Transformations in Organic Pigments

Abstract

Organic pigments are often recrystallized in various ways to effect purification and to control the particle size of the new crop of crystals. Copper phthalocyanine and indanthrene blue RS have been observed to yield crystals of varying shades, ranging from a light greenish blue to a dark reddish blue (purple). These shades have in the past been attributed only to differences in particle size because the elementary analyses strongly indicated that the samples were chemically identical. This investigation shows that both these pigments are polymorphic, and each can exist in at least two different physical forms. The shade differences, therefore, may be due to the differences in the light absorption exhibited by the structurally different polymorphic forms. The polymorphic transformations can, for both pigments, be readily observed to take place in the electron microscope. The new form of both pigments, completely stable to the illuminating beam after the transformation, can be seen to grow from the vapor state at the expense of the original metastable solid material. The ``red'' shade of indanthrene blue RS and the ``hard'' powder of copper phthalocyanine have been labeled ``metastable'' because they transform in the electron microscope. The ``red'' shade of indanthrene blue RS can also be made to transform during its molecular distillation; the ``blue'' shade did not transform under any circumstances. The ``hard'' and ``soft'' powders of copper phthalocyanine can both be made to transform during their molecular distillations (not in the electron microscope) to give the same new form which is identical with the form that grew in the electron microscope at the expense of the original ``hard'' powder. Electron diffraction patterns prove that the newly formed phases are structurally different from the original samples. Both these commercially important pigments are therefore at least dimorphic.