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- All Subjects: Molecules
- Genre: Academic theses
- Creators: Mujica, Vladimiro
- Creators: Buttry, Daniel
Description
Over the last few decades, homogeneous molybdenum catalysis has been a center of interest to inorganic, organic, and organometallic chemists. Interestingly, most of the important advancements in molybdenum chemistry such as non-classical dihydrogen coordination, dinitrogen reduction, olefin metathesis, and water reduction utilize diverse oxidation states of the metal. However, employment of redox non-innocent ligands to tune the stability and reactivity of such catalysts have been overlooked. With this in mind, the Trovitch group has developed a series of novel bis(imino)pyridine (or pyridine diimine, PDI) and diimine (DI) ligands that have coordinating phosphine or amine arms to exert coordination flexibility to the designed complexes. The research described in this dissertation is focused on the development of molybdenum catalysts that are supported by PDI and DI chelates and their application in small molecule activation.
Using the phosphine containing PDI chelate, Ph2PPrPDI, several low-valent molybdenum complexes have been synthesized and characterized. While the zerovalent monocarbonyl complex, (Ph2PPrPDI)MoCO, catalyzes the reduction of aldehyde C=O bonds, the C-H activated Mo(II) complex, (6-P,N,N,N,C,P-Ph2PPrPDI)MoH was found to be the first well-defined molybdenum catalyst for reducing carbon dioxide to methanol. Along with low- oxidation state compounds, a Mo(IV) complex, [(Ph2PPrPDI)MoO][PF6]2 was also synthesized and utilized in electrocatalytic hydrogen production from neutral water. Moreover, with the proper choice of reductant, an uncommon Mo(I) oxidation state was stabilized and characterized by electron paramagnetic resonance spectroscopy and single crystal X-ray diffraction.
While the synthesized (PDI)Mo complexes unveiled versatile reduction chemistry, varying the ligand backbone to DI uncovered completely different reactivity when bound to molybdenum. Unlike PDI, no chelate-arm C-H activation was observed with the propyl phosphine DI, Ph2PPrDI; instead, a bis(dinitrogen) Mo(0) complex, (Ph2PPrDI)Mo(N2)2 was isolated. Surprisingly, this complex was found to convert carbon dioxide into dioxygen and carbon monoxide under ambient conditions through a novel tail-to-tail CO2 reductive coupling pathway. Detailed experimental and theoretical studies are underway to gain further information about the possible mechanism of Mo mediated direct conversion of CO2 to O2.
Using the phosphine containing PDI chelate, Ph2PPrPDI, several low-valent molybdenum complexes have been synthesized and characterized. While the zerovalent monocarbonyl complex, (Ph2PPrPDI)MoCO, catalyzes the reduction of aldehyde C=O bonds, the C-H activated Mo(II) complex, (6-P,N,N,N,C,P-Ph2PPrPDI)MoH was found to be the first well-defined molybdenum catalyst for reducing carbon dioxide to methanol. Along with low- oxidation state compounds, a Mo(IV) complex, [(Ph2PPrPDI)MoO][PF6]2 was also synthesized and utilized in electrocatalytic hydrogen production from neutral water. Moreover, with the proper choice of reductant, an uncommon Mo(I) oxidation state was stabilized and characterized by electron paramagnetic resonance spectroscopy and single crystal X-ray diffraction.
While the synthesized (PDI)Mo complexes unveiled versatile reduction chemistry, varying the ligand backbone to DI uncovered completely different reactivity when bound to molybdenum. Unlike PDI, no chelate-arm C-H activation was observed with the propyl phosphine DI, Ph2PPrDI; instead, a bis(dinitrogen) Mo(0) complex, (Ph2PPrDI)Mo(N2)2 was isolated. Surprisingly, this complex was found to convert carbon dioxide into dioxygen and carbon monoxide under ambient conditions through a novel tail-to-tail CO2 reductive coupling pathway. Detailed experimental and theoretical studies are underway to gain further information about the possible mechanism of Mo mediated direct conversion of CO2 to O2.
