Photochemistry and Photobiology, 20**, **: *–* Special Issue Research Article Photochemistry of Tris(2,4-dibromophenyl)amine and its Application to Co-oxidation on Sulfides and Phosphines † Sergio M. Bonesi 1,2,3 * , Mariella Mella 4 , Daniele Merli 4 and Stefano Protti 3 * 1 Departamento de Qu´ ımica Org ´ anica, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Buenos Aires, Argentina 2 Centro de Investigaciones en Hidratos de Carbono (CIHIDECAR), CONICET-Universidad de Buenos Aires, Buenos Aires, Argentina 3 PhotoGreen Lab, Department of Chemistry, University of Pavia, Pavia, Italy 4 Department of Chemistry, University of Pavia, Pavia, Italy Received 5 January 2021, revised 9 February 2021, accepted 15 February 2021, DOI: 10.1111/php.13403 ABSTRACT The photochemistry of tris(2,4-dibromophenyl)amine was investigated via time-resolved nanosecond spectroscopy. The tris(2,4-dibromophenyl)amine radical cation (“Magic Green”) was immediately detected after the laser pulse; this interme- diate then cyclizes to N-aryl-4a,4b-dihydrocarbazole radical cation. The latter transient reacted with molecular oxygen to provide the corresponding hydroperoxyl radical, which smoothly co-oxidize sulfides into sulfoxides. On the other hand, the photogenerated “Magic Green” was exploited to promote the co-oxidation of nucleophilic triarylphosphines to triarylphosphine oxides through an electron transfer process preventing the amine cyclization. In this case, the intermedi- ate Ar 3 POO •+ was found to play a key role in phosphine oxide formation. INTRODUCTION Stable nitrogen-centered radical cations can be smoothly electro- generated in situ from triarylamines bearing substituents in the para position (1,2), as early testified by the formation of trianisy- lamine radical cation, that was in turn employed as the redox cat- alyst in the oxidation of cyanide anions (3). Throughout the years, tris(2,4-dibromophenyl)amine radical cation (also known as Magic Green, MG, today commercially available) has been widely employed as an oxidizing reagent and tris(2,4-dibro- mophenyl)amine can be considered as the precursor of the corre- sponding radical cation. Apart from the use in mechanistic studies (see below), some technological applications of MG have been recently developed, including, among the others, its use in measuring the overcharge protection in lithium-ion batteries (LIBs) (4). The species was also used as a co-reactant in producing an intense blue light emission of corannulene derivatives (5). Recently, electrochemistry has emerged as sustainable approach to access chemical intermediates (including radical ions and radicals) under mild conditions and to perform reactions that it is not possible to be accomplished by other synthetic approaches (6,7). In this context, radical cations deriving from triarylamines such as tris(4-bromophenyl)amine and tris(2,4-dibromophenyl) amine have been employed as efficient redox mediators for the gem-difluorodesulfurization reaction of dithioacetals (8) for the monodesulfurization of phenylthio-β-lactams (9) and S-aryl thiobenzoates (10) and as redox mediator in the efficient conversion of substituted S-phenyl thiobenzoate into phenyl benzoates (11). On the other hand, the electrochemical generation of tris(2,4- dibromophenyl)amine radical cation was exploited in the spectro- scopic detection of short-lived aromatic intermediates deriving from anthracene derivatives and N-methyldiphenyl- and dipheny- lamine by means of electron transfer stopped-flow (ETSF) meth- ods (12). Likewise, the same authors described in details the kinetic of the electron transfer process between a series of 9-sub- stituted anthracene and tris(2,4-dibromophenyl)amine radical cation electrogenerated in situ and the reactivity of the anthra- cene derivative radical cations against nucleophiles like water and methanol applying electron transfer stopped-flow (ETSF) methodology (13). This method was also extended to the evaluation of the reac- tivity of a series of mono- and dicationic states of meta-con- nected oligoarylamines and characterization of the species through their absorption spectra (14). When stabilized as the hexachloroantimonate salt (15), MG is often employed oxygenation reactions in apolar solvents. As an example, the reaction of 4,4-dimethyladamantylidene with molec- ular oxygen in the presence of catalytic amounts of MG (5 mol %) in dichloromethane at -78°C provided 4,4-dimethylspiro [adamantane-2,3’-[1,2]-dioxetane] as the primary photoproduct that in turn is converted at room temperature to 2-methy- ladamantyl-methyl ketone (16). Likewise, the monoelectronic oxidation of hindered olefins by the aminium salts have been investigated at both low and room temperature in dichloro- methane and a series of dioxetanes, epoxides, ketones or allylic derivatives were formed in good to excellent yields (17). *Corresponding authors email: smbonesi@qo.fcen.uba.ar (Sergio M. Bonesi) and stefano.protti@unipv.it (Stefano Protti) † This article is part of a Special Issue celebrating the career of Dr. Edward Clen- nan. © 2021 American Society for Photobiology 1