Chemistry - Theses, Dissertations, and other Required Graduate Degree Essays

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Elucidating the Electronic Structure of Transition Metal Complexes Featuring Redox Active Ligands

Date created: 
2014-10-30
Abstract: 

In this thesis a number of projects involving the design and characterization of complexes bearing redox active ligands are described. Focusing on the phenolate containing ligands, the properties and electronic structure of their corresponding metal complexes were studied by a series of experimental (i.e. electrochemistry, UV-Vis-NIR, EPR, rR etc.) and theoretical (DFT) methods. Specifically, the redox processes of these metal complexes were tuned by varying the para-ring substituents. In one study, nickel-salen (salen is a common abbreviation for N2O2 bis-Schiff-base bis-phenolate ligands) complexes were investigated, where the oxidation potentials of the ligand were predictably decreased as the electron donating ability of the para-ring substituents was increased (NMe2 > OMe > tBu > CF3). Interestingly, the oxidation of these geometrically-symmetric complexes afforded an asymmetric electronic structure in a number of cases, in which the ligand radical was localized on one phenolate rather than delocalized across the ligand framework. This difference in electronic structure was found to be dependent on the electron donating ability of the substituents; a delocalized ligand radical was observed for electron-withdrawing substituents and a localized ligand radical for strongly donating substituents. These studies highlight that para-ring substituents can be used to tune the electronic structure (metal vs. ligand based, localized vs. delocalized radical character) of metallosalen complexes. To evaluate if this electronic tuning can be applied to the metal center, a series of cobalt complexes of these salen ligands were prepared. Indeed, the electronic properties of the metal center were also significantly affected by para-ring substitution. These cobalt-salen complexes were tested as catalysts in organometallic radical-mediated polymerizations, where the most electron rich complexes displayed the best conversion rates. With a firm understanding of the role that the para-ring substituent can play in influencing the electronic structure and reactivity of metallosalen complexes in catalysis, two novel iron complexes, which contain a number of redox active phenolate fragments, were prepared. In addition, these iron-complexes feature a chiral bipyrrolidine backbone. Ligands with this chiral diamine backbone bind metals ions diastereoselectively owing to its increased rigidity, which is critical to stereoselectivity in catalysis. A symmetric (with two phenolates) ligand was prepared by reported methods, and a novel route to synthesize an asymmetric ligand (one phenolate and one pyridine) from symmetric starting materials was established. The neutral iron-complexes were found to be high spin (S = 5/2), and can undergo ligand based oxidation to form an antiferromagnetically-coupled (Stotal = 2) species. The results presented will serve as the basis for catalyst development using complexes of similar ligands.

Document type: 
Thesis
File(s): 
Senior supervisor: 
Tim Storr
Department: 
Science: Chemistry
Thesis type: 
(Thesis) Ph.D.

Controlling Photochemistry Using Molecular Switches and Upconverting Nanoparticles

Author: 
Date created: 
2014-09-30
Abstract: 

Because light can be tuned, focused and has a ‘on-off’ control, the use of light to drive photolabile compounds to unmask bioactive molecules provides spatial and temporal control required to evaluate how a specific chemical species will affect the cells in living organisms and to potentially deliver therapeutiecs on demand. However, there are still questions need to be answered when using light as a tool for applications. The research described in this thesis addresses issues related to how light can be used to release small molecules from ‘masked’ forms in complex environments such as in living cells of organisms. Four inter-related questions concerning different aspects of the topic listed below are answered in the thesis. (1) How does the user know when and where the photorelease has occurred? In Chapter 2, a ‘release and report’ concept is demonstrated using a novel photolabile compound. The compound absorbs two UV photons and undergoes two sequential reactions. The first reaction releases the protected molecule and the second reaction produces a visible colour that can be conveniently monitored without any special techniques therefore the successful release process is reported.(2) How does one deliver light that is less-damaging but still capable of inducing photoreactions? And (3) How can one maintain photoreactivity of organic compounds in an aqueous environment?In Chapter 3, a ‘plug and play’ method demonstrates the simplicity of creating a water-dispersible nanosystem through co-encapsulation of hydrophobic upconverting nanoparticles and photoactive compounds by an amphiphilic organic polymer shell. More importantly, the photoreactivity of the encapsulated compounds is well maintained in aqueous medium. (4) How are unwanted photo reactions avoided? Chapter 4 decribes how to use a UV-blocking polymer shell to encapsulate upconverting nanoparticles that prevents a one-photon driven photoreaction while still allowing multi-photon driven processes. Data will be presented to show how the isomerization of diarylethenes in the assembly can be triggered by irradiation of Near-Infrared light but not by UV light or ambient light.

