Spectral properties and structures of supramolecular complexes of naphthylpyridine with beta-cyclodextrinстатья
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Дата последнего поиска статьи во внешних источниках: 18 июля 2013 г.
Аннотация:The electronic absorption and fluorescence spectra of 4-(2-naphthyl)pyridine (1) and N-methyl-4-(2-naphthyl)pyridinium perchlorate (2(+)center dot ClO4-) were studied in aqueous solutions in the absence and presence of beta-cyclodextrin (beta-CD). In aqueous solutions and organic solvents in the presence of water or H+ ions, compound 1 exhibits intense fluorescence with a maximum at 21 270 cm(-1), and its quantum yield in an aqueous solution is 0.9 +/- 0.09. The same fluorescence spectrum was detected for an aqueous solution of 2(+)center dot ClO4-. In an aqueous solution, compound 1 and beta-CD form stable fluorescing supramolecular 2 : 2 complexes, whose structure was calculated by the quantum-chemical MNDO/PM3 method. The formation of these complexes induces a hypsochromic shift of the fluorescence maximum of 1 by 5000 cm-1. The stability constant of the complex is similar to 2 (.) 10(3) L mol(-1). A decrease in the pH results in the formation of a protonated form of 1 (1 (.) H+) and destruction of the complex, thus favoring the escape of the substrate from the beta-CD cavity. The quantum-chemical calculations showed that the insertion of I into the beta-CD cavity is thermodynamically more favorable than hydration; on the contrary, the formation of 1 (.) H+ increases dramatically the hydration energy, which promotes the escape of 1 (.) H+ from the beta-CD cavity; cation 2(+) does not form a complex with beta-CD; in the thermodynamically most favorable 2 : 2 complex, the naphthalene fragments of two molecules 1 are parallel to each other in a broad section of the beta-CD dimer constructed according to the head-to-head type.