Thesis (M.S.)--Auburn University, 2008. / Abstract. Includes bibliographical references (p. 29-30).

<p> When excited in the region 365-436 nm in the gas phase, 2,3-pentanedione is shown to emit fluorescence and phosphorescence. The absolute emission yields have been determined and the radiationless processes removing the excited singlet and triplet states have been considered. </p> <p> The radiationless processes removing the triplet state have been investigated in different temperatures and concentrations and are shown to be intersystem crossing to the ground state, a temperature dependent unimolecular reaction and a temperature dependent bimolecular self-quenching reaction. </p> <p> Excitation at 365 nm causes emission from vibrationally excited levels of the singlet state. The variation of the fluorescence yield with pressure at this excitation wavelength is compatible with either a strong or weak collision mechanism for vibrational deactivation within the singlet manifold. Phosphorescence yield measurements support this conclusion. </p> <p> A consistent mechanism is proposed to describe the system and is tested with fluorescence, phosphorescence, and lifetime measurements at various temperatures and concentrations. </p> <p> The interaction of the triplet state of pentanedione with various classes of substrate has been investigated. Emphasis has been placed on temperature dependence of the reaction modes. The possibility of energy transfer from the triplet pentanedione molecule has been investigated with 1,3-butadiene and cyclopentadiene at different temperatures. The butadiene quenching system is kinetically simple at all temperatures studied, whereas the cyclopentadiene is simple at high temperatures but complex at lower temperatures. This behaviour has been reconciled with reversible energy transfer in the gas phase triplet pentanedione/cyclopentadiene system at the lower temperature. A mechanism is proposed which demonstrates directly the intermediacy of the triplet cyclopentadiene species and, therefore, confirms the energy transfer nature of the interaction. The energy transfer rate constants are discussed with respect to the current models of energy transfer to dienes. </p> / Thesis / Doctor of Philosophy (PhD)

Thesis (Ph. D.)--University of Washington, 1996. / Vita. Includes bibliographical references (leaves [129]-134).

The ability of the nucleic acids to absorb ultraviolet light and remain relatively photostable is a property upon which life depends. The nucleobases, which are the primary chromophores, when irradiated display rapid radiationless decay back to the ground state, in general faster than is needed for photoreaction. Fluorescent analogs of these bases have structures similar to the nucleic acid bases, but display much longer excited state lifetimes. Theoretical investigations using quantum mechanical methods can provide insight into the precise mechanisms of these decay processes, and to the molecular specifics that contribute to them. The results of multi-reference configuration interaction (MRCI) ab initio investigations into these mechanisms are presented, with emphasis on cytosine and its fluorescent analog 5-methyl-2-pyrimidinone (5M2P). A comprehensive picture of the potential energy surfaces of these two bases is given, including stationary points and conical intersections, where radiationless transitions are promoted, between up to three state surfaces, as well as pathways connecting these points for each base. Cytosine is shown to have two different energetically accessible radiationless decay channels. The fluorescence of 5M2P is also demonstrated theoretically, with mechanism proposed. The potential energy surfaces of the two bases have many close similarities, with the different photophysical properties being attributed to subtle energetic differences between the two bases. Nonadiabatic coupling and the geometric phase effect are analyzed in detail near conical intersections in cytosine, including in a region close to a three-state conical intersection. A substituent effect study on the 2-pyrimidinone ring system shows that the presence, position and orientation of the amino group in cytosine is central to its photophysical properties, particularly its high absorption energy, and can be explained with a simple Frontier Molecular Orbital model. The effects of water solvent on the excitation energies of cytosine and uracil are theoretically investigated using two multi-reference ab initio methods, a quantum mechanical molecular mechanics method using MRCI (MRCI-QM/MM), and the fragment molecular orbital multiconfiguration self-consistent field method (FMO-MCSCF). The solvatochromic shifts calculated from both methods agree well with other more expensive methods and experimental data. The effects of water on the photophysical pathways of cytosine is also investigated using MRCI-QM/MM, including considerations of solvent reorganization. Results show that the overall effect of water on the decay mechanisms is small, with neither decay channel being significantly blocked or favored. / Chemistry

This work presents a new implementation of the QP algorithm, a computer method to diagonalize the extremely large matrices arising in multimode vibronic problems. Benchmark calculations are included, showing the accuracy of the program. The QP algorithm is extended to treat multiple electronic surfaces for competitive control and this is demonstrated with an Hamiltonian including three electronic states, a model of the benzene radical cation. Finally, the evolution of zeroth-order states in a simple two electronic states, two dimensional model with a conical intersection is explored, towards building a time-dependent view of overlapping resonances coherent control.

This work presents a new implementation of the QP algorithm, a computer method to diagonalize the extremely large matrices arising in multimode vibronic problems. Benchmark calculations are included, showing the accuracy of the program. The QP algorithm is extended to treat multiple electronic surfaces for competitive control and this is demonstrated with an Hamiltonian including three electronic states, a model of the benzene radical cation. Finally, the evolution of zeroth-order states in a simple two electronic states, two dimensional model with a conical intersection is explored, towards building a time-dependent view of overlapping resonances coherent control.