Electrochemical and spectroscopic behavior of iron(III) porphyrazines in Langmuir-Schafer films

Thin films of a newly synthesized iron(III) porphyrazine, LFeOESPz (L ) ClEtO, OESPz ) ethylsulfanylporphyrazine), have been deposited by the Langmuir-Scha¨fer (LS) technique (horizontal lifting) on ITO or gold substrates. Before deposition, the floating films have been investigated at the air-water interface by pressure/area per molecule (π/A) experiments, Brewster angle microscopy (BAM) and UV-vis reflection spectroscopy (RefSpec). The complex reacts with water subphase (pH 6.2) forming the μ-oxo dimer, which becomes the predominant component of the LS films (LS-Fe) as indicated by optical, IR, XPS, and electrochemical data. LS-Fe multilayers exhibit, between open circuit potential (OCP) and +0.90 V (vs SCE), two independent peak pairs with formal potentials, Esurf (I) and Esurf(II) of +0.56 V and +0.78 V, respectively. According to dynamic voltammetric and coulometric experiments the peak pair at +0.56 V is attributed to one-electron process at the iron(III) centers on the monomer, while the peak pair at +0.78 V is associated to a four-electron process involving μ-oxo-dimer oligomers. LS-Fe films prove to be quite stable electrochemically between OCP and +0.90 V. The electrochemical stability decreases, however, when the potential range is extended both anodically and cathodically outside these limits, due to formation of new species. Upon incubation with TCA solutions, LS-Fe films show remarkable changes in the UV-vis spectra, which are consistent with a significant μ-oxo dimer f monomer conversion. Addition of TCA to the electrochemical cell using a LSFe film as working electrode, results in a linear increase of a cathodic current peak near -0.40 V as the TCA concentration varies in the 0.1-2.0 mM range. This behavior is interpreted in terms of TCA inducing a progressive change in the composition of the LS-Fe films in favor of the monomeric iron(III) porphyrazine, which is responsible for the observed increase in the cathodic current near -0.40 V.