Analysis and Regulation of Unusual Adsorption of Phthalocyanine Zinc(II) into a Nafion Film as Investigated by UV−vis Spectroscopic Techniques

Phthalocyanine zinc(II) (ZnPc) was found to be adsorbed well into a Nafion (Nf) film. The kinetic analysis suggested that the adsorption of ZnPc into the Nf film is controlled by its diffusion in the Nf film with a diffusion coefficient of D = 1.9 x 10(-6) cm(2) s(-1) that is higher than those (10(-9)-10(-12) cm(2) s(-1)) of cationic redox molecules in the Nf film by 3-6 orders of magnitude. The adsorption isothermal was analyzed by a Brunauer-Emmett-Teller (BET) equation suggesting multilayer adsorption of ZnPc into the film. The BET analysis provided the amount of ZnPc for monolayer adsorption (w(m) = 1.50 x 10(-7) mol cm(-2)), from which the effective area for the ZnPc adsorption was estimated to be larger by a factor of 1.7 x 10(3) than the Nf film area (1.0 cm(2)). The absorption spectra of a Nf film adsorbing ZnPc ((Nf/ZnPc)(ads) film) exhibited two broad absorption bands at 385 and 680-750 nm without any structural features, which is significantly different from the absorption spectra of either ZnPc solution in DMF or a (Nf/ZnPc)(mix) film prepared from a DMF solution containing Nf and ZnPc by solvent evaporation. This is ascribed to the formation of a ZnPc aggregate in the (Nf/ZnPc)(ads) film. Photoluminescence data for the (Nf/ZnPc)(mix) film suggested the presence of a ZnPc monomer and dimer at equilibrium in the film with a concentration of 0.1 M and that energy transfer occurs from the monomer to the dimer in excitation of the monomer (at lambda(ex) = 609 nm) to yield emission from the dimer. By contrast, photoluminescence data for the (Nf/ZnPc)(ads) film suggested that the excited ZnPc is self-quenched significantly by the formation of the ZnPc aggregate in the film. The lesser electroactivity of ZnPc in the (Nf/ZnPc)(ads) film compared with that in the (Nf/ZnPc)(mix) film could be ascribable to more difficult diffusion of ZnPc in the former film due to the formation of the ZnPc aggregate. The adsorption of ZnPc into the Nf film was significantly regulated by simple pretreatments of the Nf film such as immersion in solvents and storage under solvent vapors. The regulation was explained by controlled physical and chemical properties of a channel for mass and ion transport that is formed by sulfonate groups, countercations, and solvent molecules in the Nf film.