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Metal Ion-induced Supramolecular pKa Tuning and Fluorescence Regeneration of p-Sulfonatocalixarene Encapsulated Neutral Red Dye

Abstract

The host-guest interactions and the consequent modulation in the prototropic equilibrium of a phenazine dye, neutral red, with p-sulfonatocalix[4]arene (SCX4) and p-sulfonatocalix[6]arene (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 cucurbit[7]uril. 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.

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Publication details

The article was received on 28 Feb 2017, accepted on 19 Apr 2017 and first published on 19 Apr 2017


Article type: Paper
DOI: 10.1039/C7OB00506G
Citation: Org. Biomol. Chem., 2017, Accepted Manuscript
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    Metal Ion-induced Supramolecular pKa Tuning and Fluorescence Regeneration of p-Sulfonatocalixarene Encapsulated Neutral Red Dye

    M. N. Shinde, R. Khurana, N. Barooah, A. C. Bhasikuttan and J. Mohanty, Org. Biomol. Chem., 2017, Accepted Manuscript , DOI: 10.1039/C7OB00506G

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