Pseudorotaxane complexes of naphthylpyridines and naphthylbipyridyl with β-cyclodextrin and hydroxypropyl-β-cyclodextrin

The electronic absorption spectra and fluorescence spectra of 4-(2-naphthyl)pyridine (1), 2-(4-methyl-2-pyridyl)-4-(2-naphthyl)pyridine (2), and 4-(2-naphthyl)-2-phenylpyridine (3) in solutions and in complexes with beta-cyclodextrin (beta-CD) and well water-soluble hydroxypropyl-beta-cyclodextrin (HP-beta-CD) were studied. Fluorescence near 475 nm observed in aqueous solutions of compounds 1-3 arises from protonated forms of these compounds produced in the excited state. Results of DFT quantum chemical calculations show an increase in proton affinity energies of excited-state naphthylpyridines 2 and 3. The formation of inclusion complexes with cyclodextrins makes protonation of compounds 2 and 3 more difficult, which manifests in large hypsochromic shifts of fluorescence band maxima. The stability constants of the complexes 1 (.) HP-beta-CD and 2 (.) HP-beta-CD determined from their fluorescence spectra are 3425 and 3760 L mol(-1), respectively. The stability constant of the complex 3 (.) HP-beta-CD (5500 +/- 600 L mol(-1)) was found from the changes in the solubility of naphthylpyridine 3 in water upon complexation. Semiempirical quantum chemical calculations of the molecular structures and thermodynamic characteristics of pseudorotaxane inclusion complexes of trans-2, cis-2, and trans-2 (.) H2O with HP-beta-CD were carried out.