Dye precursors

CgHeNj. Brown-yellow crystals m.p. 103-104 C, b.p. 256 258°C. Its solutions reduce Ag ions and it is used as a photographic developer. It is also used as a dye-precursor, for the synthesis of phenazine derivatives and for characterizing inter alia) a-diketones. 

Permanent Hair Colorants. Permanent colorants produce hair coloration that lasts until the hair grows out. Color is formed inside the hair by hydrogen peroxide-induced coupling reactions of colorless dye precursors. A hiU range of shades can be obtained with this system and the permanent or oxidative hair colorants are considered to be the most important class of hair dyes. 

Attempts to broaden the range of materials available as dye precursors have been made (34,35). Oxidative dyes based on pyridine derivatives produce less sensitization than those based on benzene derivatives (36) however, they lack tinctorial power, lightfastness, and availabihty. Derivatives of tetra am in opyrim i din e are claimed to act as primary intermediates to give intense shades with good fastness and excellent toxicological properties (37). 

Carbonless Copy Paper. In carbonless copy paper, also referred to as pressure-sensitive record sheet, an acid-sensitive dye precursor, such as crystal violet lactone or /V-hen2oy11eucomethy1ene blue, is microencapsulated with a high boiling solvent or oil within a cross-linked gelatin (76,83,84) or in synthetic mononuclear microcapsules. Microcapsules that have a starch binder are coated onto the back of the top sheet. This is referred to as a coated-back (CB) sheet. The sheet intended to receive the image is treated on the front (coated-front (CF)) with an acid. When the top sheet is mechanically impacted, the dye capsules mpture and the dye solution is transferred to the receiving sheet where the acid developer activates the dye. 

Subtractive dye precursors (couplers) that could be immobilized in each of the silver containing layers were sought, so that dye formation in all layers could proceed simultaneously rather than successively. The first of these to be commercialized were in Agfacolor Neue and Ansco Color films, introduced soon after Kodachrome film. These reversal working films contained colorless couplers that were immobilized (ballasted) by the attachment of long paraffinic chains. The addition of sulfonic or carboxyUc acid groups provided the necessary hydrophilicity to make them dispersible as micelles in aqueous gelatin. 

The dyes or dye precursors may be initially diffusible in alkaU, in which case they will be immobilized imagewise, or they may be initially immobile in alkaU and released imagewise to transfer. Positive-working processes produce dye transfer density inversely related to the developed silver density conversely, negative-working processes produce dye transfer density in direct proportion to the developed silver. 

Dye release may relate either directly or inversely to the image-related reduction of silver halide. Release of the dye or dye precursor may be accomphshed or initiated by the oxidized developing agent or the unoxidized developing agent or by alkali or silver salts. 

Release by Silver-Assisted Cleavag e. A soluble silver complex formed imagewise in the undeveloped areas of the silver haUde layer may be used to effect a cleavage reaction that releases a dye or a dye precursor. The process yields positive dye transfer images directiy with negative-working emulsions (46). An example is the silver-assisted cleavage of a dye-substituted thiazolidine compound, as shown in equation 7. 

Dihydroxy-4,8-dinitroanthraquiaone [128-91 -6] (54) is an important dye precursor for Cl Disperse Blue 56, and is prepared from... 

Aniline (aminobenzene) is an oily liquid that turns brown when exposed to air and light. The compound is an important dye precursor. 

Very recently, a sandwich assay for prostatic acid phosphatase antigen was carried out using two cascaded enzyme reactions to provide amplification of the immunochemical event. In one format, an optical readout was used whereby a forma-zan dye was generated by reaction of a dye precursor and NADH generated from the second enzyme cycle. In the electrochemical format, the NADH generated in the second enzyme cycle was used to reduce Fe(CN) to FeCCN) " which was then detected amperometrically. While the use of Fe(CN) in ECIA has appeared in the... 

Arylmethane leuco dyes are converted into di- or triarylmethane dyes on oxidation. This class of dye precursors sometimes is referred to as leuco di- or triphenylmethane dyes, or di- or triphenylmethane leuco dyes. The use of the term di- or triarylmethane dyes can be misleading as the central carbon atom is a carbonium ion. Instead, the term di- and triarylmethine dye is recommended for this class as it correlates with the well-known polymethine dyes. Nevertheless, it has not been commonly used. 

For oxalate detection, authors proposed a similar detection approach for recognition of oxalate via an immobilized oxalate oxidase/peroxidase couple and dye precursors MBTH (3-methyl-2-benzothiazolinone hydrazone) and DMAB (3-dimethylaminobenzoic acid). The peroxide generated by oxidation of oxalate to CO2 reacted with the dye precursors in a peroxidase-catalyzed reaction to yield an indamine dye with absorption maximum at 590 nm. The concentration of oxalate was correlated with increased absoiption from dye. 

Coloring adjunct, 12 31 Coloring, electrolytic, 16 221-222 Color intensity, of spices, 23 159 Colorless dye precursors, 19 295-296 Color matching, of inorganic pigments,... 

Leukos (Greek) means white, colorless . The term leuko dye is commonly used for a colorless (or largely UV-absorbing) dye precursor. 

As mentioned previously (see p. 237), the 1,2,3-benzotriazinones function as masked diazonium compounds, and the so-called Pologenowe dyes developed in Poland are operated on this principle and have been in use there since 1959. There are three major advantages associated with the use of 1,2,3-benzotriazinones as azo-dye equivalents (1) The equilibrium that theoretically exists between the heterocyclic system and the diazonium compound normally lies completely to the side of the cyclic compound. Opening of the heterocyclic ring can be easily accomplished thermally, and hence dye formation can be induced by heat treatment processes. (2) Many 1,2,3-benzotriazinones are reasonably water soluble and the sodium salts even more so consequently, complete permeation of fabric fibres by the dye precursors immediately prior to generation of the dye can be ensured. (3) The colors produced are fast, and some of them can be made highly water insoluble. ... 

The image dyes (or dye precursors) used in these materials actually transfer from an image-recording section into a separate image-receiving section of the unit. The dyes interact with a mordant contained in a layer of the receiver and become immobilized. In some versions of the processes, the receiver is a separate sheet but in others it is coated as an integral part of the unit. 

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