Effect of Confinement on Proton-Transfer Reactions in Water Nanopools

Proton transfer from the photoacid 8‐hydroxy‐1,3,6‐pyrenetrisulfonic acid (HPTS) to water is studied in reverse micelles with ionic (AOT=sodium dioctyl sulfosuccinate) and non‐ionic (BRIJ‐30=polyoxyethylene(4)lauryl ether) surfactants. The dynamics are studied by probing the transient electronic absorption and transient vibrational absorption, both with sub‐picosecond resolution. The reverse micelle sizes range from approximately 1.6 to 5.5 nm in diameter. For both surfactants it is found that the rate of proton transfer decreases with decreasing reverse micelle size, regardless of surfactant. In addition, for AOT reverse micelles, a fraction of the photoacid molecules exhibit non‐radiative decay, preventing proton transfer. Using sub‐picosecond resolution pump‐probe spectroscopy, optically triggered proton‐transfer from a photoacid to the water solvent in nanometer‐sized reverse micelles is investigated (see figure). Due to confinement, the proton transfer is slowed down. Additionally, for some photoacid molecules proton transfer is prevented by a specific interaction with the surfactant molecules of the reverse micelle.