Access Type

Open Access Dissertation

Date of Award

January 2014

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Chemistry

First Advisor

Claudio N. Verani

Abstract

LANKA D. W. WICKRAMASINGHE ARACHCHILAGE

August 2014

Advisor: Professor Cláudio N. Verani

Major: Chemistry (Inorganic)

Degree: Doctor of Philosophy

The silicon-based microelectronic industry has made remarkable technological advancements. Among these, new generation consumer electronics, telecommunication devices, and high performance data processing smart devices are of special interest. At present, electronic components existing on a single silicon chip grow rapidly, and soon the miniaturization process of electrical components will face limitations due to heat dissipation. Therefore, as an advanced alternative for silicon-based electronic components, the investigation of nanoscale molecular electronic devices is of great importance. This dissertation research is focused on the development of redox-active asymmetric metallosurfactants as potential candidates for molecule-based electronics. Asymmetric donor-acceptor type [D-A] molecules are promising candidates to study the current rectification behavior. In order to address this phenomenon, a series of ligands with phenolate moieties and, their gallium(III), iron(III), and manganese(III) complexes with [N2O3] and [N2O2] coordination environments were synthesized and structurally characterized using multiple methods. Ligands were designed with different electron-donating and electron-withdrawing substituents, to modulate structural, chemical, and physical properties, such as geometry, spectroscopic, redox, film formation, and electrical properties.

As an initial attempt, low symmetry gallium(III) and iron(III) complexes with [N2O3] binding moieties were developed. Different alkyl substituents are introduced to the phenylenediamine moiety of the main ligand structure to allow for film formation of the metal complexes. The electrochemical properties of gallium(III) complexes show ligand-based redox processes, while the iron(III) complexes show metal- and ligand-centered redox processes. The EPR data of iron(III) complexes indicate the formation of high spin species under [N2O3] coordination environment. Metal complexes with methoxy and methoxyethoxy substituents on the phenylenediamine moiety, show formation of homogeneous conformal thin films at the air/water and air/solid interfaces. However, a possible amine/imine conversion is observed for gallium(III) and iron(III) complexes at the air/water interface. This interconversion is evident in the spectroscopic data of gallium(III) and iron(III) thin films. This study exhibits promising results in merging surfactant and redox properties into a single molecule. The iron(III) complexes indicate, that phenolate moieties act as donors when coordinated trivalent metal ions and the iron(III) center acts as an electron-acceptor moiety. These data suggest further investigations for iron(III) complexes to study their electrical properties followed by device fabrication.

When designing a current rectifier, energy of frontier molecular orbitals should be comparable to energy of the metal Fermi levels. Then, the potential candidate can show an efficient electron transfer between the molecule and metal electrodes. In addition, if the molecule shows lower HOMO-LUMO energy gap, a unimolecular current rectification mechanism is applicable. Such situations facilitate efficient internal electron transfer pathways. The best possible way to modulate the HOMO-LUMO energy difference is the introduction of electron-withdrawing substituents into the molecular design. If these substituents are redox-active, then the overall redox properties of the molecule can be enhanced. In order to achieve this goal, two nitro substituted iron(III) complexes with [N2O3] and [N2O2] coordination environments were investigated. Both complexes showed pentacoordinate geometry around the metal ion. These two iron(III) complexes showed excellent redox properties with lower potential differences between the first oxidation and reduction peaks, when compared to alkyl substituted iron(III) complexes. However, the iron(III) complex with [N2O3] donor set was found to be a better redox candidate when compared to the other. The density functional theory (DFT) calculations suggest, that the first cathodic process to be iron(III)/(II) redox couple, and the other cathodic and anodic redox processes to be nitro reductions and phenolate oxidations, respectively. Both complexes form mono and multilayers at the air/solid interface. Spectroscopic and surface analyses of these two complexes show formation of well-ordered conformal LB films with intact molecular structures. Therefore, this investigation suggests that nitro substituted iron(III) complexes are successful in obtaining more redox accessible states with low HOMO-LUMO energies when compared to iron(III) complexes with electron-donating substituents. The modulation of HOMO and LUMO energies is important when designing molecular rectifiers.

Device fabrication studies were performed to investigate the feasibility of applying iron(III) complexes with [N2O3] environments in molecule-based electronics. An iron(III) complex [FeIIIL4] with methoxyethoxy substituents was investigated, and this complex showed the iron(III)/(II) reduction process at -1.49 V vs. Fc+/Fc. The iron(III) complex formed well-organized uniform thin films at air/water and air/solid interfaces with a collapse pressure of ~ 60 mN/m. The device with gold|LB-monolayer|gold configuration shows asymmetrical current responses with rectification ratios varying from 4.52 to 12 between -2 and +2 V and from 2.95 to 36.7 between -4 and +4 V, respectively. Therefore, this study showed that triphenolate coordinated iron(III) complexes are able to act as potential current rectifiers.

Further studies are performed to identify the possibility of using iron(III) complexes with [N2O2] donor sets for current rectification. This study also examined a probable current rectifying mechanism for iron(III) systems. In order to address this, two iron(III) complexes with salophen-type ligands were synthesized and structurally characterized. These iron(III) complexes have substituted phenolate moieties (tert-butyl and nitro) to facilitate different physical and chemical properties. The nitro substituted iron(III) complex forms a μ-oxo bridged species due to electronic and steric effects. The tert-butyl substituted iron(III) complex showed current rectifying properties with rectification ratios ranging from 3.99 to 28.6 between -2 and +2 V and from 2.04 to 31 between -4 and +4 V, respectively. This study also showed, that the asymmetrical nature of the iron(III) complex is fundamental for the observed current rectification. The tert-butyl substituted iron(III) complex displays metal-centered, singly occupied molecular orbitals (SOMO), and ligand-centered, highest occupied molecular orbitals (HOMO). Additionally, the electrochemical and DFT calculations of this system suggest, that the SOMO energy level is closer to the metal Fermi level when compared to the HOMO energy level. Therefore, electron transfer through SOMO energy level is energetically more favorable than electron transfer through the HOMO energy level. According to this data, an asymmetric current rectification mechanism is plausible for this [D-A] type iron(III) complex.

Manganese(III) complexes that are immobilized onto solid surfaces are another important class of compounds that can be used in electronic applications. Therefore, a series of manganese complexes with [N2O3] and [N2O2] donor sets were investigated. These complexes show metal-based manganese(III)/(II) and ligand-based oxidation redox processes in their cyclic voltammograms. The majority of manganese(III) complexes form well-ordered uniform LB films. The LB film analyses, also suggest that the molecular structure remains intact during the film formation processes. These manganese(III) complexes display possible [D-A] structures, nevertheless gold|LB-monolayer|gold devices of the most promising candidate show asymmetric current responses with poor current amplitudes, thus behaved as an insulator. The observed current insulating behavior could be due to structural and/or electronic parameters of manganese(III) complex and further investigations are necessary to identify the foundation.

This dissertation research presented new classes of saloph-type and triphenolate ligand systems and their gallium(III), iron(III), and manganese(III) complexes, which can merge redox and amphiphilic properties together. More importantly, this project facilitated the understanding of geometric, electronic, redox, and amphiphilic properties of different classes of metal complexes. Finally, this project allowed the study electrical properties of iron(III) complexes with [N2O3] and [N2O2] donor sets and reveled directional flow of electrical current to denote the rectifying behavior.

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