Solid photochemical stabilization

Photochemical Stability and the Wavelength of Excitation. Cyanines of chain length beyond Cy3 snffer from increasing photochemical instability. This can be a problem when they are nsed in conjunction with solid-state lasers operating around 630-650 nm. To overcome these problems workers at Boehringer Mannheim have developed the so-called pentacyclic fluorescent labels based on either the oxazine or rhodamine ring systems, e.g. Light Cycler Red 640 NHS (3.76). ... 

Mielcarek, J. Photochemical stability of the inclusion complexes of nicardipine with a-, y-cyclodextrin, methyl-P-cyclodextrin, and hydroxypropyl-P-cyclodextrin in the solid state and in solution. Pharmazie 1996, 51 (7), 477-479. 

The study of solid state fluorescence and photochemical stability (43) showed the following results ... 

Changes in physicochemical properties of the drug substance (e.g., color, crystal modification) may take place upon irradiation. Efforts should be made to observe such changes during the in vitro assay. The sample absorption spectrum should be recorded before and after irradiation surface color of solid samples should be evaluated by appropriate methods and the identity of the sample crystal modification should be confirmed when the drug is irradiated in the solid state. The humidity in the test chamber can influence the photochemical stability of certain solid samples, as demonstrated for mefloquine. The photoinduced yellowing of uncoated mefloquine tablets is accelerated by an increase in humidity. These tablets are mainly used in tropical countries and the real in-use conditions will include high relative humidity. In such cases, the influence of the humidity on the photostability must be taken into account (Tpnnesen et al., 1997). 

Photochemical stability of the solid compound is an important aspect when the drug is formulated as a suspension. Photochemical stability of a drug in the solid state can depend on the polymorphic/pseudo polymorphic form of the compound, which is demonstrated for chloroquine, mefloquine, and furosemide (Nord et al., 1997b Tpnnesen et al., 1997 De Villiers et al., 1992). The crystal structure, molecular conformations, and surface of the particles can thus influence photoreactivity of a suspended drug. When formulated as a suspension, the drug should be in the form of the stable polymorph. Transformation can occur between different crystal forms in the presence of a liquid, often accompanied by caking of the crystals (Martin, 1993). Photochemical stability of solids is further discussed in Chapter 16. 

The molecular mobility can be further restricted by using appropriate excipients. It is obvious that the unknowing use of a metastable crystal form can lead to solubility and stability problems. A change in crystal modification can occur during processing or as a result of a photochemical process (Briggner et al., 1994 Ahmed et al., 1996 Nord et al., 1997). The data available on photochemical stability of drugs in the solid state that include information on the effect of formulation principle and excipients are at present rather limited. 

One of the most important application of QCNs/CPs hybrids is in LEDs. The idea behind exploiting these QCNs in solid LEDs is txmable emission, photochemical stability, and low processing cost. In conventional OLEDs, charge is injected between two electrodes (one of the electrodes is transparent through which light comes out) into one molecular lumniscent... 

Such exciton trapping has been exploited by Mullen and coworkers, who prepared copolymers of dioctylfluorene- and perylene-based dyes (1-5 mol%) by Yamamoto copolymerisation (Scheme 55) [222-224]. These materials were designed so that by efficient FOrster energy transfer from the fluorene to the dye units, efficient emission across the whole visible spectrum could be obtained. Perylene dyes were chosen as the chromophores due to their high solid-state PL quantum yields, and their excellent thermal and photochemical stability. 

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. 

An interesting quaternary orange solid solution consisting of quinacridone, 4,11-dichloroquinacridone, quinacridonequinone, and 6,13-dihydroquinacridone has been prepared, in which the last component has a positive effect on the photochemical stability of the system. 

The colors of 4,11-disubstituted quinacridones, unlike the 2,9-disubstituted counterparts, are dominated by steric rather than electronic effects. As displayed in Table 18.2, the solubility of these compounds (boiling a-chloronaphthalene, b.p. 264 C) increases with increasing bulk of the substituents and correspondingly the photochemical stability in the solid state decreases. 

QAQ is a yellow compound with deficient lightfastness, but by virtue of solid solution formation with quinacridone the resulting maroon pigment demonstrates outstanding photochemical stability. Evidently, the energy of the excited... 

An orange-colored quaternary solid solution [56], composed of QAQ, quinacridone, 4,11-dichloroquinacridone, and 6,13-dihydroquinacridone, is a system of interest as the latter component contributes a positive effect to its photochemical stability. 

Some examples of multifunctional epoxy resins other than epoxy phenol novolac resins are depicted in Figure 2.39 (b), (c) and (d). Triglycidylisocyanurate is a solid trifunctional epoxy cross-linker used in powder coatings, which provides a higher cross-link density and superior photochemical stability compared to BPA epoxy resins. The use of triglycidyl isocyanurate may present toxicity hazards. 

In solid state (films), the turbidity of the samples, depending on their morphological state, can mask weak absorption bands. The determination of these structures is of practical interest, related to the problems of initial coloration and thermal and/or photochemical stability of commercial resins. 

Besides thermolysis, the photochemical decomposition of solid trihalo-methylmercury compounds RHgCCli, CF3HgOCOCF3 and Hg(OCOCF3)2 has been studied (Scheme 2). The irradiation of samples placed in an evacuated quartz tube, which was connected to a helium cryostat, was carried out at -50 to +10°C. Thus, a desorption into the gas phase of the primary products of the photolysis occurred, and consequent low-temperature matrix stabilization of them was made. As a result, the formation of only the radicals CCI3 (1 3 898 cm" ) and CF3 (vi 1084, V2 702, P2+ V4 1205, r>3 1249 cm ) or of products of their secondary reactions was observed (Mal tsev et al., 1974, 1975, 1977b). 

Figure 7.24. Solid-state photochemical decarbonylation model for ketones. The dashed path corresponds to the experimentally determined energies of acetone (in kcal/mol). The effects of substituents with radical stabilizing energies (RSEs) are illustrated by the solid line in the reaction coordinate. See color insert.

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