Document type: 
Thesis
File(s): 
Senior supervisor: 
Neil Branda
Department: 
Science:
Thesis type: 
(Thesis) Ph.D.

Modulation of Metastable Metal-Semiconductor Junctions

Author: 
Date created: 
2014-09-26
Abstract: 

The feasibility of modulating the electrical properties of metal-semiconductor (MS) junctions was examined via the preparation of self-assembled monolayers (SAMs) at the interface. In this thesis, metal-monolayer-semiconductor junctions were prepared using a hanging mercury (Hg) drop electrode in contact with an oxide-free silicon substrate (H-Si≡), where the mercury drop was subsequently modified with alkanethiolate SAMs. It has been demonstrated that the electrical properties of an Hg-S-C18|H-Si≡ junction can be tuned from rectifying to ohmic or vice versa by manual manipulation of the size and shape of the Hg drop. Evaluation of the rectification ratio (R), ideality factor (η) and barrier height (qɸeff) enables the determination of the threshold value of the surface area change of the mercury contact. In addition, the effect of variation of the alkyl chain length of the alkanethiolate SAMs on the Hg electrode was studied. Both qɸeff and R were found to depend on the alkyl chain length and changed gradually upon aging. This augments the potential for molecularly tuning the electrical properties of classical MS junctions without complicated materials assembly or device fabrication.

Document type: 
Thesis
File(s): 
Senior supervisor: 
Hua-Zhong Yu
Department: 
Science:
Thesis type: 
(Thesis) M.Sc.

Molecular Modeling of Interfacial Proton Transport in Polymer Electrolyte Membranes

Date created: 
2014-08-27
Abstract: 

The proton conductivity of polymer electrolyte membranes (PEMs) plays a crucial role for the performance of polymer electrolyte fuel cells (PEFCs). High hydration of Nafion-like membranes is crucial to high proton conduction across the PEM, which limits the operation temperature of PEFCs to <100o C. At elevated temperatures (>100o C) and minimal hydration, interfacial proton transport becomes vital for membrane operation. Along with fuel cell systems, interfacial proton conduction is of utmost importance in biology and materials science; yet experimental findings of ultrafast proton transport at densely packed arrays of anionic surface groups have remained controversial and unexplained. In the main part of this thesis work, ab initio simulations were performed on a minimally hydrated, densely packed array of sulfonic acid surface groups (SGs). This system served as a model to study the mechanism of interfacial proton transport in perfluorosulfonic acid membranes. Specifically, simulations were performed to explore the impact of the density of SGs on the mechanism of interfacial proton transport. Results reveal a mechanism of highly collective proton motion at a critical SG separation of 6.6 Å. The activation free energy of proton translocations exhibits a high sensitivity to the SG density. A spontaneous concerted proton transition was observed with low activation barrier at a surface group separation of 6.8 Å. When protons translocate concertedly, the activation barrier of the transition drops by more than a factor of two to the value of 0.25 eV compared to the case of disconcerted proton transfer. Results show that the hydrogen-bond network with long-range order that forms upon densification of SGs at the interface enables highly effective proton transport under minimal hydration conditions. These results were then incorporated in a soliton theory for describing collective proton transport through minimally hydrated and highly charged interfaces.

Document type: 
Thesis
File(s): 
Senior supervisor: 
Michael Eikerling
Department: 
Science: Department of Chemistry
Thesis type: 
(Thesis) Ph.D.

