On the Mechanism of Phenolate Photo-Oxidation in Aqueous Solution.

The photo-oxidation dynamics following UV (257 nm) excitation of the phenolate anion in aqueous solution is studied using broadband (550 to 950 nm) transient absorption spectroscopy. A clear signature from electron ejection is observed on a sub-picosecond timescale, followed by cooling dynamics and the decay of the signal to a constant offset that is assigned to the hydrated electron. The dynamics are compared to the charge-transfer-to-solvent dynamics from iodide at the same excitation wavelength and are shown to be very similar to these. This is in stark contrast to a previous study on the phenolate anion excited at 266 nm, in which electron emission was observed over longer time-scales. We account for the differences using a simple Marcus picture for electron emission in which the electron tunnelling rate depends sensitively on initial excitation energy. After electron emission, a contact pair is formed which undergoes geminate recombination and dissociation to form the free hydrated electron at rates that are slightly faster than for the iodide system. Our results show that, while the underlying chemical physics of electron emission differs between iodide and phenolate, the observed dynamics can appear very similar.