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Molecular Design and Synthesis of Ligand-to-Ligand Charge-Transfer Complexes of Ni(II) and Ru(II) Using Redox-Active Ligands

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The work described in this dissertation focuses on the rationale behind the design and synthesis of ligand-to-ligand (LL′CT) charge-transfer chromophores supported by redox-active ligands. The molecular chromophores reported herein are meant to interest scientists in materials chemistry, physics, and chemical engineering that wish to employ molecular photosensitizers in photovoltaics and/or in artificial photosynthetic systems.

Chapter 1 provides background information on redox-active ligands, their role in non-innocent coordination compounds, the importance of photo-induced excited-states, and demonstrates the ability to tune excited-state properties through molecular design. Chapter 2 introduces four Ni(II) square-planar LL′CT chromophores that are a part of a larger group of square-planar Ni(II) complexes originally published together. These complexes posses HOMO and LUMO orbitals that are localized on the redox-active ligands and that their energies can be controlled independently through chemical modification to the catecholate donor and diimine acceptor.

Chapter 3 describes new Ni(II) square-planar donor-acceptor (D-A) chromophores designed absorb near-IR photons and access excited-state oxidation potentials capable of electron injection into metal oxide (semiconductor) surfaces. Bipyridyl-type acceptor ligands are employed to maintain a LUMO at high energy. Incorporation of the strongly reducing amidophenolate donor ligand is meant to destabilize the HOMO energy and push the LLʹCT absorption into the NIR (λmax: 890 and 970 nm). According to the electro- and spectrochemical data the (amidophenolate)Ni(acceptor) dyes reported in herein are estimated to access excited-state oxidation potentials potent enough to populate the conduction band of TiO2 (-0.7 V vs. SCE).

Chapter 4 details the investigation of two square-planar Ni(II) D-A LL′CT dyes from Chapter 3 that are equipped with carboxyl anchoring groups for tethering to metal oxide surfaces. Upon optical excitation, the dyes reported herein maintain their excited-state reductive potency. Preliminary binding studies on TiO2 thin films suggest successful dye adsorption but film degradation occurred once exposed to air. Although the air sensitivity of the dye-functionalized film is problematic, future precautions such as air-free studies can be made in order to test their efficacy to inject electrons into large band gap semiconductors.

Chapter 5 introduces and investigates three six-coordinate D-A Ru(II) charge-transfer dyes with the general formula: (donor)Ru(N2N2q) [donor = 2 Cl–, (catB4)2– and (cat)2–]. The co-planar arrangement of donor and acceptor orbitals was achieved through the use of the tetradentate, redox-active, N,Nʹ-bis-(3-dimethylaminopropyl)-4,5-dimethoxy-benzene-1,2-diiminoquinone, (N2N2q). These new octahedral D-A Ru(II) charge-transfer complexes demonstrate that the tetradentate N2N2q acceptor ligand, when coordinated to a d6 metal ion, allows for the co-planar installation of redox-active donor ligand. Furthermore, the lowest energy transitions of the complexes reported herein are heavily dependent on the identity of the donor ligand, suggesting it is LL′CT in nature. Thus far, our findings suggest that the optical and ground-state redox properties of dyes based on a d6 metal ion and the N2N2q ligand can be tuned through ligand modification. This may open up new areas of research where, instead of a precious heavy metal, such as ruthenium, the earth abundant Fe(II) ion could be used to realize a truly robust and tunable earth abundant charge-transfer photosensitizer.

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