Hydroxyl Radical as an Unlikely Key Intermediate in the Photodegradation of Emerging Pollutants

Time-resolved kinetic assessment of the role of singlet and triplet excited states in the photocatalytic treatment of pollutants at different concentrations

“…Several species can mediate the indirect mechanisms, among them hydroxyl radicals, singlet oxygen, superoxide anion, inorganic radicals (chloride, sulfate, carbonate), organic radicals (carboxyl, peracyl) or excited states of dissolved or suspended organic substances. [3][4][5][6][7] An overall analysis of photocatalyzed-redox processes should consider two complementary aspects: the thermodynamic and the kinetic viability of all the competing pathways. More specifically, first thermodynamic estimations based on the redox potentials of the involved species could help to discard those pathways that result highly endergonic.…”

Photoredox catalysis for environmental and chemical applications. A mechanistically-based approach.

“…However, photochemical systems are not simple, as they involve excited states, hydroxyl radical, singlet oxygen, superoxide anion and/or organic/inorganic radicals. [104,112,[136][137][138] Therefore, in order to fully understand such a system, two complementary aspects should be taken into account: the thermodynamics and the kinetic viability of all the competing pathways. Firstly, thermodynamic estimations based on the redox potentials of the involved species could help to discard those pathways that result highly endergonic and to identify the thermodynamically allowed key steps.…”

“…10 -8 M, almost all the 1 (NMQ + ) * will be quenched by O2, and subsequent quenching of 1 O2 by the pollutants at that low concentration will be negligible compared to its intrinsic deactivation. [104] Thus, at such low concentrations, photodegradation is expected to be almost null. If the concentration is increased to the milimolar range, the electron transfer pathway would be responsible for the quenching of more than 80% of 1 (NMQ + ) * , being again Type I the main photodegradation pathway.…”

“…Thus, the difficult detection of these short-lived primary key species involved in the photocatalytic processes has hampered the elucidation of the reaction mechanisms behind these reactions. [104] Interestingly, photophysical techniques based on emission and UV-vis absorption spectroscopy, provide time-resolved data in the nano or microsecond scale. They allow direct monitoring and quantification of reactive species; therefore, they can be applied to gain insight into the mechanistic aspects of the photoredox reactions.…”

“…All these examples deal with the oxidation of the substrates, but little attention has been paid to the potential of RF as a reductant in either water remediation or organic synthesis.It is worth mentioning that reductive dehalogenation methods are used in a wide variety of synthetic contexts and these procedures have already been optimized using visible-light photocatalysts such as Ru(bpy)3Cl2 [211][212][213][214][215][216][217][218][219][220][221][222]. However, many of the recent publications lack a kinetic study of the involved excited states; therefore, a deeper mechanistic understanding of the processes based on time-resolved techniques could help to broaden the substrate scope of the photocatalyzed organic reactions [70,103,104,111,140,167]. Under this context, the objective of the present chapter is to perform reductive dehalogenation of different bromo-derivatives using RF as a metal-free photocatalyst upon visible light irradiation (460 nm), in the presence of amines and isopropanol as eand H-donors, respectively.…”