Latent pigments

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. 

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. The same method has more recently been used to solve the structure of the (3-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 a-form crystallizes (less ordered) with three conformation-ally 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 . ... 

Contact of solutions of ethylene oxide with the skin of human volunteers caused characteristic burns after a latent period of 1-5 hours, effects were edema and erythema and progression to vesiculation, with a tendency to coalescence into blebs, and desquamation. Complete healing without treatment usually occurred within 21 days with, in some cases, residual brown pigmentation. Application of the liquid to the skin caused frostbite three of the eight volunteers were said to have become sensitized to ethylene oxide solutions. The undiluted liquid or solutions may cause severe eye irritation or damage. 

Structurally modified PVF has been extruded.66 Thin films are manufactured by extrusion of a dispersion of PVF in a latent solvent.67 Such dispersion contains usually pigments, stabilizers, plasticizers, and flame retardants as well as deglossing agents if needed. The solvent is removed by evaporation. The extruded film can be biaxially oriented and the solvent is removed by evaporation only after the orientation is completed. 

Development. This is obtained through the electrical attraction of a toner (thermoplastic pigment mixed with carbon black), whose fine particles carry electrical charges that are opposite in polarity to the latent electrostatic image charges. 

Toxicological Implications for Man. Because psoralens are potent photoactive compounds, they have been used medically for treatment of skin de pigment at ion or vitiligo (16,17), and psoriasis (18). However, there has recently been concern associated with the medical use of these compounds (19). This concern is due to the observed phototoxicity during therapeutic use (17), the suspected photocarcinogenicity of xanthotoxin (20,21), and the latent onset of tumors in treated laboratory animals (22). Acute gout secondary to psoriasis also was exacerbated by psoralen and UV-A (PUVA) photochemotherapy (23). 

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