Excited-State Reaction Dynamics of Bacteriochlorophyll Dimers

点击翻译按钮获取中文摘要

9728134 Beck This research project is designed to test the hypothesis that bacteriochlorophyll dimers in asymmetric protein environments populate intradimer charge-transfer states after optical preparation of the lower exciton state. In order to test this hypothesis, the investigator will characterize the excited-state coherent wavepacket motions that arise during light-induced charge transfer in two asymmetric systems: the bacteriochlorophyll a dimer in the B820 subunit of the LH1 light-harvesting complex, as isolated from Rhodospirillum rubrum G9, and the carbocyanine dye 3,3'-diethylthiatri-carbocyanine iodide (DTTCI). Dynamic absorption spectroscopy will be used to characterize the modulations of the photobleaching and stimulated-emission signals that arise from coherent wavepacket motion on the ground and excited states, respectively. Stimulated-photon-echo and transient-grating spectroscopy will be used to examine the electronic dephasing of the initially prepared state. The wavepacket motions exhibited by B820 will be compared to those exhibited by monomeric bacteriochlorophyll a and by the intact LH1 ring system, where dimer-dimer interactions are thought to be important, to see if the surface-crossing dynamics observed in B820 are specific to bacteriochlorophyll dimers. The long-term goal of the proposed work is to obtain a detailed understanding of the dynamics of charge transfer in both target systems in terms of the potential-energy surfaces that are involved and in terms of the vibrational modes that promote the reaction. This research project will use laser spectroscopy with femtosecond pulses of light to study how electrons are transferred in pairs of bacteriochlorophyll molecules. Bacteriochlorophyll molecules are used by photosynthetic plants and bacteria in light-harvesting structures and in reaction centers, where some of the energy of the absorbed light is converted to chemical energy. The structure of the reaction center from purple bacteria contains a pair of ba cteriochlorophyll molecules that act as the trigger point for the fast events that lead to storage of energy. The work in this research project will study reactions in a simple bacteriochlorophyll-containing protein and in an important type of organic dye; both systems exhibit very fast electron-transfer reactions that occur just after absorption of light. In addition to contributing to the understanding of the mechanism of the light reactions of photosynthesis, this research project will contribute to the general understanding of how solar energy can be converted by chemical systems into stored forms of energy. In the long term, the project will help determine how light-absorbing molecules can be physically organized in materials to obtain energy-storing devices or novel catalysts.