Characterization of the first stage prototype of the TIFFIN detector

Date created: 
2014-07-16
Abstract: 

A single-sided, gridded, gas ionization chamber with digital read-out and a dedicated gas-flow control system was built as the first stage prototype of the Twin Ionization chamber for Fission Fragment Investigation (TIFFIN) detector. The detector was tested with an α-particle source, and the operating parameters of the detector were probed to establish its response. Investigation of the operating parameters of the prototype led to an energy resolution of 8.69(1)% when running the detector with a gas mixture of 90% argon and 10% methane (referred to as P10) at a gas pressure of 1800 Torr. Digital read-out of the signal allowed signal risetimes to be successfully evaluated on an event-by-event basis. However, electric field inconsistencies limited the energy resolution achievable. An electric field cage should be installed to encourage a uniform field between the detector plates. The Nuclear Science Laboratories at Simon Fraser University are embarking upon a program of research to address various topics of interest to modern science such as the origin, production, composition and structure of exotic, neutron-rich isotopes. The study of fission fragments is an effective way to investigate neutron-rich nuclei. Ionization chambers are an appropriate and versatile tool with which to study fission fragments, and can be built in such a way as to allow energy, mass and charge measurements of both fragments. The single-sided prototype is a crucial first step towards the final design which will allow such measurements.

Document type: 
Thesis
File(s): 
Senior supervisor: 
Krzyzstof Starosta
Department: 
Science: Department of Chemistry
Thesis type: 
(Thesis) M.Sc.

The reactivity of organometallic diamido-actinide complexes

Date created: 
2013-06-24
Abstract: 

This thesis explores the coordination chemistry of diamido-actinide complexes and establishes them as active catalysts for a number of transformations. The synthesis, characterization and reactivity studies of a series of diamido-ether actinide complexes are described. The focus of these studies include the use of two different ligand frameworks: a silyl ether backbone of the form [(RNHSiMe2)2O] ([RNON]H2; R = tBu, 2,6-iPr2Ph, 2,4,6-Me3Ph), and an alkyl ether backbone of the form [(RNHCH2CH2)2O] ([RNCOCN]H2; R = 2,6-iPr2Ph). A series of diamido-ether actinide halide complexes supported by these ligands can be prepared through the addition of either [RNON]Li2 or [RNCOCN]Li2 to UCl4 or ThCl4•2DME. These halide complexes can then be alkylated via the addition of appropriate alkylating reagents such as LiCH2SiMe3, LiCH(SiMe3)2 or KCH2Ph. A series of new diamido-actinide alkoxide complexes has also been prepared from the addition of alkoxide reagents such as LiOiPr or KOtBu. These diamido-actinide alkyl complexes are explored as catalysts for ethylene polymerization, lactide polymerization, and intramolecular hydroamination. In all cases, these alkyl complexes are found to be active catalysts, but in each instance, different complexes are established as the most active species. The mixed-donor amido-amino-siloxo ligands [(RN(Li)SiMe2N(R)SiMe2O] ([RNNO]Li2; R = 2,6-iPr2Ph, 2,4,6-Me3Ph), are also used to prepare a series of new actinide-based complexes. The addition of one equivalent of ligand per metal centre yields “ate” complexes that retain solvent and salt in the coordination sphere of the metal while the addition of two equivalents of ligand per metal centre yields bis(amido-siloxo)actinide-based products. These complexes, like their diamido counterparts, can be alkylated through the addition of appropriate alkylating reagents such as LiCH2SiMe3. Lastly, oxidation reactions with {[tBuNON]UCl2}2, [iPr2PhNCOCN]UCl3Li•THF and [Me3PhNNO]2U are described. The preparation of two non-uranyl U(VI) complexes from the oxidation of {[tBuNON]UCl2}2 are reported. Furthermore, the oxidation of [Me3PhNNO]2U yields a new, ligand-activated U(V) complex.

Document type: 
Thesis
File(s): 
Senior supervisor: 
Daniel B. Leznoff
Department: 
Science: Department of Chemistry
Thesis type: 
(Thesis) Ph.D.

