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.

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.

A pulsed supersonic Laval nozzle flow reactor has been employed to investigate a variety of neutral-radical reaction processes at temperatures between 53 and 188 Kelvin. These supersonic flows simulate the conditions found in the Earth's upper atmosphere as well as certain environments in the interstellar medium and outer planetary atmospheres and thus provide direct information on the chemistry and physical processes occurring in those environments. Studies of this type, in the limit of 0 Kelvin, coupled with modern astronomical observations of planetary atmospheres and dense molecular clouds provide for a global understanding of chemistry in cold environments. With this in mind, the flow reactor was used to conduct fundamental studies involving the reactivity of hydroxyl (OH) and imidogen (NH) radical species with a variety of partners. More specifically, the reactions of OH+HBr and all of the H/D isotopic variants were explored between 53 and 135 K, with the goal of elucidating the kinetic isotope effects, both primary and secondary, for a reaction system occurring over a potential energy surface without an appreciable barrier, that demonstrates inverse temperature dependence. While not of direct astronomical importance, the reaction of OH+HBr does affect the partitioning of Br in the Earth's atmosphere, and knowledge of kinetic isotope effects helps one understand the chemistry leading to H/D fractionation observed in a variety of interstellar environments. The reactions of NH radical with NO, saturated, and unsaturated hydrocarbons were also studied between 53 and 188 Kelvin in the Laval nozzle flow reactor. These species were chosen as most are important constituents in the atmosphere of Titan, which is known to possess a rich organic chemistry. The reactions of NH with the unsaturated hydrocarbons are found to display negative temperature dependence over the window investigated, and are thought to proceed through an addition mechanism. Finally, the flow reactor was also coupled to a tunable vacuum and extreme ultraviolet frequency source based on four wave frequency mixing to allow for studies of radical species with their first electronic transitions in this frequency range. A discussion of the development, implementation, and future directions is included.

Nuclear magnetic resonance spectroscopy and molecular dynamics simulation have been used to study the structure and dynamics of biologically active peptide ligands for glucagon and melanocortin receptors, providing valuable insights into the receptor ligand interactions and useful information for the further design of more potent and selective ligands for these receptors. The NMR structure of the potent glucagon antagonist [desHis¹, desPhe⁶, Glu⁹]glucagon amide consists of an unstructured N-terminal segment (2-5), an irregular helix (7-14), a hinge region (15-18), and a well-defined α-helix (19-29). The two helices form an L-shaped structure with an angle of about 90° between the helix axes. There is an extended hydrophobic cluster, which runs along the inner surface of the L-structure and incorporates the side chains of the hydrophobic residues of each of the amphipathic helices. The outer surface contains the hydrophilic side chains. This result is the first clear indication of an overall tertiary fold for a glucagon analogue in the micelle-bound state. In addition to the structural difference, molecular dynamics simulations showed both N- and C-terminal residues in the glucagon antagonist are more highly ordered than those in glucagon. The single helix obtained for glucagon in the crystal state was found to unravel in the simulation around the region approximately corresponding to the hinge region in the antagonist. These results may have important implications for the biological activities of both peptides. The conformational study of cyclic alpha-melanocyte stimulating hormone analogues by NMR showed that their overall backbone structures are similar around the message sequence (His⁶-D-Phe⁷/D-Nal(2')⁷-Arg⁸-Trp⁹). beta-Turns spanning His⁶ and D-Phe⁷/D-Nal(2')⁷ were identified in all analogues. However, a stacking between the aromatic rings of His⁶ and D-Phe⁷/D-Nal⁷ was observed for the melanocortin agonists, but not for the antagonists. Based on the NMR structure of MTII, a library of new alpha-MSH analogues was designed and synthesized with a disulfide or lactam bridge used as a conformational constraint and the pharmacophore group in Arg⁸ mimicked by Nᵅ-alkylation via the Mitsunobu reaction. These new analogues exhibited high binding affinity and selectivity for the human melanocortin-4 receptor, thus suggesting the usefulness of the NMR structural model of α-MSH peptides.

This dissertation presents the results of several investigations into vibrational and rotational relaxation of neutral molecules at low temperatures. The use of the free jet flow reactor technique for production of very low local temperatures and the determination of relaxation data and derivation of rate coefficients are discussed. Four different molecules, HBr, CO, OH and OD, are examined in the free jet with various buffer gases to determine their vibrational and rotational relaxation properties. For HBr and CO, terminal rotational temperatures are measured in various buffer gases with the resonance enhanced multiphoton ionization (REMPI) technique, and results are analyzed and compared with translational temperatures. For OH and OD, discussion includes experimental production of radicals and relaxation within the X²Π and A²Σ states is made. With the use of laser induced fluorescence (LIF) and a novel method for production of radicals in the free jet, radical relaxation rates are determined for the first time at extremely low collision energies. We have measured the low temperature (T(trans) near 1 K) rate coefficients for: OH(A²Σ,vᵢ,Nᵢ)+Ar →(k) OH(A²Σv(f)N(f)) +Ar where v and N refer to the quantum numbers for vibration and pure rotational angular momentum, while the subscripts i and f refer to the initial and final states, respectively. The experiments reported in this dissertation help lead to a better understanding of collisions on a molecular level at very low collision energies.