Generation, Characterization, and Deprotonation of Phenol Radical Cations

Abstract

A variety of methoxy- and methyl-substituted phenol radical cations have been generated and characterized by laser flash photolysis in solution under ambient conditions. The radical cations were generated by either photosensitized electron transfer using 1,4-dicyanonaphthalene with biphenyl as a cosensitizer in acetonitrile or by direct excitation of the phenol at 266 nm. The phenol radical cations have absorption maxima between 410 and 460 nm, with the exception of the 3,5-dimethoxyphenol which absorbs at 580 nm. The assignment of the observed transients to phenol radical cations is based on their spectral similarity to matrix spectra for the same species and to the corresponding methoxybenzene radical cations, as well as their characteristic reactivity. In the presence of small amounts of water the radical cations are not detected and the phenoxyl radical is the only observed transient in the photosensitized electron transfer. For those phenols for which authentic phenoxyl spectra are not available, the identity of the radical was confirmed by its generation by hydrogen abstraction by the tert-butoxyl radical. Although several of the phenols can be photoionized in either 1:1 aqueous ethanol or in acetonitrile, this is a less general route for the formation and characterization of the phenol radical cations. The rate constants for deprotonation of the phenol radical cations by water were measured and fall within the range (0.6−6) × 108 M-1 s-1; the 2-methoxyphenol radical cation is more reactive than the 4-methoxy, consistent with a recent estimate of the pK a for these species.

Could triplet-sensitised transformation of phenolic compounds represent a source of fulvic-like substances in natural waters?

“…Indeed, the difference of transient spectra obtained after excitation of AQ2S with and without phenol shows a transient species absorbing from 360 up to 420 nm, with λ max ~ 400 nm. It is compatible with the formation of phenoxyl radical (Gadosy et al, 1999). Further irradiation of phenol dimers is then expected to produce higher oligomers, as recently proposed by Net and coworkers (2010).…”

Experimental and theoretical insights into photochemical transformation kinetics and mechanisms of aqueous propylparaben and risk assessment of its degradation products

“…• (according to the calculated spectra shown in Figure 5) was formed at 390 nm ( Figure 5 and its inset plot), which agreed with previous reports that the phenoxyl radical always showed an absorption maximum of approximately 400 nm [30]. The PPB-H • was probably produced through the rapid reaction of the oxidative radicals formed (N 3…”

Phototransformation of 4-phenoxyphenol sensitised by 4-carboxybenzophenone: Evidence of new photochemical pathways in the bulk aqueous phase and on the surface of aerosol deliquescent particles

“…The 355 From literature data, the long-lived transient can be identified as the phenoxy radical of 4PP (Gadosy et al, 1999). The insert in Figure 2 shows the first-order decay constant of the CBP triplet state, Experiments under continuous irradiation CBP underwent phototransformation when irradiated alone under the UVA lamp.…”