Photocurrent generation in metal bisphosphonate multilayer thin films

ATTEMPTS to mimic the highly efficient process of photosynthesis^1-3 are of considerable interest, the goal being to design artificial systems for the efficient conversion of solar energy into chemical or electrical energy^4-14. In both natural and artificial systems, the underlying process is photoinduced charge separation, typically involving a redox reaction between a photoexcited donor molecule and an acceptor molecule. Through careful choice of the molecular arrangement, and the redox potentials of the donors, intermediate charge carriers and acceptors, it is possible to minimize the reverse electron transfer process and thereby obtain stable photoinduced charge separation. In this context, photochemical electron acceptors based on methylviologen and its derivatives have been widely studied^{5, 6, 8-12, 14-19}. Here we describe the synthesis and characterization of metal biphosphonate multilayer thin films composed of viologen-based acceptor layers and donor layers of p-phenylenediamine. Our method of film growth provides control over both the structure and composition of the multilayer films, leading to efficient photoinduced charge separation and directional electron transport. These films generate photocurrents when irradiated with ultraviolet and visible light.