O-NDI bundles is essential for activity. A halide topology VI (Cl > F > Br ∼ I, Cl/Br ∼ Cl/I > 7) implied strong anion binding along the anion–π slides with relatively weak contributions from size exclusion (F ≥ OAc). Anomalous mole fraction effects (AMFE) supported the occurrence of multiion hopping along the π-acidic O-NDI rods. The existence of anion–π interactions was corroborated by high-level ab initio and DFT calculations. The latter revealed positive NDI quadrupole moments far beyond the hexafluorobenzene standard. Computational studies further suggested that anion binding occurs at the confined, π-acidic edges of the sticky NDI surface and is influenced by the nature of the phenyl spacer between two NDIs. With regard to methods development, a detailed analysis of the detection of ion selectivity with the HPTS assay including AMFE in vesicles is provided.