ContributorsPal, Raja (Author) / Trovitch, Ryan J (Thesis advisor) / Buttry, Daniel (Committee member) / Seo, Don (Committee member) / Arizona State University (Publisher)
Created2016
Description
Sunlight, the most abundant source of energy available, is diffuse and intermittent; therefore it needs to be stored in chemicals bonds in order to be used any time. Photosynthesis converts sunlight into useful chemical energy that organisms can use for their functions. Artificial photosynthesis aims to use the essential chemistry of natural photosynthesis to harvest solar energy and convert it into fuels such as hydrogen gas. By splitting water, tandem photoelectrochemical solar cells (PESC) can produce hydrogen gas, which can be stored and used as fuel. Understanding the mechanisms of photosynthesis, such as photoinduced electron transfer, proton-coupled electron transfer (PCET) and energy transfer (singlet-singlet and triplet-triplet) can provide a detailed knowledge of those processes which can later be applied to the design of artificial photosynthetic systems. This dissertation has three main research projects. The first part focuses on design, synthesis and characterization of suitable photosensitizers for tandem cells. Different factors that can influence the performance of the photosensitizers in PESC and the attachment and use of a biomimetic electron relay to a water oxidation catalyst are explored. The second part studies PCET, using Nuclear Magnetic Resonance and computational chemistry to elucidate the structure and stability of tautomers that comprise biomimetic electron relays, focusing on the formation of intramolecular hydrogen bonds. The third part of this dissertation uses computational calculations to understand triplet-triplet energy transfer and the mechanism of quenching of the excited singlet state of phthalocyanines in antenna models by covalently attached carotenoids.
ContributorsTejeda Ferrari, Marely (Author) / Moore, Ana (Thesis advisor) / Mujica, Vladimiro (Thesis advisor) / Gust, John (Committee member) / Woodbury, Neal (Committee member) / Arizona State University (Publisher)
Created2016
Description
Understanding the interplay between the electrical and mechanical properties of single molecules is of fundamental importance for molecular electronics. The sensitivity of charge transport to mechanical fluctuations is a key problem in developing long lasting molecular devices. Furthermore, harnessing this response to mechanical perturbation, molecular devices which can be mechanically gated can be developed. This thesis demonstrates three examples of the unique electromechanical properties of single molecules.
First, the electromechanical properties of 1,4-benzenedithiol molecular junctions are investigate. Counterintuitively, the conductance of this molecule is found to increase by more than an order of magnitude when stretched. This conductance increase is found to be reversible when the molecular junction is compressed. The current-voltage, conductance-voltage and inelastic electron tunneling spectroscopy characteristics are used to attribute the conductance increase to a strain-induced shift in the frontier molecular orbital relative to the electrode Fermi level, leading to resonant enhancement in the conductance.
Next, the effect of stretching-induced structural changes on charge transport in DNA molecules is studied. The conductance of single DNA molecules with lengths varying from 6 to 26 base pairs is measured and found to follow a hopping transport mechanism. The conductance of DNA molecules is highly sensitive to mechanical stretching, showing an abrupt decrease in conductance at surprisingly short stretching distances, with weak dependence on DNA length. This abrupt conductance decrease is attributed to force-induced breaking of hydrogen bonds in the base pairs at the end of the DNA sequence.
Finally, the effect of small mechanical modulation of the base separation on DNA conductance is investigated. The sensitivity of conductance to mechanical modulation is studied for molecules of different sequence and length. Sequences with purine-purine stacking are found to be more responsive to modulation than purine-pyrimidine sequences. This sensitivity is attributed to the perturbation of &pi-&pi stacking interactions and resulting effects on the activation energy and electronic coupling for the end base pairs.
