No description available.

<p> The photoadditions of 3-phenyl-2-cyclohexenone to bicyclo [2.2.1] hepta- 2,5-diene, bicyclo [2.2.1] hept-2-ene and cyclopentene have been studied. In all cases cis fused cyclobutane products were obtained. Quenching and sensitization experiments indicated a singlet excited state to be active in photocycloaddition. Phosphorescence and fluorescence emission were observed from 3-phenyl-2-cyclohexenoneo Energy transfer to the lowest triplet of 3-phenyl-2-cyclohexenone was evident from the quenching of Michler's ketone phosphorescence. Two norbornene dimers were detected in the photolysis of 3-phenyl-2-cyclohexenone and norbornene giving evidence for a higher triplet excited state of the enone. The photoaddition of 3-methyl-2-cyclohexenone to cyclopentene was studied for comparison and both cis and trans fused adducts were obtained. In photolyses with bicyclo [2.2.1] hepta-2,5-diene or cyclopentene, 2-phenyl-2-cyclohexenone was unreactive. </p> / Thesis / Master of Science (MSc)

Photovoltaic (PV) technologies could provide abundant, clean and secure energy through the conversion of sunlight into electricity, but currently are too expensive to compete with conventional sources of power. Novel PV devices incorporating nanostructured materials, such as the dye-sensitized solar cell (DSC), have been identified as viable, low-cost alternatives to traditional solar cell designs. In spite of technological progress in the field over the last twenty years, the underlying physics governing DSC operation is still not well understood. In this thesis, first-principles (i.e. parameter-free) calculations are performed with the aim of connecting experimentally-measured photoemission data to the underlying atomistic and electronic structure of interfaces found in DSCs. The principal system under study is the interface between anatase titanium dioxide (TiO₂) and the "N3" dye molecule, one of the most widely-investigated device designs in DSC research. Atomistic models of the interface are determined within density-functional theory. Core-level spectra of these interface models are then calculated using a ∆SCF approach. Comparison of the calculations to published experimental data finds that intermolecular interactions have a significant effect on the spectra. Next, the electronic structure of bulk TiO₂ and of isolated N3 molecules is calculated using the GW approximation and ∆SCF method respectively. For the former, it is shown that including Hubbard U corrections in the initial Hamiltonian reduces the GW gap by 0.4 eV. These calculations are then used to determine the valence photoemission spectrum of the full interface. By including image-charge effects, thermal broadening and configurational disorder, quantitative agreement with experimentally-measured spectra is demonstrated. In addition to the N3/TiO₂ system, calculations of the core-level spectra of the interfaces between TiO₂ and H₂O and bi-isonicotinic acid are also presented. The thesis concludes with a study of the X₂Y₃/TiO₂ interfaces (X=Sb, Bi; Y=S, Se) found in recently-developed semiconductor-sensitized solar cells.

La théorie de la fonctionnelle de la densité dépendante du temps (TDDFT) est une méthode basée sur la densité pour calculer les états excités. Bien que la TDDFT soit une théorie exacte, on doit en pratique partir d'une approximation de la fonctionnelle d'échange-corrélation, qui reste inconnue. L'approximation adiabatique est l'approximation de la fonctionnelle la plus courante. Cette approximation donne de très bons résultats pour les propriétés spectroscopiques, mais elle est inexacte pour les simulations en photochimie. Dans cette thèse, on montre que l'origine du problème réside dans l'approximation de la fonctionnelle de corrélation. Le résultat principal de la thèse consiste en un noyau de corrélation, qui peut être utilisé dans la formulation de la réponse linéaire, noyau dérivée à partir de la théorie des perturbations à plusieurs corps. Le noyau inclut de façon générale les excitations doubles qui donnent l'effet principal à la corrélation dans les états excités. La comparaison de ce noyau avec la fonctionnelle adiabatique nous a permis d'identifier les termes manquants à ce dernier. Nous avons testé la possibilité d'ajouter ces termes comme une correction à l'approximation adiabatique. Le noyau pourrait potentiellement être appliqué à des systèmes de grosse taille. / Time-dependent density-functional theory (TDDFT) is a density-functional method for calculating excited states. TDDFT is formally exact, though in practice one has to approximate the unknown exchange-correlation functional, which contains all the unknown many-body effects. The adiabatic functionals are the most commonly used. Although they are very successful for spectroscopy, the adiabatic functionals are too inaccurate to be applied to photochemistry. In this thesis, we show that the main problem is due to the approximations in the correlation functional. The main result of the thesis is a correlation kernel for linear-response TDDFT, derived using many-body perturbation theory techniques, which generally includes double excitations, thus introducing the leading correlation effects in the excited states. The comparison of this kernel with the adiabatic functionals allowed us to identify which correlation effects are missing in these approximation. We tested the possibility of improving the description of correlation by adding the missing terms from many-body theory to the adiabatic functionals. This mixed kernel is more efficient than the full many-body kernel, and can potentially be applied to systems of medium to large size.

