Effect diffractive pigments

The latest category of special-effect pigments is diffractive pigments, in which interference is produced by diffraction technology. These pigments are produced by vacuum deposition on specially patterned surfaces. 

Linear trans-quinacridones exhibit multiple crystal modifications. This phenomenon is reflected in the different reflection angles which are observed in the X-ray diffraction spectra of the pigment powders. This effect may be exemplified by describing the polymorphous phases of unsubstituted quinacridone. 

The subject of energy transfer in phycobilisomes and their sub-structures already has a large literature (see Ref. 65 for a review), mostly beyond the scope of this chapter. However, two of these sub-structures - trimeric C-phycocyanin from the thermophilic cyanobacterium Mastigocladus laminosus and hexameric C-phycocyanin from the cyanobacterium Agmenellum quadruplicatum-have very recently become respectively the third and fourth photosynthetic pigment-protein complexes for which structural models based on single-crystal X-ray diffraction near atomic resolution are now available (Refs. 66,67 and Chapter 11). Since these are presently the only such complexes, in addition to the two already discussed (Sections 5 and 6), it seems appropriate to conclude this review of exciton effects with some brief remarks on these C-phycocyanin structures. 

Later, it was shown that the phenomenon of solid solution formation is widespread in this family of pigments. The previously discussed scarlet pigment 4,11-dichloroquinacridone was greatly improved in weatherfastness by solid solution formation (P.R. 207, CAS 71819-77-7 ) with the parent compound. In this case the approximate composition is also two moles of quinacridone and one mole of the substituted counterpart, retaining largely the scarlet color of the latter and the photochemical stability of the former. The X-ray diffraction pattern of the solid solution is different from that of a mixture of the two ingredients (Figure 18-12) or those of the individual components. As in most cases, while the X-ray diffraction pattern of a mixture is simply additive and predictable, the solid solution shows a non-additive diffraction pattern and an unpredictable color effect. 

Neutron diffraction, 4, 7, 369 pH-effect, see Low temperature Photosynthetic growth, 255, 262, 268, 286, 290, 310 Photosystem-II, see Native reaction centers Pigments,... 

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