INCLUSION OF FLUORESCEIN AND HALOGENATED DERIVATIVES IN ALPHA-CYCLODEXTRIN, BETA-CYCLODEXTRIN, AND GAMMA-CYCLODEXTRINS - A STEADY-STATE AND PICOSECOND TIME-RESOLVED STUDY

The inclusion of fluorescein (FL), erythrosin B (EB), and rose bengal (RB) in alpha, beta, gamma-cyclodextrins (alpha-, beta-, and gamma-CDs) was studied with several techniques. Induced circular dichroism (icd) absorption spectroscopy and steady-state and picosecond time resolved fluorescence spectroscopy were used to derive stoichiometry, association constants, and structural information on the inclusion complexes formed. A stoichiometry of 1:1 was found for the range of concentration explored (xanthenes, 10(-6)-10(-4) M; CD, 10(-3)-10(-2) M). From icd measurements, FL is found to bind weakly with alpha- and gamma-CDs (K(A) = 35 M-1) and associate preferably with beta-CD (K(A) = 360 M-1). EB binds preferably to gamma-CD (K(A) = 110 M-1) and more weakly with alpha- and beta-CDs (K(A) = 20 M-1, K(A) = 40 M-1, respectively). RB binds only with gamma-CD with an association constant similar to that of EB (K(A) = 100 M-1, respectively). Fluorescence enhancement and different luminescence lifetimes with respect to that of the free chromophore in water are found only upon association of CD with EB and RB, the halogenated dyes. The nonradiative rate constant of their emitting state is known, in fact, to increase with the hydrogen bonding ability of the media. There is indication from two distinct fluorescence lifetimes, paralleled by different signs of icd bands, that at least two different complexes are formed, depending on the dimensions of the cavity and the degree of halogenation of the dye. The contemporary presence of the two complexes in gamma-CD systems could explain some inconsistency in the association constants determined by the different methods. FL complexes do not show fluorescence enhancement (except at high concentration of the dye where CD inclusion disrupts the nonluminescent aggregates), and a distinct emission lifetime with respect to water is not observed, as expected on the basis of the invariance of FL fluorescence yield and lifetimes with the nature of the medium.