Understanding excited states and energy transfer in highly ordered organic molecular assemblies

π-stacked organic materials are tunable light absorbers with many potential applications in optoelectronics such as light emitting diodes, solar cells, and photocatalysts. Their optical properties are highly dependent on the nature and energy of electron-hole pairs or excitons formed upon light absorption, which are in turn determined by the intra- and inter-molecular electronic and vibrational excitations. In this dissertation, first principles methods such as density functional theory (DFT), time-dependent DFT (TDDFT), and a recently developed time-resolved non-adiabatic dynamics approach are used to understand excitons and their interactions with atomic vibrations. Perylene diimide (PDI) molecules are studied as a model system to gain physical insight about these phenomena. This class of materials is highly suitable for solar energy conversion because of the strong optical absorbance, efficient energy transfer, and chemical tunability. TDDFT, including vibronic effects, was applied to macromolecular DNA-based surrogates composed of one to three stacked PDI molecules, in order to understand the influence of electronic coupling to vibrational modes on the exciton. This approach is validated by comparison to experimental measurements and it was determined that intra- and inter-molecular interactions result in distinct vibrational, electronic, and optical properties. Additionally, exciton dynamics within these macromolecules is studied, simulating the internal energy decay from a high to lower energy excitonic state due to coupling of the excitation with atomic vibrations. It is shown that stacking leads to enhanced energy decay because of decreased energy spacing between states. Additionally, a new approach is presented to identify the vibrational modes that assist energy transfer, revealing that interactions between stacked molecules modulate the normal modes that couple to the exciton. Lastly, by studying the dynamics of the transition density, it is demonstrated that stacking impacts the localization of the exciton, a key feature of interest for solar energy conversion. For the dimer, the exciton quickly localizes and oscillates between two monomers, while the trimer can host long-time delocalization of the exciton. In summary, by applying first-principles theory, the coupling of inter-/intra-molecular electronic and vibrational excitations and their effects on energy transfer is identified. These findings provide fundamental understanding of the atomic-scale process associated with energy conversion, and provide insight towards rational design of new optoelectronic organic assemblies.2023-08-26T00:00:00