Water molecules, distinguished from other liquids, form an intricate network of intermolecular hydrogen (H-) bonds, which strongly influence the physical and chemical properties of water. As an omnipresent solvent in numerous chemical and biochemical reactions, the water structure and dynamics are of fundamental importance and practical significance. Among all the reactions, proton transport in water is particularly interesting, and has attracted extensive experimental and theoretical investigations. As a model system, the excited state proton transfer (ESPT) between a photoacid pyranine (HPTS) and a solvent water molecule, triggered by a photoexcitation pulse, has been comprehensively studied by a variety of ultrafast electronic and vibrational spectroscopic techniques. Nonetheless, due to limitations of current experimental methods to dissect an ESPT reaction, it remains unclear about how the solvent molecules accept a proton from the photoacid, and how this nascent proton impacts the H-bonding of the first solvation shell as well bulk water. In this contribution, we tackled the challenge by applying tunable femtosecond stimulated Raman spectroscopy (FSRS) with a broad detection window from ca. 400—4000 cm-1, so we could track the evolution of vibrational modes from both HPTS and water molecules. The dynamics of the O–H stretching bands of water during the photoinduced ESPT process were clearly resolved. Meanwhile, we performed several control experiments varying the solute concentration, replacing H2O with D2O, and examining similar systems without ESPT. Given that the ground state bleaching peaks are negligible to the Raman signal, we surmise that the much enhanced water signal arises from the resonance enhancement of near-field water molecules within the first solvation shell of the excited state photoacid. By tracking the water O–H band, different dynamics associated with different H-bonding configurations offer the crucial link to a more complete understanding of photoinduced proton transfer in water.

Simultaneous observation of donor and acceptor vibrational modes during excited state proton transfer in water

2019 Fall National Meeting

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