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Excited-State Proton Transfer in Confined Medium. 4-Methyl-7-hydroxyflavylium and β-Naphthol Incorporated in Cucurbit[7]uril B

Basílio, Nuno, Laia, César A. T., Pina, Fernando
The Journal of physical chemistry 2015 v.119 no.6 pp. 2749-2757
Cucurbitaceae, absorption, acidity, aqueous solutions, fluorescence, mathematical models, nuclear magnetic resonance spectroscopy, phenol, titration
Excited-state proton transfer (ESPT) was studied by fluorescent emission using a mathematical model recast from the Weller theory. The titration curves can be fitted with three parameters: pKₐ (acidity constant of the ground sate), pKₐₚ* (apparent acidity constant of the excited state), and ηA*, the efficiency of excited base formation from the excited acid. β-Naphthol and 4-metyhl-7-hydroxyflavylium were studied in aqueous solution and upon incorporation in cucurbit[7]uril. For all the compounds studied the interaction with the host leads to 1:1 adducts and the ground-state pKₐ increases upon incorporation. Whereas the ESPT of 4-methyl-7-hydroxyflavylium practically does not change in the presence of the host, in the case of β-naphthol it is prevented and the fluorescence emission titration curves are coincident with those taken by absorption. The position of the guest inside the host was investigated by NMR experiments and seems to determine the efficiency of the ESPT. The ESPT decreases for the guest, exhibiting a great protection of the phenol to the bulk water interaction.