Dissertation Defenses

Doctoral students who have an upcoming dissertation oral defense are posted here. So why not take this opportunity to learn about the research that our graduate students are doing!

Dissertation Defense for Jianyu Zhao

Program: CHEMISTRY: PHD

Department Contact Email: cindi.rohwer@unh.edu

Defense Title: LINEARLY EXTENDED PYRYLIUM SALTS (LEPS) AND LINEARLY EXTENDED THIOPYRYLIUM SALTS (LETS) AS ORGANIC SEMICONDUCTORS

Defense Date and Time: 06/02/16 10:00 am

Defense Location: Parsons Hall, Room W131

Defense Advisor: Professor Glen Miller


Defense Abstract: Research was focused on the development of a new class of n-type organic semiconductors called linearly extended pyrylium salts (LEPS) and linearly extended thiopyrylium salts (LETS). While a lot of progress has been made on p-type organic semiconductors over the last decade, less attention has been paid to n-type organic semiconductors.Pyrylium and thiopyrylium cations are aromatic structures akin to benzene but with one methine carbon(CH) replaced by either a positively charged oxygen or sulfur atom, respectively. This makes these aromatic π-systems very electron-deficient. With pentacene-like backbones,LEPS and LETS compounds combine linearly conjugated π-systems of acenes with the highly electron-deficient nature of pyrylium and thiopyrylium salts.The LEPS and LETS compounds described here were synthesized efficiently using new reactions. Their properties were calculated through high-level computation and studied experimentally by UV-Vis-NIR spectroscopy, cyclic voltammetry, single crystal X-ray diffraction, ESR and variable temperature NMR(VT-NMR) experiments.
The LEPS and LETS compounds described here show moderate to high solubilities in organic solvents like acetonitrile and dichloromethane. While they are resistant to oxidation due to their electron-deficient backbones, they are sensitive to nucleophiles like water. LEPS and LETS compounds with a mesityl or 2’,6’-dimethylphenyl substituent have much higher resistance to moisture compared to other LEPS and LETS compounds due to steric shielding of the reactive site via the o-methyl groups of the phenyl substituents. LEPS and LETS compounds have broad absorptions in the UV-Vis-NIR region and possess small HOMO-LUMO gaps close to that of pentacene. They also exhibited reversible electrochemical behavior including remarkably easy reductions. Single crystal X-ray diffraction studies show that LEPS and LETS compounds form intermolecular face-to-face π-π stacking thin films. Broad and indiscernible NMR signals in the aromatic region and strong ESR signals were detected for LEPS and LETS compounds bearing mesityl substituent. Weak to moderate ESR signals were also observed for most of other LEPS and LETS compounds. The broad aromatic NMR signals for LETS 106 bearing a mesityl substituent in CD2Cl2 sharpened upon gradually decreasing temperature in VT-NMR experiments. This indicates a switch from a paramagnetic triplet state to a diamagnetic singlet state. We propose that the LEPS and LETS compounds showing strong ESR signals and broadened NMR spectra have paramagnetic, triplet excited state that lie close in energy to their corresponding diamagnetic ground states. In these cases, populating the paramagnetic excited states via the thermal excitation is possible at or near room temperature.


 

Dissertation Defense for Narges Ahmadi

Program: PHYSICS: PHD

Department Contact Email: narges.ahmadi@unh.edu

Defense Title: Nonlinear Evolution of Mirror Instability in the Earth's Magnetosheath in PIC Simulations