Determining local viscoelastic properties of collagen systems using optical tweezers

Author: 
Date created: 
2014-01-13
Abstract: 

In this work, I aimed to develop and apply a technique capable of measuring the viscoelastic properties of collagen at different levels of hierarchy. Collagen is the predominant structural protein in vertebrates, and its self-assembly into well-defined structures including fibrils underlies the formation of a wide variety of biological structures with a broad range of functions. Here, in order to understand the correlation between collagen’s structure and its mechanical properties, the viscoelastic properties of different collagen systems were characterized, ranging from solutions of molecules to self-assembled forms of fibrillar gels and gelatin. To determine rheological properties, optical tweezers were used to trap and monitor thermal fluctuations of an embedded micron-sized particle, producing measurements of viscoelastic response of collagen systems at a high bandwidth (> 10^4 Hz). To validate these measurements, I reproduced results on a previously characterized system (polyethylene oxide). The obtained viscoelastic response is affected by the timescales of the interactions between polymers, which play a critical role in conferring elasticity to the system. To provide guidance to the microrheology experiments, the structure of collagen in acidic solution was probed using dynamic light scattering. My microrheology studies of collagen molecules in acidic solution showed that elastic response becomes comparable to viscous response at the highest concentration studied here, 5 mg/ml. Here, the significant elasticity observed at frequencies above ~200 Hz is due to collagens’ intermolecular interactions, which I found were not due to electrostatic interactions. However, elasticity was found to decrease following the removal of collagen’s telopeptides, consistent with their role in facilitating fibril formation. At the fibrillar level, unlike in solutions of collagen, I observed spatial heterogeneity in viscoelastic properties. The elastic modulus varies by an order of magnitude at different locations within fibrillar collagen gels. By making measurements over 100-minute timescales as collagen self-assembled into fibrils, I probed the development of microscale heterogeneity and concluded that heterogeneity appears during early phases of fibrillar growth and continues to develop further during this growth phase.

Document type: 
Thesis
File(s): 
Senior supervisor: 
Dr. Nancy R. Forde
Department: 
Science: Department of Chemistry
Thesis type: 
(Thesis) Ph.D.

Examining Structure-Property Relationships of Proton Exchange Membranes through the Study of Model Sulfonated Graft Copolymers

Date created: 
2014-03-24
Abstract: 

The role of polymer nanostructure on morphology, crystallinity, water sorption and proton conductivity was investigated using a model solid polymer electrolyte. Poly([vinylidene difluoride-co-chlorotrifluoroethylene]-graft-styrene) [P(VDF-co-CTFE)-g-PS], which consists of a hydrophobic, fluorous backbone and styrenic graft chains of varied length was synthesized with controlled chain architecture and chemical composition. The polystyrene graft chains were sulfonated to different degrees to provide three series of polymers with controlled ion exchange capacity (IEC). Due to chemical dissimilarity of the hydrophobic fluorous segments and the hydrophilic sulfonated polystyrene segments, the copolymers phase separate into ionic and non-ionic domains. The ionic domains allow transport of water and protons; the hydrophobic domains provide mechanical integrity, preventing the membranes from dissolving in water. The design of the model graft copolymers allows systematic examination of the effects of graft length and graft density on water sorption and proton conductivity. One of the major features of this work is that the sulfonated graft copolymers with shortest graft chains exhibit highest degree of crystallinity and highest PVDF content, which restrict excessive swelling and alleviate acid dilution, leading to a wider IEC operating range for high proton conductivity. Furthermore, the short graft copolymers allow access to very high IEC membranes that are insoluble in water. These short graft polymers with high IECs exhibit exceptionally high proton conduction under reduced humidity and elevated temperatures. In addition, for a given PVDF content, the lower graft density copolymers were observed to possess higher crystallinity and more contiguous PVDF domains that allow high IEC membranes to be prepared that possess lower degrees of swelling. Another important finding is that blending fully sulfonated graft copolymers with high molecular weight PVDF yields membranes with overall low IECs that exhibit highly localized ion content. This promotes the interconnection of ionic domains for effective proton transport while the more extended hydrophobic domains significantly reduce excessive swelling which serve to maintain the mechanical property of the membranes. This thesis describes a systematic approach, demonstrating the design, synthesis, characterization of model polymers, followed by the analysis of structure-property relationships in proton exchange membranes.

