The photoreactions of aromatic carbonyl compounds with amines. Part II. The reactions of triplet benzophenone and other ketones with primary and secondary aromatic amines

Abstract

Triplet benzophenone does not react efficientlywith aniline, o-toluidine, and diphenylamine to give isolable products. However, flash-photolysis studies showed that the ketone quite efficiently abstracts hydrogen from the amines.From a consideration of the kinetics of decay of the a-hydroxydiphenylmethyl radical, pathways were suggested whereby the radicals may react so as to produce the starting compounds. These results lead to a questioning of the assumption that primary and secondary aromatic amines act purely as physical quenchers for triplet ketones and the conclusion is reached that deactivation may be a result of either or both electron transfer and hydrogen-atom transfer. 9.1 O-Dihvdroacridine reacts with excited benzophenone, benzylideneacetophenone, and benzylideneacetone to give isolable reduction products.AN intriguing aspect of the photoreactions of aromatic carbonyl cornpodnds with aromatic tertiary amines l a p is that they take place by an electron-transfer mechanism. The nature of the intermediates produced by the electron transfer process, i.e., an exciplex or radical ions (see Scheme l), depends upon the polarity of the solvent. Kinetic measurements have shown that deactivation of the triplet ketone by the electron-transfer process is very efficient lb92 and flash-photolysis experiments have verified that radical-ion formation takes place in polar solvents.la9Backstrom and Sandros have reported3 upon the quenching of biacetyl phosphorescence by primary andsecondary aromatic amines and have shown that the rate constants for quenching approach the diff usion-controlled limit. In contrast, triphenylamine was found to be a less efficient quencher. Turro and Engel confirmed these results 4 and interpreted the quenching as occurring by electron transfer from the amine to the triplet ketone. If this mechanism is correct the efficiency of quenching should be related to the ionisation potential of the amine and consequently triphenylamine should be as good a quencher as aniline. In a related study, Cohen and Davis have reported that aniline and diphenylamine quench excited fluorenone much more readily than tri-~henylamine.~" However these workers did find that the quenching ability of a series of anilines was related to their ionisat ion PO t entialssb 1 (a) R.

Consequence of one-electron oxidation and one-electron reduction for aniline

“…However, for unknown reason, other internal effect changes this trend for the reduced mixture for which the less delocalized imine isomers 2a / 3a (89%) and 2b / 3b (11%) dominate. This variation of the tautomeric preference and π-electron delocalization may be helpful in understanding the mechanisms of spontaneous point mutations in DNA [64–66] as well as the mechanisms of aniline oxidation polymerization and polycondensation [52–62]. …”

“…These processes may pass through different intermediates among which the radical cation of aniline (PhNH 2 +• ) is possible [8–14, 52–57]. Oxidation of aniline by oxygen from air, catalyzed by sunlight or UV light, is also a very complex process [13, 60–62]. It may occur even in a closed bottle of aniline when colored products appear.…”

Thioxanthone‐Based Hydrophilic Visible Photoinitiators for Radical Polymerization

“…Back electron transfer procedure is a term often used to indicate thermal reversal of the excited state electron transfer restoring the donor and acceptor in their original oxidation level . It may lead the photoinitiator to keep its structure . Upon visible light excitation of the n–π* transition, the generation of the triplet was very fast in aqueous solution (presumably as fast as in homogeneous solution) .…”

“…With the decrease of the oxygen, more and more exciplex took part in the proton transfer procedure which produced much more active radicals. The radicals could be quenched by oxygen or initiate radical polymerization in the presence of monomer or undergo coupling reactions, such as dimerization, etc . Thus, the photolysis was faster after 0.5 h. Another possible reason may result from the fact that THF is a kind of hydrogen donor which could participate in the production of free radicals.…”

Vanadia-catalyzed solar photooxidation of aniline

“…V 2 O 5 (E g = 2.8 eV) [3] is susceptible to photoactivation with visible light of wavelength less than 443 nm and here we report solar photocatalysis by V 2 O 5 ; reports on solar photocatalysis are few and preliminary [4][5][6]. An air-equilibrated solution of aniline yields azobenzene on irradiation at 365 nm with benzophenone sensitizing the oxidation [7][8][9]. Aniline is oxidized to azobenzene on ZnO with UV light [10][11][12] and photomineralized on TiO 2 [13].…”