2-Naphthol Complexation by β-Cyclodextrin: Influence of Added Short Linear Alcohols

Abstract

2-Naphthol (NOH) in its ground state forms a 1:1 complex with β-cyclodextrin (β-CD) both in the absence and presence of linear alcohols. Association constants, K_app, were measured using a steady-state fluorescence method. K_app decreases linearly with an increasing number of carbon atoms in the chain of the alcohol, n_C, up to n_C = 5. We attribute this to a competition between NOH and alcohol for the β-CD cavity. Fluorescence studies confirm the redistribution of NOH from the CD environment to the aqueous phase when alcohols are present. NOH fluorescence is quenched by iodide in all the systems studied. At 2 mM β-CD, alcohols increase the Stern?Volmer constant above the value found in the absence of alcohols. These results suggest that alcohols occupy space within the β-CD cavity with the result that the aqueous NOH concentration is increased. This was further investigated by dynamic fluorescence measurements on the system β-CD:NOH:pentanol. Global biexponential analysis of fluorescence decay data shows that the Stern?Volmer constants correlate inversely with the fraction of NOH complexed by β-CD. By global compartmental analysis of the fluorescence decays, values for the excited-state association and dissociation rate constants were determined. The dissociation rate constant increases from approximately 500 s^-1 in the absence of pentanol to about 14?000 s^-1 at a pentanol concentration of 0.1 M. The association rate constant increases from 2.5 × 10⁹ to 5.8 × 10⁹ M^-1 s^-1 upon addition of pentanol. The more pronounced increase of the dissociation rate constant leads to an exclusion of complexed NOH into the aqueous bulk phase. As the complexed NOH is shielded against iodide quenching, this explains the increase of the Stern?Volmer constant when an alcohol is added to the aqueous β-CD:NOH system.