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Access Type
WSU Access
Date of Award
January 2024
Degree Type
Dissertation
Degree Name
Ph.D.
Department
Chemistry
First Advisor
Claudio N. Verani
Abstract
This dissertation research focused on two main objectives: (i) developing newly substituted redox-active metallosurfactants and hydrophobic metal complexes for current rectification, and (ii) creating a new immobilization strategy to form hierarchical self-assembled bilayers on a conductive substrate. These bilayers, which include a chromophore and a catalyst, are intended for use in water-splitting catalysis. The first objective was probed based on the hypothesis that halogen substitution of the ligand framework can reduce the electron density around the metal center of a complex thereby reducing the Au Fermi-SOMO that will facilitate directional electron transport. Furthermore, it was hypothesized that the redox activity and film formation ability due to bent spatial geometry by a hydrophobic metal complex may promote directional electron transport. For the second objective, we hypothesized that the introduction of alkoxy chains in a catalyst would lead to more stable chromophore/catalyst bilayer structures due to the incorporation of stronger dipole interactions between the alkoxy groups on the catalyst and the alkyl moieties on the anchoring chromophore.As such, new iodo-substituted phenolate-based amphiphilic and redox-active amino catechol-based hydrophobic iron(III) complexes were synthesized and characterized for current rectification studies. On the other hand, alkyl and alkoxy-substituted amido-based cobalt(III) complexes were designed, synthesized, and characterized. These water oxidation catalysts were used together with ruthenium(II) bpy-based chromophore synthesized by Prof. Javier Concepcion to develop and characterize self-assembled bilayers for future catalytic studies. First, we evaluated the effects of peripheral changes and the development of 3d5 FeIII systems where iodo groups replace the usual tert-butyl groups attached to phenylenediamine-bridged phenolate donors. Two new metallosurfactants were designed, namely [FeIII(L1)Cl] (1) and [FeIII(L2)Cl] (2), where 1 incorporates shorter hydrophilic alkoxy chains than 2. These species had their electronic and electrochemical properties evaluated by experimental and computational methods. We also considered ancillary changes, such as subphase polarity effects on their interfacial properties of Pockels-Langmuir monolayer films on water and Langmuir-Blodgett monolayer films on gold electrodes, evaluated by multiple surface-dedicated methods and probed for directional electron transfer in Au|LB|Au and EGaIn|LB|Au junctions, prior to comparison to the previously published t-Bu substituted standard [FeIII(LtBu)Cl] (3). These modifications led to significant modulation of metal-based SOMO energy levels towards the Fermi energy levels of the electrode from 1.0 eV in complex 3 to ca. 0.5 eV in complex 1 and 2, which resulted in better currents observed than those for complex 3. To further explore the possible modulation of the Fermi/SOMO/HOMO gaps, we synthesized and structurally characterized an unprecedented hydrophobic 3d5 HSFeIII complex with o-iminobenzosemiquinonate and o-imino-phenolate derivatives, designated as [Fe(L)(Cl)] (1), both in solid state and solution. The electronic structure and redox behavior of this species were thoroughly investigated through experimental techniques and density functional theory (DFT) calculations, revealing the availability of energetically accessible ligand-based molecular orbitals relevant for directional electron transport. This species successfully forms stable Pockels–Langmuir films at the air|water interface, regardless of its hydrophobic nature enabling the deposition of thin films on gold and eutectic gallium indium electrodes to create nanoscale Au|LB|Au and EGaIn|LB|Au junctions for current-voltage (I/V) measurements. Remarkably, this species demonstrated a proposed rare asymmetric or unimolecular electron transport with a maximum rectification ratio of 32, attributed to possible electron tunneling through the HOMO and LUMO of the aminocatechol moiety. To probe the new catalyst immobilization strategy, we designed two CoIII-based molecular catalyst candidates, namely [CoIIIL1(pyrr)2]ClO4 (Co1) and [CoIIIL2(pyrr)2]ClO4 (Co2) where L1 and L2 are the respective deprotonated forms of N,N′-[4,5-bis(dodecyloxy)-1,2-phenylene]dipicolinamide, and N,N′-[4,5-bis(methoxyethoxy)-1,2-phenylene]dipicolinamide and characterized their electrochemical, electronic, and film formation properties. Species Co1 and Co2 were deposited by means of a hierarchical self-assembled strategy in which an anchor molecule such as octylphosphonic acid (OPA) or the chromophoric [RuII(bpyPO3H)2(bpyC7)]Cl2 (Ru) was deposited onto conductive FTO prior to receiving the cobalt species. Four hierarchical films were obtained, namely, FTO|OPA-Co1, FTO|OPA-Co2, FTO|Ru-Co1 and FTO|Ru-Co2, and the role of dipole-dipole interactions between anchor and catalyst candidate was assessed. These newly-synthesized hierarchical films were characterized by a host of surface-specific methods that include X-ray photoelectron spectroscopy, spectroscopic ellipsometry, X-ray fluorescence, and contact angle, thus enabling an unprecedented level of analysis. Compared to the weak C—H van der Waals interactions exhibited by Co1, the presence of alkoxy chains in Co2 ensures stronger dipole-dipole interactions with the alkyl chain of the anchors due to O–H formation. The persistence of their redox properties, which include metal oxidation and directionality of electron transport, were probed, suggesting direct relevance to catalytic processes such as water oxidation. This dissertation research presented new classes of substituted bisphenolate, aminocatechol, and amido ligand-based systems and their iron(III), and cobalt(III) complexes, that exhibit amphiphilic or hydrophobic and redox properties being merged to facilitate directional electron transport. This dissertation project facilitated the understanding of structural, electronic, redox, spatial geometric, and interfacial properties of different classes of metal complexes and their specific roles in electron transport behavior determined from electrical property studies. Furthermore, this dissertation facilitated the understanding of a new strategy for immobilizing molecular water-splitting catalysts on conductive substrates using phosphonated anchors like octylphosphonic acid and phosphonated ruthenium(II) bpy-based chromophores. The bilayers containing substrate|chromophore-catalyst showed electrochemical and structural stabilities in different media, suggesting the possible applications in water oxidation or reduction.
Recommended Citation
Kirui, Gibson, "Studies On Electron Transport In Ordered Mono- And Multilayered Films Of New Metal Containing (fe, Co, Ru) Amphiphiles, Hydrophobes And Hierarchical Self Assemblies" (2024). Wayne State University Dissertations. 4097.
https://digitalcommons.wayne.edu/oa_dissertations/4097