A review of thirty years of research on quinacridones. X-ray crystallography and crystal engineering

The various stages of the development of quinacridones are described progressing from the bonding hypothesis in the molecular lattice (1969), through light-microscopy examinations, to the study of the morphology of certain crystal faces (habit) and their representation by means of lego blocks (1972). Intermediate work was concerned with Cromophtal Red A3B, the molecular lattice of which was represented using Stuart¯Briegleb space-filling models, and the bridging intermolecular H-bonds and pi...pi bonds were studied (1984). The long-range order (Bravais lattice) and the space group of 2,9-dimethylquinacridone were determined and a certain analogy to smectic liquid crystals was proposed (1985). Moreover, close-packing of molecules in the lattice was minimised and compared with the reflection intensities of the experimentally obtained powder diagrams. The positions of the molecules were determined independently of this. The finishing treatment by technological methods was described crystallographically (1987). Other work was concerned with the structure of betaquinacridone (1983), alpha-quinacridone (1996) and a study on the structure and hue of all quinacridones known at that time (1997). The central theme of this 30 year work has been to provide a better comprehension of the bonding character of extended organic mesomeric molecules (pi...pi bond) in stacks with NH and CO groups capable of forming intermolecular H-bonds. The pi-sigma correlation in the crystal lattice was proposed for certain important organic molecules, and this is capable of inducing both light absorption and insolubility at the same time.