Non-continuum contributions to the energetics of optically induced intramolecular electron transfer

Date of Award




Degree Name

Doctor of Philosophy (Ph.D.)



First Committee Member

Nita A. Lewis, Committee Chair


The Marcus-Hush models for optically induced intramolecular electron transfer assume no specific solute-medium interaction contributions to the energy barrier for the process. This assumption is shown to be inadequate and various non-continuum contributions are investigated.Using the IT band energy of a number of mixed-valence complexes as probes it was established that the observed band energy is ionic strength dependent, but concentration independent. The ionic strength was adjusted by adding such external salts as tetra-n-butylammonium hexafluorophosphate. These observations are rationalized in terms of net stabilization of the ground state potential surface, as a consequence of ion pair formation.From the ionic strength dependence data one can extract, with the Guntelberg's modification of the Debye-Huckel equation, the energy barrier to the electron transfer process at infinite dilution. This quantity, E$\sp0\sb{\rm op}$, was found to be weakly dependent on the dielectric properties of the solvent. The slope of the E$\sp0\sb{\rm op}$ dependence on the dielectric continuum parameter, (1/n$\sp2$ $-$ 1/D$\sb{\rm s}$), plot was found to be 2.02 compared with 20.7 as predicted by the Marcus-Hush models. The poor agreement may be due to the high degree of non-adiabaticity in the probe complex ($\mu$-2,6-dithiaspiro (3.3) heptanedecaamminediruthenium(II, II) hexafluorophosphate) used and/or the extensive solute-solvent interaction between the probe and the solvents. The E$\sp0\sb{\rm op}$ values were also found to depend on the redox potential of the oxidant used in generating the mixed-valence complexes for their isovalent precursors. Electrochemical measurements indicate coaggregation of the probe complex with the oxidant, used in generating the mixed valence species, to form a supercomplex which then exhibits the observed IT band.The slopes of the E$\sb{\rm op}$ dependence on the ionic strength curves were found to be solvent acceptor number dependent, which is an indication of very strong specific solute-solvent interaction. Increasing pressure was found to promote solute-solvent interaction, especially in D$\sb2$O.Finally, as the probe became non-adiabatic it became more sensitive to medium effects, as the non continuum term became more important to the energy barrier of the electron transfer process.


Chemistry, Inorganic

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