UBC Theses and Dissertations
Sulfur-bridged chromophore dimers Christensen, Peter R.
Controlling both energy and electron transfer processes is necessary for the performance of both light harvesting, and light emitting devices. The efficiency of both energy and electron transfer depends on the degree, or magnitude of the electronic coupling between molecules. In photosynthetic organisms such as plants and bacteria, long range energy transfer occurs between electronically coupled assemblies of chlorophyll molecules. The remarkable energy transfer efficiencies of light harvesting complexes employed in photosynthesis is intimately related to the orientation and intermolecular spacing between neighboring, identical chlorophyll molecules. In an attempt to develop inexpensive artificial light harvesting systems and devices researchers have synthesized an impressive catalogue of organic small molecules and polymers to generate free electrons and holes from absorbed light energy. The most commonly employed design principle for synthesizing both small molecule and polymer light absorbing systems is to link together an electron deficient (acceptor) molecule to an electron rich (donor) molecule through a conjugated linker. The conjugated bridge between donor and acceptor units enables charge transfer states that offer band-gap tuning and more easily separated electron/hole pairs. However, exciton migration in these synthetic donor-acceptor systems only partially mimics the assemblies of light harvesting molecules found in nature. As a result, exciton migration is limited to very short distances in synthetic light harvesting organic molecules. The following thesis presents a new approach for controlling the electronic coupling between neighboring identical molecules. By bridging two identical molecules about a sulfur atom, the electronic coupling between the two molecules can be increased by increasing the oxidation state of the “sulfur-bridge”. The broader implication of this work is that we may not need to use strongly coupled, conjugated donor-acceptor systems to achieve efficient charge separation and excitation migration. Instead, by using a non-conjugated linker, such as sulfur or perhaps even a saturated carbon atom, between molecules with identical electronic structure we may be able to enhance exciton transport in synthetic light harvesting assemblies.
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