Thesis (Ph. D.)--University of Illinois at Urbana-Champaign, 2008. / Source: Dissertation Abstracts International, Volume: 69-05, Section: B, page: 3022. Adviser: Todd J. Martinez. Includes bibliographical references. Available on microfilm from Pro Quest Information and Learning.

<p>Elementary energy and electron transfer processes are ubiquitous in the renewable energy science of the last half of the 20th century. As global energy demands increase, researchers are inclined to explore chemical physics that is outside the scope of the single electron paradigm using new theoretical concepts and methods. This thesis advances theories of two specific condensed phase phenomena: singlet fission, and energy transfer in photosynthetic light harvesting complexes. Some photoactive organic molecules relax through a multielectron process known as singlet fission, where a photon excites a chromophore that can down-convert the energy of a singlet excitation by relaxing to two triplet excitations. Singlet fission may lead to unprecedented solar power conversion efficiencies, but its many-body chemical physics can be challenging to model. We explore the fundamental role of thermal energy in singlet fission in liquids and solids over multiple timescales. Using quantum master equations and diabatic representations of the single and double electronic excitations, we study the scope of the Markovian approximation for the chemical environment's response to singlet fission. To better understand how singlet delocalization and triplet localization impact quantum yields in molecular crystals, we develop a theory for delocalized singlets interacting with a dense band of two triplet excitations that includes biexciton interactions. We use the Bethe Ansatz for the two triplets and calculate an entanglement for indistinguishable bipartite systems to analyze the triplet-triplet entanglement born out of singlet fission.

<p> A theoretical model describing photophysics of &pi;-conjugated aggregates, such as molecular crystals and polymer thin films, is developed. A Holstein-like Hamiltonian expressed with a multi-particle basis set is used to evaluate absorption and photoluminescence (PL) spectra of neutral excitons as well as charge modulation spectra (CMS) and transient absorption spectra (TAS) of positively charges hole-type polarons. The results are used to develop a better theoretical understanding of the organic electronics being studied and their photophysics, and also to probe the morphology of poly(3-hexylthiophene) (P3HT) thin films, which are used in photovoltaic devices.</p>

The first pure-rotational spectra of two transition-metal sulfides, copper sulfide (CuS) and manganese sulfide (MnS) have been determined. In addition, the millimeter/sub-millimeter spectra of SrS, SrNH₂ and SrND₂ have been determined for the first time. Electronic ground states have been confirmed with experimental evidence. Results indicate that the bonding trends of the transition-metals sulfides are slightly different than their isovalent analogs, the transition-metal oxides. Furthermore, the study of MnS has allowed for experimental testing of current high spin-state quantum theory, by analysing the spectra using third and fourth-order corrections to the effective Hamiltonian. Highly accurate spectroscopic constants and rotational transition frequencies have been determined for all species.

<p> Quantum dots (QDs) are the foundation of many optoelectronic devices because their optical and electronic properties are synthetically tunable. The inherent connection between synthetically controllable physical parameters, such as size, shape, and surface chemistry, and QD electronic properties provides flexibility in manipulating excited states. The properties of the ligands that passivate the QD surface and provide such synthetic control, however, are quite different from those that are beneficial for use in optoelectronic devices. In these applications, ligands that promote charge transfer are desired. To this end, significant research efforts have focused on post-synthetic ligand exchange to shorter, more conductive ligand species. Surface ligand identity, however, is a physical parameter intimately tied to QD excited state behavior in addition to charge transfer. A particularly interesting group of ligands, due to the extraordinarily thin ligand shell they create around the QD, are the chalcogenides S2-, Se2-, and Te2-. While promising, little is known about how these chalcogenide ligands affect QD photoexcited states. This dissertation focuses on the impact of chalcogenide ligands on the excited state dynamics of cadmium chalcogenide QDs and associated implications for charge transfer. This is accomplished through a combination of theoretical (Chapters 2, 3, and 6) and experimental (Chapters 2, 4, 5 and 6) methods. We establish a theoretical foundation for describing chalcogenide capped QD photoexcited states and measure the dynamics of these excited states using transient absorption spectroscopy. The presented results highlight the drastic effects surface modification can have on QD photoexcited state dynamics and provide insights for more informed design of optoelectronic systems.</p>

