An Investigation of the Electronic Coupling in Some Dimeric Ruthenium (II) Polypyridine Complexes
Date of Graduation
Doctor of Philosophy in Chemistry (PhD)
Chemistry & Biochemistry
Second Committee Member
Third Committee Member
A detailed understanding of respiration at the molecular level requires an understanding of the many electron transfer steps involved in the process. These electron transfer processes are extremely fast and are impossible to measure by simple rapid mixing techniques. In order to get around this problem, scientists have used laser flash photolysis. This technique relies on the fact that under proper conditions, a reactant can be generated by a very short laser pulse. Once generated, the course of the reaction can be monitored by various techniques capable of very rapid time response. Many applications of this methodology rely on the use of ruthenium (II) polypyridine complexes to initiate the reactions of interest. This approach has been used to study the rates of electron transfer between cytochrome c, and cytochrome b5, cytochrome peroxidase and cytochrome oxidase and the bc1 complex. The latter are key components in the respiration process. In these investigations special emphasis was placed on the design of ruthenium complexes that were efficient and compatible with the biological components. A thorough understanding of the design parameters are critical to continued success in this area.
Dimeric ruthenium complexes at the current time appear to be among the best candidates for photochemical initiators. The photophysical properties of these complexes, however, have not yet been examined. In particular the excited-state lifetime of some of the monomers of interest appears to be comparable or even longer than the corresponding dimers. This observation is inconsistent with the single covalent bond that links the two monomeric units which would provide strong electronic coupling and rapid excited state decay. Preliminary observations suggest a very weak electronic coupling. The underlying basis of this inconsistency is important in future design endeavors and may provide useful information for the use of these complexes in other areas such as solar energy conversion.
In order to investigate the magnitude of the electronic coupling, both symmetric and asymmetric ruthenium (II) dimeric complexes were synthesized. The ligands used in the synthesis of these dimers were limited to either those commercially available or those that could be easily synthesized. The symmetric ruthenium (II) bipyridine dimer ([Ru(bpy)2diphen(bpy)2](PF6)4)and ([Ru(TAP)2diphen(TAP)2](PF6)4 were synthesized through a nickel catalyzed coupling reaction . The asymmetric dimer ([Ru(bpy)2diphen(dmbpy)2](PF6)4 ) on the other hand was synthesized by decarbonylating [Ru(dmbpy)2(CO)2](PF6)2 with three fold of excess trimethylamine N-oxide in the presence of 2-methoxy ethanol and reacting it with [Ru(bpy)2diphen](PF6)2.
Emission measurements confirmed that there is no significant difference in the excited state lifetime of the monomers and the dimers (both symmetric and asymmetric) used in this study. The result from our electrochemical studies showed that the mixed dimer complex was made up of two metal centers with different redox potentials. The symmetric dimer on the other hand has the same redox potential for each of the two metal centers and they do not interact with each other thus giving a single two electron oxidation at the same potential. Finally, our result from the emission study of the mixed dimer showed that the emission energy of the mixed dimer was equal to the average of the bpy and dmbpy dimers.
From the photochemical studies, one can conclude that the mixed dimer and the symmetric dimers behaved as the monomers because there was no significant change in the excited state life time This indicates that the metal center of both the mixed dimer and the symmetric dimers are weakly coupled by the bridging ligand and there is no significant coupling between the two metal centers.
Ngebichie Njabon, Roland, "An Investigation of the Electronic Coupling in Some Dimeric Ruthenium (II) Polypyridine Complexes" (2013). Theses and Dissertations. 678.
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