Estudo espectroscopico da fluoresceina em poli (alcool vinilico) e poli (acetato de vinila)




Using the eletronic absorption (UV/VIS) and fluorescence emission spectroscopies, it was studied the photophysics of fluorescein in several systems: (a) aqueous solutions at different pH s, (b) protic solvents (ethanol and ethylene glycol ), (c) poly (vinyl alcohol) (PVA), (d) aprotic solvents (ethy acetate, dioxane and acetone) and (e) poly (vinyl acetate) (PVAc). In this way it was possible to understand the behavior of all dissociation forms, as well as the characteristics of the aggregates of the dye molecule. The studies of the fluorescein in low viscosity liquid solvents, allowed: the interpretation of the results obtained for fluorescein dispersed in rigid polymeric matrixes and the identification of the nature of the different absorption sites of the dye in the polymeric films. In aqueous dilute solutions, the dissociation forms present in solution depend on the pH and all the species exist in the ground and excited electronic states, except the neutral forms: lactonic and ampho-ion, that are note present in the later state. Utilising the electronic excitation spectroscopy, it was verified that in the saturated aqueous solutions in acid (pH <4,0) and, neutral and basic (pH >6,0) media there is the formation of a type sandwich aggregates. In the aprotic dilute system, only lactonic form is present in the ground and excited electronic states However, in more concentrated solutions, there is a shift of the dissociation equilibrium to form the ampho-ion in electronic states. For fluorescein dispersed in PVAc, the lactonic form is present in both, dilute and concentrated films, in the electronic ground state. In the excited state, there is a shift of the dissociation equilibrium toward the neutral form ampho-ion, which is responsible for the fluorescence. The emission spectrum of the concentrated polymeric film showed two highly overlapped bands. When this film was heated to 50°C (annealing process) and slowly cooled, the two bands collapsed, resulting in a single broader band; indicating that the dye exists in different adsorption sites of the polymeric matrix. There is a dissociation equilibrium between the neutral lactonic and quinonoid forms of fluorescein (both in the ground electronic state) in protic dilute systems. It was suggested that in the excited electronic state, there is an equilibrium between the neutral quinonoid form and monoanion. In concentrated solutions, the neutral quinonoid form, the monoanion and, probably, the dianion are present in equilibrium, in both, ground and excited electronic states. In the dilute films of PVA containing fluoresceinc, it was seen that the quinonoid and monoanionic forms are present in the ground and excited electronic states. The infrared spectra showed that the neutral quinonoid form ocupy the adsorption sites in crystallite interfaces and the monoanion is dispersed in the adsorption sites of the amorphous fase. Concentrated films of PVA containing fluorescein result in the formation of the structure aggregates. These results allow the following conclusions: (A) the studies on fluorescein solutions permitted the identification of the photophysical behavior of the dissociation forms; (B) utilising these studies it could be possible identify the forms present in the polymeric matrixes; (C) the spectral differences were ascribed to different interaction forces for polymer-polymer and polymer-dye system in the different adsorption sites.


analise espectral

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