The Nature of the Excimer and Exciton States in Self-Assembled Organic Chromophore Systems

Myong, Michele

doi:https://doi.org/10.21985/n2-04b0-7336

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The Nature of the Excimer and Exciton States in Self-Assembled Organic Chromophore Systems

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Creating sustainable and clean sources of energy is an outstanding problem of importance for the current generation of scientists to solve. Organic photovoltaics (OPVs) are a promising renewable energy source but require improvements to their architectures and better fundamental understanding of the effects of organic chromophore morphology on the resulting photophysical properties. To address these questions, we constructed chromophore assemblies and probed their photophysics with optical spectroscopies. Self-assembly is an effective tool to create ordered systems through simple synthetic methods. Using anodic aluminum oxide (AAO) membranes, we built ordered assemblies of various organic chromophores that are relevant to light harvesting. These nanoscale architectures represent the mesoscale region between the solution and solid-state phases, providing an exciting opportunity to elucidate the behavior of the exciton and excimer states in these systems. We extended our studies of the exciton to the solid-state, using cocrystallization strategies to investigate the differential exciton diffusion processes between electronically similar cocrystals. In PDI assemblies on AAO with different degrees of disorder, we demonstrate that the solvent environment can still modulate the excimer dynamics, even when the molecules are very strongly coupled to another through π-stacking interactions. The kinetics of excimer formation, relaxation, and decay are also influenced by the degree of disorder in the self-assembled chromophore system. BPEAs can be ordered in assemblies on AAO as well, and pump power dependent transient absorption studies reveal that the excimer state is diffusive. This challenges the assumption that the excimer is immobile because of its lower energy as a trap state and extends the examples of a diffusive excimer exciton to supramolecular systems. The nature of the charge transfer exciton is probed in two electronically similar cocrystals composed of PDI and pyrene that have markedly different packing structures. In the final study, we outline the challenges and possibilities associated with probing exciton coherence lengths in a series of AAO assemblies with well characterized photophysical behavior. These studies further our understanding of the exciton and excimer in ordered organic chromophore systems and offer future pathways to create organic materials with desirable energetics for energy conversion.

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