Sensitization merocyanine

Solvent Influence. Solvent nature has been found to influence absorption spectra, but fluorescence is substantiaHy less sensitive (9,58). Sensitivity to solvent media is one of the main characteristics of unsymmetrical dyes, especiaHy the merocyanines (59). Some dyes manifest positive solvatochromic effects (60) the band maximum is bathochromicaHy shifted as solvent polarity increases. Other dyes, eg, highly unsymmetrical ones, exhibit negative solvatochromicity, and the absorption band is blue-shifted on passing from nonpolar to highly polar solvent (59). In addition, solvents can lead to changes in intensity and shape of spectral bands (58). 

Several types of nitrogen substituents occur in known dye stmetures. The most useful are the acid-substituted alkyl N-substituents such as sulfopropyl, which provide desirable solubiUty and adsorption characteristics for practical cyanine and merocyanine sensitizers. Patents in this area are numerous. Other types of substituents include N-aryl groups, heterocycHc substituents, and complexes of dye bases with metal ions (iridium, platinum, zinc, copper, nickel). Heteroatom substituents directly bonded to nitrogen (N—O, N—NR2, N—OR) provide photochemically reactive 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). 

Fig. 3. Spectral sensitizing dyes for silver haUdes. (a) Blue sensitizers (400—500 nm) are designated BN (b) green sensitizers (500—600 nm) are designated GN (the ring oxygen may be replaced by N(R)) (c) red sensitizers (600—700 nm) are designated RN (d) MN designates a merocyanine dye and (e),...

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. 

A merocyanine dye, l-ethyl-4-(2-(4-hydroxyphenyl)ethenyl)pyridinium bromide (M-Mc, 2), exhibits a large spectral change according to the acid-base equilibrium [40, 41]. The equilibrium is affected by the local electrostatic potential and the polarity of the microenvironment around the dye. Hence, this dye is useful as a sensitive optical probe for the interfacial potential and polarity when it is covalently attached to the polyelectrolyte backbone. 

The merocyanine dye mentioned above shows solvatochromism, which means that the absorption band maximum of the quinoid form (D form) is sensitive to solvent polarity [40,41]. In Fig. 3, the absorption maximum of the solvatochromic band for M-Mc (a low molecular weight merocyanine analog) is plotted against the dielectric constant of 1,4-dioxane/water mixtures [42]. With the relationship... 

Fig. 2 Examples of the structures of a few fast-response electric filed sensitive dyes N-(4-sulpho-butyl)-4-(4-(4-(dipentylamino)phenyl)butadienyl)pyridinium inner salt (RH421, a styrylpyridinium dye), ANNINE 5 (an annellated hemicyanine dye), merocyanine 540, and N-[(4 -dimethylamino)-3-hydroxy-6-flavonyl mcthyl-N,N-trimcthyl ammonium (F4N1, a 3-hydroxychromone dye)...

Merocyanine Dye Method for Acid Analysis. Resist photochemistry can often be monitored by the changes in ultraviolet absorption spectra associated with a bleaching of the sensitizer absorbance. In the case of resist systems with triphenylsulfonium salts, no change in the film absorption is observed on irradiation. In order to determine the amount of acid produced, a direct method for acid analysis was required. A highly sensitive method was desirable since the amount of acid produced is approximately 10 6 mmol for a 1 micrometer thick film on a 2 inch wafer. Furthermore a nonaqueous technique is preferred in order to avoid hydrolysis of the hexafiuoroantimonate salt. Hydrolysis gives hydrogen fluoride (14) which makes accurate acid determination more difficult. 

Changes in the shape of the absorption spectrum correspond very well with micelle formation. The ratio of absorbance at 550 nm to that at 500 nm(both are absorptions of merocyanine) is constant below the CMC whereas the value increases continuously with concentration above CMC. This indicates that the merocyanine is a sensitive probe to detect micelle formation. During the photoirradiation experiment shown in Figure 2, the ratio of absorbance started to increase at the A /Aq value where the surface tension showed a sudden drop. 

Abe R, Sayama K, Arakawa H (2002) Efficient hydrogen evolution from aqueous mixture of E and acetonitrile using a merocyanine dye-sensitized Pt/Ti02 photocatalyst under visible light irradiation. Chem Phys Lett 362 441-444... 

Cyanine and merocyanine dyes have also been tested as dye photosensitizers [132-137]. A nanocrystalline 2 solar cell sensitized by the special merocyanine dye as shown in Fig. 15 revealed a good efficiency of 4.2-4.5% (7 =... 

Related to the cyanine dyes are the merocyanines — a group of compounds which is of equal importance as sensitizers. A general structure is depicted in formula (11) wherein A represents the atoms necessary to complete a heterocyclic nucleus, Y = atoms to complete a carbonyl-containing heterocycle, m = 0 or 1, and n = 0 or a small positive integer. 

Sophisticated derivatives of cyanines and merocyanines have been synthesized. For example, either the conjugated chain or the heterocyclic nuclei themselves may contain further substituents. Thus, the dyes (12) and (13) are sensitizers for the red and infrared, respectively. 

Both the well-known photochromism453,454 and the phosphorescence454 of spiropyrans are enhanced by sensitizers such as benzo-phenone. Consequently the photochemical opening of these compounds to the deeply colored merocyanines can proceed via triplet states. 

Color sensitizers are dyes added to silver halide emulsions to broaden their response to various wavelengths. Unsensitized emulsions are most responsive in the blue region of the spectrum and thus do not correctly represent the light spectrum striking them. Widely used sensitizers include the cyanine dyes, the merocyanines. the benzooxazoles, and the hcn/n11iki/oIl1 s Cryplocyaniiie sensitizes the extreme red and infrared. 

The merocyanines are also a valuable class of photographic sensitizers. Because of their nonionic structure they are more soluble in nonpolar solvents than the ionic cyanines. Merocyanines 21-23 can also sensitize from the blue to the near-infrared. They are most useful as blue and green sensitizing dyes. 

A number of dyes belonging to the polyene class - oxonols, merocyanine, rho-damines, and cyanines - have been used in 31P-NMR as potential-sensitive probes to investigate their effects on the membrane and their location in PC vesicles [115], A substantial broadening of the 31P resonances and a reduction in spin-lattice and... 

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