Surface uptake and intercalation of fluorescein anions into Zn-Al-hydrotalcite. Photophysical characterization of materials obtained

The xanthene dye fluorescein was immobilized, via anion-exchange reactions, in the interlayer region and/or on the surface of a synthetic Zn-Al hydrotalcite, prepared by the thermal hydrolysis of urea. Surface uptake was investigated by equilibrating [Zn0.67Al0.33(OH)(2)][CO3](0.165)0.4H(2)O with diluted solutions of fluorescein disodium salt. Only carbonates present on the surface of microcrystals were exchanged by fluoresceinate anions to reach a maximum uptake of 50 mu mol/g. Intercalation compounds were obtained by equilibrating the perchlorate form of hydrotalcite [Zn0.67Al0.33(OH)(2)][ClO4](0.33)0.6H(2)O (interlayer distance 11.0 Angstrom) with 10(-2) mol/dm(3) fluorescein disodium salt solution. Intercalation of fluoresceinate anions occurs with a first-order phase transition from the ClO4- phase to a phase containing 0.11 mol of fluoresceinate per mol of the host and an interlayer distance of 16.5 Angstrom. The materials were characterized by chemical and thermal analysis and by X-ray powder diffraction and FT-LR techniques. Computer models based on the structure of the host, the composition and interlayer distance of the intercalation compound, and the. van der Waals dimensions of the guest showed that fluorescein anions are incorporated in the interlayer region as a bilayer of species, with the main axis parallel to the layer plane. We performed the photophysical characterization by determining the absorption, excitation, and fluorescence spectra and the fluorescence lifetimes of the dye in different experimental conditions. The emission properties of the guest adsorbed or intercalated in the inorganic host were compared to those of the dye as a pure solid or in solution. Diffuse reflectance laser flash photolysis experiments allowed us to determine the absorption spectra and decay lifetimes of the triplet state of the dye bound to the inorganic matrix. A description of the guest arrangement and the type of interaction with the hydrotalcite is based on the structural and photophysical characteristics of these materials. This description is in agreement with the computer model based on the structure of the host and the dimensions of the guest.