Crystal violet radical

A metal-free reductant that is strong enough to produce the Cgg -anion was found with the crystal violet radical 7 [161,162] 7 is a non-dimerizing, persistent radical that can be readily prepared by treatment of crystal violet iodide with zinc. It does not, as expected for a radical, add to Cgg but reduces it to give Cgg as the brovm... 

Among possible alternatives to DPMR we discarded the cx.cx-elimination of benzidine 12 from biradical-zwitterions 2 and 3 and formation of carbene 23 based on the absence of trapped carbene products in experiments with cyclohexene and ethanethiol (Xgxc = 300 nm). However, norcaradiene 6 produces carbene 23 when subjected to 350-nm irradiation. Another plausible alternative to DPMR involves initial C-CN bond homolysis, which is energetically favorable in both excited states, followed by a photochemical reaction of Crystal Violet radical. We discarded this mechanism because none of the expected products of the photochemical reactions of Crystal Violet radical has been detected during the course of our work. Nevertheless, isolation of MGCN in the trapping experiment with ethanethiol points to the possible involvement of the C-CN bond homolysis in S. Finally, although norcaradiene, cycloheptatriene, and norbomadiene are valence tautomers and are easily interconverted into each other [188], products of the latter two kinds have been neither detected nor isolated in our study [66, 30]. 

Incorporation of an ionic component into a donor/acceptor molecule is a very effective way of suppressing electron back-transfer. One interesting example consists of the photo-oxidation of leuko crystal violet (LCV) to crystal violet (CV, the dye) by benzophe-none bearing a quaternary ammonium ion (Tazuke Kitamura 1984). In this case, the cation radical of LCV formed is repulsed by the ammonium positive charge. At the same time, the benzophenone anion radical remains stabilized by the attached cationic atmosphere (Scheme 5-16). As shown in the scheme, two favorable results are achieved the stabilization of an ion radical pair by counterion exchange and the charge separation by coulombic repulsion between the two positive charges. This leads to 100% efficiency of the photo-oxidation. With unsubstituted benzophenone itself, the efficiency does not exceed 20%. 

The Dylux system operates somewhat like the above photoxidation of leuco crystal violet, but the initiation step is different. Triarylimidazolyl radicals are generated by ultraviolet (365 nm) irradiation of their stable dimers (HABI s hexaarylbiimidazoles, 11) [44]. The persistent free radicals (called lophyl radicals because they were historically derived from the parent lophine, triphenylimidazolyl... 

Creed, 1978a). It was found that as the charge-transfer character in the transition state increased the rate constant for cycloaddition decreased. The oxidation of crystal violet to its cation radical can be initiated by reaction of the dye with the excited singlet states of many polycyclic aromatic hydrocarbons. This reaction was found to be far less efficient for polymer-bound pyrene than for free pyrene and this was attributed to excimer formation occurring in the polymer system which ultimately led to energy wastage (Tazuke et al., 1979). 

In the second case a violet-colored species (crystal violet) is produced by the photogenerated imidazole radical (Scheme 2). 1,6-Hydroxypyrene which is produced from 1,6-pyrenequinone by visible irradiation quenches the imidazole radical, and the system is fixed. 

Mayo et al. [422] found that methylene blue, 3-ethoxy-5-methoxy-xanthone, crystal violet and acridine red sensitize the photo-oxidation of polyisoprene in chlorobenzene on exposure to UV light. The reaction rate is highest when the concentration of the dye is 10-4 mole 1 1. The addition of 2,6-di-tert-butyl-4-methylphenol to the solution does not stop the photo-oxidation reaction but merely reduces the quantity of oxygen absorbed. It may be concluded that the oxidation cannot occur by the free radical mechanism. 

