Лако-красочные материалы — производство

In the process of mapping molecular functionality to the physical and technical properties of a pigment, crystal structure is key information which can be looked at as a molecular property itself. Therefore, the determination and simulation of crystal structures provide core know-how for all aspects of designing high perfor­mance pigments. About a hundred crystal structures of organic pigments are pub­lished in the literature, but many more are kept as proprietary know-how in the archives of pigment manufacturers.

In former times, most crystal structures of organic pigments have been solved classically from single-crystal X-ray diffraction data. However, because oftheir lim­ited solubility and high sublimation enthalpy, single crystals of pigments suitable for crystal structure determination are difficult to grow, or require special tech­niques such as high vacuum/high temperature sublimation [14], controlled cool­ing crystallization from high boiling solvents [13, 28], or gel crystallization.

In cases of metastable polymorphs (susceptible toward crystal growth condi­tions), twinned crystals, and materials of extreme insolubility, electron diffraction or X-ray powder data can be used as a basis for structure determination.

Electron diffraction has been used to investigate bulk structures of pigments [29, 30, 79] as well as thin layers [11, 80]. A strength of this method is the spatial resolution which allows the structural characterization of single microcrystals even in mixtures of polymorphs. Even more striking, the occurrence of two poly­morphs in the same sub-micron crystal may be analyzed, as in the case of a/b copper phthalocyanine [29].

Throughout past decades, the number of methods giving access to structure so­lutions from polycrystalline powders has been grown significantly [31]. However, most of these methods still require samples of appropriate crystallinity, yielding high resolution X-ray powder patterns suitable for indexing. At present, indexing of lower resolution powder patterns in real-life situations limits, in many cases, the access to structure information, because of low quality of data obtainable from routine samples. To obtain high resolution powder data, considerable effort must be put into the crystallization (usually ripening) of the material and in the diffrac­tion experiments. Again, electron diffraction is an alternative tool to determine unit cell constants from microcrystals, and is in many ways complementary to X-ray powder diffraction.

Despite such restrictions, real-space crystallographic methods based on genetic algorithms [32], Monte-Carlo methods [33], or simulated annealing techniques [34] have proved to be powerful means for structure solutions from X-ray powder patterns. Provided with the unit cell, the composition and configuration of the

R Y 14 (R=H)

asymmetric unit, and sufficiently texture-free diffraction data, refinable structure models can be obtained within minutes to hours on a personal computer, even for molecules with multiple internal degrees of freedom [35]. The resulting structure models are then refined by Rietveld techniques, which use the whole profile of the X-ray diffraction pattern for refinement [36].

A polymorph of the quinoid red crystal form of fluorescein was one of the first examples of a complex molecule whose structure was determined by a real-space approach based on the Monte-Carlo method [37]. The same method has been used to solve the structure of the b-form of the latent pigment boc-DPP (Figure 8.6). The kinetics of the thermal fragmentation to DPP differs for both forms. The more reactive а-form crystallizes (less ordered) with three conforma­tionally different half-molecules in the asymmetric unit. This structure was initial­ly solved from single-crystal data. However, it could be improved substantially by Rietveld refinement, thus demonstrating the potential of this technique [38].

Apart from purely diffraction-based structure determinations, minimization of the packing energy offers an alternative route to the solution of structures from indexed powder patterns (Figure 8.7). In addition to R-factors, packing energy may be taken as an additional criterion for the correctness of a structure model. Packing energy minimizations within fixed lattices yield structure models of high geometrical accuracy. This approach has been successfully applied to a variety of pigments, e. g., x-H2Pc [39], the perinone DHP [40], P. Y.13 and P. Y.14 [41, 76] (Fig­ure 8.6). In the case of the perinone DHP, minimum energy packings have been calculated for all possible conformations of the hydroxyl functions in two space groups (P na21, P 212121). The experimental structure corresponds to the lowest energy packing in space group P na21. However, several minima in space group

P 2j2j2j were found to be lower in energy, which may be explainable by inadequa­cies in the force field.

The minimization of the packing energy allows the determination of crystal structures even from nonindexable, lowquality X-ray powder diagrams (see Sec­tions 8.4.2 and 20.3.3.1) [74, 82].

8.4