Search for: All records

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. 

Proton transfer reactions are ubiquitous in chemistry, especially in aqueous solutions. We investigate photo-induced proton transfer between the photoacid 8-hydroxypyrene-1,3,6-trisulfonate (HPTS) and water using fast fluorescence spectroscopy and ab initio molecular dynamics simulations. Photo-excitation 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 emission wavelength and proton-HPTS distance over the simulations’ time scale, suggesting that emission wavelength can be used as a ruler for 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. 

Binary mixtures of ethylammonium nitrate and acetonitrile show interesting properties that originate from the structural and dynamical nano-heterogeneity present in ionic liquids. These effects are most pronounced when the ionic liquid is the minority compound. In this study the transport properties of such mixtures are studied, including viscosity, self-diffusion and conductivity. The results strongly support the presence of structural inhomogeneity and show an interesting composition-dependent behaviour in the mixtures.