Other Mechanisms Other three-component systems involve different mechanisms (see Refs [li,m] and references therein). For example, in the dye (crystal violet, phenosaffanine, methylene blue, thiopyronine)/amine/ketone (acetophenone, benzo-phenone, thioxanthone, 4,4 -bis-dimethylamino benzophenone) combi-nation, the mechanism is quite complex. In the camphorquinone CQ/amino-benzophenone ABP two-component system, both CQ and ABP absorbs the light an aminoalkyl radical on ABP and a ketyl radical on CQ are formed. 

The addition takes place on a (6,6)-bond. Here as well, dehydrogenated products are obtained as the amino protons present in the first stage are removed by oxidative elimination. Usually mono- or bisadducts are formed. Tertiary amines cannot react with C ) to yield analogous products. They give charge-transfer complexes instead, with some of them being considerably stable, for example, the Ca)-complex with the lewco-form of crystal violet. Apart from that, they may also enter into photochemical or thermal radical processes (Section 2.5.5.5). 

Reaction between iV,JV-dimethylaniline and chloranil in polar solvents yields a violet material with an absorption spectrum corresponding to that of the crystal violet cation. The diamagnetic complex (D A) and semiquinone radical were identified as intermediates. The overall course of the reaction is apparently as given above. The intermediate steps must involve an intermolecular methyl migration. 

When a molecule is photoexcited, the excited state is better electron donor as well as better electron acceptor than the ground state. In the presence of donor and acceptor which are not capable of conducting dark electron transfer reaction with each other or with sensitizer, the sensitizer may be recycled via radical cation or radical anion(Fig. 2). Detailed kinetic study on electron transfer sensitization of photooxidation of leuco crystal violet(LCV) in the presence of perylene(Pe), and 9-cyanoanthracene(CNA) or 1,4-dicyanobenzene(DCB) as sensitizer and acceptor, respectively, (3) provides the following information, i) Reaction between Pe and DCB is ca. 2.5 times faster than reaction between Pe and LCV. ii) The turnover number of Pe could be infinite in vacuo, iii) Oxygen could substitute the role of DCB. iv) The quantum efficiency of Pe-LCV-DCB ternary system is better than that by direct excitation of LCV-DCB system. v) When other solvents are used, solvents having high... 

It also became evident before long, that the application of HABIs in photochromic windows would be a daunting challenge. We noted that the radicals were excellent hydrogen abstractors, and we could not develop media that lacked abstractable hydrogen. Additionally, the color of the generated free radicals was weak, compared to real dyes, with extinction coefficients under 10,000, compared to dyes, such as Crystal Violet, with over 140,000. 

A method was devised to determine extinction coefficients of radicals. A flash photolysis unit with filters to confine irradiation to a spectral region where only biimidazoles absorbed was utilized to generate radicals whose initial absorbance was recorded photographically from an oscilloscope. In the same cell another solution was then photolyzed as above. This second solution contained an identical concentration of biimidazole to the first plus leuco Crystal Violet dye, tris-(4-dimethylamino-phenyl) methane, and toluenesulfonic acid. The absorbance of dye produced by oxidation of leuco dye by was observed at an unobstructed wavelength. The extinction of the dye at this wavelength in this medium was determined experimentally. The stoichiometry of radical attack on dye was and the quantum yield of this reaction under... 

As the presence of acid increased the autooxidation of Leuco Crystal Violet, tribro-momethyl phenylsulfone was included in the formulation. This cleaved on photolysis or thermolysis, and the presence of bromine radicals enhances the image color formation. 

Isolation of salts of both the cation radicals and dications has been achieved for a number of systems. Thus, reaction of 2,3,4,5-tetraphenyl-l,4-dioxin with antimony pentachloride in chloroform gives the cation radical (63) as blue-violet crystals, and this can be oxidized by voltammetry to the green dication salt (64). Tetraphenyl-l,4-dithiin with antimony pentachloride in benzene affords the dication salt (65) directly as dark violet crystals (70ZC147). 6-Methyl-1,4-benzodithiin reacts with the same reagent to give a cation radical salt, which decomposes in air to a black resin (62JCS4963). 

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