Experimental Studies of Electron Tunneling Through Non-Covalent Contacts in Supermolecules

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The Organic and Macromolecular Chemistry Program at the National Science Foundation supports Professors David H. Waldeck and Christian E. Schafmeister at the University of Pittsburgh for a project that will investigate how nuclear fluctuations of chemical reactants and solvent (nuclear motion) effect the electron's motion in reactions that occur over large distances (greater than or equal to one molecule intervening between the electron donor and electron acceptor). While the understanding of electron transfer is well developed when either the electron's movement or the nuclei's movement is rate controlling, the intermediate case, in which both electron motion and nuclear motion are important, has eluded experimental study. This project will use time-resolved fluorescence spectroscopy and fluorescence correlation spectroscopy to follow the electron transfer in well-defined supermolecules that are designed to juxtapose electron donating and electron accepting units at some distance from each other so that solvent molecule(s) can occupy the intervening space ('gap'). An important extension from earlier work is the creation and study of supermolecules that are soluble in water. This feature will allow the study of electron tunneling through water molecules. A second goal is to understand how the solvent friction and the strength of electronic interaction between the reaction's partners (electronic coupling) affect the reaction rates. The model systems studied here can have their properties (distance, electronic coupling, etc) tuned so that features of theoretical models can be probed. By performing studies in water and aqueous solutions, the conditions relevant to those in biological and electrochemical systems will be studied.This project will develop the understanding and control of electron motion in supramolecular structures and encounter complexes of reactants, which is important in many areas of chemistry and biology. For example, the efficiency of electron tunneling through water molecules is essential to a mechanistic understanding of important bioenergetic processes. The results of this work will be communicated through scientific conferences and publications. The training and education of graduate and undergraduate students is the primary mechanism through which this research work will be accomplished. The students will gain expertise in modern laser methods, spectroscopy, and nanotechnology, and learn how to design experiments that test theoretical models as a method to extend our scientific understanding. A special effort to attract students from under-represented groups in chemical research will be pursued at all levels: graduate, undergraduate, and high school (via the PECAP-Investing Now program at the University of Pittsburgh and the ACS Seed Program).