Light harvesting proteins in photosynthetic organisms contain highly ordered arrays of chromophores responsible for the collection of energy from solar photons. The organization of the chromophores may lead to collective excitations (excitons) that are delocalized over many molecules in the array. The delocalized excitations allow for coherent, or wavelike, energy transfer between the chromophores, rather than a particle-like, incoherent, energy transfer process. It has been proposed that these collective excitations may direct the flow of energy along the most efficient pathway to enhance the fitness of photosynthetic organisms. Photosynthetic organisms may also favor closely packed chromophore arrays because the structure is compact whilst optimizing large optical cross sections for absorption. Control of the coupling between chromophores may lead to a photoregulatory mechanism, which could control the energy transfer rate as a function of ambient light intensity fluctuations. Broad-band two- dimensional electronic spectroscopy (2DES) can be used to elucidate donor–acceptor pathways and mechanisms for both coherent and incoherent excitation energy transfer (EET) in photosynthetic light-harvesting proteins. In this dissertation, 2DES is applied to determine how quantum coherent energy transfer occurs between carotenoids and chlorophylls (Chls) in the peridinin-chlorophyll protein (PCP), a mid-visible peripheral light-harvesting protein in marine dinoflagellates that delivers excitation energy to photosystem II. PCP is unique in that it uses a carotenoid, peridinin, as the main light harvesting chromophore and that it can be reconstituted with different chlorophylls to change the energy landscape without causing structural changes. Through 2DES experiments on native PCP with Chl a, we show that although the collective excitations of chromophores are very short lived, they lead to an enhanced quantum yield compared to that for conventional, incoherent energy transfer mechanisms. Replacing the native Chl a acceptor chromophores with Chl b slows energy transfer from peridinin to Chl despite narrowing the donor–acceptor energy gap. The formyl substituent on the Chl b macrocycle hastens decoherence by sensing the surrounding electrostatic noise, leading to lower EET efficiencies. This work is significant because it improves our understanding of the role of coherent energy transfer in photosynthetic light harvesting. This information may prove useful when designing materials featuring strongly interacting electronic chromophores for the collection of solar energy for the generation of fuels or for use in photocatalysis.
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