Photoelectrochemical characteristics of a self-assembled monolayer of porphyrin-mercaptoquinone coupling molecules

A molecule which has porphyrin as a photoactive group, quinone as an electron transport group and thiol group as a surface-active group, 5-(4-(3-mercapto-1,4-benzoquinone)oxy-phenyl)-10,15,20-tris(p-tolyl)porphyrin, was newly synthesized and immobilized on gold surfaces by the self-assembly (SA) method. The redox potential and the adsorbed quantity of the coupling molecules on a gold electrode were determined by cyclic voltammetry. The redox potential of the quinone moiety of the gold electrode modified with this molecule was slightly more positive than that of a mercaptoquinone-modified gold electrode. The number of adsorbates was much less than that of mercaptoquinone (only 5%). In an electrolyte solution containing methylviologen and EDTA as electron acceptor and donor, respectively, anodic and cathodic photocurrents were observed at potentials more positive and more negative, respectively, than the redox potential of the quinone moiety, indicating that the photoinduced electron transfer in the SA monolayer took place through the quinone group. The action spectrum of the photocurrent was in good agreement with the absorption spectrum of the modi tied ITO electrode, confirming that the porphyrin group acted as a photoactive group.