Electron Transfer. Shunichi Fukuzumi

1 Introduction

The rapid consumption of fossil fuel has already caused unacceptable environmental problems such as the greenhouse effect by CO₂ emission, which is predicted to lead to disastrous climatic consequences [1]. Moreover, the consumption rate of fossil fuels is expected to increase further at least twofold relative to the present by midcentury because of population and economic growth, particularly in the developing countries. It is becoming more and more obvious that fossil fuels will run out eventually in the next century despite the recent shale gas revolution. Thus, renewable and clean energy resources are urgently required in order to solve global energy and environmental issues [2,3]. Among renewable energy resources, solar energy is by far the largest exploitable resource [1–3]. Nature harnesses solar energy for its production by photosynthesis, and fossil fuels are the product of photosynthesis [4]. Fossil fuels range from volatile materials with low carbon:hydrogen ratios such as methane, to liquids such as petroleum, and to nonvolatile materials composed of almost pure carbon, such as anthracite coal. The age of the organisms and their resulting fossil fuels is typically millions of years, and sometimes exceeds 650 million years. The consumption rate of fossil fuels is becoming much faster than the production rate by nature. Thus, it is quite important to develop artificial photosynthetic systems for production of solar fuels, which are hopefully simpler and more efficient than natural systems. The conversion of photon energy to chemical energy in photosynthesis is achieved by electron transfer from the excited state of an electron donor (D^*: ^* denotes the excited state) to an electron acceptor (A) to produce the charge‐separated state (D^·+–A^·−). The high oxidizing power of D^·+ in D^·+–A^·− is used for four‐electron oxidation of water to produce dioxygen, whereas the high reducing power of A^·− is used to reduce nicotinamide adenine dinucleotide phosphate (NADP⁺) coenzyme to NADPH [5]. NADPH reduces CO₂ by multi‐electron reduction to produce sugar [4,5]. Thus, electron transfer plays essential roles in photosynthesis. In order to develop artificial photosynthesis systems, it is quite important to control electron‐transfer systems to maximize the energy conversion.

Electron transfer is the most fundamental chemical reaction in which only electron is removed or attached. However, it is quite important to recognize the fundamental difference of electron‐transfer reactions from other chemical reactions in which chemical bonds are cleaved and formed during the reactions. Because electron transfer occurs based on the Franck–Condon principle, nuclear configurations remain the same before and after the electron transfer [6]. In contrast, nuclear configurations are changed significantly in chemical reactions associated with the cleavage and formation of chemical bonds. The fastest electron transfer is achieved with no activation energy when the nuclear configurations are the same before and after the electron transfer with a large driving force of electron transfer. When the driving force of electron transfer is further increased, the nuclear configurations are not the same any more before and after the electron transfer. In such a case, activation energy is again required to change the nuclear configurations before the electron transfer to be the same as that after the electron

Скачать книгу