Malachite green, similarity

The valence-bond (resonance) description of the triphenylmethine dye Malachite Green (125) is illustrated in Figure 6.5. Comparison with Figure 6.4 reveals their structural similarity compared with cyanine dyes. Formally, the dye contains a carbonium ion centre, as a result of a contribution from resonance form II. The molecule is stabilised by resonance that involves delocalisation of the positive charge on to the p-amino... 

The technical preparation of crystal violet and of its methyl-free parent substance, parafuchsine, almost the oldest of the triphenylmethane dyes, is not so easily explained. As is well known, in this process aniline and p-toluidine are united by oxidation in an acid melt. (In the preparation of fuchsine itself, which contains a methyl group attached to one of the benzene rings, o-toluidine is an additional ingredient.) Although all the phases of this important synthesis have not yet been experimentally established, we may nevertheless explain it on the basis of a dehydrogenation similar to that involved in the formation of malachite green. Moreover, the union of several molecules of base proceeds exactly according to the principle on which indamines are formed (p. 321) (Bucherer). 

On the other hand, the photoconductivity of a poly(vinyl chloride) copolymer and of sucrose benzoate each containing the leucobase of malachite green was characteristic of the dyestuff and independent of the polymeric host (137). This is similar to the behavior of conventional impurity semiconductors. However, a distinction must be drawn between an impurity which acts solely as an impurity providing an easier path to the conduction band, one which acts only as a plasticizer and one which may act as both impurity and plasticizer. 

In commercial preparations, several fractions of the dye are always separated since different consumers have quite different requirements as to the appearance of the product. The oxalate of malachite green corresponds to the formula, 2 C2 )H24N2 + 3 CaHi-O. The crystallization requires several days and frequently crystals of great beauty are obtained. The addition of ammonium oxalate to promote crystallization is reminiscent of similar procedures in alkaloid chemistry and was discovered purely empirically. 

Like most ammonium bases, methylene blue does not dye wool easily, but is readily fixed on silk and tannined cotton. It has also a slight affinity for unmordanted vegetable fibres. Methylene blue is principally used in cotton-dyeing. It dyes a greenish shade of blue, which shows a dull tone similar to that of indigo. It is very fast to light, and the shades may be readily modified by other basic dyestuffs, such as methyl violet or malachite-green. 

Absorption and fluorescence spectra of 7-aminocoumarin derivatives have been recorded and the photophysics of coumarin inclusion complexes with 0- and y-cyclodextrins investigated. Solvent effects on the photophysical behaviour of pyrrolocoumarin derivatives are similar to those on the psorolens . Vibronic exciton bands and the absorption spectra of Eosin Y dimers have been analysed theoretically and compared with experimental data. Picosecond spectroscopic studies have been made on the ionic photodissociation dynamics of malachite green leucocyanide . The lowest state produces ions within 0.1 to 0.5 ns whilst for higher states 6-13 ps are sufficient for the development of charge separation. The photophysics of crystal violet in a series of n-... 

Besides the Rhodamine 6G-SnCl2 flotation-spectrophotometric method described above, similar methods using other basic dyes, e.g., Victoria Blue B, Victoria Blue 4R, Capri Blue [41], Crystal Violet (e = 2.1-10 ) [42], or Nile Blue A [43], have been proposed. An ion-associate of the chloride platinum complex with Methyl Green has been extracted with a mixture of 1,2-dichloroethane and CCI4 (e = 1.45-10 ) [44]. The ion-associate of the Pt-thiocyanate complex with Malachite Green has been extracted with benzene [45], and a thiocyanate- or iodide- Pt complex associated with Crystal Violet has been extracted into xylene or toluene [46]. 

No triphenylmethyl cation or radical was detected. Malachite green leuco-methoxide, 51, in cyclohexane behaved similarly. In contrast, 51 showed rapid growth of the triaryl cation from excited Si with a rise time of 1 ns in 9 1 acetonitrile/water, and an even faster rise time in methanol. No evidence for the triarylmethyl radical was obtained. Also, the dithioketal, 52, has been used to generate the corresponding relatively long-lived cations by photoheterolysis of the carbon-oxygen ether bond the half-life for cation decay in 1 1 ethanol/water is 0.17 s. Rate constants for the reactions of this cation with a number of amine and anionic nucleophiles were measured. 

Employing a reflection technique, Kortiim and Vogel (75) examined the spectrum of the lactone of malachite green-o-carboxylic acid adsorbed on the surfaces of a series of alkali halides and alkaline earth sulfates. Chemisorption of this compound resulted in formation of a zwitter-ion by ring cleavage [Eq. (8)] as evidenced by the similarity in... 

From these spectral data it was possible to draw some conclusions concerning the bonding involved in this chemisorption. The similarity of the spectrum of the adsorbate with that of the malachite green cation dictates that the resonance system shown in structure B must remain... 

In 1902 Norris and Kehrman independently discovered that colorless triphenyl-methanol gave deep yeUow solutions in concentrated sulfuric acid (Scheme 2.1). Triphenylmethyl chloride similarly formed orange complexes with aluminum and tin chlorides. Adolf von Baeyer recognized in 1902 the salt-Hke character of the compounds formed. He dubbed the relationship between color and salt formation halochromy, of which malachite green is a prime example. 

In fact, the earliest application of kinetic methods was to determine trace levels of substances exerting catalytic activity in oxidation-reduction reactions involving multiple electron transfers (1885-trace level V on its catalysis of the oxidation of aniline). For example, the reduced form of many triphenylmethane dyes is colorless , and loses two electrons on oxidation to the dye. The rate of reaction with such oxidants as 104 is relatively slow, but can be catalyzed by trace levels of transition metal ions which involve single electron transfer in their own redox steps. Thus, trace levels of manganese can be determined by the proportionality of the rate of oxidation of leuco-malachite green by iodate at less than micromolar concentrations. Similarly, trace levels of Cu ", < 10 M, can be determined from the catalytic effect on the atmospheric oxidation of ascorbic acid. Such systems can be written as a generalized redox reaction... 

Polyhalo-styrene-based films containing similar dyes (e.g., malachite green methoxide) have similar characteristics as mentioned above, but these are free of humidity effects (Bishop et al. 1973). 

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