The Application of Electrochemical Impedance Techniques in Analyzing the AC Response of Some Two-electron Transfer Dye Systems

The Electrochemical Impedance Spectroscopic techniques (EIS) were used to investigate the behavior of some dye compounds (quinoid systems) characterized with 2e-transfer processes. For this purpose, Alizarin Red S (ARS), Alizarin Cyanine (AC), Alizarin Viridin (AV) and carminic acid were chosen for the measurements. The EIS experiments were performed using a small AC amplitude (10 mV p-p) in addition to a relatively wide frequency range (0.01 Hz ≤ f ≤ 10⁵ Hz). The investigations were carried out at room temperature in aqueous media (HClO₄, NaClO₄ and KNO₃) on the Hanging Mercury Drop Electrode (HMDE) and for comparison one experiment only was measured in aprotic solvent (DMF) on the Pt-disc electrode. The EIS diagrams of these systems were characterized in the complex plane by two fundamental observations, the first of which is a straight line crossing the real axis at an angle of 45° (or at least nearly so) and the second one is two semicircles beside each other corresponding to high-frequency and low-frequency regions, which are implying the presence of well-separated time constants. The EIS characteristic parameters for these dye systems were calculated and discussed.