Molecular‐Recognition‐Assisted p K a Shifts and Metal‐Ion‐Induced Fluorescence Regeneration in p ‐Sulfonatocalix[6]arene‐Encapsulated Acridine

The host–guest interactions of cationic (AcH + ) and neutral (Ac) forms of the dye acridine with the macrocyclic host p ‐sulfonatocalix[6]arene (SCX6) were investigated by using ground‐state absorption, steady‐state and time‐resolved fluorescence, and NMR measurements. The cationic form undergoes significant complexation with SCX6 ( K eq =2.5×10 4   M −1 ), causing a sharp decrease in the fluorescence intensity and severe quenching in the excited‐state lifetime of the dye. The strong binding of the AcH + form of the dye with SCX6 is attributed to ion–ion interactions involving the sulfonato groups (SO 3 − ) of SCX6 and the positively charged AcH + at pH of approximately 4.3. Whereas, the neutral Ac form of the dye undergoes weak complexation with SCX6 ( K eq =0.9×10 3   M −1 ) and the binding constant is lowered by one order of magnitude compared with that of the SCX6–AcH + system. The strong affinity of SCX6 to the protonated form leads to a large upward p K a shift (≈2 units) in the dye. In contrast, strong emission quenching upon SCX6 interaction and the regeneration of fluorescence intensity of the dye in the presence of Gd 3+ through competitive binding have also been demonstrated.