Proton Transfer from a Photoacid to a Water Wire: First Principles Simulations and Fast Fluorescence Spectroscopy

Proton
transfer reactions are ubiquitous in chemistry, especially
in aqueous solutions. We investigate photoinduced proton transfer
between the photoacid 8-hydroxypyrene-1,3,6-trisulfonate (HPTS) and
water using fast fluorescence spectroscopy and ab initio molecular
dynamics simulations. Photoexcitation causes rapid proton release
from the HPTS hydroxyl. Previous experiments on HPTS/water described
the progress from photoexcitation to proton diffusion using kinetic
equations with two time constants. The shortest time constant has
been interpreted as protonated and photoexcited HPTS evolving into
an “associated” state, where the proton is “shared”
between the HPTS hydroxyl and an originally hydrogen bonded water.
The longer time constant has been interpreted as indicating evolution
to a “solvent separated” state where the shared proton
undergoes long distance diffusion. In this work, we refine the previous
experimental results using very pure HPTS. We then use excited state
ab initio molecular dynamics to elucidate the detailed molecular mechanism
of aqueous excited state proton transfer in HPTS. We find that the
initial excitation results in rapid rearrangement of water, forming
a strong hydrogen bonded network (a “water wire”) around
HPTS. HPTS then deprotonates in ≤3 ps, resulting in a proton
that migrates back and forth along the wire before localizing on a
single water molecule. We find a near linear relationship between
the emission wavelength and proton-HPTS distance over the simulated
time scale, suggesting that the emission wavelength can be used as
a ruler for the proton distance. Our simulations reveal that the “associated”
state corresponds to a water wire with a mobile proton and that the
diffusion of the proton away from this water wire (to a generalized
“solvent-separated” state) corresponds to the longest
experimental time constant.