pH Jumps in a Protic Ionic Liquid Proceed by Vehicular Proton Transport

[Image: see text] The dynamics of excess protons in the protic ionic liquid (PIL) ethylammonium formate (EAF) have been investigated from femtoseconds to microseconds using visible pump mid-infrared probe spectroscopy. The pH jump following the 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 (∼70%) occurs over picoseconds through a preformed hydrogen-bonded tight complex between HPTS and EAF. We investigate the longer-range and longer-time-scale proton-transport processes in the PIL 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 nearly unity (k(H)/k(D) ≈ 1) suggests vehicular transfer and the transport of excess protons in this PIL. Our findings provide comprehensive insight into the complete photoprotolytic cycle of excess protons in a PIL.