Complexation of Naphthylethanols with β-Cyclodextrin

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.

Fluorescence Studies of the Hydrogen Bonding of Excited‐State Molecules within Supramolecular Host–Guest Inclusion Complexes

“…In this case, interaction with the alcohol shields the BPT guest from the quenching interaction with water, and a very dramatic increase in the S 2 fluorescence results. Hydrogen bonding has also been shown to have significant effects on the properties of triplet states included within CD cavities, using phosphorescence and time-resolved absorbance studies [81][82][83][84][85]. Bohne et al [81][82][83] have shown that triplet xanthone is much more weakly bound in b-CD than is ground-state xanthone, with measured binding constants K of 1100 and 48 M À1 for the ground and triplet states respectively [81].…”

“…Hydrogen bonding has also been shown to have significant effects on the properties of triplet states included within CD cavities, using phosphorescence and time-resolved absorbance studies [81][82][83][84][85]. Bohne et al [81][82][83] have shown that triplet xanthone is much more weakly bound in b-CD than is ground-state xanthone, with measured binding constants K of 1100 and 48 M À1 for the ground and triplet states respectively [81]. They proposed this to be a result of stronger hydrogen bonding abilities of the triplet as compared with the ground state [83], in agreement with the findings described earlier that indicated that increased hydrogen bond abilities of a guest will lead to lower binding [48].…”

“…Bohne et al [81][82][83] have shown that triplet xanthone is much more weakly bound in b-CD than is ground-state xanthone, with measured binding constants K of 1100 and 48 M À1 for the ground and triplet states respectively [81]. They proposed this to be a result of stronger hydrogen bonding abilities of the triplet as compared with the ground state [83], in agreement with the findings described earlier that indicated that increased hydrogen bond abilities of a guest will lead to lower binding [48]. (In this same study [83], they also found a lower binding of 1-naphthyl-1-ethanol in b-CD than that of corresponding alkylnaphthalenes, which they also attributed to its hydrogen bond acceptor ability.)…”

Application of cyclodextrin-mediated capillary electrophoresis to determine the apparent binding constants and thermodynamic parameters of the alkylnaphthalene derivatives

“…The UV detector was operated at 224 nm. For temperature studies, compounds migration times were determined at the temperatures 15,20,25,30,35, and 407C (the precision of each temperature was 60.17C). The electrophoretic system was equilibrated at each temperature for at least 6 h prior to each experiment.…”

“…Their review revealed that both neutral and charged CDs could be exploited in CE to optimize the achiral separations of small molecules and various positional isomers. To elucidate the mechanisms of achiral separation by CDs, spectrophotometric methods and calorimetric titration approaches have been applied to determine the apparent binding constants of guest-CD complexation, as well as the thermodynamic parameters and stoichiometries in the binding of the various positional and structural isomers [16][17][18][19][20].…”