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 been reported recently for Ir-2 excited state donors linked to substituted pyridinium accepters. These data pose a substantial paradox. Simple analysis suggested that donor-acceptor coupling matrix elements differ by 1 order of magnitude for the excited triplet and singlet states. Yet, there is no fundamental reason to expect this large electronic coupling dependence on spin state. We offer an alternative self-consistent interpretation based on a hybrid theoretical analysis that includes ab initio quantum calculations of electronic couplings, molecular dynamics simulations of molecular geometries, and Poisson-Boltzmann computations of reorganization energies. Taken together the analysis provides a detailed comprehensive interpretation of these reactions. In our analysis, we reach the conclusions: (1) that reorganization energies in 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 molecules have access to multiple conformations differing both in reorganization energy and electronic coupling, and explicit treatment of this flexibility is crucial to interpret the rate data, and (4) that a considerable dip is expected in the donor-acceptor coupling dependence on tunnelling energy, associated with destructively interfering electron and hole-mediated coupling pathways, which probably leads to a small observed ET rate in one of the compounds. Taken together, this analysis explains most of the experimental data. Fundamental arguments and detailed computations show that the influence of donor spin state on long-range electronic interactions is relatively weak. Many of the molecular aspects that establish the ET characteristics of these molecules exist in other semirigid model compounds, making this hybrid theoretical strategy of general interest.