Photophysical and theoretical studies on the stereoselective complexation of naphthylethanols with beta-cyclodextrin

Stereoselective binding of the l-naphthyl-l-ethanol (1-NpOH) and 2-naphthyl -1-ethanol (2-NpOH) enantiomers with beta -cyclodextrin (beta -CD) was inve stigated by photophysical and theoretical studies. The latter are based on structural calculations of the complexes and their respective induced circu lar dichroism spectra. For the enantiomers of 1-NpOH, where the complexes w ith beta -CD have a 1:1 (guest:CD) stoichiometry, a mixture of structures w ith different geometries was observed for each enantiomer. No stereoselecti vity was apparent for these complexes. (R)- and (S)-2-NpOH form complexes w ith beta -CD that have both 1:1 and 2:2 stoichiometries. Structural calcula tions indicate that the naphthyl moiety of both enantiomers is preferential ly included in the CD cavity for the 1:1 complex. In the case of the comple xes with 2:2 stoichiometry, a mixture of complexes is present that either h ave the naphthyl moieties at a distance or in close proximity. The former g eometry 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 beta -CD is due to the different contributions of 2:2 complexes with different geometries.