Electron transfer (ET) is one of Nature's most important reactions, playing key roles in respiratory and photosynthetic pathways that are of fundamental bioenergetic significance. Though relatively well understood as a reaction class, ET reactions are not understood in detail in terms of the molecular features that define reactivity: ultrafast vibrational spectroscopy (infrared or coherent optical) will provide the first direct experimental probe that will help decipher how molecular motions are coupled to actual ET events. The reorganization energy (l),comprised of inner-sphere (high-frequency vibrational modes of the donor, acceptor, and medium) and outer-sphere (low-frequency vibrational modes of the solvent or bath) contributions, is of primary significance in determining how fast ET occurs in a given donor-acceptor system. Our goal to actually identify and measure the energy of vibrational modes that are key to an ET reaction coordinate represents an unexplored experimental frontier: the potential to add to our fundamental knowledge of charge transfer chemistry as well as influence the theoretical framework that has been developed to describe ET reactions is enormous. Our work focuses on simple donor-acceptor complexes that undergo fast charge separation and/or change recombination reactions (kET greater than 2 x 10[12] s-1); the requirement of fast ET ensures that vibrational dephasing of excited-state reactants as well as ground- and excited-state ET products does not occur on our experimental timescale. Model systems that allow pair-wise examination of reaction center-type chromophores (such as porphyrins and quinones) feature prominently in this proposal. Similar to the intimate coupling of reorganization energy and reaction free energy, it is believed that the vibrational modes that activate chromophores for ET and accept energy in the photosynthetic reaction center play a central role in the highly efficient separation of an electron from a hole across a membrane during early photosynthetic events. Work completed to date has concentrated on utilizing optical pump-coherent optical probe experiments to study ET reactions that occur faster than the longitudinal relaxation time of the solvent. Such a coherently controlled ET reaction has recently been implicated in the long-range photosynthetic charge separation event. In addition to our experimental efforts designed to develop a fundamental understanding of coherence phenomena in ET reactions, other work exploits optical pump-IR probe experiments to probe the relationship between the readily identifiable high and low frequency chromophore vibrational spectroscopic tags (C-O and C-N stretching modes, arene and pyridinium ring breathing modes, porphyrin ruffling modes, etc.) and the ET reaction coordinate. Such detailed studies of how molecular vibrations are coupled to ET will both provide a new level of understanding of biological charge separation reactions as well as aid in the design of artificial systems that mimic photosynthesis, potentially impacting new energy storage technologies.
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