This thesis presents the results of a series of experiments in which electrons are superelastically scattered from various laser excited states of sodium. The atoms, once in the optically prepared state, are forced to relax via the superelastic collision with an electron. The rate of detection of superelastically scattered electrons was measured as a function of the laser polarisation which enabled pseudo Stokes parameters to be determined. These pseudo Stokes parameters are functions of both optical pumping parameters and atomic collision parameters. The optical pumping parameters describe the laser-atom interaction and the atomic collision parameters describe the electron-atom collision process. Three different laser excitation mechanisms were used to optically pump the atoms into various excited states. The first of these used a single laser tuned to the 32S 112(F'=2 hyperfine state)-~32P312 transition. The excited atoms underwent a superelastic collision with an electron leaving the atom in the ground state and pseudo Stokes parameters were measured as a function of both scattering angle and incident electron energy. The second superelastic experiment, utilised a folded step excitation mechanism which employed two lasers tuned from the two hypethne states of the 32S112 ground state respectively to the 32P312 excited state. Power broadening effects in the single laser experiment cause the atoms to be optically pumped into the F= 1 hyperfine ground state. The laser powers used were not great enough to power broaden the hyperfine ground states and as such the F'= 1 sublevel effectively acted as a sink. The folded step excitation method enabled the excited state population to be increased so that data at larger scattering angles could be obtained. Stokes parameters from both of these experiments which had an incident energy range of 10eV to 30eV and an angular range of 5°-25° were compared to three current electron-atom scattering theories and previous experimental data. Overall, fair to good agreement was found between theory and experiments for the individual Stokes parameters. Losses of coherence was observed at small scattering angles (50.200) at 20eV and 25eV incident electron energies which were poorly modelled by the three different theories. The third superelastic experiment involved the use of two lasers of specified polarisation to stepwise excite the atoms to the 32D512 excited state. Superelastic collisions with incident electron energies of 20eV from the 32D512-*32P312~312 collision were studied at three different scattering angles and pseudo Stokes parameters for the case where the polarisations of the radiation from the lasers were parallel were measured. The single step and folded step laser-atom interactions for it excitation were modelled using a full quantum electrodynamical treatment so that the optical pumping parameters from the single and folded step experiments could be investigated. Equations of motion were derived in the Heisenberg picture and it is shown that for the single laser case 59 equations of motion are required to fully model the interaction and for the folded step ease 78 equations of motion are required. The results of calculations demonstrated that the optical pumping parameters were sensitive to laser intensity, laser detuning and the Doppler width of the atomic beam. The theoretical quantum electrodynamical calculation results were in good agreement with the experimental results.

Density functional theory is used to calculate the core excitation spectra of titanium structures. Specifically, the core-hole approximation is used. In this scenario, the excitation energies of core electrons are calculated using the approximation that the core energy level be frozen throughout the relaxation process of the orbitals. This allows a more acurate determination of the resulting X-ray spectra. The method described has been implemented in an NWChem module.

No description available.

This thesis is dedicated to studies of the excited-state dynamics in organic molecular systems for solar energy conversion by employing time-resolved experimental techniques. Organic photovoltaic (OPV) devices have received significant attention in the past decade and reaching record high power conversion efficiencies (PCE) above 17%. An essential step towards reaching the predicted PCE limit of 25.5% is to develop a comprehensive picture of the photophysical processes, specifically the loss processes, in OPV devices. It is the aim of this thesis to investigate and understand the fate of excited-states in organic electron donor/acceptor systems by ultrafast spectroscopic techniques, specifically, to reveal the interplay between energy and charge transfer processes. The first part deals with the identification of different polymorphs in a diketopyrrolopyrrole-based (DPP) polymer. Applying time-resolved photoluminescence (TRPL) measurements to the polymer dissolved in different solvent mixtures and using multivariate curve resolution (MCR) to deconvolute the ground-state absorption spectra reveals the co-existence of an amorphous (α) and two semi-crystalline (β1 and β2) polymer phases. The OPV device performance is shown to increase by the additional absorption of the β2 phase. The second part compares the efficiency of direct and energy transfer-mediated charge generation in prototypical donor-acceptor dyads that use as the electron donor triangulene derivatives chemically linked to the electron acceptor perylenediimide (PDI) block via oligophenylene spacers of different lengths. Charge generation efficiencies are found to be similar and increase with the donor-acceptor spatial separation. A combination of transient absorption (TA) measurements and computation of the dyad’s excited-state landscape revealed the presence of “optically-dark” excited-states that are populated by ultrafast donor-acceptor energy transfer prior to hole (back) transfer. The last part of the dissertation uses TRPL, TA, and time-delayed collection field (TDCF) measurements alongside MCR analysis to provide a comprehensive analysis of the yield of individual photophysical processes in OPV devices. A systematic methodology is proposed and tested on two all-polymer BHJ devices used as model systems. The experimental findings are supported by successful simulation of the solar cells’ JV characteristics using the spectroscopically-determined kinetic parameters. More generally, this approach can be used to quantify efficiency-limiting processes in other donor-acceptor BHJs.

