Complexation of naphthylethanols with β-cyclodextrin

The complexation behavior of 1-naphthyl-1-ethanol (1-NpOH) and 2-naphthyl-1-ethanol (2-NpOH) with beta-cyclodextrin (beta-CD) was studied by employing several spectroscopic techniques. In the case of 1-NpOH, only a complex with 1:1 stoichiometry is formed with beta-CD, which. has an equilibrium constant that is smaller than that observed for the 1:1 complex between beta-CD and 2-NpOH. Excimer emission was observed in the presence of beta-CD for solutions containing high 2-NpOH concentrations. This excimer emission was ascribed to a complex with 2:2 beta-CD/2-NpOH stoichiometry. In addition, H-1 NMR data suggest that 2-NpOH is axially incorporated into the beta-CD cavity. Only in the case of 2-NpOH was a broadening of the signals corresponding to the aromatic protons observed in the presence of beta-CD. This broadening was attributed to the formation of the 2:2 complex. The dynamics of NpOH complexation was investigated by using the quenching methodology for triplet states. The entry rate constants for the 1:1 complex of 1-NpOH and 2-NpOH are (4.7 +/- 1.9) x 10(8) M-1 s(-1) and (2.9 +/- 1.6) x 10(8) M-1 s(-1), respectively, whereas the exit rate constants for the two compounds are (4.8 +/- 1.8) x 10(5) s(-1) and (1.8 +/- 0.7) x 10(5) s(-1). In the case of 2-NpOH, we were able for the first time to estimate the rate constant for the dissociation of a beta-CD 2:2 complex ((0.2-2.5) x 10(3) s(-1)), showing that the dynamics for complexes including more than one cyclodextrin are remarkably slower than the dynamics observed for 1:1 complexes.