Advances in spectroscopic measurement techniques enabling highly accurate measurements of trace gases in the atmosphere are critical for furthering our understanding of the chemical processes that impact both climate and human health. This dissertation presents the development and deployment of laser-based instruments for measuring parts per trillion (pptv) concentrations of iodine monoxide and glyoxal. Iodine, which is primarily released from oceanic sources, is highly reactive in the atmosphere. Despite its trace concentrations, iodine plays a potentially important role in ozone destruction, the catalysis of mercury deposition, and the formation of marine clouds. An in situ instrument to detect iodine monoxide (IO) using laser-induced fluorescence was developed and then validated during a deployment to the Shoals Marine Laboratory (Appledore Island, ME) in August and September 2011. Mixing ratios up to 10 pptv of IO were observed with a strong tidal dependence. The instrumental detection limit \((3\sigma)\) of 0.36 pptv in 1 minute is indicative of unprecedented sensitivity. Glyoxal, the smallest alpha-dicarbonyl, serves as an atmospheric tracer of both the oxidation of biogenic volatile organic compounds in forest environments as well as secondary organic aerosol. Modeling studies indicate that production of glyoxal on a global scale is driven primarily by biogenic emissions, specifically emissions of isoprene. However, measurements of glyoxal in environments where isoprene dominates its production are limited. An instrument to detect glyoxal in situ by laser-induced phosphorescence was developed. The 3σ limit of detection of this instrument was 3.9 pptv in 1 minute. During July and August 2009, gas-phase measurements of glyoxal were made during the Community Atmosphere-Biosphere Interactions Experiment at the PROPHET tower in an isoprene-dominated forest site in northern Michigan. Additional measurements made throughout the campaign have been used to constrain a box model using the Master Chemical Mechanism. The model over-predicts glyoxal relative to the observed mixing ratios. Theoretically predicted reaction pathways implemented in many isoprene oxidation schemes exacerbate this disagreement. / Chemistry and Chemical Biology

Spatially explicit knowledge of land cover is increasingly important for environmental modeling and decision support for land managers. Such knowledge is often provided over large regions by thematic maps produced from remotely sensed satellite data. Remote sensing of vegetation in semi-arid areas is complicated, however, by high levels of landscape spatial heterogeneity, resulting in large part from spatially varying soils, topography, and microclimates. Increased understanding of spatial distributions of vegetation and the factors affecting them will enhance our ability to inventory and monitor natural resources, and to model potential consequences of land management alternatives and larger issues such as global climate change. In addition, the uncertainty in spatial knowledge must be made spatially explicit in order to determine where more information is needed and where predictions maybe less reliable. Geostatistical kriging and multiple linear regression interpolation were used to map climate spatial distributions over the San Pedro River watershed, southeastern Arizona. Both methods used climate station location and elevation and climate data. Although mean interpolation errors were similar, kriging climate with elevation as external drift was preferred due to the patterns of spatial bias in regression errors. Interpolation results provided a step toward understanding climate influence on vegetation in this area. Accuracies of four land cover maps covering the upper San Pedro watershed, mapped from remotely sensed data, were determined using aerial photography, digital orthophoto quadrangles, and airborne video data reference data sets as alternatives to contemporaneous ground-collected data. Overall map accuracies were 67--75%; class accuracies varied more for smaller classes than for larger ones. Finally, the uncertainty of occurrence of the low-accuracy Mesquite Woodland class was mapped using simple indicator kriging with locally varying means and data derived from accuracy assessment information. Enhanced class discrimination in an independent validation data set confirmed the utility of this procedure. The results of these analyses can provide direct input for use in environmental modeling and can inform land management decision making, and the methods can be employed in other settings where spatial variability and uncertainty play large roles in the landscape.

This dissertation presents results on the study of temperature dependence on neutral - neutral reactions. In particular OH radical reactions were studied taking advantage of the uniform supersonic expansions produced via a convergent-divergent Laval nozzle. The relative concentration of the OH radical was followed via laser induced fluorescence. The technique and the considerations of the uniform flow reactors used are considered. The method of analysis of the data obtained is also presented. The measurement of rate coefficients for several reactions is reported. These studies demonstrate the utility of this technique as both bimolecular and termolecular reactions have been studied in low temperature environments. Specifically, the temperature dependence of the rate coefficient for the bimolecular reaction, OH + HBr has been investigated at low temperatures (T = 48-224 K) using both the pulsed and continuous uniform supersonic flow reactors. This reaction shows interesting temperature dependence, which can be accurately described with two forms of global fits: UNFORMATTED FORMULA FOLLOWS: k(T) = 1.11 x 10⁻¹¹ [T/298]⁻⁰˙⁹¹ and k(T) = 1.06 x 10⁻¹¹ [T/298]⁻¹˙⁰⁹ exp [(-10.5K)/T] cm³3s⁻¹. UNFORMATTED FORMULA END To aid in understanding this interesting temperature dependence investigations were performed on reactions of OH and OD radicals with HBr and DBr between 120--224 K, using the pulsed uniform supersonic flow reactor. Though the rates are measured by hydroxyl loss, the lack of observed isotopic scrambling indicates the reaction occurs by H/D transfer at all temperatures. The current work provides unequivocal experimental evidence of inverse primary and secondary kinetic isotope effects (kH/kD < 1) at low temperatures. Also reported are measurements of the temperature and pressure dependence of the recombination rate of the atmospherically important reaction OH + NO₂ in both the pulsed and uniform supersonic flow reactor. Employing fall-off behavior analysis, the rate coefficients are compared with the most recent sets of recombination rate measurements obtained at higher temperatures and pressures as well as various rate recommendations. This current recommendation predicts a 20% reduction in the current JPL recommendation for stratospheric conditions.

