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Concerted electron-proton transfer in the optical excitation of hydrogen-bonded dyes.


ABSTRACT: 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.

SUBMITTER: Westlake BC 

PROVIDER: S-EPMC3102414 | biostudies-literature | 2011 May

REPOSITORIES: biostudies-literature

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Concerted electron-proton transfer in the optical excitation of hydrogen-bonded dyes.

Westlake Brittany C BC   Brennaman M Kyle MK   Concepcion Javier J JJ   Paul Jared J JJ   Bettis Stephanie E SE   Hampton Shaun D SD   Miller Stephen A SA   Lebedeva Natalia V NV   Forbes Malcolm D E MD   Moran Andrew M AM   Meyer Thomas J TJ   Papanikolas John M JM  

Proceedings of the National Academy of Sciences of the United States of America 20110509 21


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 proc  ...[more]

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