Proton Abstraction Mediates Interactions between the Super Photobase FR0-SB and Surrounding Alcohol Solvent

J Phys Chem B. 2019 Oct 10;123(40):8448-8456. doi: 10.1021/acs.jpcb.9b06580. Epub 2019 Sep 27.

Abstract

We report on the motional and proton transfer dynamics of the super photobase FR0-SB in the series of normal alcohols C1 (methanol) through C8 (n-octanol) and ethylene glycol. Steady-state and time-resolved fluorescence data reveal that the proton abstraction dynamics of excited FR0-SB depend on the identity of the solvent and that the transfer of the proton from solvent to FR0-SB*, forming FR0-HSB+*, fundamentally alters the nature of interactions between the excited molecule and its surroundings. In its unprotonated state, solvent interactions with FR0-SB* are consistent with slip limit behavior, and in its protonated form, intermolecular interactions are consistent with a much stronger interaction of FR0-HSB+* with the deprotonated solvent RO-. We understand the excited-state population dynamics in the context of a kinetic model involving a transition state wherein FR0-HSB+* is still bound to the negatively charged alkoxide, prior to solvation of the two charged species. Data acquired in ethylene glycol confirm the hypothesis that the rotational diffusion dynamics of FR0-SB* are largely mediated by solvent viscosity while proton transfer dynamics are mediated by the lifetime of the transition state. Taken collectively, our results demonstrate that FR0-SB* extracts solvent protons efficiently and in a predictable manner, consistent with a ca. 3-fold increase in dipole moment upon photoexcitation as determined by ab initio calculations based on the equation-of-motion coupled-cluster theory.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Alcohols / chemistry*
  • Hydrogen Bonding
  • Hydrogen-Ion Concentration
  • Models, Molecular
  • Molecular Conformation
  • Protons*
  • Solvents / chemistry*
  • Thermodynamics

Substances

  • Alcohols
  • Protons
  • Solvents