The study of two-photon absorbing (TPA) chromophores has previously shown that intramolecular charge transfer (ICT) between electron donors (D) and acceptors (A) symmetrically substituted on a π-conjugated bridge can result in large values of the two photon cross section (δ). This work addresses the role of torsion in the π-backbone on δ, modulation of δ by solvent polarity, and development of two-photon excitable molecular sensors. Investigations of the one- and two-photon properties of di(styryl)benzene derivatives with terminal donor groups and cyano acceptors substituted either on the central phenyl ring or on the adjacent vinyl groups show that the position of the cyano group has a significant effect on the geometry and optical properties of the molecules. δ for vinyl substituted molecules is half that for phenylene substituted molecules and is similar to the values obtained for molecules with no acceptor groups. The one- and two-photon spectroscopic differences can be related to the donor-acceptor distance in these molecules and to the degree of torsion in conjugated backbone. Torsional effects on the electronic coupling of multi-chromophore molecules are also investigated. For quadrupolar TPA chromophores, solvent polarity weakly affects the linear absorption but strongly modifies the fluorescence spectral position and quantum yield (η). The TPA peak position does not shift with solvent polarity, however the magnitude of δ increases by up to a factor of two in acetonitrile relative to toluene. Analysis of the data in terms of the stabilization of intramolecular charge transfer by polar solvents is explored. Two-photon absorbing molecular sensors are investigated in which an ion binding group is incorporated as one of the donor groups (D-A-D' ) in a TPA molecule. When one monoaza-15-crown-5-ether macrocycle is bound to the chromophore, the two-photon induced fluorescence signal (ηδ) decreases seven-fold upon addition of magnesium ions, in part as delta is modulated, due to decreased ICT from the nitrogen lone pair involved in ion binding. In molecules incorporating 1,2-bis(o-aminophenoxy)-ethane- N,N,N',N'-tetraacetic acid, a 5-fold enhancement of ηδ is observed upon calcium ion binding in water at 720 nm. Changes in the TPA spectrum upon binding of Ca²⁺ in micellar systems are also observed.

Understanding the nature of chemical bonds constitutes a major theme of this thesis. This thesis investigates the gas phase rotational spectra, electronic charge distributions and molecular structures of organometallic and organic molecules, using high-resolution pulsed beam Fourier transform microwave spectroscopy (PBFTMS) and computational methods. High-resolution rotational spectra and structural parameters were obtained for the following organometallic molecules in the singlet electronic state, including three symmetric and five asymmetric top complexes: C₅H₅Nb(CO)₄, CH₃Mn(CO)₅, MnRe(CO)₁₀, C₅H₅Mo(CO)₃H, C₅H₅W(CO)₃H, C₅H₅NiC₃H₅, C5H₄(CH₃)FeC₅H₅ and (C₅H₄(CH₃))₂Fe. High-resolution rotational spectra and structural parameters were obtained for three organic molecules in the singlet electronic state: ortho-benzyne (C₆H₄) and the keto-enol tautomers, 2-hydroxypyridine and 2-pyridone (C₅H₅NO). In addition to the tautomeric forms, pure rotational spectra of the H-bonded dimer, 2-hydroxypyridine:2-pyridinone, were also obtained. These detailed spectral investigations yielded novel and useful information about the molecular properties of these molecules. Primarily, these results provided information regarding chemical bonding, vibrational ground state structures, structural isomers, conformational behavior, metal-hydrogen bonding and electronic charge distributions. Density functional theory (DFT) and ab-initio calculations were carried out in conjunction with the experiments, providing additional insights into further understanding the equilibrium structures, structural isomers and the electric field gradient distributions for these molecules.