Photoinduced Electron Transfer

Abstract— Given the pre‐eminent roles of photoinduced electron transfer and energy transfer as primary events in photobiology, it is incumbent on practitioners of the science to understand those principles which govern these elementary events. Recent developments in both theory and experiment on photoinduced electron transfer have allowed for important insights into understanding the factors governing such steps. For example, shown in Fig. 1 is a representation of the positions of the chromophores in the photosynthetic reaction center of Rhodopseudomonas sphaeroides , as determined from crystallographic measurements (Chang et al. , 1986), showing that its three dimensional arrangement within an intact membrane is analogous to that observed earlier in Rhodopseudomonas viridis (Deisenhofer et al. , 1984). In both systems, the critical event of photosynthesis is the transfer of an electron from the photoexcited special pair of bacteriochlorophylls located at the top via the pheophytin at the far right to the quinone at the bottom which acts as an ultimate repository for the separated charge. Of great importance is a determination of those factors which govern the efficiency and the rate of electron transfer through this photoinduced cascade and a better understanding of how this fixed, prearranged structure maximizes the efficiency of photochemical energy storage. Although the reaction center represents nature's most graphic and most detailed connection between the experimental and theoretical models of photoinduced electron transfer, much of the physical insight into this process has been developed on simpler organic molecules. Rattier than focusing on the details of photosynthesis, this article will concentrate on recent developments in model systems which evaluate the relative importance of factors influencing the efficiency and rates of excited state induced electron transfer. We introduce these concepts at a level appropriate for the scientifically literate biologist who has not heretofore been concerned with such details. Since this is an overview article, few citations of the original literature will appear, and the interested reader should refer to any number of excellent topical reviews which consider specific aspects of photoinduced electron transfer for further investigation (Fox and Chanon, 1989; Pac and Oshitani, 1989).