Defense Date and Time: 06/02/16 10:00 am

Defense Location: Morse Hall, Room 301

Defense Advisor: Kai Germaschewski


Defense Abstract: Mirror modes are large amplitude non-propagating structures frequently observed in the magnetosheath and they are generated in space plasma environments with proton temperature anisotropy of larger than one. The proton temperature anisotropy also drives the proton cyclotron instability which has larger linear growth rate than that of the mirror instability. Linear dispersion theory predicts that electron temperature anisotropy can enhance the mirror instability growth rate while leaving the proton cyclotron instability largely unaffected. Contrary to the hypothesis, electron temperature anisotropy leads to excitement of the electron whistler instability. Our results show that the electron whistler instability grows much faster than the mirror instability and quickly consumes the electron free energy, so that there is no electron temperature anisotropy left to significantly impact the evolution of the mirror instability.
Observational studies have shown that the shape of mirror structures is related to local plasma parameters and distance to the mirror instability threshold. We investigate the nonlinear evolution of mirror instability using particle-in-cell expanding box simulations. We change the plasma conditions by artificially enlarging the simulation box over time to resemble the plasma expansion in the vicinity of the magnetopause which leads to changing the plasma conditions toward a more mirror stable state and investigate the final nonlinear state of the mirror structures. We show that the direct nonlinear saturation of the mirror instability leads to magnetic peaks while in expanding box simulation, mirror structures saturate to deep magnetic holes.


 

Dissertation Defense for Casey Grenier

Program: CHEMISTRY: PHD

Department Contact Email: cindi.rohwer@unh.edu

Defense Title: Rapid, High Affinity Binding by Molecularly Imprinted Poly(N-isopropylacrylamide) Copolymers with Mostly Non-Covalent Crosslinks for Chemical Sensing and Separation Applications

Defense Date and Time: 06/03/16 10:00 am

Defense Location: Room N108, Parsons Hall

Defense Advisor: Professor W. Rudolf Seitz


Defense Abstract: A new kind of molecularly imprinted copolymer (MIP) based on poly(N-isopropylacrylamide) has been developed and used to rapidly bind polar organic compounds with high sensitivity. Imprinting occurs when polymerization is done in the presence of template. They involve three distinct types of crosslinks. Hydrophobic crosslinks are formed when poly (N-isopropylacrylamide) collapses to form a globule above its lower critical solution temperature (LCST). Acid-base crosslinks are formed by adding both acidic and basic monomers during polymerization. Minimal covalent crosslinker is included in the formulation so that the imprinted copolymer will return to the same conformation, even below the LCST. After free radical or reverse addition fragmentation chain transfer (RAFT) polymerization is complete, template is removed by dialysis.
The MIP binds with highest affinity at the LCST, where it collapses to a globule. The presence of the templated molecule was shown to reduce MIP aggregation above the LCST. This can be used as a physical signal to follow binding. Initial experiments used 4-nitrophenol as a template to establish optimum conditions to achieve rapid, selective, and high binding. Distribution ratios (([Template on Polymer])/([Template in Solution] )) as high as 4.57 ±0.0058 and selectivity to isomers of 4-nitrophenol as low as 1.33 ±0.0058 were measured by equilibrium dialysis.
Further experiments templated with fluorescein were conducted because it could be detected with higher sensitivity. Fluorescein binding constants were measured by equilibrium dialysis. A maximum value of 8.8x1010 L/mol was measured when working at low template concentrations. The value decreases as template concentration increases, indicating that not all sites bind with the same affinity. Kinetic experiments showed that binding is complete in less than on second with 0.0355 mg/L of MIP and non-templated polymer shows no fluorescein binding.
This material is attractive as a chemical sensor because it can be tuned to any polar organic molecule. Capable of being a liquid, below the LCST, makes it possible to inkjet print on to paper based substrates. Attaching a fluorophore to the MIP enables a visual response to binding the template, α-tocopherol. Equilibrium dialysis revealed distribution ratios for α-tocopherol, 35.96, and its selectivity compound 4-hydroxycoumarin, 7.11.
Chemical separations can be achieved by removing compounds from solution using MIPs attached to gold nanoparticle (AuNP). The MIP’s RAFT end groups were reduced to a thiol which allows easy attachment to gold substrates. The binding constant of fluorescein was determined by the equilibrating fluorescein with MIP on AuNP and then removing them. A maximum value of 3.3x1012 L/mol was reported.


 

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