Energy and Charge Transfer in Chlorophyll- and Porphyrin-Based Multichromophore Arrays

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This thesis consists of the synthesis and photophysical characterization of chlorophyll- and porphyrin- based multichromophore arrays designed for the purpose of studying energy and charge transfer therein. The first series of arrays consists of donor-acceptor dyads in which various arylene diimide electron acceptors are attached through a phenyl group to the 20-position of Chlorophyll a. Photoexcitation of the chlorophyll components results in efficient electron transfer to the arylene diimides. Comparisons with the analogous porphyrin donor-acceptor dyads reveal similar rates of photoinduced charge separation and charge recombination. The second set is comprised of two, three-fold symmetric chlorophyll-based molecules which were constructed using both ethynyl and phenyl linkages to the chlorophyll macrocycle. Photoexcitation of the chlorophylls results in rapid energy transfer between chromophores within the trefoils. Characterization of the chemically-generated, mono cations reveal that unpaired electrons are shared between the three redox centers in only one of the arrays. The third series involves linear chlorophyll dimers in which the positioning of the phenyl spacer in the linkage is varied. Photoexcitation of the chlorophylls results in rapid energy transfer between chromophores within the dimers. Characterization of the chemically-generated, mono cations reveals that direct phenyl linkage to the chlorophyll macrocycle prohibits charge transfer between covalently linked chlorophylls. The fourth array involves a chlorophyll derivative that self-assembles into a cyclic tetramer via metal coordination. This non-covalent assembly exhibits intramolecular energy transfer rates that are comparable to those observed previously only for covalent ring structures. The fifth group of arrays contains butadiyne-linked chlorophyll and porphyrin dimers organized in non-covalent, prismatic assemblies following the addition of trigonal, metal-coordinating ligands. Photoexcitation of the prismatic assemblies reveal efficient energy transfer occurs between the assembled dimers with minimal distance dependence. The final array is constructed from four zinc porphyrins attached to the various positions of perylenediiimide (PDI). The asymmetry of PDI results in two distinct pathways for efficient electron transfer. Using a combination of metal-ligand binding with a bidentate ligand and - stacking, the array forms a supramolecular assembly in toluene. The structure of this hierarchical assembly demonstrates both efficient light harvesting and facile charge separation and transport.

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  • 06/26/2018
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