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Excited-State Proton Transfer of Photoexcited Pyranine in Water Observed by Femtosecond Stimulated Raman Spectroscopy

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  • We use femtosecond stimulated Raman spectroscopy (FSRS) to illuminate the choreography of intermolecular excited-state proton transfer (ESPT) of photoacid pyranine (8-hydroxypyrene-1,3,6-trisulfonic acid, HPTS) in water. The multidimensional reaction coordinate responsible for photoacidity is revealed to involve sequential activation of characteristic skeletal motions during the ca. 1 ps preparation stage preceding ESPT. The initial ring-coplanarity breaking follows in-plane ring breathing (191 cm⁻¹), and is facilitated by HPTS ring wagging (108 cm⁻¹) and ring-H out-of-plane motions (321, 362, 952 cm⁻¹), which largely decay within ~1 ps. ESPT then occurs with intrinsic inhomogeneity via various number of intervening water molecules over relatively larger distances than those in acetate-water system. The intricate relationship between the time-resolved excited-state vibrational modes of HPTS reveals the essential role of coherent low-frequency skeletal motions gating ESPT, and the multi-staged proton-transfer process having the kinetic isotope effect (KIE) value of 3–4 in aqueous solution on the 5–200 ps timescale.
  • Keywords: Photoacid, Excited-state proton transfer, Molecular conformational dynamics, Femtosecond stimulated Raman spectroscopy, Low-frequency skeletal motions, Hydrogen bond dynamics
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  • F. Han, W. Liu, C. Fang, Excited-State Proton Transfer of Photoexcited Pyranine in Water Observed by Femtosecond Stimulated Raman Spectroscopy, Chemical Physics (2013), doi: http://dx.doi.org/ 10.1016/j.chemphys.2013.03.009
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  • 422
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  • We thank the financial support from the Oregon State University Faculty ResearchStartup Fund, and the College of Science Venture Fund Award to C. Fang.
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