Theory and computational modeling: Medium reorganization and donor/acceptor coupling in electron transfer processes

摘要

The continuing goal is to convert the rapidly accumulating mechanistic information about electron transfer (ET) kinetics (often representable in terms of simple rate constants) into precise tools for fine-tuned control of the kinetics and for design of molecular-based systems which meet specified ET characteristics. This treatment is limited to the kinetic framework defined by the assumption of transition state theory (TST). The primary objective of this paper is to report recent advances in the theoretical formulation, calculation, and analysis of energetics and electronic coupling pertinent to ET in complex molecular aggregates. The control of ET kinetics (i.e., enhancing desired processes, while inhibiting others) involves, of course, both system energetics, especially reorganization energies E(sub r) and free energy changes Delta G(sup 0) and electronic coupling of local D and A sites, which for thermal processes is most directly relevant only after the system has reached the appropriate point (or region) along the reaction coordinate (i.e., the transition state). The authors first discuss TST rate constant models, emphasizing genetic features, but also noting some special features arising when metal electrodes are involved. They then turn to a consideration of detailed aspects of medium reorganization and donor/acceptor coupling. With these theoretical tools in hand, they examine the results of recent applications to complex molecular systems using the techniques of computational quantum chemistry and electrostatics, together with detailed analysis of the numerical results and comparison with recent electrochemical kinetic data.