Of dyes

White crystals, m.p. 114" C. Manufactured by reacting aniline with excess ethanoic acid or ethanoic anhydride. Chief use is in the manufacture of dye intermediates such as p-nitro-acetanilide, p-nitroaniline and p-phenylene-diamine, in the manufacture of rubber, and as a peroxide stabilizer. 

Obtained by fusing o-t nzenedisulphonic acid with NaOH. It is used as a photographic developer, for preparation of dyes and pharmaceuticals, and as an antioxidant. 

Although 2-naphthol is employed chiefly for the manufacture of dyes, considerable quantities are used in the manufacture of anti-oxidants. 

As already mentioned, electronically resonant, two-pulse impulsive Raman scattering (RISRS) has recently been perfonned on a number of dyes [124]. The main difference between resonant and nom-esonant ISRS is that the beats occur in the absorption of tlie probe rather than the spectral redistribution of the probe pulse energy [124]. These beats are out of phase with respect to the beats that occur in nonresonant ISRS (cosinelike rather tlian sinelike). RISRS has also been shown to have the phase of oscillation depend on the detuning from electronic resonance and it has been shown to be sensitive to the vibrational dynamics in both the ground and excited electronic states [122. 124]. 

Experimental investigations of the model system of dye molecules adsorbed onto surfaces of polystyrene spheres have finuly established the sensitivity and surface specificity of the SHG method even for particles of micrometre size [117]. The surface sensitivity of die SHG process has been exploited for probing molecular transport across the bilayer in liposomes [118], for measurement of electrostatic potentials at the surface of small particles [119] and for imaging... 

Zeisel D, Deckert V, Zenobi R and Vo-Dinh T 1998 Near-field surface-enhanced Raman spectroscopy of dye molecules adsorbed on silver island films Chem. Phys. Lett. 283 381... 

Murakami H, Kinoshita S, Hirata Y, Okada T and Mataga N 1992 Transient hole-burning and time-resolved fluorescence spectra of dye molecules in solution evidence for ground-state relaxation and hole-filling effect J. Chem. Phys. 97 7881-8... 

Weiner A M and Ippen E P 1985 Femtosecond excited state relaxation of dye molecules in solution Chem. Phys. Lett. 114 456-60... 

Sugi M, Salto M, Fukui T and lizima S 1983 Effeot of dye oonoentration in Langmuir multilayer photooonduotors Thin Solid Films 99 17-20... 

Sohoeler U, Tews K FI and Kuhn FI 1974 Potential model of dye moleoule from measurements of the photoourrent in monolayer assemblies J. Chem. Phys. 61 5009-16... 

Since then, the fundamental physicochemical aspects of the synthesis and properties of ev anines have been exhaustively reviewed by Heseltine and Stunner in the fourth edition of Mee s treatise (3) and by Sturmer in Weissberger s edition of the Chemistry of Heterocyclic Compounds (4). So the purpose of this section dealing especially with thiazolomethine dyes is to give, apart from a complete and recent list of dyes and references, a description of the particularities of their chemistry and chiefly of the reaction mechanisms involved in their synthesis that have remained unknown or have not been discussed until now. 

It appears now that, whatever its usefulness, the resonance theory is somewhat inadequate in explaining and predicting either chemical or physical characteristics of dyes compared to more or less sophisticated molecular orbital calculations. 

In aprotic conditions acetic anhydride sodium acetate induces formation of a fused ring through an intra molecular condensation. It results in a pyrrolo[2,l-fc]thiazole (39), which constitutes an interesting intermediate for the synthesis of dyes (Scheme 18) (40). 

This class of dye has offered in the past two matters of interest. It has been chosen bv Mills as the material for demonstrating the mechanism of... 

Monoazathiazolocyanines (Tables 2171A.B). The same reactive intermediates and the same conditions as in the case of thiazolomethine dyes are used in the synthesis of this class of dyes, except that the CH group is replaced by a -NH group (Schemes 46 and 47). No j3-azatrimethine thiazolocyamne has been described, in spite of an access method applied with success to other rings (65). 

Examples of different kinds of dyes obtained by usual synthesis starting from bis-thiazolium salts are listed in Tables 41A,BtC, 42, 43A.B-C, 44. 

Since the very beginning of chemistry, many efforts have been devoted to find out basic relationships between the characteristics of absorption spectra and the molecular structure of dyes. 

Most of the qualitative relationships between color and structure of methine dyes based on the resonance theory were established independently during the 1940 s by Brooker and coworkers (16, 72-74) and by Kiprianov (75-78), and specific application to thiazolo dyes appeared later with the studies of Knott (79) and Rout (80-84). In this approach, the absorptions of dyes belonging to amidinium ionic system are conveyed by a group of contributing structures resulting from the different ways of localization of the 2n rr electrons on the 2n l atoms of the chromophoric cationic chain, rather than by a single formula ... 

Likewise, quantum mechanical calculation succeeds in giving a theoretical explanation of some facts that the resonance theory could not explain, for example, why bis(pyridine-2)monomethine cyanine and bis(pyridine-4)monomethine cyanine possess the same lowest energy transition contrary to the 2,2 - and 2,4 -quinoline monomethine dyes, together with a molecular coefficient extinction lower than that of the 4,4 -quinoline dye (11). Calculation shows also that there is no theoretical reason for observing a relationship between and pK in a large series of dyes with different nuclei as it has been postulated, even if limited observations and calculations in short homogeneous series could lead to this conclusion (105). 

The relations between structure and electrochemical potential are an important aspect in the study of dyes, since effective sensitizers require both the correct absorption wavelength and suitable electrochemical potentials. 

The use of sensitizing dyes in photography has been the subject of many studies and constitutes. still now. one of the most studied areas in specialized periodic publications (125, 126) or in textbooks (88. 127). It can be ascertained that one hundred years after Vogel s discovery of spectral sensitization, the basic mechanisms of action of dyes on their silver halide support still remain not fully understood. However, the theoretical reasons explaining why among many other dye families practically only cyanine methine dyes appear to be spectral sensitizers (128) are better known. 

The bactericidal and enzymatic action of dyes, particularly of vinyl derivatives of 3,4,5-substituted thiazolium, for example, 45 (Scheme 70) (139), have been systematically studied to know more about the basic mechanisms involved (140). 

Acridine is a heterocyclic aromatic compound obtained from coal tar that is used in the syn thesis of dyes The molecular formula of acndine is C13H9N and its ring system is analogous to that of anthracene except that one CH group has been replaced by N The two most stable reso nance structures of acridine are equivalent to each other and both contain a pyndine like struc tural unit Wnte a structural formula for acridine... 

In this experiment mixtures of dyes are used to provide a means for determining spectrophotometrically a sample s pH. 

One characteristic property of dyes is their colour due to absorption from the ground electronic state Sq to the first excited singlet state Sj lying in the visible region. Also typical of a dye is a high absorbing power characterized by a value of the oscillator strength/ (see Equation 2.18) close to 1, and also a value of the fluorescence quantum yield (see Equation 7.135) close to 1. 

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