Organic Photoelectrochemical Cell Mimicking Photoinduced Multistep Electron Transfer in Photosynthesis: Interfacial Structure and Photoelectrochemical Properties of Self-Assembled Monolayers of Porphyrin-Linked Fullerenes on Gold Electrodes

Self-assembled monolayers (SAMs) technique has been applied to porphyrin-linked fullerene systems to mimic supramolecular photoinduced charge-separation (CS) events in photosynthesis. Porphyrin-linked C-60 molecules with a sulfide group were designed to self-assemble on a gold surface using S-Au interaction. The structure of the SAMs formed on gold electrodes was investigated by spectroscopic methods including X-ray photoelectron spectroscopy (XPS), ultraviolet (UV)-visible spectroscopic ellipsometry, UV-visible absorption spectroscopy, and Fourier transform infrared (FTIR) spectroscopy as well as electrochemical studies. These results indicate that the SAMs have loosely packed structure; the porphyrin-Coo molecules are tilted and nearly parallel onto the gold surface. Short-circuit photocurrent with a level of sub-mu A cm(-2) was observed for the photoelectrochemical cell in the presence of methylviologen (MV) under illumination with ca. 4 mW cm(-2). The maximum intensity of the photocurrent in free-base porphyrin-C-60 system is five times larger than that in the corresponding free-base porphyrin system, indicating that C-60 is an effective mediator in multistep electron transfer (ET) processes. A possible mechanism for the photocurrent generation is discussed in terms of the photodynamics of porphyrin-linked fullerenes in solution.