Spectroscopic properties, structure, and photoinduced motion of 4-(2-naphthyl)pyridine in cyclodextrin cavitiesстатья

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[1] Spectroscopic properties, structure, and photoinduced motion of 4-(2-naphthyl)pyridine in cyclodextrin cavities / V. B. Nazarov, V. G. Avakyan, S. P. Gromov et al. // Russian Chemical Bulletin. — 2010. — Vol. 59, no. 5. — P. 941–953. Spontaneous and photoinduced protonation of 4-(2-naphthyl)pyridine (1) in solutions and in complexes with beta-cyclodextrin (beta-CD) and 2-hydroxypropyl-beta-cyclodextrin (HP-beta-CD) was studied using the absorption and fluorescence spectroscopies. The structures and stabilities of complexes of compound 1 and its quaternized derivative, 1-methyl-4-(2-naphthyl)pyridinium perchlorate (3), with beta-CD and HP-beta-CD were examined by (1)H NMR titration (logK = 1.5-2.3). The molecule of naphthylpyridine 1 is always in the cyclodextrin cavity, regardless of the pH value of the solution. 2-Hydroxypropyl-beta-cyclodextrin binds better the neutral form of compound 1 than does beta-CD, while naphthylpyridinium salts exhibit nearly equal affinities to both cavitands. According to spectroscopic data, pK (a) (1) is 5.12 in water, which favors protonation of the N atom both in the ground and excited states; as a result, the fluorescence spectrum exhibits only the band of the protonated form with a lifetime of 15 ns. The addition of HP-beta-CD to a solution of naphthylpyridine 1 results in the formation of inclusion complex 1@HP-beta-CD, lowers pK (a) to 4.62, and gives rise to a fluorescence band of the nonprotonated form of compound 1 with a lifetime of 1.25 ns. Therefore, the presence of compound 1 in the HP-beta-CD cavity precludes its protonation in the excited state. The initial portions of the fluorescence curves for compound 1 in solution and in its complex with HP-beta-CD obtained upon pulsed excitation were compared to propose the initiation mechanism of short-lived fluorescence of the nonprotonated form of naphthylpyridine 1. Quantum chemical modeling of the protonation and complexation of compound 1 in the presence of water was performed. Based on the results obtained, a reversible photoinduced mechanical motion of naphthylpyridine 1 in the HP-beta-CD cavity was suggested. [ DOI ]

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