Chemical dynamics of radical pairs formed by benzophenone photoreduction in solid polymers

Abstract

Strong reducing agents, such as phenols and amines, decrease substantially the quantum yield of aromatic ketone consumption in solid polymers. A significant quenching of ketone phosphorescence and an increase in the yield of radical pairs are unambiguous evidence for the efficient primary intermolecular transfer of H atoms under these conditions. The effect in question is related to a sharp change in the ratio of recombination and disproportionation rates of geminate radical pairs in solid polymers in comparison with solutions and micellar systems. The main reason for the phenomenon is the low molecular mobility, which provides no sufficiently fast reorganization of reacting species and their environment needed for the forma tion of radical pair recombination products.

Photosensitized Heterogeneous Chemistry of Ozone on Organic Films

“…24,25,30 The reaction leads to the formation of ketyl radical and phenoxy radicals. [23][24][25]27,28 In organic solutions, in the presence of oxygen, the ketyl radical is mainly reoxidized to ketone and O 2 •-/HO 2 • are generated, 25 so additional reactions could take place between the organic film and the oxidants formed. The phenoxy radical is also known to react with ozone and form ozonide radical (O 3 •-) followed by O 2 and O •formation, which in presence of water leads to hydroxyl radical formation.…”

“…This is caused by the low molecular mobility that strongly hinders the rearrangement of species, which is necessary for the formation of radical recombination products. 28 Contact Angle Measurements and UV-vis Analysis. As already mentioned above, the surface of the organic film is changing when exposed to ozone and light.…”

“…The benzophenone triplet state (285 kJ/mol) is not able to undergo any energy transfer with phenol, which has a higher triplet state energy (342 kJ/mol). In solution, however, oxidation of phenol by benzophenone triplet state occurs through charge-transfer (CT) and/or hydrogen abstraction, depending on the polarity of the solvent. ,, The reaction leads to the formation of ketyl radical and phenoxy radicals. − ,, In organic solutions, in the presence of oxygen, the ketyl radical is mainly reoxidized to ketone and O 2 •- /HO 2 • are generated, so additional reactions could take place between the organic film and the oxidants formed. The phenoxy radical is also known to react with ozone and form ozonide radical (O 3 •- ) followed by O 2 and O •- formation, which in presence of water leads to hydroxyl radical formation .…”

“…This activates a chain reaction until transfer to, e.g., a final electron acceptor occurs. [23][24][25][27][28][29][30][31][32] Despite the existence of a significant body of literature on photoinduced charge or/and energy transfer in organic molecules related to biochemistry 24,26,30,33 or to the degradation of dissolved organic matter in aquatic chemistry, [34][35][36][37][38][39] only a little is known about photochemistry of atmospheric particles and organic solid films. Nevertheless, in view of the presence of light-absorbing organic material in atmospheric particles, it should be assumed that such processes will significantly affect the aerosols chemical and physical properties during aging.…”

“…The presence of light-absorbing material can promote photosensitized processes. A photosensitizer is a compound that absorbs light and can transfer the excitation energy (from its triplet state) to another compound that hardly absorbs the available radiative energy. − Depending on the redox properties of the reaction partners and the medium, charge or energy transfer can occur to another molecule. This activates a chain reaction until transfer to, e.g., a final electron acceptor occurs. − ,− …”