Three different kinds of mixed self-assembled monolayers have been prepared to mimic photosynthetic energy and electron transfer on a gold surface. Pyrene and boron-dipyrrin were chosen as a light-harvesting model. The mixed self-assembled monolayers of pyrene (or boron-dipyrrin) and porphyrin (energy acceptor model) reveal photoinduced singlet-singlet energy transfer from the pyrene (or boron-dipyrrin) to the porphyrin on the gold surface. The boron-dipyrrin has also been combined with a reaction center model, ferrocene-porphyrin-fullerene triad, to construct integrated artificial photosynthetic assemblies on a gold electrode using mixed monolayers of the respective self-assembled unit. The mixed self-assembled monolayers on the gold electrode have established a cascade of photoinduced energy transfer and multistep electron transfer, leading to the production of photocurrent output with the highest quantum yield (50 +/- 8%, based on the adsorbed photons) ever reported for photocurrent generation at monolayer-modified metal electrodes and across artificial membranes using donor-acceptor linked molecules. The incident photon-to-current efficiency (IPCE) of the photoelectrochemical cell at 510 and 430 nm was determined as 0.6% and 1.6%, respectively. Thus, the present system provides the first example of an artificial photosynthetic system, which not only mimics light-harvesting and charge separation processes in photosynthesis but also acts as an efficient light-to-current converter in molecular devices.

Disulfides, with a systematic series of alkyl spacers containing porphyrins at both terminals, were prepared to investigate the effect of the spacer length on the structure and photoelectrochemical properties of self-assembled monolayers (SAMs) of the porphyrins on a gold electrode. The structure of the SAMs was studied using ultraviolet (UV)−visible absorption spectroscopy in transmission mode, cyclic voltammetry, UV−visible ellipsometry, and fluorescence spectroscopy. These measurements showed that as the length of the spacers increases, the SAMs tend to form highly ordered structures on the gold electrodes. In addition, the structures of the monolayers vary depending on the even and odd number of the methylene spacers (n). From these measurements a porphyrin dimer model is proposed in that the two porphyrins take J-aggregate-like partially stacked structures in the monolayers. Photoelectrochemical studies were carried out in argon-saturated Na2SO4 aqueous solution containing methyl viologen as an electron carrier using the modified gold working electrode, a platinum wire counter electrode, and a Ag/AgCl reference electrode. The quantum yield increases in a zigzag fashion with an increase in the spacer length up to n = 6 and then starts decreasing slightly as the chain lengths become longer. A plausible explanation for the photocurrent trend comes from the following points:  (i) there are two competitive deactivation pathways for the excited singlet state of the porphyrin dimer, i.e., the quenching by the electrode via energy transfer and by the electron carrier via electron transfer, (ii) the porphyrin aggregation enhances the rate of nonradiative pathway in the excited state, and (iii) the electron transfer rate from the gold electrode to the resulting porphyrin cation radical decreases with an increase of the spacer lengths. These results will provide basic information for the construction of molecular assembly with photoactive function on surface.

Disulfides, with a series of alkyl spacers containing porphyrins at both ends, were prepared to evaluate the effect of the spacer length on the interfacial structure and photoelectrochemical properties of selfassembled monolayers (SAMs) on a gold electrode. The structure of the SAMs was investigated using UV-visible absorption spectroscopy, cyclic voltammetry, and photoelectrochemical studies. These measurements showed that as the length of the spacers increases, the SAMs tend to form a highly ordered structure on the gold electrode. Photoelectrochemical studies, using modified Au and Pt electrodes, were carried out in the presence of methyl viologen as an electron carrier. The photocurrents decrease dramatically with a decrease in the spacer length, indicating that there are two competitive deactivation pathways for the excited porphyrin, i.e., the quenching by the electrode and electron carrier.