Please do not adjust margins hydrogen bond to chloride is less deshielding than the intramolecular interaction with the carbonyl oxygen). At higher host/guest ratios, the equilibrium shifts to a 1:2 receptor:guest complex via another conformational switch. This time both urea carbonyl groups flip inwards, and each bind a chloride anion through N-H hydrogen bonds on the exo-face of the complex (Scheme 2a bottom most structure). The acridinone N-H proton resonance begins to shift back downfield due to the reformation of intramolecular hydrogen bonding interactions with the urea carbonyl oxygens. A similar conformational change was proposed for the binding modes of carbazole-based bis-ureas and bis-thioureas. The thiourea groups of compounds 6-10 are initially held closer to perpendicular to the acridinone motif (Scheme 2b, top structure), in a manner similar to the ground state computational studies performed by Dias et al. on their anthracene-based thiourea compounds. At lower equivalents of TBACl, a 1:1 receptor:guest complex is preferred with a chloride anion held within the central cavity (Scheme 2b, middle structure). Hydrogen bonding interactions between chloride and all thiourea N-H protons and the acridinone N-H proton cause the 1 H-NMR resonances for these protons to collectively shift downfield. At higher equivalents of guest, each thiourea group binds a chloride anion individually as the equilibrium shifts towards a 1:2 receptor:guest complex (Scheme 2b, bottom most structure). The bulky nature of the sulfur atoms means the thiourea groups are oriented perpendicular to the acridinone motif. An intramolecular bonding interaction does not form between the thiourea sulfur atoms and the acridinone N-H proton due to the steric bulk and poorer hydrogen bond acceptor ability of sulfur, and hence the signal attributed to the acridinone N-H proton begins to plateau. The acridinone N-H resonance should in fact move upfield, but the minimal shift experienced by this signal reflects the small amount of 1:2 complex that forms. The interaction parameter, α (Table ), was calculated for receptors 1-10 and found to be less than 1 in each case, indicating that the 1:1 complex is heavily favoured over the 1:2 complex (negative cooperativity). This, in concurrence with the proposed binding modes presented in Scheme 2, suggests a cooperative binding site featuring multiple hydrogen bonding interactions encapsulating chloride is present in the 1:1 binding complex. The shift to a 1:2 binding complex removes this cooperativity, as reflected in the much smaller K12 values compared to K11 values for the acridinone series. An assessment of the K11 values suggests that the bis-urea compounds exhibit a higher affinity towards chloride than their bis-thiourea analogues, despite the enhanced acidity of the thiourea protons. The higher binding affinity can be attributed to the planarity in the molecule necessary for optimal binding in the 1:1 complex. The smaller oxygen atoms of the bis-urea receptors are more capable of achieving the required planarity than the bulky thiourea sulfur atoms, which distort the shape of the binding site. A similar phenomenon was observed for bis-(thio)urea based receptors. The trend in K11 values for both series of receptors is in agreement with the expected trend when increasingly electron-withdrawing substituents are appended to the peripheral phenyl rings. The parent phenyl receptors 1 and 6 exhibit the lowest affinities for chloride. The K11 values for receptors 2 and 7 are smaller than the K11 values for 5 and 10, respectively, highlighting the enhanced electron-withdrawing ability of the para-SF5 group compared to a para-CF3 group. The para-NO2 (3 and 8) and 3,5-bis-CF3 (4 and 9) derivatives displayed the highest affinities for chloride, which is consistent with the increased acidity of their (thio)urea protons, induced by the strong electron-withdrawing nature of these substituents.