Molecular-Recognition-Assisted pKa 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=2.5x10(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 (SO3-) 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=0.9x10(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 pK(a) shift (approximate to 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 Gd3+ through competitive binding have also been demonstrated.