| Excitation energy transfer (EET) between organic molecules is a common phenomenon in nature, for example, in photosynthesis, and it is a key process in the working mechanism of modern organic optoelectronic devices, for example, in organic solar cells, or light emitting diodes; however, the development of organic materials for any type of electronic application requires a detailed understanding of energy transfer processes amongst the organic molecules. Energy transfer processes in organic molecules are dependent on conformations of the conjugated polymers, and structure of the donor-acceptor dyads. Poly-(para-Phenylenevinylene) (PPV), and its derivatives such as poly-(2-methoxy,5-(2-ethyl-hexoxy)-1,4-phenylene vinylene) (MEH-PPV), are extensively studied conjugated polymers, and LPPP5 (Ladder-type poly-(para-phenylene)) - PDI (Perylenediimide) is a typical donor-acceptor species. In this thesis, we explore non-radiative excitation energy transfer (EET) in conjugated polymers. First, we address the non-adiabatic state-to-state energy transfer to study energy migration, and fluorescence depolarization in model PPV chains. Our result reinforces experimental understanding of the fluorescence depolarization that the excitation energy transfer is sensitive to the structural morphology of the polymer chain.;In addition, we study non-adiabatic excitation energy transfer between co-joined donor-acceptor species consisting of a perylenediimide unit (PDI) linked to a ladder-type poly-(para-phenylene) (LPPP5) oligomer. An intramolecular energy transfer in the co-joined species is shown to be efficient in the absence of spectral overlap, which shows a non-Forster behavior due to strong coupling that is mediated by through bridge interactions between donor and acceptor; however, substituting a long spacer molecule, or increasing the length of bridge between donor and acceptor may revert the mechanism of energy transfer to the Forster limit. We describe the experimental evidence of bridge-mediated EET pathways where we show that the covalent linkage, or any interchromophoric bridge between donor and acceptor species may influence EET due to delocalization of pi-electrons through the covalent linkage. Here, we have also analyzed how different types of vibrational modes participate in the reorganization relaxation process, and in the direct non-adiabatic coupling. We also predict a hitherto unobserved isotope effect in these systems, which is consistent with isotope effects observed in related systems. |
