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Influence of Protonation State on the Electronic Properties of Ruthenium Complexes Containing Hydroxyl-Substituted Terpyridine Ligands

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Description: In Proton Coupled Electron Transfer (PCET) mechanisms, electron transfer reactions are frequently accompanied by a change in proton content. Metal complexes with ligands capable of donating or accepting protons could be developed for PCET catalysts driven by the absorption of light. This would help give promise towards solar energy production. The complexes [Ru(tpy)(tpyOH)] 2+ and [Ru(tpyOH) 2 ] 2+ (tpy = 2,2':6', 2''-terpyridine, tpyOH = 4'-hydroxy- 2,2':6', 2''-terpyridine) have been synthesized and studied along with the widely studied [Ru(tpy) 2 ] 2+ complex as a control. Electrochemically, the tpy=O ligand (2,6-bis(2-pyridyl)- 4(1 H )-pyridone), which is a tautomer of the tpyOH ligand, undergoes an irreversible reduction processes. When the ligand is complexed to ruthenium, reversible redox waves are seen for both the [Ru(tpy)(tpyOH)] 2+ and [Ru(tpyOH) 2 ] 2+ complexes. The increase of electron donating tpyOH ligands on the ruthenium center leads to a decrease in the reduction potential of the Ru II/III waves. A full solvent study (acetonitrile, acetone, 1,2-dicholorethane, methanol and water) utilizing UV-Visible spectroscopy was done to understand effects of solvent on the absorption profiles of the complexes. pH studies of the [Ru(tpy)(tpyOH)] 2+ (pK a = 5.60) and [Ru(tpyOH) 2 ] 2+ (pK a = 5.78) complexes in buffer solutions show shifts to lower energies as the pH becomes basic. Spectroelectrochemical studies of the complexes reveal a reversible decrease in the metal-to-ligand charge transfer (MLCT) band as a potential is held.
Language: English
Format: Degree Work