Open Access Dissertation
Date of Award
The work presented in this dissertation is focused on the design and synthesis of new redox-active amphiphilic architectures to optimize and understand the redox, electronic, and film formation properties for applications in current rectification, corrosion mitigation, and water oxidation. As such, new redox-active, and amphiphilic iron(III), cobalt(III), nickel(II), copper(II), and zinc(II) complexes were synthesized as precursors for Langmuir-Blodgett films used for the aforementioned applications.
For the use in molecular rectification, amphiphilic copper(II) and nickel(II) complexes with [N2O2] ligand environment were synthesized. Homogeneous film formation ability was observed between ~20-25 mN/m pressure, with a film collapse observed at ~35 mN/m for both complexes. The presence of both ligand- and metal-based redox processes made the copper(II) complex a viable candidate for device fabrication. However, current vs. voltage (I/V) measurements obtained for Au|LB|Au assemblies resulted in a flat I/V curve, diagnostic of an insulating nature. Comparison of the frontier molecular orbital energy between the insulating copper(II) complex and a similar rectifying iron(III) complex provided insight as to the rectification mechanism. Considering the orbital arrangement, the singly occupied molecular orbital (SOMO) of the Cu(II) complex is energetically high to enable electron transfer, thus making it an insulator. For comparison purposes, the SOMO of Fe(III) is situated 1.0 eV above the Fermi level of the Au electrode allowing for easy electron transfer from electrode to molecule. Furthermore, the highest occupied molecular orbital (HOMO) of Cu(II) is situated 1.0 eV below the Au Fermi level and the insulating nature of the device confirms that the HOMO is not involved in electron transfer. Therefore, only the SOMO of the Fe(III) complex should be involved in electron transfer following an asymmetric current rectification mechanism. This observation confirms that the SOMOs of such molecules can act as electron acceptors to facilitate electron transfer and assist in current rectification. In this regard, to enhance the energy compatibility between the SOMO and the Fermi level of the Au electrode, a series of new asymmetric iron(III) complexes with [N2O2], [N3O], and [N3O2] coordination environments were synthesized. According to isothermal compression data, new complexes exhibited amphiphilic nature with collapse pressures between ~35-40 mN/m. Calculations performed based on the redox potentials of the complexes showed that the SOMO energies of the Fe(III) complexes with [N2O2] coordination environment were situated 0.4 eV above the electrode Fermi level, while the SOMO energies of other complexes were situated 0.8 eV above the Fermi level. The energetic compatibility of these SOMOs with electrode Fermi levels makes these complexes viable candidates for molecular current rectification. Therefore, Au|LB|Au assemblies can be used to identify the rectification behavior of theses complexes.
Based on the knowledge that Cu(II) salophen-based complexes can insulate electron transfer and therefore preclude electron transfer, a series of amphiphilic Fe(III), Cu(II), and Zn(II) complexes were synthesized to be used as protective coatings in corrosion mitigation. Cyclic voltammetry experiments revealed that 11-layer LB films of the complexes can effectively passivate electron transfer to the surface in a better way than the ligand alone. Agar experiments revealed a low corrosion rate for LB film-coated iron plates, with no blue coloration in the complex-coated area after one week. The presence of K3Fe(CN)6 in the agar medium can form a Prussian blue complex with Fe2+ ions produced due to oxidation. Optical micrograph and SEM images showed considerably low rust formation on complex-coated iron plates compared to the bare iron plate. Weight loss measurement studies confirmed that 11 layers of the Fe(III) and Zn(II) complexes demonstrate the best corrosion mitigation ability, with respective corrosion inhibition efficiencies of 27 % and 30 %, while the ligand alone showed an inhibition efficiency of only 6 %. This observation suggests that metal coatings can function as passivating barriers to electron flow between the electrolyte and the iron plate.
Similarly, the use of LB films in heterogeneous water oxidation was investigated using a phenolate-rich cobalt(III) complex. This complex demonstrated homogeneous film formation ability at ~30-35 mN/m, with a collapse pressure of 42 mN/m. Monolayer-deposited FTO electrodes supported water oxidation at an overpotential of 0.50 V. Gradual enhancement of catalytic activity was observed in up to 9 deposited layers. Upon application of a potential bias, the monolayer yielded an estimated turnover number of 54,000 ± 1,500 after one hour with a Faradaic efficiency of ~ 100 %. Although the molecular species were rearranging into an ultrathin catalytic layer, the presence of symmetric and asymmetric C-H vibrations in IRRAS spectra suggests that carbon-based residues act as modifiers, thus confirming that the ligand choice is relevant to obtain efficient and robust catalytic films for water oxidation.
In summary, this dissertation research presented a new series of ligand designs varying from [N2O2], [N3O], and [N3O2] and their iron(III), cobalt(II), nickel(II), copper(II), and zinc(II) complexes, which exhibit both redox and amphiphilic character. Further, this project investigates their LB film formation ability and use of these films on solid substrates in molecular electronics, corrosion mitigation, and heterogeneous water oxidation.
Jayasundara Mudiyanselage, Sunalee Gonawala, "Electron Transfer Studies In Langmuir-Blodgett Films Of Metallosurfactants For Current Rectification, Corrosion Mitigation, And Water Oxidation" (2016). Wayne State University Dissertations. 1546.