Document type: 
Thesis
File(s): 
Senior supervisor: 
Steven Holdcroft
Department: 
Science: Department of Chemistry
Thesis type: 
(Thesis) Ph.D.

Rational Design of Dithienylethenes with Photoelectrochromic Properties

Date created: 
2014-01-13
Abstract: 

1,2-Dithienylethene compounds (DTEs) reversibly interconvert between two isomeric forms, referred to as the colourless (ring−open isomer) and coloured (ring−closed isomer). This interconversion can be achieved through photochemical, electrochemical and thermal means. The photochemical toggling of DTEs is well characterized in the literature. However, their electrochromic behaviour is seldom reported and almost all known examples degrade through electropolymerisation. The fully electroactive DTEs are unstable in the coloured form at room temperature and undergo ring−opening in the dark. Depending on the application, DTEs may require a “single time use” (biological application / imaging) or must undergo extensive cycling (ophtalmics, smart windows). The goal of this work was to find the structural demands that allow integration of three desirable features, photo-, electrochemical and thermal stability. The first part of this thesis addresses the synthetic manipulation of the DTE core capable of catalytic oxidative ring−opening reactions. Key functional groups were appended in the α−positions of the electroactive DTE framework. These groups were carefully chosen in the context of thermal stability of the coloured−form and the mitigation of the undesired electropolymerisation. These substituents range from electron donating (through induction) methyl and t-butyl, to electron withdrawing fluorine atoms and electron rich thiophene rings. It was found that the DTEs studied presented an additive driving force, π-π stacking, for the undesired thermal ring−opening in the dark, a first observation among these type of compounds. The second part of the thesis is concerned with the mechanism of the ring−closing reaction triggered by the reduction of the uncoloured form. A new hypothesis regarding the mechanism of the process is presented. The preliminary studies suggest that the intermediate for the ring−closing reaction is the doubly reduced form of the ring−open isomer rather than the mono-reduced form as previously thought.

Document type: 
Thesis
File(s): 
Senior supervisor: 
Neil R. Branda
Department: 
Science: Department of Chemistry
Thesis type: 
(Thesis) Ph.D.

Studies toward the Total Synthesis of Biselides & Radical Fluorination of Aliphatic Compounds

Author: 
Date created: 
2014-01-24
Abstract: 

Modern pharmaceuticals originate predominately either from natural products or totally synthetic compounds. The rich variety of marine life found in the Earth’s oceans have especially allowed our access to increasingly more and intriguing biologically active marine natural products, such as haterumalides and biselides. The lack of materialistic return associated with natural product isolation from marine organisms has prompted the need for practical laboratory synthesis. The same demand holds true for synthetic drugs, as more novel synthetic methods are required to complement the increasingly target- and diversity-oriented approach to synthetic drug discovery. Biselides, isolated from the Okinawan ascidian Didemnidae sp., are marine macrolides which have demonstrated potent cytotoxicity towards a variety of human cancer cell lines while being non-toxic towards brine shrimp. They contain a 2,5-disubstituted-3-oxygenated tetrahydrofuran functionality and a (Z,Z)-1,4-diene as part of a 14-membered macrocyclic molecular skeleton. A proposed total synthesis of biselide A involves the cyclization of chloropolyols to form the substituted tetrahydrofuran and metathesis for construction of the (Z,Z)-1,4-diene. Specifically, direct ring-closing metathesis (RCM), relay ring-closing metathesis (RRCM), and cross metathesis (CM) strategies were examined, with only the CM strategy allowing us to construct the 1,4-diene with desired geometry. The second part of this thesis describes the development of a novel methodology for the direct conversion of C(sp3)-H bonds to C(sp3)-F bonds. This radical-based, photocatalytic process yielded monofluorinated products from various small molecule aliphatic substrates, and has showed further potential with other chemical systems. Improvement and expansion of this methodology could hold positive future implications with regards to fluoropharmaceuticals.

Document type: 
Thesis
File(s): 
Senior supervisor: 
Robert Britton
Department: 
Science: Department of Chemistry
Thesis type: 
(Thesis) M.Sc.