Dynamics of Intramolecular Electron Transfer in Polar Solvents

Abstract

When the electronic interaction between an electron donor (D) and acceptor (A) is very weak, the electron transfer (ET) process is considered to be non-adiabatic. When the interaction becomes fairly strong, the reaction will become adiabatic. If the electronic interaction becomes sufficiently strong and the energy gap relations are also favorable, the ET process will become barrierless. In such a case it is believed that the ET process is governed mainly by the orientational motions of polar solvent molecules or polar groups in the environment surrounding D and A, and the longitudinal dielectric relaxation time τ L will be important as a factor controlling the ET rate. In the limit of strong interaction between D and A groups combined by a rigid spacer or single bond, its excited singlet state can be regarded as a very polar single molecule and we can observe a large fluorescence Stokes shift due to the solvation in polar solvents. For the elucidation of the above mechanisms, especially the interactions of D and A with solvent including its dynamical effects on the ET process, systematic femtosecond-picosecond laser photolysis studies on various combined D and A systems with different degrees of electronic interaction between them are of crucial importance.