Applications of response theory to excited electronic states : metal oxide clusters and non-linear absorption in squaraine dyes

Abstract

Over the past one hundred years, an incredible range of electronic structure methods have been developed providing great insight into the quantum chemistry involved in chemical processes, including reaction pathways, structure and dynamics. The quantum mechanics of excited states provides a challenge for electronic structure methods. Initially, this thesis inspects the ability of coupled cluster methods and time dependent density functional theory (TD-DFT) to accurately characterize the excitation energies and properties of titanium oxide clusters. By utilizing a hierarchy of coupled cluster methods, the convergence of excited state properties in terms of systematic electron correlation has been presented along with an examination of convergence towards the complete basis set limit. Major deficiencies were seen in second order CC2 for the clusters, attributed to a large singles amplitudes in the Hartree-Fock orbitals. TD-DFT allowed for the study of larger clusters, too demanding for CC. Chromium oxide clusters yielded similar results. Having established the successes and limitations of electronic structure methods to calculate excited state properties of transition metal oxide clusters, attention was directed towards organic molecular systems - squaraine dyes. Squaraines, which have an application in photodynamic therapy, exhibit interesting photochemistry which was explored using linear and quadratic response functions in TD-DFT. The one-photon and two-photon absorption spectra were determined in this way, while the a posteriori Tamm-Dancoff Approximation (ATDA) was also utilized to determine two photon absorption cross sections using a three state model. Vertical excited states from the first excited state geometry gave rise to the ESA spectra determined by ATDA TD-DFT. To summarise, the power, scope and limitations of electronic structure methods has been explored in two very different chemical systems - transition metal oxide clusters and organic molecular squaraines, with interesting results.