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A: Kinetics, Dynamics, Photochemistry, and Excited States
Excited-State Proton Transfer from the Photoacid 2Naphthol-8-sulfonate to Acetonitrile/Water Mixtures Oren Gajst, Luís Pinto da Silva, Joaquim C.G. Esteves da Silva, and Dan Huppert J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b04417 • Publication Date (Web): 06 Jul 2018 Downloaded from http://pubs.acs.org on July 11, 2018
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The Journal of Physical Chemistry
Excited-State Proton Transfer from the Photoacid 2-Naphthol-8sulfonate to Acetonitrile/Water Mixtures Oren Gajst,γ Luís Pinto da Silva,a,b Joaquim C.G. Esteves da Silvab,c and Dan Huppert γ,* γ
Raymond and Beverly Sackler Faculty of Exact Sciences, School of Chemistry, Tel Aviv University, Tel Aviv 69978, Israel
a
Chemistry Research Unit (CIQUP), Department of Chemistry and Biochemistry,
Faculty of Sciences of University of Porto, R. Campo Alegre 687, 4169-007 Porto, Portugal. b
LACOMEPHI, GreenUP, Department of Geosciences, Environment and Territorial
Planning, Faculty of Sciences of University of Porto, R. Campo Alegre 687, 4169-007 Porto, Portugal. a
Chemistry Research Unit (CIQUP), Department of Geosciences, Environment and Territorial Planning, Faculty of Sciences of University of Porto, R. Campo Alegre 687, 4169-007 Porto, Portugal. *Corresponding author: Dan Huppert E-mail:
[email protected] Phone: 972-3-6407012 Fax: 972-3-6407491
Abstract Steady-state and time-resolved fluorescence techniques were used to study the excited-state proton transfer (ESPT) to water of the reversible photoacid 2-naphthol8-sulfonate (2N8S) in acetonitrile/water mixtures. In acetonitrile-rich mixtures, up to χwater≤0.12, we found a slow ESPT process on the order of nanoseconds. At χwater≈0.15, the RO- fluorescence band intensity is at minimum, whereas at χwater≈0.030, it is at the maximum. The steady-state fluorescence spectra of these mixtures show that the intensity of the RO- fluorescence band at χwater≈0.030 is about 0.24 of that of the ROH band. We explain this unusual phenomenon by the presence of water clusters that exist in the acetonitrile-rich CH3CN/H2O mixtures. We propose that a water bridge forms between the 2-OH and 8-sulfonate by preferential solvation of 2N8S and this enables the ESPT process between the two sites of the molecular 1 ACS Paragon Plus Environment
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structure of 2N8S. In mixtures of χwater≥0.25, the ESPT process takes place to water clusters in the bulk mixture. The higher the χwater in the mixture, the greater the ESPT rate constant. In neat water, the rate constant is rather small - 4.5×109s-1. TD-DFT calculations show that a single water molecule can bridge between 2-OH and 8sulfonate in the excited-state. The activation energy for the ESPT reaction is about 9 kcal/mol and the RO-(S1) species is energetically above the ROH(S1) species by about 1.6 kcal/mol.
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The Journal of Physical Chemistry
Introduction Photoacids are a class of photoactive molecules based on hydroxy aryl compounds like naphthols. In their electronic ground-state, they are weak acids with pKa in the range of 5-10. In their electronic excited-state they are much stronger acids with pKa* in the range of -8
