Organic xanthene dyes

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. 

Various reports of the photoreduction of organic dyes have appeared. The photoreduction of methylene blue in the presence of several electron donors,93 thionine in the presence of allylthiourea,94 fluorescein,95 Rhodamine B, and other xanthene dyes have all been investigated by spectrophotometric or e.s.r. techniques. 

It is worthwhile to refer here to the use of rhodanune B and rhodamine 6G and other xanthene dyes as probes for the study of organic crystals, calcium fluoride, and quartz plates [74]. 

The strongest commonly observed absorption bands in organic molecules are exhibited by laser dyes such as rhodamine 6G, a xanthene dye with a rigid chromophore which exhibits e ax 10 L/mol cm at 5300 A (Chapter 9). Its fluorescence lifetime t is in the neighborhood of 5 ns (depending on solvent), and is dominated by t,ad because (2f nearly unity under conditions in which stimulated emission is suppressed. At the other extreme, for Ij in an inert solvent like cyclohexane is about 7 x 10 L/mol cm for the spin-forbidden -V B ITou transition. In accordance with Eq. 8.50, T d for this transition is in excess of 10 /xs. Little I2 -> emission can be seen in solution,... 

PO can be made degradable by means of additives. The types of additives include aromatic ketones (benzo-phenone and substituted benzophenones [47], qui-none), aromatic amines (trisphenylamine), polycyclic aromatic hydrocarbons (anthracene, certain dyes such as xanthene dyes), or transition metal organic compounds. The transition metal compounds of Fe, Co, Ni, Cr, Mn are widely used. Organo-soluble acetyl acetonates of many transition metals are photooxidants and transition metal carboxylates are also thermal pro-oxidants. Co acetylacetonate appears to be an effective catalyst for chemical degradation of PP in the marine environment. The preferred photoactivator system is ferric dibutyldithiocarbamate with a concentration range of 0.01. 1%. Scott has patented the use of organometallic compounds hke iron (ferric) dibutyldithiocarbamate or Ni-dibutyl-dithiocarbamate [48]. Cerium carboxylate [49] and carbon black are also used in such materials [50]. 

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. 

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. 

Basic dyes (triphenylmethane, xanthene, azine, etc., dyes) form ion-association compounds (ion-pairs) with anionic halide complexes of metals and non-metals e.g., SbCl, AuBr4 , TaFe , BF4 ). The resulting compounds, that may be extracted into non-polar organic solvents, may serve as a basis for sensitive spectrophotometric methods [62-65]. 

Macrocycles (crown ethers, cryptands) with chromogenic groups combine the natural selectivity of macrocycles with the possibility of direct spectrophotometric determination of some metals (e.g., K, Ca) in an organic phase after extraction [127-129]. 4-Picrylamlnobenzo-15-crown-5 crown ether (formula 4.41) is applied in the extraction and spectrophotometric determination of potassium. The determinations are based on extractable ion-associates of metals (e.g., Li, Na, K, Pb) with crown ethers and xanthene or sulphophthalein dyes [130]. 

The other dye classes are nitroso, carotene, xanthene, acridine, methine and polymethine, thiazole, indamine and indophenol, azine, oxazine, thiazine, sulfur, lactone, aminoketone, hydroxy-ketone, and natural organic coloring matters. 

Fluorescence lifetime measurements provide invaluable information about the processes involved in the dissipation of excitation energy within organic molecules. Some of the fastest excited state relaxation rates are observed in solvated ionic dyes, such as the xanthene, triphenylmethane and polymethine dyes. The stereochemistry of these dyes and the close proximity of the ground and excited states make it possible for radiationless transitions to occur at rates far in excess of those observable in most aromatic molecules. Consequently, the fluorescence lifetimes of these dyes are strongly dependent upon the rate constants of the various radiationless relaxation processes. 

Some of the early studies of spectral sensitization of semiconductors have been on ZnO and CdS. The first report was published by Putzeiko and Terenin in 1949 when they reported sensitization of pressed ZnO powder by adsorbed Rhodamine B, Eosin, Erythrosin and by Cyanine dyes [12]. Hauffe and Gerischer coworkers made some pioneering studies in the seventies and these have been extended by many others [13-18]. Most of these early studies focussed on organic dyes of interest to photographic industry (e.g. Xanthenes such as Rhodamine B or Eosin or Cyanines). 

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