Sensitizers xanthene dyes

A useful classification of sensitizing dyes is the one adopted to describe patents in image technology. In Table 1, the Image Technology Patent Information System (ITPAIS), dye classes and representative patent citations from the ITPAIS file are Hsted as a function of significant dye class. From these citations it is clear that preferred sensitizers for silver haUdes are polymethine dyes (cyanine, merocyanine, etc), whereas other semiconductors have more evenly distributed citations. Zinc oxide, for example, is frequendy sensitized by xanthene dyes (qv) or triarylmethane dyes (see Triphenylmethane and related dyes) as well as cyanines and merocyanines (see Cyanine dyes). 

Neady every significant class of dyes and pigments has some members that function as sensitizers. Toxicological data are often included in surveys of dyes (84), reviews of toxic substance identification programs (85), and in material safety data sheets provided by manufacturers of dyes. More specific data about toxicological properties of sensitizing dyes are contained in the Engchpedia under the specific dye classes (see Cyanine dyes Polymethine dyes Xanthene dyes). 

ZnO (suspension) sensitizes the photoreduction of Ag" by xanthene dyes such as uranin and rhodamine B. In this reaction, ZnO plays the role of a medium to facilitate the efficient electron transfer from excited dye molecules to Ag" adsortei on the surface. The electron is transferred into the conduction band of ZnO and from there it reacts with Ag. In homogeneous solution, the transfer of an electron from the excited dye has little driving force as the potential of the Ag /Ag system is —1.8 V (Sect. 2.3). It seems that sufficient binding energy of the silver atom formed is available in the reduction of adsorbed Ag" ions, i.e. the redox potential of the silver couple is more positive under these circumstances. 

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. 

Recently some quantitative analyses of the kinetics of type II direct and indirect photooxygenation reactions have been made. These dealt with the photooxygenation of 2,5-dimethylfuran7 61 and allylthiourea86 sensitized by xanthene dyes and chlorophyll, respectively, and with the... 

With rose bengal and erythrosin as xanthene dye sensitizers, the... 

The photooxygenation of 2,5-dimethylfuran in alcoholic solution at 20°C sensitized by xanthene dyes has been kinetically studied by Schenck and Gollnick.7,81 The rate constants of the various reaction steps have been determined according to the following procedure. 

Dianions of Xanthene Dyes as Sensitizers of Photooxygenation Reactions... 

Beside xanthene dyes, methylene blue,127 porphyrins and por-phins,128-130 zinc-tetraphenylporphin,128 a-hydroxyanthraquinones in alkaline alcoholic or pyridine solutions,112,131 vitamin A, /9-carotene, hypericin, rubrene, 3,4-benzpyrene, 20-methylcholanthrene, chlorophyll, and many other compounds have been found to be sensitizers of photooxygenation reactions.70,132,133 Some groups of dyes, however, such as porphyrins and porphins containing transition metals (e.g., Co), azo dyes, cyanine dyes, and triphenylmethane dyes either did not show any or only very poor sensitizer capabilities in the photooxygenation of a-terpinene.128,134... 

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... 

Xanthene dyes have also been reported as sensitizers for the reduction of... 

One of the limitations in using the xanthene dyes as photoinitiators is their relatively low solubility in nonpolar media. Solubility considerations become important in dye-sensitized photopolymerization of multifunctional acrylates in the absence of solvent. In the search for faster initiators for three-... 

It is reported that the oxidation of several excited xanthene dyes at n-ZnO electrodes occured via the triplet state. This was concluded from the result that the quantum efficiency of the sensitization current decreased upon addition of a typical triplet quencher [27]. Another interesting example is Ru(bipy)3, which shows in solution an intersystem crossing yield of unity [11]. Nearly all electron transfer reactions between the Ru complex and an electron acceptor in solution occur via the triplet state. In the case of adsorbed Ru(bipy)3, electron transfer from the excited singlet and from the triplet to the semiconductor electrode is possible depending on the coupling of the dye to the semiconductor (see Section 10.2.6). It should be mentioned here that the energy levels of the excited Ru complex given in Fig. 10.4, were calculated for a triplet state. 

Chemiluminescence.—It has been suggested that problems which occur in the determination of yields of bio- and chemi-luminescence may be due to the sample cell. Errors of 25% may be caused by reflection and refraction from interfaces, and, consequently, frosted containers and point-source geometries were recommended. Several authors have concentrated on the use of sensitizers for the enhancement of chemiluminescence. The heavy-atom effect was found to operate in the energy transfer from enzyme-generated acetone to xanthene dyes. 9,10-Diphenylanthracene (9,10-DP A) has been suggested to be a poor singlet counter for chemiluminescence as some triplet states were also counted. In another report, 9,10-dibromoanthracene was found to be a more effective enhancer, when compared with 9,10-DPA, for chemiluminescence from a cyclic peroxide. Luminol chemiluminescence was employed in the analysis of Cr" ions in sea-water. Enhancement with bromide ions enabled detection limits of 3.3 X 10 m to be achieved. 

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... 

Elaborate kinetic analyses of the photo-oxygenation of 2,5-dimethyl-furan (22, 56) and allylthiourea (40) sensitized by xanthene dyes and chlorophyll, respectively, and of the direct photo-oxygenation of anthracene and 9,10-diphenylanthracene (41) have revealed that Type II direct and indirect (sensitized) photo-oxygenation reactions proceed with... 

The second family of xanthene dyes is fluorescein and its derivatives. Fluorescein itself is only slightly fluorescent in alcohol solution. In contrast, the alkali salt obtained by addition of alkali exhibits the well-known yellow-green fluorescence characteristic of the fluorescein dianion (uranin). Fluorescein and its derivatives, e.g. eosin Yand erythrosin Y, are known to be very sensitive to pH and can thus be used as pH fluorescent probes (see Chapter 10). 

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) ,... 

---