title: Ultrafast intermolecular proton transfer to a proton scavenger in an organic solvent.
abstract: Proton transfer reactions are functionally important in numerous chemical and biological processes. To unravel proton scavengers in action with atomistic details, we studied excited-state proton transfer (ESPT) from photoacid pyranine to the weak base acetate in methanol using transient absorption and wavelength-tunable femtosecond stimulated Raman spectroscopy (FSRS). Proton transfer is inhibited in neat methanol, but coherent proton motions and the formation of a charge-separated state occur on the sub-picosecond (sub-ps) timescale, accompanied by chromophore solvation wherein the longitudinal relaxation time of methanol (∼9 ps) dominates. With acetate ions added, bimolecular diffusion-controlled ESPT from the photoacid to acetate occurs on the ∼30 ps timescale, followed by ∼600 ps diffusion-assisted charge separation and solvation in the methanol H-bonding network. Besides intensity dynamics, frequency redshift and blueshift of the transient ∼285 and 1525 cm-1modes track ESPT after 400 nm photoexcitation. Tunable FSRS exploits resonance Raman enhancement with optimal wavelengths, extends the detection window of excited-state vibrational modes to low frequency, and enables a deeper mechanistic understanding of the proton transfer reaction to proton scavengers in an organic solvent.
authors.affiliation: Oregon State University, Department of Chemistry, 263 Linus Pauling Science Centre (lab), 153 Gilbert Hall (Office), Corvallis, OR 97331, USA. Chong.Fang@oregonstate.edu.
authors.forename: Breland G
authors.initials: BG
authors.lastname: Oscar
authors.affiliation: Oregon State University, Department of Chemistry, 263 Linus Pauling Science Centre (lab), 153 Gilbert Hall (Office), Corvallis, OR 97331, USA. Chong.Fang@oregonstate.edu.
authors.forename: Weimin
authors.initials: W
authors.lastname: Liu
authors.affiliation: Oregon State University, Department of Chemistry, 263 Linus Pauling Science Centre (lab), 153 Gilbert Hall (Office), Corvallis, OR 97331, USA. Chong.Fang@oregonstate.edu and School of Chemical, Biological and Environmental Engineering, 103 Gleeson Hall, Corvallis, OR 97331, USA.
authors.forename: Nikita D
authors.initials: ND
authors.lastname: Rozanov
authors.affiliation: Oregon State University, Department of Chemistry, 263 Linus Pauling Science Centre (lab), 153 Gilbert Hall (Office), Corvallis, OR 97331, USA. Chong.Fang@oregonstate.edu.
authors.forename: Chong
authors.identifier: http://orcid.org/0000-0002-8879-1825
authors.initials: C
authors.lastname: Fang