On the Mechanism of Phenolate Photo-Oxidation in Aqueous Solution

Abstract

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 timescales. 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.

Ultrafast structural rearrangement dynamics induced by the photodetachment of phenoxide in aqueous solution

“…The former appears as a prominent absorption band at 400 nm alongside the vibronic feature at 382 nm, in good agreement with the previously reported absorption spectrum of PhO∙ in aqueous solution, whereas the latter gives rise to the increasing Δ A signal that appears towards the long-wavelength region of the probe spectral range. Previous studies have shown that 4.7-eV photoexcitation of phenoxide to the S 1 ( ππ *) state yields the phenoxyl radical via delayed electron ejection 24,32 . In our work, we rule out the formation of PhO∙ via the dense manifold of electronic excited states 33 of PhO – because the pump-power dependence measurement shows that photodetachment occurs directly via a four-photon process (see Supplementary Fig.…”

Liquid-microjet photoelectron spectroscopy of the green fluorescent protein chromophore

“… eV, which is consistent with that of a partially solvated electron 37 . Transient electronic absorption spectroscopy measurements of the phenolate moiety of p -HBDI − in aqueous solution propose electron appearance timescales of a few hundred femtoseconds 38 to picoseconds 39 . We plan time-resolved photoelectron spectroscopy measurements to confirm if solvated electrons are formed following photoexcitation of the higher-lying state of solvated p -HBDI − .…”

Photooxidation of the Phenolate Anion is Accelerated at the Water/Air Interface

“…The relevant rate coefficients (expressed as lifetimes, k –1 ) are given in Table 1 and are in good agreement with pervious measurements. 22 …”

“…Assuming that the absorption spectrum of [Ph:e – ] (aq) and e – (aq) are indistinguishable, then the total signal is the sum of both these contributions, with the escape yield of e (aq) – equating to the long-time (100 ps) offset observed in Figure . The relevant rate coefficients (expressed as lifetimes, k –1 ) are given in Table and are in good agreement with pervious measurements …”