pH-jumps in a Protic Ionic Liquid Proceed by Vehicular Proton Transport

TL;DR

This study investigates how excess protons move in a protic ionic liquid after a pH-jump, revealing that proton transport occurs mainly via vehicular mechanisms with limited diffusion before recombination.

Contribution

It provides detailed femtosecond to microsecond insights into proton transfer mechanisms and dynamics in a protic ionic liquid, highlighting vehicular transport over hydrogen-bond hopping.

Findings

Proton transfer occurs within picoseconds via hydrogen-bonded complexes.

Protons diffuse only a few solvent shells before recombining.

Kinetic isotope effect near unity indicates vehicular proton transport.

Abstract

The dynamics of excess protons in the protic ionic liquid ethylammonium formate (EAF) have been investigated from femtosecond to microseconds using visible pump mid-infrared probe spectroscopy. The pH-jump following visible photoexcitation of a photoacid (8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt, HPTS) results in proton transfer to the formate of the EAF. The proton transfer predominantly occurs over picoseconds through a pre-formed hydrogen-bonded complex between HPTS and EAF. We investigate the longer range and longer timescale proton transport processes in the ionic liquid by obtaining the ground-state conjugate base (RO-) dynamics from the congested transient-infrared spectra. The spectral kinetics indicate that the protons diffuse only a few solvent shells from the parent photoacid before recombining with RO-. A kinetic isotope effect of near unity (kH/kD~1) suggests vehicular transfer and transport of excess protons in this ionic liquid. Our findings provide a comprehensive insight into the complete photoprotolytic cycle of excess protons in a protic ionic liquid.