Anion exchange of methyl orange into Zn-Al synthetic hydrotalcite and photophysical characterization of the intercalates obtained

The intercalation via ion exchange of the azoic dye methyl orange (MO-) int o the hydrotalcite-like compound Zn0.67Al0.33(OH)(2)Cl-0.33. 0.6H(2)O has b een investigated. X-ray diffraction patterns of samples with increasing dye loading showed that Cl-/MO- exchange occurs with a first-order phase trans ition from the Cl- phase (interlayer distance 7.74 Angstrom) to the MO phas e with an interlayer distance of 24.2 Angstrom. A sample of composition [Zn 0.67Al0.33(OH)(2)][MO0.31Cl0.02]. 0.85H(2)O has been studied by thermogravi metric analysis and by X-ray powder diffraction at different temperatures. The loss of hydration water between 80 and 120 degrees C causes a decrease of the interlayer distance from 24.2 to 21.5 Angstrom. Computer models and calculations based on the structure of the host showed that MO anions are a rranged in the interlayer space as a monolayer of species with the main axi s perpendicular to the layer plane. Emission fluorescence spectra of the dy e only exchanged on the external surface of the host or intercalated, at di fferent loading, were compared with the fluorescence spectra of MO as micro crystals or dissolved in ethanol. By changing the experimental conditions, MO fluorescence emission can cover the whole visible wavelength range. The spectrum in ethanolic solution, lambda(max) = 480 nm, is at the highest ene rgies, while that of microcrystals is shifted toward the red (lambda(max) = 690 nm). The fluorescence of MO-intercalated samples is near that of micro crystalline methyl orange but shifted at higher energies. A further shift i s observed for the sample containing only surface-exchanged MO. The energy difference between the fluorescence spectra of the MO in different environm ents has been attributed to the change of the emitting state energy caused by interactions of the excited species with neighboring unexcited species. The fluorescence measurements can thus be considered a valuable tool for st udying the microenvironment of the dye.