Binding of 2-Acetylnaphthalene to Cyclodextrins Studied by Fluorescence Quenching

The molecule 2-acetylnaphthalene (2-AN) exhibits significant room-temperature fluorescence only in strong hydrogen-bonding solvents, such as water and fluorinated alcohols. As a consequence, the fluorescence of 2-AN is strongly quenched when it binds in the relatively nonpolar cavity of cyclodextrins (CDs). Fluorescence lifetime data confirm that the quenching is static rather than dynamic, and thus the slope of the Stern-Volmer quenching plot (F°/F vs . [CD]) is equal to K, the binding constant. We have used these plots to measure K for 1:1 complexes formed between 2-AN and several CDs, including unsubstituted α;-, β-, and γ-CD and a dimethyl substituted β-CD. In addition, we have determined Δ H° and Δ S° of binding from the temperature dependence of these plots. The room-temperature binding constants vary over a 50-fold range for the 2-AN:CD complexes studied herein, and the variability stems primarily from the Δ S° term. We report here the results of this work and also show evidence for observations other than 1:1 complexes, including emission from the excimer of 2-AN in the presence of γ-CD.