2,2 -Cyanines

Perhaps the most widely studied chemical class of near-IR absorbers are the cyanines. The chemistry and synthetic routes to cyanines are discussed in other parts of this book (Chapter 2, section 2.3.1.4 and Chapter 3, section 3.5.1.7). The factors influencing the position of their absorption maxima and molar absorption (extinction) coefficients areC  

---

Structurally, the molecule is formed by the union of one atom of the metal with four molecules of phthalonitrile. Copper phthalo-cyanine is typical, and has the formula... 

A. Schmidt, L. K. Chau, A. Back, and N. R. Armstrong, Epitaxial Phthalocyanine Ultrathin Films Grown by Organic Molecular Beam Epitaxy (OMBE), in Phthalo-cyanines, Vol. 4, C. Leznof and A. P. B. Lever, eds., VCH Publications, 1996. 

Kneipp K, Kneipp H, Deinum G, Itzkan I, Dasari R R and Feld M S 1998 Single-molecule detection of a cyanine dye in silver colloidal solution using near-infrared surface-enhanced Raman scattering App/. Spectrosc. 52 175-8... 

In the first chapter, devoted to thiazole itself, specific emphasis has been given to the structure and mechanistic aspects of the reactivity of the molecule most of the theoretical methods and physical techniques available to date have been applied in the study of thiazole and its derivatives, and the results are discussed in detail The chapter devoted to methods of synthesis is especially detailed and traces the way for the preparation of any monocyclic thiazole derivative. Three chapters concern the non-tautomeric functional derivatives, and two are devoted to amino-, hydroxy- and mercaptothiazoles these chapters constitute the core of the book. All discussion of chemical properties is complemented by tables in which all the known derivatives are inventoried and characterized by their usual physical properties. This information should be of particular value to organic chemists in identifying natural or Synthetic thiazoles. Two brief chapters concern mesoionic thiazoles and selenazoles. Finally, an important chapter is devoted to cyanine dyes derived from thiazolium salts, completing some classical reviews on the subject and discussing recent developments in the studies of the reaction mechanisms involved in their synthesis. 

IX. Cyanine Dyes Derived from Thiazolium Salts... 

Cyanine Dyes Derived from Thiazoliurn Saits... 

According to general usage, the term cyanine designates any cationic dye in which two nuclei of different or same nature are linked by a mono or polymethine chain. When these groups (-CH= are replaced partially or totally by one or several nitrogen atoms, the cationic dye is called azacyanine. 

The name found its origin in the fact that the first dyes known were all derived from quinoline as, for example, 2,2 -cyanine (Scheme 1). In the case of an asymmetrical or symmetrical dye involving one or two nuclei that are different from the quinoline ring, the name of the nucleus becomes the prefix in the name of the dye. So 3.3 -dimethyl-4-phenyl thiazolo cyanine is the dye of structure 1 and 3.1 -dimethyl 4-phenyl thiazolo 2 -cyanine is represented by the formula structure 2 (Scheme 2). 

Applying this rule, the preceding dyes are written (3-methyl-4-phenyl-thiazole-2)(3-methylthiazole-2)methine cyanine iodide (3a) and (3-methyl-4-phenylthiazole-2)( l-methylquinoline-2)methine cyanine iodide (4), respectively. Any substituent in the chain is named and its position designated by a, ft or y, for example, (3-methyl-4-phenylthia2ole-2)(3-methylthiazole-2)-/S-methyltrimethine cyanine iodide (3b). 

Rhodacyanines possess two chromophoric systems. They are at the same time neutrocyanine derivatives, which involves position 5 of the ketomethylene, and methine cyanine, which involves position 2. Following lUPAC s standard nomenclature rules, structure 7 is named 3-ethyl-4-phenyl-2- 4-oxo-3-ethyl-5-[2-(3-ethy]-2,3-dihydro-benzo-l,3-thiazo-lylidene)ethylidene]-tetrahydro-l,3-thiazolylidene-methyl -1.3-thiazolium iodide (Scheme 5). It implies that the 4-phenyl thiazole ring having the... 

In this last case, the main skeleton considered could be the cationic trimethine cyanine and the third ring with its independent chain could be named as a substituent at the a or j3 position. When this substituent is of an ionic nature, it could be named after the corresponding ring, for... 

In 1923, Mills introduced thiazole for the first time in the synthesis of methine dyes through a somewhat indirect route. In order to demonstrate the 2,4 -cyanine mechanism of formation by quinoline and quinaldine quaternary salts reacting together, Mills used other pairs of quaternary salts as 2-methylthiazolium with either quinolinium or benzothiazolium (42, 43). 

Even if the specific role attributed to benzothiazolium was not confirmed later (24), all these syntheses account for the significant and common behavior of quaternary salts, carbocations giving either symmetrical or asymmetrical reactive anhydrobases. They constitute the mam step in cyanine dye formation. 

In the case of a pyridinium salt, an indirect proof of the correctness of this mechanism can be found in the presence of a heptamethine thiazolo-cyanine, together with monomethine. which results unambiguously from 19 by the known opening of the pyridine ring (Scheme 25) (52). 

The symmetrical dye, bis-(3-methyl-4-phenylthiazole)monomethine cyanine (not mentioned by Mills), has also been identified, and its formation is explained by the enamine character of 20 (Scheme 27). 

When a quinaldinium salt and 2-unsubstituted thiazolium are condensed together in the presence of a basic agent, the resulting bis-(methylquinoline-2)trimethine cyanine is issued from the cleavage of the thiazolium ring of the anhydrobase (25). It is induced by the -CHj attack of quinaldinium according to a process already described (Scheme 28). 

As an illustration of the real complexity of Mill s reaction, when two molecules of heterocycloammoniums of different nature, one of them being thiazolium (2-substituted or not), are put together in a basic medium, nine dyes theoretically can be produced (depending on the nature of the substituent in the ring) three thiazolomonomethine cyanines (two symmetrical, one asymmetrical) and six trimethine cyanines (two symmetrical, two symmetrical mesosubstituted. one unsymmetrical, one unsymmetrical mesosubstituted). One cannot separate such a mixture by usual chromatographic means. 

The nucleophilic carbon of ketomethylene compounds can react with anhydrobases of different species in a basic medium. This reaction presents a narrow similitude with -CHj attack. The resulting dye, neut-rodimethine cyanine either mesomethyl-substituted or not. varies with the nature of the anhydro base (Scheme 30) (53. 54). 

Aniiinovinyl derivatives condense with methyl or methylene reactive groups of heterocycloammonium or ketomethylene. and are useful intermediates in the syntheses of thiazolotnmethine cyanines and thiazolodimethine neulrocyanine. They are prepared according to the following methods ... 

A mesophenyl-substituted trimethine cyanine has been identified as a by-product in syntheses leading to styryl dyes it is also obtained when the anhydrobase reacts with the styryl dye (26). 

III. Syntheses of Thiazolocyanines C. Dimethine Cyanines (Tables 1122A,B)... 

Thiazolomonomethine cyanines result from the condensation of 2 moles of 2-alkylmercaptothiazolium on 1 mole of malonic acid in pyridine (method C) but could not be obtained from this intermediate in acetic anhydride as is the case for other rings (26). 

---