Proton NMR titrations were conducted with compounds 1 -10 in DMSO-d6/0.5% D2O and tetrabutylammonium chloride (TBA Cl). Solutions were titrated with up to 300 equivs. of TBACl. Large downfield shifts in the urea or thiourea NH resonances were observed upon addition of TBACl for all the compounds. Interestingly, the resonance attributed to the acridinone N-H proton of bis-urea compounds 1-5 responded differently to the addition of TBACl compared to the corresponding resonance in the bis-thiourea compounds 6-10. Stacked plots of the titrations of each compound with TBACl are shown in the supplementary information. For titrations conducted with the bis-urea compounds the acridinone signal initially shifted slightly upfield, before reversing and moving downfield above approx. 10 equiv. of the TBACl. During the Cl -titrations of the bis-thiourea compounds, the acridinone N-H proton resonance shifted downfield initially before plateauing at higher equivalents.