ABSTRACT
Among many types of chemical reactions, an electron-transfer reaction is undoubtedly the most fundamental one because the electron is the minimal unit of the change in chemical reactions and any chemical bond is formed via electrons. This simplicity of the reaction has permitted the development of a relatively simple detailed quantitative theory, known as the Marcus theory of electron transfer [1-3]. With the aid of such an analytical theory, the field of electron-transfer reactions has undergone a remarkable expansion not only in chemistry but also in physics, biology, and advanced technology during the past half-century [4-7]. In particular, electron transfer has been recognized as the key step in a number of biological processes which are essential for life, such as photosynthesis and respiration [8-10]. Photoexcited states as well as the ground states are involved in photosynthesis, where multistep electron-transfer processes are remarkably well designed to optimize the efficiency of solar energy conversion [10].
