Study of Spectral Sensitization and Supersensitization by Means of the Rotating Ring‐Disk Electrode Technique

Rotating ring‐disk electrode (RRDE) technique, using a ZnO single crystal as the disk electrode, was applied to the study of spectral sensitization by rhodamine B (RhB) and supersensitization by hydroquinone (HQ) in acetonitrile. The primary product of spectral sensitization, i. e., RhB radical cation (RhB + · ), was detected electrochemically at the Pt ring electrode. Upon addition of HQ to the electrolyte solution, the sensitized photocurrent was enhanced (supersensitization), and concomitantly the collection efficiency for RhB + · at the Pt ring electrode showed a decrease. Assuming that the supersensitization results from the reduction of RhB + · by HQ, the rate constant for this process was calculated to give 10 6 − 10 7 M −1 s −1 . Taking into account the singlet lifetime of RhB* as well as the rate constant for the quenching of RhB* by HQ, the mechanism of spectral sensitization and supersensitization is discussed.