Metal Ion-induced Supramolecular pKa Tuning and Fluorescence Regeneration of p-Sulfonatocalixarene Encapsulated Neutral Red Dye
The host-guest interactions and the consequent modulation in the prototropic equilibrium of a phenazine dye, neutral red, with p-sulfonatocalixarene (SCX4) and p-sulfonatocalixarene (SCX6) macrocyclic hosts have been investigated. Both the neutral (NR) and cationic (NRH+) forms of the dye formed inclusion complexes with SCX6, with the larger binding constant for the later (K = 8.6 × 105 M–1 versus 4.8 × 103 M–1) due to the cation receptor behavior of the calixarenes. The distinct differences in the binding constant of NR and NRH+ provided a finite tunning of the pKa between 6.5 and 8.8, through a competitive binding with metal ions. Importantly, the fluorescence quenching observed in the SCXs-Neutral Red interactions stand contrasting to the fluorescence enhancement observed with other macrocyclic hosts, such as -cyclodextrin and cucurbituril. This is due to the distinct portal stacking interaction of NRH+ with the SCXs, as compared to the axial inclusion geometry documented for the other macrocycles. Electron transfer from SCX to neutral red dye is adjudged to be the effective excited-state relaxation pathway leading to fluorescence quenching. In combination with the metal-ion induced fluorescence regeneration and tuning in the pKa value, the SCXs-Neutral Red system finds potential applications in drug delivery, photodynamic therapy, catalysis, and sensor applications.