The Electronic and Optical Properties of Graphene Flakes and Similar Systems

Organic materials have garnered interest from physicists, chemists, and engineers due to their remarkable electronic properties and potential optical applications. To accurately describe the electronic and optical properties of these systems, electron correlations beyond the mean-field level must be included. In this thesis, we focus on three systems: dibenzoterrylene, rectangular graphene flakes, and -acene molecules. We use the Pariser-Parr-Pople model to describe the electrons and apply a configuration interaction strategy to solve for the many-body wavefunctions of these systems. We first investigate the optical properties of dibenzoterrylene (DBT); DBT is of interest as a possible single-photon source. We calculate the linear absorption spectrum and the spatial distributions of the electrons involved in several bright transitions. We determine the two-photon absorption spectrum and show there are several states that are bright due to two-photon absorption. We predict that there are no competing optical processes that might hinder the single-photon source application of DBT. Next, we focus on the electronic and optical properties of rectangular graphene flakes. We find that the lowest energy bright transition has a transition dipole moment that is polarized along the long axis of the flake, but the charge distributions involved in these transitions are invariably localized on the zigzag edges. We investigate the role of impurities on the optical properties of these graphene flakes; we find that impurities located on the zigzag edges can have a significant - and often dramatic - impact on the optical properties of these systems. Finally, we study the optical properties of tetracene, pentacene, and hexacene, known as the -acenes. We compute the one-photon absorption spectra of the -acenes and compare our results with other quantum chemistry calculations. We calculate the charge densities involved in the bright transitions. We determine the two-photon absorption spectra and show that there are two two-photon active states in the low energy regime. We identify simple qualitative features in the optical properties of the -acenes which can be easily understood.Ph.D