Photophysical and Theoretical Studies on the Stereoselective Complexation of Naphthylethanols with β-Cyclodextrin

Abstract

Stereoselective binding of the 1-naphthyl-1-ethanol (1-NpOH) and 2-naphthyl-1-ethanol (2-NpOH) enantiomers with β-cyclodextrin (β-CD) was investigated by photophysical and theoretical studies. The latter are based on structural calculations of the complexes and their respective induced circular dichroism spectra. For the enantiomers of 1-NpOH, where the complexes with β-CD have a 1:1 (guest:CD) stoichiometry, a mixture of structures with different geometries was observed for each enantiomer. No stereoselectivity was apparent for these complexes. (R)- and (S)-2-NpOH form complexes with β-CD that have both 1:1 and 2:2 stoichiometries. Structural calculations indicate that the naphthyl moiety of both enantiomers is preferentially included in the CD cavity for the 1:1 complex. In the case of the complexes with 2:2 stoichiometry, a mixture of complexes is present that either have the naphthyl moieties at a distance or in close proximity. The former geometry explains the observation of long-lived excited triplet states for 2-NpOH, whereas the latter geometry is responsible for the excimer emission. The stereoselectivity observed for the monomer to excimer intensity ratios when (R)- and (S)-2-NpOH are complexed to β-CD is due to the different contributions of 2:2 complexes with different geometries.