<p> In order to gain mechanistic information of photoaddition reactions of 2-cyclohexenones, the additions of 2- and 3-methyl-2-cyclohexenone and 2-cyclohexenone to bicyclo[2.2.1]hepta-2,5-diene have been studied. Substituted 2-cyclohexenone products are obtained in addition to cyclobutane derivatives, and it is proposed that the former adducts arise from diradical intermediates and intramolecular hydrogen shifts. The photo-rearrangement, of 4, 4-dimethyl-2-cyclohexenone, and its photocycloaddition to cyclopentene, was studied in order to identify the enone excited state responsible for cycloaddition. These results, and naphthalene quenching experiments, indicate that all of the additions studied proceed via triplet excited states.</p> / Thesis / Doctor of Philosophy (PhD)

The field of theoretical chemistry has provided undeniably useful insights about molecular systems that otherwise, through experiment, would not be obtainable. We are constantly developing new and improved methods to fill in the gaps about how various factors including the electronic structure can affect the chemistry seen experimentally. The goal of most quantum chemistry methods is to develop a method that is widely applicable, has low computational costs, but with as much accuracy as possible. Some of the most challenging systems in our field include those that are considered strongly correlated. Strong correlation is usually referring to the need for a large number of configurations to properly model the chemistry. These systems can not be solved exactly, thus various approximations must be made. A set of methods that take advantage of truncating only the unimportant configurations to solve these challenging systems are selected configuration interaction methods. Even though these selected CI methods can often provide accurate results, their general application is limited by memory bottlenecks. In 2020, our group developed the Tensor Product Selected Configuration Interaction (TPSCI) method to overcome these memory bottlenecks. We take advantage of the local character of these strongly correlated systems by doing a change of basis into tensor products, then do a selected CI algorithm in that basis. In this dissertation, we discuss how we have extended TPSCI to compute excited states. We first test on a set of polycyclic aromatic hydrocarbons that were previously studied with TPSCI. We find very high accuracy and dimension reduction as compared to state of the art selected CI approaches. We then validate TPSCI's ability to study the electronic structure involved in the singlet fission process in tetracene tetramer with extending analysis using a Bloch effective Hamiltonian. This effective Hamiltonian allows for intuitive analysis of the singlet fission process. We also show how accurate and interpretable TPSCI can be on an open-shell biradical transition bimetallic complex, in addition to, hexabenzocoronene that is not straightforward clustering due to the conjugated benzene rings. To alleviate the previous system size limitations, we recently implemented a Restricted Active Space Configuration Interaction as a local solver for clusters. We present novel results of using this new solver on a tetracene dimer. We comment on specific coupling strengths and show the electronic dynamics of our TPSCI effective Hamiltonian which support a CT-mediated mechanism for the tetracene dimer singlet fission. / Doctor of Philosophy / The field of theoretical chemistry has used some of the fundamental principles in quantum mechanics to study the electronic structure of molecular systems for many years. The power of computational resources has increased over the years, facilitating the increased complexity and accuracy of quantum chemistry methods. This dissertation lies in the realm of pushing past previous molecular system computational limits with rewarding accuracy and increased interpretability. We achieve these goals by taking advantage of the localized nature in most of our chemistry vocabulary by using tensor product methods. Tensor product methods are able to separate a large problem into smaller units to overcome previous system size limitations while maintaining the desired accuracy. The main method focused on in this dissertation is a tensor product method called Tensor Product Selected Configuration Interaction (TPSCI) established by our research group in 2020. This dissertation covers the required background information including basic terminology and previously developed methods then presents very recent and novel research using TPSCI. We first focus on extending TPSCI to excited states since excited states are extremely important for many photochemical processes, spectral analysis, and chemical sensing. We then test TPSCI on a spectrum of systems that range from very local character (separated molecular units) to bimetallics to very delocalized (carbon-based conjugation) chemistry. We find TPSCI is able to handle this variety of systems with very high accuracy and allows for very in-depth qualitative analysis. Finally, we present novel results incorporating an additional approximation in the local solver to further extend TPSCI's applicability. To test this new local solver, we focus on a process called singlet fission which is promising to help overcome solar cell efficiency limits. We are able to match previously reported results for the tetracene dimer which supports the use of TPSCI to study larger singlet fission systems in future work. With the work presented in this dissertation, we have aimed to highlight the potential utility of TPSCI, motivating further developments and research in this direction.