Formation and Dynamics of Molecular Excitons and their Fingerprints in Nonlinear Optical Spectroscopy

Abstract: An efficient transfer of energy in molecular systems has proven to be of fundamental importance both in nature and industrial applications. The ability of molecules to work together forming collective excitations, so-called excitons, plays a key role in for example the extraordinary fast energy transfer involved in the first steps of photosynthesis. The characterization of excitonic states in conjugated polymers is furthermore of importance in the hunt for new materials in light emitting devices and solar cells. This thesis deals with different properties of molecular excitons. Their formation, size and dynamics are discussed both in the context of molecular aggregates and conjugated polymers. The dynamics of Frenkel excitons in a small molecular aggregate was explored. A quantum-classical approach was used to describe the interplay of nuclear dynamics and delocalization of excitons. The coupling of the excitonic state to the surrounding environment results in relaxation. An approach based on a Langevin equation for the dissipative surrounding was used to examine delocalization and localization effects as a function of time. Exciton dynamics was also studied in the case of a weak electron-vibrational coupling, in which case Redfield relaxation theory can be used. In particular the signatures of exciton dynamics in non-linear spectroscopy were analyzed. The three-pulse photon echo experiment is a non-linear method that provides some especially interesting features. Simulations were done to provide nsight on echo signals both in time and frequency domain. Collective excitations may also be studied by means of the electronic structures obtained by quantum chemical methods. Time-dependent density functional theory was used to study neutral and charged conjugated polymers. The spatial localization and character of the elementary excitation was obtained using different representations. Time-dependent density functional theory was also used to investigate the influence of the local environment on the energetic structure of molecules. Transition energies of a bacteriochlorophyll subjected to strong electric fields were determined to address the role of the field in molecular static disorder.