Polypyrrole films functionalized with pendant titanocene dichloride complexes: Ellipsometric study of the electropolymerization process

Electrochemical and ellipsometric methods have been used to study the electropolymerization process of a functionalized monomer, Tc3Py (in which pyrrole is covalently bonded with titanocene dichloride complex) on platinum electrode from a dilute monomer solution in acetonitrile. The deposition has been performed in the potentiodynamic regime. A new method for the acquisition and treatment of ellipsometric data has been proposed which allowed us to analyze the evolution of the film properties at each potential. The model of a single uniform layer was unable to describe the ellipsometric experimental observations with the necessary precision. A proper fitting of the data has been achieved considering a model of two layers having identical values of the refractive index but different values of the extinction coefficient (higher absorption for the outer layer). The analysis of the calculated values of the optical parameters of these layers has led to the assumption that both layers represent morphologically the same medium (i.e. that the whole polymer film is uniform) while the higher extinction coefficient of the external layer was due to a penetration into it of solute oligomers possessing an intensive absorption at the wavelength of the observation, 633 nm. The dependences of the thicknesses of each layer and of the whole film on the cycle number have been established, the increment of the growth (5.5 nm per cycle) being constant within the whole deposition procedure. The variation of the optical constants of the layers during the potential cycling was determined and interpreted. The calculated values for the extinction coefficients and the layer thicknesses have been used to estimate how the film absorbance changes during the potential cycling. Its comparison with the same characteristic measured directly in a spectroelectrochemical cell for a film deposited at the ITO electrode surface has provided an extra evidence in favor of the above hypothesis on the film being a morphologically uniform material, with incorporated solute oligomers in its outer layer.