Fluorescence of the complexes of 2-methylnaphthoate and 2-hydroxypropyl-α-, -β-, and -γ-cyclodextrins in aqueous solution

Fluorescence techniques were employed to study the inclusion complexes of 2-methylnaphthoate (MN) with 2-hydroxypropyl-alpha-cyclo-dextrin (alphaHPCD), 2-hydroxypropyl-beta-cyclodextrin (betaHPCD), and 2-hydroxypropyl-gamma-cyclodextrin (gammaHPCD). Emission spectra of MN show two vibronic bands whose intensity ratio R is very sensitive to the polarity of the medium. The stoichiometry and formation constants of these complexes were investigated by obtaining R as a function of the cyclodextrin (CD) concentration. Results showed identical stoichiometry (1/1) for the three MN/alphaHPCD, MN/beta-HPCD, and MN/gammaHPCD complexes. Formation constants at 25 degreesC were 780 +/- 15, 2700 +/- 130, and 165 +/- 10 M-1, respectively. DeltaH(0) and DeltaS(0) were obtained from linear van't Hoff plots. Results reveal that the complexation of MN with alphaHPCD is enthalpy driven. With betaHPCD, both the entropy and enthalpy terms favor the process, whereas the formation of the complex with gammaHPCD is entropically governed. The extrapolation of R at infinite CD concentration allows us to estimate the effective dielectric constants of the inner CD cavities, which are around 50, but which differ from their counterparts, the naturally occurring alpha-, beta-, and gamma-CDs. Fluorescence anisotropy, quencher lifetimes, and average lifetimes can also give additional information about the structure and driving forces accompanying the formation of such complexes.