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Because of their commercial importance, different forms of many phthalocyanines are readily available. In keeping with their intended uses, however, these are carefully formulated mixtures that may contain surface adsorbed components and/or small amounts of chemically modified phthalocyanines. Thus these are usually not suitable for use as generation layers. Isolated polymorphic forms of different phthalocyanines are commercially available. Because of the severe synthetic conditions, these usually contain high concentrations of impurities. Purification and characterization are therefore of considerable importance in the use of these materials as generation layers.