pH‐Responsive Interaction of Fluorogenic Antimalarial Drug Quinine with Macrocyclic Host Cucurbit[7]uril: Modulations in Photophysical and Acid‐Base Properties

Intriguing changes in photophysical and acid‐base properties of fluorogenic antimalarial drug, quinine (QN), are presented on its interaction with macrocyclic cucurbit[7]uril (CB7)host. While dicationic QNH22+ form shows large enhancement in both steady state and time‐resolved fluorescence, monocationic QNH+ and neutral QN forms show significant reductions. The pKa(1) and pKa(2) values of the drug undergo ∼1 and ∼1.7 units of upward shifts, respectively, on its binding to CB7 host, substantiating the differential binding strengths of various prototropic forms. Whereas the QNH22+ and QNH+ forms undergo 1:2 (dye to CB7) inclusion complex formation, the neutral QN is suggested to form 1:1 inclusion complex with CB7.Interestingly, the observed results in the present study are substantially different and in some cases quite contrasting to the results reported in a recently published paper (Phys. Chem. Chem. Phys., 2016, 18, 30520). In the present study we could unequivocally establish that there were many serious inconsistencies in the above paper, which we could now convincingly rectify, designing appropriate experimental and sample conditions. With the intriguing modulations observed in the photophysical and acid‐base properties, the quinine‐CB7 system is suggested to act as an improved fluorimetric standard than the free quinine system. Additionally, the modulations have implications in drug stabilization, drug delivery, sensors and other applications. Interaction of different prototropic forms of fluorogenic antimalarial drug, quinine, with cucurbit[7]uril leads to intriguing modulations in photophysical and acid‐base properties of the drug. The results have direct implications towards drug stabilization, drug delivery, sensor applications and also as improved fluorimetric standard.