Xanthene dyes formation

Macrae and Wright (96) demonstrated that visible light irradiation of xanthene dyes (eosin, erythrosin, rhodamine B, or RB) in ethanolic solutions of 4-(N,N-diethylamino)benzene-diazonium chloride (as the zinc chloride double salt) resulted in decomposition of the diazonium salt. Electron transfer from the dye excited state(s) to the diazonium salt was postulated and dye-diazonium salt ion pair formation in the ground state was shown to be important. Similar dyes and diazonium salts were claimed by Cerwonka (97) in a photopolymerization process in which vinyl monomers (vinylpyrrolidone, bis(acrylamide)) were crosslinked by visible light. Initiation occurs by the sequence of reactions in eqs. 40-42 ... 

In this family of the xanthene dyes, excitation of oxygen free aqueous solution of the dye alone is sufficient to induce the formation of the semioxidized and semireduced form of the dianionic starting material and the electron transfer steps always concern the triplet dye X 3 [167-173]. Electron exchange may occur between triplet and ground state or between two triplet excited states (steps 23 and 24). 

The formation of donor-acceptor complexes between bipyridinium salts (electron acceptors) and xanthene dyes (electron donors) (e.g., eosin. Rose Bengal) has been studied extensively. Crystal structures of these complexes have been identified, and the structural features of the donor-acceptor complexes in solutions have been characterized using NMR spectroscopy. The xanthene dye/bipyridinium donor-acceptor complexes are stabilized by... 

The formation and the dissociation of the supramolecular donor-acceptor complex between the xanthene dye and the bipyridinium unit can therefore be triggered by the light-induced transformation of the latter component to photoisomers exhibiting high or low affinities for the electron donor, respectively. 

Pulse Radiolysis of Aqueous Fluorescein Dyes. The xanthene dyes of the fluorescein type were investigated using l-/xsec. pulses of 30 Mev. electrons (4, 5, 8). The transient spectra obtained with deaerated fluorescein solutions show three characteristic sets of bands. A prominent peak that shifts from 355 m/x in neutral solutions to 395 m/x in alkaline solutions corresponds with the semiquinone monoanion (pKa = 9.5) (13). This band is quenched by e q scavengers, such as oxygen or H202, and was attributed to reduction of the dye by eaq. A band at 415-420 m/x which does not change with pH was identified with the semioxidized radical monoanion, a phenoxyl derivative first observed in flash photolysis also (13). This band is quenched by formate and was attributed to the oxidative attack of OH. The remaining transient consists of a diffuse... 

El-Brashy, A. M., Metwally, M. E. and El-Sepai, F. A. 2004. Spectrophotometiic determination of some fluoroquinolone antibacterials by binary complex formation with xanthene dyes. II Farmaco. 59 809-817. 

Other examples include acridine dyes (with absorption peaks aroimd 475 nm), xanthene dyes ( 500-550 nm), fluorone dyes ( 450-550 nm), coiunarin dyes (" 350-450 nm), cyanine dyes ( 400-750 nm), and carbazole dyes ( 400 nm) (12,19-21). The oxidation or reduction of the dye is dependent on the co-initiator for example, methylene blue can be photoreduced by accepting an electron from an amine (22) or photo-oxidized by transferring an electron to benzyltrimethyl-stannane (12). Either mechanism will result in the formation of a free-radical active center capable of initiating a growing polymer chain. For a more detailed discussion of the mechanisms, see Reference 12. 

In these cases it is usually possible to explain why the color is formed. The reactions can be further classified, on the basis of the type of colored compound formed, into reactions leading to the formation of azo dyes, di- or triphenylmethane dyes, xanthene dyes, polymethine dyes, indophenols, etc. Azo dyes are formed, for example, in the reaction of diazonium salts and phenols (p. 192) or amines (p. 324), azomethines in the reaction of primary aromatic amines with aromatic aldehydes (p. 215), di- and triphenylmethane dyes in the reaction of aromatic aldehydes with aromatic hydrocarbons in concentrated sulfuric acid (p. 213), triphenylmethane dyes in the reaction of phenols with aromatic aldehydes or oxalic acid (p. 196), xanthene dyes in the reaction of anhydrides of dicarboxylic acids with resorcinol (p. 196), polymethine dyes are formed after the cleavage of the pyridine ring in the reaction of the glutaconaldehyde formed and barbituric acid (p. 378), indophenols on reaction of phenols with Gibbs reagent (p. 195), or 4-aminoantipyrine according to Emerson (p. 194), or on the Liebermann reaction (p. 195). 

