Upon excitation, the pKa of the photoacid pyranine decreases from 7.4 to 1.3, so short pulse laser excitation enables time-resolved studies of proton transfer kinetics. In addition, the excited deprotonated and protonated forms of pyranine are both luminescent and their peaks are well-separated. It has been hypothesized that for excited-state proton transfer to occur, several (2-4) water molecules must be clustered near the OH group of the photoacid, and that the water molecules must be able to reorient into a correct conformation to accept a proton. From the experimental point of view, this is a challenging problem because the number of water molecules around pyranine cannot be controlled in pure water, and water orientation cannot be slowed enough by decreasing temperature since the freezing point of water is relatively high.  This work solves the aforementioned problems by selecting ethylene glycol-water solutions as the reaction medium because (a) unlike water, ethylene glycol cannot accept a proton from pyranine, (b) the spatial distribution of molecules is thought to be homogeneous, and (c) the solutions can be supercooled, allowing low-temperature experiments on how water mobility affects the proton transfer. Emission spectra of pyranine in ethylene glycol-water solutions were collected to quantify the steady-state ratios of deprotonated and protonated pyranine. Decreasing water concentration was found to decrease the proportion of deprotonated pyranine. Deprotonation was not detectable for solutions with water mole fractions less than 0.20. This confirms that to serve as a proton acceptor, more than one water molecule must be in close proximity to the OH group of pyranine. Decreasing temperature was found to decrease pyranine deprotonation. Proton transfer stopped around -55°C. Near this temperature, crossover of viscosity was observed (5 Pa*s). This indicates that for proton transfer to occur, water molecules must be able to reorient to achieve the conformation required to serve as a proton trap. To fully quantify our data, molecular simulations of the water distribution around pyranine are needed, and that work is in progress.

Molar composition and temperature effects on proton transfer from excited state pyranine to water in ethylene glycol-water solutions

2019 Spring National Meeting

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