Complexation of Naphthylethanols with β-Cyclodextrin

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Abstract

The complexation behavior of 1-naphthyl-1-ethanol (1-NpOH) and 2-naphthyl-1-ethanol (2-NpOH) with β-cyclodextrin (β-CD) was studied by employing several spectroscopic techniques. In the case of 1-NpOH, only a complex with 1:1 stoichiometry is formed with β-CD, which has an equilibrium constant that is smaller than that observed for the 1:1 complex between β-CD and 2-NpOH. Excimer emission was observed in the presence of β-CD for solutions containing high 2-NpOH concentrations. This excimer emission was ascribed to a complex with 2:2 β-CD/2-NpOH stoichiometry. In addition, 1H NMR data suggest that 2-NpOH is axially incorporated into the β-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 β-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) × 108 M-1 s-1 and (2.9 ± 1.6) × 108 M-1 s-1, respectively, whereas the exit rate constants for the two compounds are (4.8 ± 1.8) × 105 s-1 and (1.8 ± 0.7) × 105 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 β-CD 2:2 complex ((0.2−2.5) × 103 s-1), showing that the dynamics for complexes including more than one cyclodextrin are remarkably slower than the dynamics observed for 1:1 complexes.

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