Acid xanthene dyes

The acid xanthene dyes comprise Eosin (tetrabromofluorescein, formula 4.34) [79,80], Erythrosin (tetraiodofluorescein) and Rose Bengal B (formula 4.35) [79]. All these reagents are available as sodium salts. 

Most xanthene dyes are classified as basic dyes by their method of appHcation acid dyes can be produced by introduction of sulfonic acid groups. The fluoresceins, which contain carboxy and hydroxy substituents, are also acid dyes for coloration of silk. Some of the fluoresceins in which the carboxy group has been esterified, are soluble in alcohol or other organic solvents and can be classified as solvent dyes. Mordant dyes can be produced by introducing o-dihydroxy or sahcyhc acid groups (2), which when metallised can have very good lightfastness. 

A series of water-soluble fiber-reactive xanthene dyes has been prepared from the reaction of ben2oxanthenedicatboxylic acid anhydride disulfonic acid with, for example, 3-aminophenyl-P-hydtoxyethyl sulfone to yield dyes, with high brilliance and good fastness properties for dyeing of or printing on leather, wool, sHk, or ceUulosic fibers (53). 

Xanthene dyes (qv) can be either acidic or basic. Acid xanthenes are known to exist in two tautomeric forms. The phenoHc type, or fluorans, are free-acid stmctures such as D C Orange No. 10 (17b) and D C Red No. 21 (17c). Most have poor water solubHity. In contrast to these, the quinoids or xanthenes are usuaHy the highly water-soluble sodium salt counterparts of the fluorans such as D C Orange No. 11 (18) and D C Red No. 22 (21a). Presendy, there are no certifiable basic xanthene colorants. 

Xanthene Dyes. This class is best represented by Rhodamine B. It has high fluorescent brilliance but poor light and heat stabihty it may be used in phenohcs. Sulfo Rhodamine is stable and is useflil in nylon-6,6. Other xanthenes used in acryhcs, polystyrene, and rigid poly(vinyl chloride) are Solvent Green 4, Acid Red 52, Basic Red 1, and Solvent Orange 63 (see Xanthene dyes). 

SNARF and SNAFL indicators are benzo[c]xanthene dyes that can be described as semi-naphthofluoresceins and semi-naphthorhodafluors, respectively, depending on whether the benzo[c]xanthene ring is substituted at the 10-position with oxygen or with nitrogen, respectively (Whitaker et al., 1991). These indicators, whose p K l values are in the physiological range, exhibit distinct emission bands for the pro-tonated and deprotonated forms so that emission ratio measurements are possible. In SNAFL, the acidic form is more fluorescent, whereas in SNARF, the basic form is more fluorescent. 

Nonchelating dyes include basic triphenylmethane dyes (e.g., Brilliant Green, Malachite Green, Crystal Violet), xanthene dyes (e.g., Rhodamine B, Rhodamine 6G), azine dyes (e.g., Methylene Blue), and acid dyes (e.g., Eosin, Erythrosin). These are intensely colored and when paired with an oppositely charged analyte ion lead to high sensitivities. 

Oster [174] proposed the second hypothesis to explain his results on the photopolymerization of acrylonitrile in aqueous solution, buffered at pH 7.0, and sensitized by xanthene dyes and riboflavin using ascorbic acid as the reducing agent. Whereas the monomer is efficiently polymerized when the solution is illuminated in the presence of oxygen, irradiation in its absence leads to photoreduction of the dye to its leuco form but no polymer is formed. Therefore, the author suggests that the leuco dye reacts with atmospheric... 

Comparison of these new compounds with the common xanthene dyes reveals that substitution of H at C-9 has only a minor influence in the absorption and emission spectra. Table 15 summarizes the spectral properties of HF, TBHF, and TIHF as well as the acid dissociation constants and electrochemical potentials. The color, fluorescence quantum yield, ap-... 

Fluorescein (703) (Acid Yellow 73 C.I. 45350) is possibly the best known xanthene dye. The sodium salt is soluble in water to which it imparts an intense yellow-green fluorescence, detectable even at a dilution of 0.02 p.p.m. under UV irradiation. This property leads to the use of fluorescein as a location marker for aircraft lost at sea, as a tracer for the detection of a source of contamination in drinking water, and in a number of related situations. The use of fluorescein to detect abrasions of the cornea is also based on its fluorescence. 

Synthesis of Xanthene Dyes. Phtbalic anhydride reacts with 3-dietbylaminophe-nol in the molten state to give rhodamine B, and with resorcinol to give fluorescein. For unsymmetrical xanthene dyes, dihydroxybenzoyl- (24) or an aminohy-droxybenzoylbenzoic acid (25) is required. 