Photopolymerization reactions are widely used for printing and photoresist appHcations (55). Spectral sensitization of cationic polymerization has utilized electron transfer from heteroaromatics, ketones, or dyes to initiators like iodonium or sulfonium salts (60). However, sensitized free-radical polymerization has been the main technology of choice (55). Spectral sensitizers over the wavelength region 300—700 nm are effective. AcryUc monomer polymerization, for example, is sensitized by xanthene, thiazine, acridine, cyanine, and merocyanine dyes. The required free-radical formation via these dyes may be achieved by hydrogen atom-transfer, electron-transfer, or exciplex formation with other initiator components of the photopolymer system. 

The literature of the xanthenes provides many examples of compounds with various substituted aromatic nuclei at C-9, and other examples with different substituents on the xanthene ring. The basic skeleton, however, has not been modified in recent years. It is apparent that the crowded aromatic subunit will slow reactions involving the formation of intermediates involving reaction at C-9 and may retard the subsequent reactions of the intermediates as well. This observation predicts slower rates of electron transfer, for example, and subsequent photoreduction than would be expected for unsubstituted xanthenes. Our interest was to synthesize a new series of dyes without substituents at that position and to investigate the effect of C-9 substitution on photochemical and photophysical properties. 

Aryliodonium salts have been found to be coinitiators for photooxidizable dye sensitization (105). Smith polymerized aerylamide-bis(acrylamide) mixtures using acridine, xanthene, or cyanine dyes and, for example, diphenyllodonium chloride as an electron acceptor. Reduction of the salt results in the formation of phenyl radicals. 

Basic (cationic) dyes. Basic dyes are water-soluble and produce colored cations in solution. They are mostly amino and substituted amino compounds soluble in acid and made insoluble by the solution being made basic. They become attached to the fibers by formation of salt linkages (ionic bonds) with anionic groups in the fiber. They are used to dye paper, polyacrylonitrile, modified nylons, and modified polyesters. In solvents other than water, they form writing and printing inks. The principal chemical classes are triaryl methane or xanthenes. Basic brown 1 is an example of a cationic dye that is readily protonated under the pH 2 to 5 conditions of dyeing [5]. 

In the initial description of the cationic dye-borate system [24, 76], it was postulated that electron transfer was possible because, in nonpolar solvents, dye/borate salts exist predominantly as ion pairs. Since the lifetime of the cyanine singlet excited state is quite short [24, 25], this prerequisite is crucial for eflfective photo-induced electron transfer. Recently initiator systems in which neutral dyes are paired with triarylalkylborate anions have appeared in the literature [77]. In the latter case, the borate ion acts as the electron donor while neutral merocyanine, coumarin, xanthene, and thioxanthene dyes act as the electron acceptors. It is obvious that these initiating systems are not organized for effective electron transfer processes. The formation of an encounter complex (EC) between excited dye and electron donor is required. 

It has been reported that y3-CD could improve the selectivity of the color reactions of various metal ions with triphenylmethane, xanthene acid dyes and some other coloring reagents. The effect of fi-CD on the association compound system of metal (Mo, Zn, Co)-thiocyanate basic dyes such as malachite green, crystal violet, rhodamine B, rhodamine 6G and butyhhodamine B, has been investigated and the result shows that /3-CD could contribute to a more sensitive and stable system which improve the solubility of the basic dyes and produce a favorable microenviromnent for the color reactions [63]. /3-CD could be employed to solubilize the 1,2-amino anthraquinone in water due to the formation of inclusion complex which acts as a ligand for metal ions could be used for the determination of palladium at trace levels by spectrophotometry. In the spectrophotometric determination of microamounts of Zn based on the Zn-dithizone color reaction, -CD could increase the apparent molar absorptivity at 538 nm by 8.37 times. In the presence of cr-CD, the determination sensitivity of copper in leaves based on the color reaction of Cu(II) and mesotetrakis (4-methoxy-3-sulfophenyl) porphyrin was enhanced by 50% in the spectrophotometric analysis [64,65]. 

The best-known reaction of uronic acid is that with naphthoresorcinol in cone, hydrochloric acid. The dyes formed are probably xanthene derivatives soluble in benzene (in contrast to derivatives of pentoses and hexoses) (126). Characteristic colors are also formed with carbazole in sulfuric acid, based on the formation of 5-formylfuroic acid, which subsequently reacts with carbazole. 

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