Journal
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
Volume 108, Issue 21, Pages 8554-8558Publisher
NATL ACAD SCIENCES
DOI: 10.1073/pnas.1104811108
Keywords
electron transfer; proton-coupled electron transfer
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Funding
- National Science Foundation [CHE0957215, CHE0809530, CHE0809045]
- US Department of Energy, Office of Science, Office of Basic Energy Sciences [DE-SC0001011]
- Direct For Mathematical & Physical Scien
- Division Of Chemistry [1111873] Funding Source: National Science Foundation
- Direct For Mathematical & Physical Scien
- Division Of Chemistry [0809045, 0957215] Funding Source: National Science Foundation
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The simultaneous, concerted transfer of electrons and protons-electron-proton transfer (EPT)-is an important mechanism utilized in chemistry and biology to avoid high energy intermediates. There are many examples of thermally activated EPT in ground-state reactions and in excited states following photoexcitation and thermal relaxation. Here we report application of ultrafast excitation with absorption and Raman monitoring to detect a photochemically driven EPT process (photo-EPT). In this process, both electrons and protons are transferred during the absorption of a photon. Photo-EPT is induced by intramolecular charge-transfer (ICT) excitation of hydrogen-bonded-base adducts with either a coumarin dye or 4-nitro-4'-biphenylphenol. Femtosecond transient absorption spectral measurements following ICT excitation reveal the appearance of two spectroscopically distinct states having different dynamical signatures. One of these states corresponds to a conventional ICT excited state in which the transferring H+ is initially associated with the proton donor. Proton transfer to the base (B) then occurs on the picosecond time scale. The other state is an ICT-EPT photoproduct. Upon excitation it forms initially in the nuclear configuration of the ground state by application of the Franck-Condon principle. However, due to the change in electronic configuration induced by the transition, excitation is accompanied by proton transfer with the protonated base formed with a highly elongated H+-B bond. Coherent Raman spectroscopy confirms the presence of a vibrational mode corresponding to the protonated base in the optically prepared state.
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