title: 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.
authors.affiliation: Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA.
authors.forename: Brittany C
authors.initials: BC
authors.lastname: Westlake
authors.forename: M Kyle
authors.initials: MK
authors.lastname: Brennaman
authors.forename: Javier J
authors.initials: JJ
authors.lastname: Concepcion
authors.forename: Jared J
authors.initials: JJ
authors.lastname: Paul
authors.forename: Stephanie E
authors.initials: SE
authors.lastname: Bettis
authors.forename: Shaun D
authors.initials: SD
authors.lastname: Hampton
authors.forename: Stephen A
authors.initials: SA
authors.lastname: Miller
authors.forename: Natalia V
authors.initials: NV
authors.lastname: Lebedeva
authors.forename: Malcolm D E
authors.initials: MD
authors.lastname: Forbes
authors.forename: Andrew M
authors.initials: AM
authors.lastname: Moran
authors.forename: Thomas J
authors.initials: TJ
authors.lastname: Meyer
authors.forename: John M
authors.initials: JM
authors.lastname: Papanikolas
mesh_terms: D001713:Biphenyl Compounds; D004396:Coloring Agents / Q000737:chemistry*; D003374:Coumarins; D004583:Electrons; D006860:Hydrogen Bonding; D009596:Nitrophenols; D055668:Photochemical Processes; D011522:Protons; D013057:Spectrum Analysis
chemical_list: D001713:Biphenyl Compounds; D004396:Coloring Agents; D003374:Coumarins; D009596:Nitrophenols; D011522:Protons; C030123:coumarin