| To transform solar energy into usable chemical energy, photosynthetic organisms use pigments bound within light harvesting complexes to absorb sunlight and transfer the electronic excitations to reaction centers, where the conversion begins. Carotenoid pigments aid in light harvesting by absorbing sunlight and transferring the energy to chlorophyll pigments. The electronic states through which this transfer occurs depends greatly on the energy levels and lifetimes of "dark" carotenoid states, which are undetectable by standard absorption measurements. The nature of these "dark" states can, however, be explored by two-photon excitation spectroscopies as described in this dissertation. Work is presented that clarifies the energies and dynamics of the lowest excited states of carotenoids in solution and in photosynthetically relevant proteins, such as Light Harvesting Complex II from higher plants, Peridinin - Chlorophyll a - Protein from dinoflagellates, and Light Harvesting Complex 2 from purple bacteria. These studies have helped elucidate the mechanisms of photosynthetic light harvesting by carotenoids. |
