Solution Adsorption of Fluorinated Zinc Phthalocyanines on Titania: Combined XPS, UV-Vis, and Contact Angle Study

The equilibrium solution adsorption of perfluorinated metal phthalocyanines FPcZn (x = 16, 64) on titania was investigated. This method was explored as an alternative to the frequently used vapor deposition technique for the preparation of solid-supported phthalocyanines for applications such as sensitizers, catalysts, and sensors. According to X-ray photoelectron spectroscopy (XPS), UV-vis, and water contact angles, the adsorption of phthalocyanines from acetone solution occurred readily at room temperature resulting in the formation of hydrophobic surfaces of the solid-supported phthalocyanines. The adsorption isotherms (298 K) were of the Langmuir-type with saturation plateau. The effective thickness of the adsorbed layers at the plateau regions was estimated at 0.17 nm (F₁₆PcZn) and 0.47 nm (F₆₄PcZn), which, assuming the face-down orientation of phthalocyanines, corresponded to ∼52 and ∼77% of the complete monolayers, respectively. In the case of F₆₄PcZn, the state of the adsorbed molecules was similar to that of bulk F₆₄PcZn, suggesting only weak adsorption interactions of dispersive type. In contrast, F₁₆PcZn showed strong interactions with the surface of titania including the dissociation of C-F bonds, i.e., chemisorption. The difference in the adsorption interactions of F₁₆PcZn vs F₆₄PcZn was attributed to the presence of eight i-C₃F₇ groups decorating the perimeter of the F₆₄PcZn molecule. These bulky substituents in the peripheral positions sterically protected the nonperipheral fluorine atoms, thereby preventing their substitution and any other specific interactions between the macrocycle and the surface OH groups.