The Unexpected High Solubility of Fluorinated Zinc Phthalocyanines in Aqueous Solutions and Their Use for the Preparation of Photodynamic Coatings on Various Substrates

We report the results of the investigation of aqueous solutions of perfluorinated metal phthalocyanines F<i>_x</i>PcZn (<i>x</i> = 16, 64) and their adsorption at solid-liquid interfaces. Specifically, we focused on the development of environmentally benign methods for the preparation of coatings of fluorinated photosensitizers that are capable of generating singlet oxygen and degrading organic contaminants in water. Counterintuitively, F₁₆PcZn and F₆₄PcZn demonstrated good solubility (∼10^-4 M) in water/ethanol mixtures containing up to 95-98% water by volume. The amphiphilic properties of the perfluorinated macrocycles were attributed to the presence of a highly polar metal porphyrin center surrounded by the nonpolar peripheral groups (F and CF₃). A thermodynamic model of the interfacial interactions upon solvation of the amphiphilic phthalocyanines was proposed, rationalizing their solubility in water-ethanol. As assessed by UV-vis and fluorescence, aqueous solution of F₁₆ and F₆₄PcZn contained aggregates; however, the size of the aggregates was small and not detectable by DLS. The adsorption of phthalocyanines from the water-ethanol mixtures occurred readily on various substrates, including PET plastic, polyester and nylon textiles, mica, silica gel, and talc. According to XPS, solid-state UV-vis, and contact angles, the adsorption from water-ethanol solutions produced hydrophobic, ∼monomolecular coatings of the phthalocyanines. As compared to solutions, the fraction of the monomeric phthalocyanines significantly increased in the adsorbed state. The shift of the self-association equilibrium toward monomers was particularly notable for the adsorption on talc, where both phthalocyanines were monomeric even after the adsorption from solutions with high water content. The photodynamic activity of the coatings was tested in the reactions of degradation of methyl orange dye under radiation with visible light. Both F₁₆PcZn and F₆₄PcZn adsorbed on talc produced active, stable, and reproducible coatings capable of degrading methyl orange dye in water.