The Phthalocyanines

Why are phthalocyanines blue or green and not red like the porphyrins The phthalocyanines have a similar macrocyclic structure to porphyrins so you might expect the same light-absorbing properties. They are not and one look at the UV-visible absorption spectrum of a phthalocyanine will show how it differs from a porphyrin. 

Like the porphyrins, phthalocyanines absorb in the near-ultraviolet and visible region, but the intensities of the absorptions are entirely different. It is the visible absorption bands that are more intense than the near-ultra-violet bands, not the other way round, as with porphyrins. The reasons for this are perturbations to the phthalocyanine ir-system caused by, (a) the nitrogen atoms in the meso-positions (they are more electronegative than carbon atoms so that they tend to attract -ir-electron density towards themselves) and, (b) the fused benzene rings on the pyrrole 3-positions, which extend the -iT-system (they increase the size of the electron box ). 

Apart from their use as dyes and pigments, phthalocyanines are also being used as catalysts for oxidations, high temperature lubricants, semiconductors, and liquid crystals. Their robust chemical structures, which make them, as dyes and pigments, fast to light and heat stable up to 500 °C, means that phthalocyanines will find many more applications in the future. In a later part of this chapter, we shall see how phthalocyanines are being used as novel electrical conductors. 

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It has a limited use in the preparation of the phthalocyanine pigments into which it is readily converted on heating with metallic salts. 

The phthalocyanines must be suitably dis-p>ersed to be used as pigments or they can be sulphonated to water-soluble forms for dyeing and for precipitation as lakes. The dispersion is carried out by solution in sulphuric acid, followed by precipitation in water. 

The phthalocyanine molecule is remarkably stable to heat and chemical reagents. The metal-free and heavy metal compounds sublime practically unchanged at 550-580 C. 

The chemical properties of phthalocyanines depend mosdy on the nature of the central atom. Phthalocyanines are stable to atmospheric oxygen up to approximately 100°C. Mild oxidation may lead to the formation of oxidation iatermediates that can be reduced to the original products (29). In aqueous solutions of strong oxidants, the phthalocyanine ring is completely destroyed and oxidized to phthalimide. Oxidation ia the presence of ceric sulfate can be used to determine the amount of copper phthalocyanine quantitatively (30). 

Oxidation can also occur at the central metal atom of the phthalocyanine system (2). Mn phthalocyanine, for example, can be produced ia these different oxidation states, depending on the solvent (2,31,32). The carbon atom of the ring system and the central metal atom can be reduced (33), some reversibly, eg, ia vattiag (34—41). Phthalocyanine compounds exhibit favorable catalytic properties which makes them interesting for appHcations ia dehydrogenation, oxidation, electrocatalysis, gas-phase reactions, and fuel cells (qv) (1,2,42—49). 

Approximately 90% of the phthalocyanines (predominantly copper phthalocyanine) are used as pigments (qv). In addition, they have found acceptance in many types of dyestuffs, eg, direct and reactive dyes, water-soluble and solvent-soluble dyes with physical and chemical binding, a2o-reactive dyes, a2o nonreactive dyes, sulfur dyes, and vat dyes (1) (see Dyes Dyes, reactive). 

Phthalocyanine Dyes. In addition to their use as pigments, the phthalocyanines have found widespread appHcation as dyestuffs, eg, direct and reactive dyes, water-soluble dyes with physical or chemical binding, solvent-soluble dyes with physical or chemical binding, a2o reactive dyes, a2o nonreactive dyes, sulfur dyes, and wet dyes. The first phthalocyanine dyes were used in the early 1930s to dye textiles like cotton (qv). The water-soluble forms Hke sodium salts of copper phthalocyanine disulfonic acid. Direct Blue 86 [1330-38-7] (Cl 74180), Direct Blue 87 [1330-39-8] (Cl 74200), Acid Blue 249 [36485-85-5] (Cl 74220), and their derivatives are used to dye natural and synthetic textiles (qv), paper, and leather (qv). The sodium salt of cobalt phthalocyanine, ie. Vat Blue 29 [1328-50-3] (Cl 74140) is mostly appHed to ceUulose fibers (qv). 

Phthalocyanines. The pyrrole ring system is also the fundamental stmctural unit of the important group of blue and blue-green pigments known as the phthalocyanines (see Phthalocyanine compounds). 

LB films of 1,4,8,11,15,18-hexaoctyl-22,25-bis-(carboxypropyl)-phthalocyanine (2), an asymmetrically substituted phthalocyanine, were stable monolayers formed at the water—air interface that could be transferred onto hydrophilic siUca substrates (32—34). When a monolayer film of the phthalocyanine derivative was heated, there was a remarkable change in the optical spectmm. This, by comparison to the spectmm of the bulk material, indicated a phase transition from the low temperature herringbone packing, to a high temperature hexagonal packing. 

Phthalocyanine Dyes. The phthalocyanine molecule is much too big to be used on hydrophobic fibers and therefore is only used in its sulfonated form as the basis for direct and reactive dyes (see Phthalocyanine compounds). Its forces of attraction are different from a small linear yeUow a2o dye with which it is used to form bright greens. CompatibiHty between the two is likely to be a problem in practice and to overcome this, green dyestuffs containing a phthalocyanine dye linked via a saturated chromophore blocker (—x—) have been made, eg,... 

