STRUCTURAL FLUCTUATIONS, SPIN, REORGANIZATION ENERGY, AND TUNNELING ENERGY CONTROL OF INTRAMOLECULAR ELECTRON-TRANSFER - THE SURPRISING CASE OF ELECTRON-TRANSFER IN A D(8)-D(8) BIMETALLIC SYSTEM

A considerable body of unimolecular electron-transfer rate data has be en reported recently for Ir-2 excited state donors linked to substitut ed pyridinium accepters. These data pose a substantial paradox. Simple analysis suggested that donor-acceptor coupling matrix elements diffe r by 1 order of magnitude for the excited triplet and singlet states. Yet, there is no fundamental reason to expect this large electronic co upling dependence on spin state. We offer an alternative self-consiste nt interpretation based on a hybrid theoretical analysis that includes ab initio quantum calculations of electronic couplings, molecular dyn amics simulations of molecular geometries, and Poisson-Boltzmann compu tations of reorganization energies. Taken together the analysis provid es a detailed comprehensive interpretation of these reactions. In our analysis, we reach the conclusions: (1) that reorganization energies i n these systems (similar to 1.3-1.7 eV) are larger than expected from simple analysis of experiments, (2) that electronic couplings (similar to 0.005-0.02 eV) are also larger than previously believed and differ only by a factor of 2 for singlet and triplet states, (3) that the mo lecules have access to multiple conformations differing both in reorga nization energy and electronic coupling, and explicit treatment of thi s flexibility is crucial to interpret the rate data, and (4) that a co nsiderable dip is expected in the donor-acceptor coupling dependence o n tunnelling energy, associated with destructively interfering electro n and hole-mediated coupling pathways, which probably leads to a small observed ET rate in one of the compounds. Taken together, this analys is explains most of the experimental data. Fundamental arguments and d etailed computations show that the influence of donor spin state on lo ng-range electronic interactions is relatively weak. Many of the molec ular aspects that establish the ET characteristics of these molecules exist in other semirigid model compounds, making this hybrid theoretic al strategy of general interest.