Лако-красочные материалы — производство

Even in the days before X-ray crystallography, color chemists have recognized polymorphism by color changes. In contrast to molecularly dissolved dyes, excita­tions in pigment crystals are affected by supramolecular interactions in the solid state. As a consequence, the absorption spectrum of a pigment crystal differs from the spectrum of the isolated pigment chromophores (Figure 8.10). This phe­nomenon is widely known as crystallochromy, and examples of many pigment classes are well documented in the literature, e. g., the polymorphs of phthalocya — nine and of quinacridone [11].

For several pigments, the origins of crystallochromy could be derived from accu­rate structure data in combination with spectroscopic investigations. Primarily, the distortion of the molecule due to packing effects may change the absorption spectrum of the solid. Molecular distortions have been identified to be the reason

for the solid-state absorption properties in various phthalocyanines, especially in titanyl phthalocyanine [14, 53].

Perylene pigments (perylene tetracarboxylic acid diimides, PTCDIs) probably show the phenomenon of crystallochromy in its most pronounced form. PTCDI molecules form regular stacks in the crystal with an average distance of 340 pm (picometers) between them. For different substituents of the imido nitrogen the overlap between the chromophores varies within the stacks (Figure 8.5). As a con­sequence, the solid-state colors of PTCDIs range from orange to black depending on the stacking arrangement in the crystal packing, while the color in solution always stays orange.

Firstly, Klebe et al. established an empirical correlation for uniformly stacked perylene pigments [54]. Later, a 1d-band-structure based explanation derived from extended Hueckel calculations was published by Hoffmann and Kazmaier [55]. Recently, Mizuguchi has investigated crystalline films of perylene pigments by temperature-dependent spectroscopy. These experiments demonstrate the excitonic nature of the longest wavelength absorption band [56]. Similar analyses allow for the explanation of the absorption properties of diketopyrrolopyr — roles, which can be correlated to the molecular arrangement in the crystal struc­ture [57].

8.4.4