Лако-красочные материалы — производство

Creative concepts in crystal engineering make use of supramolecular synthons to define specific interactions in molecular solids, which may be employed to design crystal packings [1, 4]. The synthon approach has been used to produce close packed structures, and, on the contrary, to build open network structures [21]. In the packing motifs of high-performance pigments several types of specific interac­tions (synthons) can be identified.

The chlorine-chlorine interaction is somewhat controversially discussed in the literature [22]. In high-performance pigments it supports the packing energy and hence the stability, as can be seen by comparing P. R.255 to P. R.254 in Table 8.2. Excessive and favorable use of chlorine-chlorine interactions is made in P. G.7, P. G.36, P. Y.110, and P. Y.138 (Figure 8.2a).

However, chlorination does not in all cases lead to an improvement of the pig­mentary character. Tetra — and octachlorinated derivatives of perylenetetracar — boxylic acid are twisted and exhibit packing in many cases less favorable than that with planar unsubstituted perylene pigments (Figure 8.2b) [23].

Hydrogen bonds deserve special emphasis. Their strength depends on the acid­ity of the H-donor and the basicity of the hydrogen-bond acceptor involved. Supra — molecular synthons based on hydrogen bonds can be found in many organic pig­ments. Quinacridone pigments of the а-type (a-P. V.19, b-P. V.19, P. R.122, P. R.202) [10, 11, 78, 81], the perylene pigment P. V. 29, diketopyrrolopyrrole pig­ments [7, 8], and the thiazine-indigo pigment P. R. 279 [24] form ladder-like arrangements induced by the two-point recognition of the molecules via hydrogen bonds (Figure 8.3a). In contrast, y-P. V.19 forms a criss-cross pattern [9-11] (cf. Figure 8.13).

Other pigment classes, where hydrogen bond functionality was introduced in order to achieve lower solubility, are the Naphthol AS and benzimidazolone pig­ments. A striking example for combining hydrogen bonds with proper molecular symmetry is P. Y.139 [6] (Figure 8.3b, see also Figure 14.11, in Chapter 14). All hydrogen bond donors and acceptors in the molecule participate in an extensive hydrogen bond network, which is essential for the superior properties (e. g., tem­perature stability) ofthis isoindoline pigment.

The packing of aromatic hydrocarbons has been subjected to intense investiga­tions [25]. Many organic pigments are derived from polycyclic aromatic hydrocar­bons, and their basic packing behavior often reflects this relationship.

Generally, aromatic systems either form stacks with strong van der Waals (and Coulomb) interactions of the p-systems or are arranged herringbonelike with close contacts between the p-system of one molecule and the peripheral hydrogen atoms of adjacent molecules [26]. p-Stacking is common for polycyclic pigments and contributes largely to the packing energy. In contrast, herringbone-like inter­actions contribute only marginally to the packing energy oflarge molecules. Various phthalocyanine polymorphs differ mainly in herringbone-type interactions (Fig­ure 8.4). However, this type of interaction occurs not so frequently among other pig­ments andcanbefound, e. g., in P. B.60 [12], P. V.19 [9-11], a-P. Y.12 [75, 76], P. Y.83 [76, 77], and P. V.23 [69] (see Chapter 20).

Connecting heteroatoms to aromatic systems changes the polarity of the mole­cules considerably. This is also reflected by preferences in the stacking arrange­ment oflarge polycyclic systems. In order to find out which stacking arrangement in perylene-tetracarboxylic acid diimide (PTCDI) pigments is preferred energeti­cally, the interaction energy of two unsubstituted PTCDI molecules was calcu­lated. This was done by plane-parallel alignment of the molecules at a distance of 340 pm (picometers) and moving them relative to one another in the molecular plane. Van der Waals and Coulomb energies (derived from the electrostatic poten­tial [27]) of the molecular pair were calculated after each 2 pm shift.

As expected, van der Waal interactions merely reflect the degree of overlap be­tween the molecules, whereas the addition of Coulomb interactions gives rise to a structured interaction map (Figure 8.5), which yields information about the stack­ing arrangement of minimum energy preferred by the perylene fragments.

Apparently, for most of the 22 known perylene structures, the PTCDI fragments actually do determine the molecular arrangement in the stacks (see Figure 8.5). It is noteworthy that this is not true of the technical pigments used in commercial products (e. g., Pigment Red 123, 178, and 179). Stacking arrangements with highly attractive interactions between perylene systems yield mainly bluish-red pigments. However, throughout the coloristic evolution of perylenes “brilliant

red” or black pigments (e. g., Pigment Black 32) were of course selected. In order to engineer solids of that type, predictions of full three-dimensional crystal struc­tures are needed.

8.3