EAPSI:: Investigation of Distance and Orientation Dependence for Electron Transfer Between Organic Molecules

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The organic molecule, pyrene, is used in a multitude of applications, including molecular recognition and sensing, molecular electronics, light emitting diodes, determination of micro-environments of polymers and liquids, etc. Fluorescence quenching of pyrene with amines proceeds by an electron-transfer mechanism where an amine may donate an electron to the pyrene molecule. The process of electron-transfer must meet specific criteria for the orientation of and distance between the lone pair of electrons of an amine quencher and pyrene. In general, the approach and distance of quenchers to aromatic molecules are inferred from less than definitive data in which the existence of a favorable mutual orientation and distance is assumed. This award supports research (1) to examine in greater detail the distance and orientation dependence of this type of electron/charge transfer; and (2) to analyze this process by equipment not accessible to Georgetown University. Particularly, laser flash spectrophotometry under the expertise of Prof. Xian-Fu Zhang at Hebei Normal University of Science and Technology in Heibei, China. The LFS will be used to examine directly the degree of and separation distance between radical ions immediately after electron-transfer.Our studies examine the approach of a variety of amines and the distance needed for them to undergo electron/charge transfer to tetrakisarylpyrene (TKPy) molecules in which access to the ð-system is restricted. The presence of a charge-transfer, excited-state complex (exciplex) is observable in emission from the interaction between the TkPys and aromatic amines. The degree of charge- or electron-transfer is dependent on the distance between quencher and TkPys at the moment of complex formation, and it varies with the type and shape of the amine. The conclusions obtained from these studies are clearly applicable to a wide variety of other systems in which fluorescence from an aromatic moiety is being quenched.This award under the East Asia and Pacific Summer Institutes program supports summer research by a U.S. graduate student and is jointly funded by NSF and the Ministry of Science and Technology of China.