Please do not adjust margins added as a DMSO solution (10 µL to a vesicle suspension of 5 mL) at t = 0 s, and chloride efflux was measured using a chloride ion-selective electrode (ISE). Experiments were conducted for 300 s before detergent was added to the vesicles providing the 100% Cl -efflux reading, which was used for calibration of the results. Dose response studies were performed followed by Hill analysis of Cl -efflux at 270 s to quantify transport activity. Dose response studies of compounds 1, 2, 6 and 7 could not be completed due to the insolubility of the receptors in aqueous solution at the required concentrations. These compounds proved less active than the other receptors tested, and addition as a DMSO solution at concentrations of 0.5 mol% or higher resulted in precipitation of the receptors. Solubility issues were also found with the para-SF5 receptors 5 and 10. The receptors both reached a maximum of 75% efflux (270 s) at the highest concentration tested, and convergence in the chloride efflux curves at higher transporter loadings was observed. These findings suggest that receptor 5 reaches its solubility limit at around 0.1 mol%, and 10 at closer to 1 mol%. However, as the final efflux is above 50%, an EC50 value could still be reliably calculated for these compounds. The calculated EC50 values, which represent the concentration required to facilitate 50% transport, and Hill coefficients, which provide an indication of the stoichiometry required for the transport process, are shown in Table . Efflux plots and fitting to the Hill equation for each compound are available in the supplementary information. Bis-urea compounds 3-5 were found to be considerably more active than their thiourea analogues 8-10, in contrast to trends previously observed for many families of anionophores. Typically, sulfur atoms enhance the acidity of the thiourea NH protons relative to the urea oxygen atoms, which leads to stronger coordination of a chloride anion and enhanced lipophilicity, allowing more efficient diffusion from the water/membrane interface. The EC50 values can again be rationalised by considering the planarity afforded in the 1:1 binding mode of the bis-urea compounds (molecular modelling can be viewed in the supplementary information, Figure ). The planar conformation offers a more complete encapsulation of a bound chloride anion and better shielding of the negative charge from the polar head groups of the lipid bilayer. Additionally, the bis-urea oxygens are held almost parallel to the acridinone plane due to the presence of intramolecular hydrogen bonds. The 1:1 binding mode of the thioureas is distorted due to the bulk of the sulfur atoms (Figure ). The bound chloride anion is more exposed to the external environment, and the bis-thiourea sulfur atoms sit almost perpendicular to the acridinone plane. The sulfur atoms are also exposed to the polar phospholipid head groups and the nonpolar bilayer interior, making transport less efficient. A similar conformation was observed for the anthracene-based bis-(thio)urea transporters which was attributed to the instability of the thiourea compounds in solution. The acridinone bisthiourea compounds in DMSO solution were found to slowly decompose but this does not happen on the time scale of the transport experiments. The potential for tautomerism of the bis-urea receptors to occur during the transport process cannot be discounted. The beneficial effect of strongly electron-withdrawing groups on transport activity is apparent for both the bis-urea and bisthiourea series of receptors. para-NO2 and 3,5-CF3 groups increased the binding affinity of the (thio)urea groups towards chloride, shown in Table and also facilitate a greater degree of delocalisation of the negative charge around the molecule, thereby enhancing the lipophilicity of the receptor:guest complex. Compounds 3 and 4 are highly effective chloride transporters. The EC50 value calculated for compound 3 of 0.0011 mol% ranks it among the most effective transporters reported by our group. For comparison, the most potent ortho-