To further verify the nature of interactions between ADBr with THF and TEA, respectively, an isothermal titration calorimetry analysis was carried out for both systems of ADBr /THF and and ADBr at a 1:1 ratio (Figure ). It has to be noted that there is a trace amount of impurities, the NMR spectrum of which were compared with that of acridone (Figure ) in the aromatic region giving a positive match. This could provide a clue to the whereabout of the TEA addition site, i.e., the carbon para to the acridinium nitrogen, given that acridone has been known to form following nucleophile addition and subsequent oxidization in the presence of water and oxygen. To substantiate the hypothesis of TEA addition induced acridone formation, 1 H-NMR and UVvis spectra were employed. As shown in Figure , the 1 H-NMR spectrum of ADBr/TEA in d6-DMSO after 10 min could be restored to the initial state, by way of neutralization from a strong acid such as trifluoroacetic acid (TFA) or hydrochloric acid (HCl). However, the acridone impurity was stable with TEA, which was also verified with 1 H-NMR (Figure ).