Date: Thursday, Jan 24th
Presenter: Prof. Chong Fang, OSU Chemistry
Chemistry in general concerns a change in substance or structure on various timescales. Mechanistic studies are thus needed to decipher the actual process going from reactants to products via intermediates, or commonly referred to, across the reactive potential energy surface. To obtain this bottom-up knowledge that bridges microscopic structure to macroscopic function, the molecular conformational dynamics leading to chemical reactivity need to be resolved both spatially and temporally. An emerging powerful technique is femtosecond stimulated Raman spectroscopy (FSRS), which is capable of elucidating structural dynamics of condensed phase molecules in situ on their intrinsic timescales. Recent FSRS results on photoacidity of pyranine (HPTS, 8-hydroxypyrene-1,3,6-trisulfonic acid) in water will be discussed, in the context of excited state proton transfer (ESPT) and multidimensional reaction coordinate. The addition of acetate ions expedites ESPT with enhanced driving force through pre-existing H-bonding network. Several key low frequency modes below 400 cm^-1 are revealed to play a gating role particularly on the sub-picosecond timescale in guiding the photoacid proton through the H-bonding chain with various lengths, involving the acetate and/or water molecules as proton acceptors, well before fluorescence to the electronic ground state.