Molecular species containing two or more paramagnetic centers in close proximity may exhibit properties that are distinct from those of the individual centers. In these spin-coupled systems, the spin exchange interaction creates an array of new spin states within both the ground- and excited-state manifolds, thereby impacting magnetic as well as optical properties of the system. Research in our group focuses on the influence that changes in electronic structure due to spin exchange have on reactivity, specifically with regard to photo-induced electron and energy transfer processes. Previous work in our group has suggested a connection between the strength of spin exchange and the rate constant for bimolecular electron transfer. To circumvent the problems inherent to bimolecular systems (diffusion-limited kinetics, poorly defined donor-acceptor distance and/or relative orientation) we have designed donor-acceptor (D-A) assemblies using a μ-oxo μ-carboxylatodiiron(III) core acceptor covalently linked to a Ru(II) polypyridyl donor, where the strength of spin coupling within the diiron(III) core can be modified by protonating the oxo bridging group. Thus, Heisenberg spin exchange may be used to alter the spin states available to the acceptor without changing its composition and/or structural properties.In this dissertation, the synthesis and characterization of several D-A systems are discussed; steady-state and time-resolved absorption and emission spectroscopies were used to explore the effect of spin exchange in electron and energy transfer processes taking place within these assemblies. Our results show that electron transfer into excited spin states may be invoked to explain some of the reactivity trends observed. The major drawback of the synthetic design used to prepare the D-A systems studied in this work is their lability, which in some cases led to rapid decomposition of the assemblies in solution. Synthetic efforts to build more robust D-A complexes are also described.
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