Current experimental efforts, both with isolated binuclear metal complexes and with modified proteins, are focusing on the role of distance and of bridging groups in intramolecular electron transfer. After a brief overview both of standard and nonadiabatic electron transfer rates (in Hopfield's formulation) and of the current experiments. We consider the possiblility of bridge‐assisted intramolecular electron transfer in three specific situations. For the bridged binuclear transition‐metal complexes studied by Taube and his students, the tunneling integral T can be deduced by study of the optical intervalence transfer band. Here we discuss semiempirical calculations which suggest that bridge assistance occurs, and determine the size of T. The assistance is via a superexchange‐type mechanism, and we suggest that hole‐type or electron‐type superexchange should dominate in saturated or unsaturated bridges, respectively. For the very strongly coupled Creutz–Taube ion, involving a pyrazine bridge, the electronic structure study of Ondrechen et al. requires the invoking of a three‐site model to understand the optical and EPR data; this involves bridge‐assisted transfer with a vengeance. Finally, for an oxide‐bridged phthalocyanine dimer, the transfer takes place via ring π‐π overlap, and no bridge assistance occurs. Thus bridge assistance in T will depend on the geometry and energetics of the specific case under study. One generally suspects, however, that bridge assistance will be found in very long‐range (>6 Å) transfer.
ASJC Scopus subject areas
- Atomic and Molecular Physics, and Optics
- Condensed Matter Physics
- Physical and Theoretical Chemistry