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Aqueous solvation from the water perspective


Ahmed, Saima; Pasti, Andrea; Fernández-Terán, Ricardo J; Ciardi, Gustavo; Shalit, Andrey; Hamm, Peter (2018). Aqueous solvation from the water perspective. Journal of Chemical Physics, 148(23):234505.

Abstract

The response of water re-solvating a charge-transfer dye (deprotonated Coumarin 343) after photoexcitation has been measured by means of transient THz spectroscopy. Two steps of increasing THz absorption are observed, a first similar to 10 ps step on the time scale of Debye relaxation of bulk water and a much slower step on a 3.9 ns time scale, the latter of which reflecting heating of the bulk solution upon electronic relaxation of the dye molecules from the S-1 back into the S-0 state. As an additional reference experiment, the hydroxyl vibration of water has been excited directly by a short IR pulse, establishing that the THz signal measures an elevated temperature within similar to 1 ps. This result shows that the first step upon dye excitation (10 ps) is not limited by the response time of the THz signal; it rather reflects the reorientation of water molecules in the solvation layer. The apparent discrepancy between the relatively slow reorientation time and the general notion that water is among the fastest solvents with a solvation time in the sub-picosecond regime is discussed. Furthermore, non-equilibrium molecular dynamics simulations have been performed, revealing a close-to-quantitative agreement with experiment, which allows one to disentangle the contribution of heating to the overall THz response from that of water orientation. Published by AIP Publishing.

Abstract

The response of water re-solvating a charge-transfer dye (deprotonated Coumarin 343) after photoexcitation has been measured by means of transient THz spectroscopy. Two steps of increasing THz absorption are observed, a first similar to 10 ps step on the time scale of Debye relaxation of bulk water and a much slower step on a 3.9 ns time scale, the latter of which reflecting heating of the bulk solution upon electronic relaxation of the dye molecules from the S-1 back into the S-0 state. As an additional reference experiment, the hydroxyl vibration of water has been excited directly by a short IR pulse, establishing that the THz signal measures an elevated temperature within similar to 1 ps. This result shows that the first step upon dye excitation (10 ps) is not limited by the response time of the THz signal; it rather reflects the reorientation of water molecules in the solvation layer. The apparent discrepancy between the relatively slow reorientation time and the general notion that water is among the fastest solvents with a solvation time in the sub-picosecond regime is discussed. Furthermore, non-equilibrium molecular dynamics simulations have been performed, revealing a close-to-quantitative agreement with experiment, which allows one to disentangle the contribution of heating to the overall THz response from that of water orientation. Published by AIP Publishing.

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Additional indexing

Item Type:Journal Article, refereed, original work
Communities & Collections:07 Faculty of Science > Department of Chemistry
Dewey Decimal Classification:540 Chemistry
Uncontrolled Keywords:Physical and Theoretical Chemistry, General Physics and Astronomy
Language:English
Date:21 June 2018
Deposited On:05 Feb 2019 13:09
Last Modified:05 Feb 2019 13:11
Publisher:American Institute of Physics
ISSN:0021-9606
OA Status:Closed
Publisher DOI:https://doi.org/10.1063/1.5034225
Project Information:
  • : FunderSwiss National Science Foundation (SNF) through the NCCR MUST
  • : Grant ID200021_165789/1
  • : Project Title

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Embargo till: 2019-06-09