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

Journal Article · Tue May 24 00:00:00 EDT 2011 · Proceedings of the National Academy of Sciences of the United States of America

DOI:https://doi.org/10.1073/pnas.1104811108· OSTI ID:1065648

Westlake, Brittany C. ^[1]; Brennaman, Kyle M. ^[1]; Concepcion, Javier J. ^[1]; Paul, Jared J. ^[1]; Bettis, Stephanie E. ^[1]; Hampton, Shaun D. ^[1]; Miller, Stephen A. ^[1]; Lebedeva, Natalia V. ^[1]; Forbes, Malcolm D. E. ^[1]; Moran, Andrew M. ^[1]; Meyer, Thomas J. ^[1]; Papanikolas, John M. ^[1]

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1. Univ. North Carolina, Chapel Hill, NC (United States)

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.

• Research Organization: Energy Frontier Research Centers (EFRC) (United States). Center for Solar Fuels (UNC EFRC)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES)
• DOE Contract Number: SC0001011
• OSTI ID: 1065648
• Journal Information: Proceedings of the National Academy of Sciences of the United States of America, Vol. 108 (21), Issue 21; Related Information: UNC partners with University of North Carolina (lead); Duke University; University of Florida; Georgia Institute of Technology; University; North Carolina Central University; Research Triangle Institute; ISSN 0027-8424
• Publisher: National Academy of Sciences, Washington, DC (United States)
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
catalysis (homogeneous)
catalysis (heterogeneous)
solar (photovoltaic)
solar (fuels)
photosynthesis (natural and artificial)
hydrogen and fuel cells
electrodes - solar
charge transport
materials and chemistry by design
synthesis (novel materials)
synthesis (self-assembly)