Isomorphism of pigments

On the other hand diketopyrrolo-pyrrol, prototypically 8-VI (R, R = H, DPP), is a relatively new family of red pigments (Herbst and Hunger 1997), having been discovered serendipitously by Farnum et al. (1974). The most widely used is P. R. 254 (R = 4-Cl, R = H), which is dimorphic (Hao et al. 1999a). The wi-chloro (R = 3-Cl, R = H) and m-methyl (R = 3-CH3, R = H) derivatives have also recently been shown to be polymorphic (Hao et al. I999b,c). The parent compound (R, R = H) has been shown to be trimorphic. 

MacLean et al. (2000) have recently smdied the dimorphic behaviour of the pigment precursor ( latent pigment) derivative of 8-VI (R = COOr-but, R = H) (abbreviated DPP-Boc). The latency is due to the thermal decomposition reaction of both polymorphs resulting in the commercially important pigment DPP. The a form of DPP-Boc contains three half molecules in the asymmetric unit (see also Ellern et al. 1994) while the form contains one half molecule per asymmetric unit. Hence, they are easily distinguishable by solid state NMR as well as by X-ray powder diffraction. The crystal structure solution from powder data and Rietveld refinement of both polymorphs is an exemplary smdy demonstrating the potential of these methods in determining the detailed crystal structure of these compounds which are often difficult to crystallize. 

R = -C4H9) has been reported to crystallize in four polymorphic forms (Brandt et al. 1982). In the copper phthalocyanines Pigment Blue 15, containing no Cl, is isomorphous with Pigment Blue 15 1, which on an average contains 0.5-1.0 atoms of chlorine per molecule (Hao and Iqbal 1997). Recognition of this phenomenon can be useful, for instance, in attempting to work out the crystal structures of unknown polymorphic forms. 

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