First, the electromechanical properties of 1,4-benzenedithiol molecular junctions are investigate. Counterintuitively, the conductance of this molecule is found to increase by more than an order of magnitude when stretched. This conductance increase is found to be reversible when the molecular junction is compressed. The current-voltage, conductance-voltage and inelastic electron tunneling spectroscopy characteristics are used to attribute the conductance increase to a strain-induced shift in the frontier molecular orbital relative to the electrode Fermi level, leading to resonant enhancement in the conductance.
Next, the effect of stretching-induced structural changes on charge transport in DNA molecules is studied. The conductance of single DNA molecules with lengths varying from 6 to 26 base pairs is measured and found to follow a hopping transport mechanism. The conductance of DNA molecules is highly sensitive to mechanical stretching, showing an abrupt decrease in conductance at surprisingly short stretching distances, with weak dependence on DNA length. This abrupt conductance decrease is attributed to force-induced breaking of hydrogen bonds in the base pairs at the end of the DNA sequence.
Finally, the effect of small mechanical modulation of the base separation on DNA conductance is investigated. The sensitivity of conductance to mechanical modulation is studied for molecules of different sequence and length. Sequences with purine-purine stacking are found to be more responsive to modulation than purine-pyrimidine sequences. This sensitivity is attributed to the perturbation of &pi-&pi stacking interactions and resulting effects on the activation energy and electronic coupling for the end base pairs.
ContributorsBruot, Christopher, 1986- (Author) / Tao, Nongjian (Thesis advisor) / Lindsay, Stuart (Committee member) / Mujica, Vladimiro (Committee member) / Ferry, David (Committee member) / Arizona State University (Publisher)
Created2014
Description
Studying charge transport through single molecules is of great importance for unravelling charge transport mechanisms, investigating fundamentals of chemistry, and developing functional building blocks in molecular electronics.
First, a study of the thermoelectric effect in single DNA molecules is reported. By varying the molecular length and sequence, the charge transport in DNA was tuned to either a hopping- or tunneling-dominated regimes. In the hopping regime, the thermoelectric effect is small and insensitive to the molecular length. Meanwhile, in the tunneling regime, the thermoelectric effect is large and sensitive to the length. These findings indicate that by varying its sequence and length, the thermoelectric effect in DNA can be controlled. The experimental results are then described in terms of hopping and tunneling charge transport models.
Then, I showed that the electron transfer reaction of a single ferrocene molecule can be controlled with a mechanical force. I monitor the redox state of the molecule from its characteristic conductance, detect the switching events of the molecule from reduced to oxidized states with the force, and determine a negative shift of ~34 mV in the redox potential under force. The theoretical modeling is in good agreement with the observations, and reveals the role of the coupling between the electronic states and structure of the molecule.
Finally, conclusions and perspectives were discussed to point out the implications of the above works and future studies that can be performed based on the findings.
First, a study of the thermoelectric effect in single DNA molecules is reported. By varying the molecular length and sequence, the charge transport in DNA was tuned to either a hopping- or tunneling-dominated regimes. In the hopping regime, the thermoelectric effect is small and insensitive to the molecular length. Meanwhile, in the tunneling regime, the thermoelectric effect is large and sensitive to the length. These findings indicate that by varying its sequence and length, the thermoelectric effect in DNA can be controlled. The experimental results are then described in terms of hopping and tunneling charge transport models.
Then, I showed that the electron transfer reaction of a single ferrocene molecule can be controlled with a mechanical force. I monitor the redox state of the molecule from its characteristic conductance, detect the switching events of the molecule from reduced to oxidized states with the force, and determine a negative shift of ~34 mV in the redox potential under force. The theoretical modeling is in good agreement with the observations, and reveals the role of the coupling between the electronic states and structure of the molecule.
Finally, conclusions and perspectives were discussed to point out the implications of the above works and future studies that can be performed based on the findings.
ContributorsLi, Yueqi, Ph.D (Author) / Tao, Nongjian (Thesis advisor) / Buttry, Daniel (Committee member) / Mujica, Vladimiro (Committee member) / Arizona State University (Publisher)
Created2017