Correlations between Heisenberg spin exchange and photo-induced electron/energy transfer in polynuclear transition metal complexes.

摘要

This dissertation focuses on the phenomenon of electron exchange coupling and its effects on the kinetics and mechanisms of electron and energy transfer processes.; Bimolecular quenching between photosensitizers and exchange-coupled transition metal complexes has been studied in an effort to establish experimentally a link between Heisenberg spin exchange and chemical reactivity. The acceptors are members of the μ-(oxo/hydroxo)bis(μ-carboxylato) class of dinuclear Fe(III) compounds, where protonation of the oxo bridge provides a means for modulating the magnitude of spin exchange within the cluster (J_oxo ≅ −120 cm−1 to J_hydroxo ≅ −17 cm −1). Photoexcitation of solutions containing Ru(II)-polypyridyl sensitizers and the Fe(III) complexes resulted in quenching of emission from the 3MLCT excited state of the Ru(II) chromophores. Decoupling electron transfer driving force (ΔG₀ET) from changes in the magnitude of spin exchange was achieved by varying the bridging carboxylate to afford a series of complexes of the form [Fe₂O(H)(O ₂CR)₂(Tp)₂]n+ (n = 0, 1, 2).{09}A comparison of quenching rates for the two extended series firmly establishes that neither reorganization energy nor electron transfer driving force considerations can account for differences in reactivity between oxo-bridged (large spin exchange) and hydroxo-bridged (small spin exchange) quenchers.; In addition to studying homodinuclear Fe(III) clusters, heterodinuclear clusters were examined to understand the reactivity of these structural motifs in the absence of exchange coupling. A comparative study between the Fe(III)Fe(III) and Fe(III)Cr(III) oxo-bridged systems showed that although the Fe(III)Cr(III) electron transfer reaction was endothermic, the rate of quenching the sensitizer excited state was larger than that of the homodinuclear cluster (k_q = 2.9 × 108 M−1 s−1 and 1.8 × 108 M−1 s −1 for the Fe(III)Cr(III) and Fe(III)Fe(III) complexes, respectively). In comparing the reactivity between the oxo- and hydroxo-bridged Fe(III)Cr(III) systems, the differences in the absorption profiles led to a possible link between exchange coupling and Förster energy transfer. (Abstract shortened by UMI.)