  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.
