OLIGOPEPTIDE DERIVATIZED [Ru(bpy)3]2+ COMPLEXES AS SCAFFOLDS FOR ARTIFICIAL PHOTOSYNTHETIC PROCESSES

Open Access
Author:
Sun, Sha
Graduate Program:
Chemistry
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
October 12, 2015
Committee Members:
  • Mary Beth Williams, Dissertation Advisor
  • Tom Mallouk, Committee Member
  • Benjamin James Lear, Committee Member
  • John H Golbeck, Committee Member
Keywords:
  • [Ru(bpy)3]2+
  • Artificial Photosynthesis
Abstract:
The design of supramolecular systems with controlled arrangement of chromophores, electron/energy donors and acceptors remains a challenge in artificial photosynthesis. Biological systems have been known to accomplish the intricacy presented simply by self-assembly. With this inspiration, biomimetic structures that utilize self-assembly to tether donors and acceptors together have been developed and studied. Using oligopeptides as the donor-acceptor bridging linkers and metal-ligand interactions as the only driving force for self-assembly, a variety of compounds with unique photophysical properties could be synthesized. Study of these compounds would provide useful information to fine tune the structures to create long-lived charge separated state and promote efficient solar energy conversion. This thesis describes the synthesis, characterization and application of a series of oligopeptide derivatized [Ru(bpy)3]2+ complexes for artificial photosynthesis. Factors affecting photophysical properties are investigated and the first example of using this motif to photocatalyze chemical reactions is thoroughly studied. A series of [Ru(bpy)3]2+ compounds linked to 1-3 Pd2+ by oligoaminoethylglycine(aeg) were synthesized for use in photocatalytically dimerizing α-methylstyrene . This is the first example using the aeg chain between a photosensitizer and a reaction center. The products had faster reaction rates and better selectivity than conjugated linkers described in the literature. Catalytic efficiency when linking [Ru(bpy)3]2+ with 1, 2 and 3 Pd2+ centers was also compared and it was found the complex with three Pd centers had the lowest catalytic activity because of lower chemistry stability. The excited state quenching mechanism is likely to be an electron transfer process based on solvent study, variable temperature study and control experiments using added sacrificial electron donor. A series of [Ru(bpy)32+] hairpin structures were designed to study impact of side chain change of different oligopeptide linkers (e.g. aminoethyglycine (gly), aminoethyvaline (val), aminoethyleucine (leu) and aminoethyphenylalanine (phe) ) on the emission photodynamics of [Ru(bpy)3]2+-[M(bpy)2]2+ complexes (M = Cu, Pd or Zn). For the Ru-Cu and Ru-Pd complexes, the non-radiative decay rate decreases and the energy barrier for non-radiative decay process increases as the steric bulk of the side chain increases. However, no trend was observed for the Ru-Zn complexes. Solvent study showed all the Ru-Cu and Ru-Pd complexes have a negative trend of non-radiative decay rates vs. Pekar factor but no trend was observed for the Ru-Zn complexes, which indicates Ru-Cu and Ru-Pd have an electron transfer non-radiative decay mechanism and is different from the mechanism of Ru-Zn. The impact of side chain change on the non-radiative decay of Ru-Cu and Ru-Pd is probably due to bulky side chains restrict the necessary geometry change that accompanies the electron transfer. To expand our study to hexacoordinate metal ions, we have developed a series of [Ru(bpy)3]2+ complexes with three pendant bpy ligands that can directly coordinate to a hexacoordinate metal ion such as Fe2+, Co2+, Ni2+ or Mn2+ to form [Ru(bpy)3]2+-[M(bpy)3]2+. Spectrophotometric emission and absorption titrations confirmed a 1:1 metal binding stoichiometry and this was further supported by mass spectrometry and elemental analysis. The formation of [Ru(bpy)3]2+-[M(bpy)3]2+ also caused quenching of the excited state emission of [Ru(bpy)3]2+ by ~ 50% - 90%. Energy transfer mechanism is speculated to be the plausible explanation for this nonradiative quenching because of high energy barrier for electron transfer and spectrum overlap. Finally, synthetic attempts towards multi-metallic, multi-functional mixed ligand complexes for redox cascade is described in the last chapter. Ru hairpin structures with 8-hydroxyquinoline as the pendant ligands were synthesized. Binding Cu2+ quenched the emission intensity of [Ru(bpy)3]2+ by electron transfer. Binding Zn2+ formed a two-chromophore complex, and the quenching mechanism is still under investigation. Structures that could potentially be used for redox cascade have been designed and synthesized. Experiments using Ru-Co complexes to split water were proposed.