Design and regulation of efficient photoinduced electron transfer in macromolecular and photosynthetic systems

The factors that govern efficient electron transfer in the initial steps of photosynthetic charge separation are discussed. The dependence of the electron transfer rate constant on free energy, temperature, and distance are described both in theory and in numerous experiments on photosynthetic and macromolecular systems, with particular attention to those aspects of macromolecular charge transfer systems that show strong analogy to characteristics of photosynthetic charge transfer reactions. The unique features of the primary charge separation reaction in photosynthesis are emphasized in light of recent experimental data, including time-resolution of excited state vibrational motion, the electric field dependence of the quantum yield, and resonance Raman and hole-burning experiments that probe the nature of the initially formed excited state. The experimental results indicate the need for further development of electron transfer theory to include nonequilibrium vibrational populations and more explicit models of higher-order electronic coupling.