Direct experimental comparison of the theories of thermal and optical electron-transfer: Studies of a mixed-valence dinuclear iron polypyridyl complex

The spectral parameters for the optically induced intervalence charge transfer and the rates of thermal electron transfer as a function of temperature have been measured for a rigid, triply linked mixed-valence dinuclear tris(2,2'-bipyridine)iron complex. The total reorganizational energy associated with the intramolecular electron exchange in this complex is almost exclusively outer-sphere in nature and comes from thermal. fluctuations of the solvent. Thus, the system can be treated rigorously at the Classical level, where in this context classical refers to treatments of the nuclear modes. The theories developed to describe the optical electron transfer and the thermal electron. transfer are evaluated by analysis of the spectral and rate data, respectively. The quantities common to both theories are the donor-acceptor coupling matrix element, H₁₂, and the total reorganizational energy. Applying the respective theories to the appropriate corresponding sets of data yields reorganizational energies that are inexcellent agreement irrespective of the manner in which the temperature dependence is treated; however, if the reorganizational energy is assumed to be temperature independent, H₁₂(th) (from the rate data) and H₁₂(op) (from the spectral data) differ by a statistically significant factor of ~2.5. If the theoretically predicted temperature-dependent reorganizational energy composed of orientational reorganization of permanent dipoles and reorganization of solvent density is used in the calculations, the agreement between H₁₂(op) and H₁₂th improves dramatically. To our knowledge, this work constitutes the first attempt to experimentally compare these two classical theories with this level of rigor. Supplementing the experimental comparisons, we have conducted self-consistent-field (SCF) and configuration interaction (CI) calculations to obtain theoretical values of H₁₂(op) and the donor-acceptor orbital separation, r, for comparison with experimentally determined values.