The shift in the bis-urea acridinone NH proton resonance and evidence of a 1:2 receptor:guest binding process can be rationalised by considering that an intramolecular hydrogen bond may form in the free receptor when the urea carbonyl is orientated inwards (Scheme 2a, top structure), akin to the solidstate structure of compound 3 (Figure ). The same intramolecular hydrogen bond may not be possible in the bisthiourea compounds 6-10 because the bulk of the sulfur atom means the thiourea cannot flip inwards as effectively and sulfur is a poorer hydrogen bond acceptor than oxygen (Scheme 2b). Therefore, the 1 H-NMR signal attributed to the acridinone NH proton is shifted further downfield for the bis-urea compounds compared to the bis-thiourea compounds due to deshielding of the proton caused by the intramolecular hydrogen bond. At low equivalents of TBACl, a 1:1 receptor:guest complex is preferred where the urea carbonyl group flips back to the exterior of the molecule and chloride is bound through N-H hydrogen bonds within the central cavity (Scheme 2a, middle structure). This binding event induces a downfield shift in the signals of the urea protons, which form cooperative hydrogen bonds with chloride, and an upfield shift in the acridinone N-H proton signal due to disruption of the intramolecular hydrogen bond formed with the urea carbonyl oxygen (presumably the