Polymorphism of dyes and pigments

The representatives of organic chemistry are coming more and more to the conclusion that the formulae of limiting states which we have been using so far fail to reflect the real conditions prevailing in nature, for there exist such subtle differences in the state of matter compared to which our methods of description that still are very simple sometimes appear wholly inefficient. (Ismailsky 1913) 

Regarding the polymorphism of colourants, another feature common to them and high energy materials (Chapter 9) is that much of the information may be found in sources of limited accessibility, in the case of colourants in company records, or in old or not widely circulated literature. 

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History. Braun and Tschemak [23] obtained phthalocyanine for the first time in 1907 as a byproduct of the preparation of o-cyanobenzamide from phthalimide and acetic anhydride. However, this discovery was of no special interest at the time. In 1927, de Diesbach and von der Weid prepared CuPc in 23 % yield by treating o-dibromobenzene with copper cyanide in pyridine [24], Instead of the colorless dinitriles, they obtained deep blue CuPc and observed the exceptional stability of their product to sulfuric acid, alkalis, and heat. The third observation of a phthalocyanine was made at Scottish Dyes, in 1929 [25], During the preparation of phthalimide from phthalic anhydride and ammonia in an enamel vessel, a greenish blue impurity appeared. Dunsworth and Drescher carried out a preliminary examination of the compound, which was analyzed as an iron complex. It was formed in a chipped region of the enamel with iron from the vessel. Further experiments yielded FePc, CuPc, and NiPc. It was soon realized that these products could be used as pigments or textile colorants. Linstead et al. at the University of London discovered the structure of phthalocyanines and developed improved synthetic methods for several metal phthalocyanines from 1929 to 1934 [1-11]. The important CuPc could not be protected by a patent, because it had been described earlier in the literature [23], Based on Linstead s work the structure of phthalocyanines was confirmed by several physicochemical measurements [26-32], Methods such as X-ray diffraction or electron microscopy verified the planarity of this macrocyclic system. Properties such as polymorphism, absorption spectra, magnetic and catalytic characteristics, oxidation and reduc-... 

As polymorphism has become an increasingly important factor in the commercial aspects of many solid materials, the number of patents relating to the discovery and use of particular polymorphic forms has increased. This is particularly important for pharmaceuticals, pigments and dyes, and explosive materials, which are discussed in Chapters 7-9. Some examples of the role of polymorphism in patent litigation are described in detail in Chapter 10. The patent literature is readily searchable using terms such as crystal form , polymorph etc., and since polymorphic behaviour often forms the basis of a patent (as opposed to many journal publications, where it may be peripheral to the main point of the paper) instances of polymorphism are relatively straightforward to locate. 

Ti02 is composed of TiOg octahedra, and is found in the polymorphic anatase, rutile, and brookite phases, which all feature different octahedral connectivity. Traditional appHcations of Ti02 have been as white pigment in many materials such as paints, as well as in electronics. In recent times, exciting appHcations for Ti02 in materials such as dye-sensitized solar cells [97],... 

Applications The general applications of XRD comprise routine phase identification, quantitative analysis, compositional studies of crystalline solid compounds, texture and residual stress analysis, high-and low-temperature studies, low-angle analysis, films, etc. Single-crystal X-ray diffraction has been used for detailed structural analysis of many pure polymer additives (antioxidants, flame retardants, plasticisers, fillers, pigments and dyes, etc.) and for conformational analysis. A variety of analytical techniques are used to identify and classify different crystal polymorphs, notably XRD, microscopy, DSC, FTIR and NIRS. A comprehensive review of the analytical techniques employed for the analysis of polymorphs has been compiled [324]. The Rietveld method has been used to model a mineral-filled PPS compound [325]. 

Molecular complexes, such as the complex formed between poly(N-vinylcaibazole) and 2,4,7-trinitro-9-fluorenone, and dye-polymer aggregates were widely used as generation materials in many early applications. Since these materials are not infrared sensitive, there has been increasing emphasis on pigments. The more widely studied are various azo, phthalocyanine, squaraine, and peiylene diimide derivatives. A common feature of all of these materials is that they are polymorphic and exist in many different crystal forms. The properties are thus very sensitive to the conditions used in their preparation. Further, the sensitivity of these materials is strongly field dependent as well as dependent on the transport material. For a review of generation materials, see Law (1993). 

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