Spectral properties and structures of supramolecular complexes of naphthylpyridine with β-cyclodextrin

Abstract

The electronic absorption and fluorescence spectra of 4-(2-naphthyl)pyridine (1) and N-methyl-4-(2-naphthyl)pyridinium perchlorate (2+·ClO4−) were studied in aqueous solutions in the absence and presence of β-cyclodextrin (β-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+·ClO4−. In an aqueous solution, compound 1 and β-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 ∼2·103 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 β-CD cavity. The quantum-chemical calculations showed that the insertion of 1 into the β-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 β-CD cavity; cation 2+ does not form a complex with β-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 β-CD dimer constructed according to the head-to-head type.