Potential Dependence of the Stretching Frequency of Carbon Monoxide Bound to the Surface of Platinum Dispersed in an Electrode-Confined Redox Polymer

Variation in the electrochemical potential of the electrode results in a change in the stretching frequency of CO bound to elemental Pt dispersed in an electrode-confined redox polymer, (PQ^2+/+)„, derived from an N, N′-dialkyl-4, 4′-bipyridinium monomer. The stretching frequency of the CO, about 2000 cm^-1, can be monitored by transmission infrared spectroscopy using a Fourier transform infrared (FTIR) spectrometer and moves to lower values with movement of the potential to more negative values. Spectroelectrochemical studies were conducted by using an IR-transparent n-Si/[(PQ^2+/+PPt(CO)_x)_n] electrode assembly in an electrochemical cell with the derivatized surface exposed to the electrolyte solution. Experiments have been carried out with both natural-abundance and 99% ¹³C-enriched CO, and the potential dependence is the same, about 20 cm^-1 V^-1, over the potential range studied, approximately 0.0 to -1.4 V vs. Ag⁺/Ag in CH₃CN/2 M LiClO₄. The stretching frequency of natural-abundance CO in the same assembly has been studied in H₂O/3 M LiCl and shows a slightly larger potential dependence, about 35 cm^-1 V^-1, in the range -0.2 to -1.0 V vs. AgCl/Ag. The data for the CO bound to the Pt dispersed in the polymer is in accord with similar data obtained for CO bound to smooth Pt electrode surfaces.