Pigment molecules

A. Loss as heat. The energy can be dissipated as heat through redistribution into atomic vibrations within the pigment molecule. 

The pigment identification is performed by the recognition of its characteristic absorption spectra. The pigment quantification is revealed by the extent of absorption (optical density). Each of these properties may be modulated by the direct environment into which the pigment molecule is incorporated. We look at this in some detail. 

A close relationship exists between physicochemical properties of pigment molecules and their ability to be absorbed and thus to exhibit biological functions. Carotenoids are hydrophobic molecules that require a lipophilic environment. In vivo, they are found in precise locations and orientations within biological membranes. For example, the dihydroxycarotenoids such as lutein and zeaxanthin orient themselves perpendicularly to the membrane surface as molecular rivets in order to expose their hydroxyl groups to a more polar environment. 

The degree of lipophilicity of a pigment molecule can play a major role in its bioaccessibility. Obviously, a compound with a lower lipophilic character will be... 

Figure 9.4.2 General structure of an anthocyanin pigment molecule in hydrangeas.

In order to be photochemically active, light must be absorbed by a specific pigment molecule. In the case for physiological bluelight-action, the most favorable photoreceptor candidate is a flavin, as discussed so far. Sun et al.172) assign a it -> rr character to all major flavin transitions (S0 St 450nm Sj—-> S0 530nm ... 

Substitution patterns, especially that of the diazotized aromatic amine, determine the color of a pigment to some extent but empirical data do not lead to unambiguous conclusions as to the exact influence of a particular substituent on the shade. The problem is intricate, since the substitution pattern also has a bearing on the size and orientation of a pigment molecule and therefore on its crystal structure, including all the interactions associated with it. 

The hue of a red azo pigment lake carrying sulfonic acid functions is determined to a considerable extent by the metal ion. In the series Na->-Ba->Sr->Ca->Mn the shift of hue from yellowish to bluish red increases in the order in which they are listed. The complex correlation between chemical constitution and color in pigment molecules poses a quantum mechanical challenge. This is complicated by interactions within the crystal lattice and by the contribution of intermolecular and... 

Questions about such interactions could only be resolved by knowledge of the exact geometry of the atoms of a pigment molecule in its unit cell and the relative position of each individual molecule within the crystal lattice. This is elucidated through three dimensional X-ray diffraction analysis of single crystals [8]. 

Additives may exert not only a physical but also a chemical influence on the coupling suspension long-chain aliphatic or cycloaliphatic amines RNH2, which react partially with the pigment molecules, will enhace the effect of other additives. The carbonyl function of the acetyl group is converted to a ketimine (azomethine) or, in the presence of the enol form, it reacts to form an alkylammonium enolate [5],... 

Enlarging the pigment molecule, as with disazo condensation pigments (Sec. 2.9). 

Introducing substituents into the pigment molecule which reduce the solubility of their host structure. 

Later, 5- and 6-membered heterocyclic rings were introduced into the pigment molecule. The most effective among these are tetrahydroquinazoline-2,4-dione (23) and tetrahydroquinoxaline-dione (24)... 

Introducing additional carbonamide moieties into the pigment molecule (Sec. 2.8). 

The commercially interesting metal complex pigments usually contain the co-ordinative tetravalent Cu+ + or Ni+ + ions, less commonly Co++ ions. The fourth coordination site is typically occupied by a solvent molecule with a free electron pair. It may also be engaged by the second nitrogen atom of a different pigment molecule, a phenomenon which is observed in azo complexes and similar materials. In the latter case, sandwich structures are obtained [5]. The copper and nickel complexes are mostly planar molecules. 

P.Y.129 is not employed in plastics. The type of metal in the stabilizer defines how much of a color change is observed as the chelated metal in the pigment molecule is exchanged. Tin stabilizers produce red complexes, which, like corresponding lead or zinc chelates, respond very poorly to light and weather (see also Sec. 1.6.7). 

Partial sulfonation or sulfamide formation may stabilize a pigment toward flocculation. Combining partial sulfonation and chlorination will further improve the effect of each individual type of modification. Similar results are observed if carboxylic acid groups are introduced into the pigment molecule. 

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