The industrial manufacture of copper phthalocyanine began in 1935 by ICI, which developed its production from phthalic anhydride, urea and metal salts. In 1936 and 1937. the I.G. Farbenindustrie and Du Pont followed, and the most important of the phthalocyanines. PcCu, is now produced worldwide. Due to its favorable properties such as light, chemical and... 

Some additional derivatives containing extended 7t-systems in place of the benzene nucleus are naphthalene, anthracene (2,3-Ac) and phenanthrene (9,10-Phc). They also belong to the phthalocyanine family. For the naphthalene system two types of macrocyclcs, the 1,2-naph-thalocyanine (1,2-Nc) and the 2,3-naphthalocyanine (2,3-Nc), are known. 

Electrochemical methods 78,79 and reactions under high pressure have also been investigated.80 Due to the high resonance stabilization of the macrocyclc, the formation of the phthalocyanine is strongly exothermic. Nevertheless, a high thermal activation and therefore usually a high temperature is necessary. 

Dipotassium phthalocyanine (PcK2) can be prepared analogously to the dilithium compound by refluxing phthalonitrile and potassium pentoxide in pentan-l-ol.58 With additional oxygen-donor ligands (e.g., crown ethers) it forms crystals with the potassium bulging outside of the phthalocyanine ring.133134... 

Zirconium phthalocyanine (PcZrCl2) is prepared in a high-boiling solvent like 1-chloro-naphthalene using phthalonitrile and zirconium(IV) chloride.222 If the reaction is carried out without solvent, chlorination of the phthalocyanine ring may occur as a side reaction.223,224... 

Due to its commercial importance, the synthesis of copper phthalocyanine (PcCu) is the best investigated of all the phthalocyanines. Copper phthalocyanine is prepared from phthalonitrile and copper(I) chloride without solvent137 and also in a melt of urea.229,277 Additionally, the insertion of copper into metal-free phthalocyanine in butan-l-ol and pentan-l-ol is possible. The copper salts used in this case are copper(I) chloride112 and copper(II) acetate.290 Starting from copper(II) acetate, copper phthalocyanine can also be prepared in ethylene glycol.127 As mentioned above, copper phthalocyanine often occurs as a byproduct of the Rosenmund-von Braun reaction. To increase the yield of the phthalocyanine the solvent dimethylformamide can be substituted by quinoline. Due to the higher boiling point of quinoline, the copper phthalocyanine is the main product of the reaction of copper(I) cyanide and 1,2-dibromoben-zene.130... 

Almost every metal atom can be inserted into the center of the phthalocyanine ring. Although the chemistry of the central metal atom is sometimes influenced in an extended way by the phthalocyanine macrocycle (for example the preferred oxidation state of ruthenium is changed from + III to + II going from metal-free to ruthenium phthalocyanine) it is obvious that the chemistry of the coordinated metal of metal phthalocyanines cannot be generalized. The reactions of the central metal atom depend very much on the properties of the metal. 

The phthalocyanines, e.g. 1, are formed as a mixture of structural isomers (see introduction,... 

A mixture of metal-free phthalocyanine PcH2 (0.3 g, 0.6 mmol), anhyd DMSO (40mL) and a solution of BuLi (0.075 g, 1.2 mmol) was heated to reflux and kept at this temperature until no more bubbles of butane came o(T. To the resulting solution of the phthalocyanine dianion I successive additions were made of solutions of Ca(OAc)2 (0.1 g, 0.6 mmol) in DMSO (2 mL)and of Lu(OAc)3 (0.5 g, 1.4 mmol) in DMSO (3 mL). The mixture was refluxed for about 1 min, cooled, and diluted with twice the volume of H,0. The precipitate formed was filtered, carefully washed with H20, and dried at 150 C/5 Torr lo give the title compound yield 0.4g (92%). 

In the preparation of lutetium(III) 2,9,16,23-tetra-ter -butylbis(phthalocyanine) from lutetium(III) acetate, dilithium phthalocyanine and dilithium 2,9,16,23-tetra-terr-butyl-phthalocyaninc in refluxing 1 -chloronaphthalene for one hour, only one of the phthalocyanine moieties carries all of the substituents (yield 20%).185... 

Although the substitution of a preformed phthalocyanine always leads to a complex mixture of more- or less-substituted products, the reaction is of major industrial importance. Besides the chloro- and bromocopper phthalocyanines, also polysulfonated phthalocyanines, which are used as water-soluble dyes, are produced by the reaction of copper phthalocyanine with the respective reactant. While typical aromatic reactions of the Friedel-Crafts type are also possible,333 direct nitration of the macrocycle commonly results in oxidation of the phthalocyanine. However, under mild conditions it is possible to introduce the nitro group directly into several phthalocyanines.334... 

An example of hydrolysis is the hydrolytic ring opening of the imide units in the phthalocyanine... 

Acyl chloride units on the phthalocyanine may be transformed into amides, e.g. 3, under standard conditions.342... 

Radical phthalocyanine species, e.g. 6, can be prepared by the reaction of thionyl chloride or thionyl bromide with metal phthalocyanines. In these cases, the phthalocyanine ligand only carries the charge — l.349... 

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