These compounds can condense and cyclize with resorcinol or substituted 3-aminophenols. Nucleophilic substitution of chlorine by amines leads to xanthene dyes which contain a sulfonic acid group [10] or hydrogen [11] instead of the carboxyl group. 

Third, reaction products of acid azo dyes acid 1 1 metal-complex azo dyes, or 1 2 metal complex azo dyes without acid groups with organic bases or cationic dyes. Cyclohexylamine, dodecylamine, and sulfonium or phosphonium compounds serve as bases, and derivatives of the xanthene range (rhodamines) are mainly used as cationic dyes. Example C.I. Solvent Red 109 [53802-03-2] is composed of Solvent Yellow 19, 13900 1 [10343-55-2] (3) and Solvent Red 49, 45170 1 [81-88-9] (4). These dyes are saltlike compounds of a metal-complex azo dye acid and a base. 

Like phenolphthalein, xanthene dyes are prepared in a condensation reaction involving phthalic anhydride. However, resorcinol is employed instead of phenol. The simplest representative of this family is C.I. Acid Yellow 73 (fluorescein), which is made via the sequence of steps shown in Fig. 13.109. Similarly, C.I. Acid Red 92 is made by the condensation of tetrachlorophthalic anhydride and resorcinol followed by bromination. 

A simple example of an intramolecular Lewis acid/base reaction (thus avoiding the association step) is the xanthene dye rhodamine B, which exists in solution either in the red-coloured zwitterionic form (26a) or as the colourless lactonic form (26b) [175, 221, 222]. Solutions of rhodamine B in non-HBD solvents such as dimethyl sulfoxide,... 

Analogous solvent-dependent intramolecular Lewis acid/base equilibria between laetonie and zwitterionie forms have also been found for the xanthene dyes fiuoreseein and eosin [176]. Strueturally related eolourless 3,3-diarylphthalides form highly eoloured triarylearbenium ions by interaetion with acidic compounds such as phenols. This aeid/ base reaetion has been used to design optochemical sensors suitable for the deteetion of vapours of polar solvents in the air or in waste water [271]. 

Thallium(III), in the form of the TICU or TlBr4" complex ions, reacts in acid media (1-2 M HCl) with the basic xanthene dye, Rhodamine B (formula 4.29), to form a red-violet, slightly fluorescent ion-associate complex, which is soluble in benzene, DIPE, and isoamyl alcohol. These extracts have been used for the spectrophotometric determination of thallium [8]. A mixture of CeHe and CCI4 (2+1) is a convenient extractant since it is denser than water. 

Use has been made of ion-pairs formed by the cationic complex of zinc with 1,10-phenanthroline or 2,2 -dipyridyl and acid dyes, such as eosin, Erythrosin, Rose Bengal, dibromofluorescein (xanthene dyes) [72-74], Bromophenol Blue, Bromophenol Red (triphenylmethane dyes) [75,76]. In some of these methods, molar absorptivities are -10. Chloroform is the usual extraction solvent. 

The dye-sensitized photo-oxidation of phenols has been examined using CIDNP techniques.148 It is concluded from the observed polarizations of nuclear spin that sensitization by xanthene dyes (such as Rose Bengal) is the result of reversible hydrogen abstraction by the triplet dye molecule from the phenolic hydroxyl group. Any resulting photochemical reactions (e.g. with oxygen) arise from irreversible reactions of the phenoxy-radicals produced, rather than from direct reactions of the phenols with 102. Dye-sensitized photo-oxidation of p-hydroxyphenylpyruvic acid (94) in solution at pH 7 (conditions under which... 

Adsorption of sulfophthalein and xanthene dyes by protamine sulfate/carboxymethylcellulose (PtS/CMC) layer-by-layer (LbL) films has been studied. The adsorbed amount of a dye depends on its nature, solvent, conditions of polyelectrolyte multilayers (PEMs) preparation. Addition of ethanol into solution and preliminary drying of PEMs increase adsorption of the dyes. Kinetics of a dye release from (PtS/CMC)2o films in acidic solution (pH 3.0) has been investigated. 

Phthalide derivatives are of major importance in the dye industry, particularly in the area of recording material color formers. Pressure-sensitive carbonless copy paper and thermal recording paper are typical applications. Five principal structural classes have been developed extensively the xanthene dyes (fluorans (208)), 3,3-diarylphthalides (209), spirofluorenes (210), 3,3-bis(di-arylethylene)phthalides (211), and 3-substituted phthalides <84Mi 208-03>. Some of these structures are common to many familiar acid/base indicators, dyes, biological stains, or laser dyes such as fluorescein (212) and phenolphthalein (213) ,... 

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