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 (Keq =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 (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 (Keq =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 pKa 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.