PHOTOCHEMICAL STABILITY

Such functionality can also be of great practical importance since functional initiators, transfer agents, etc. are applied to prepare end-functional polymers (see Section 7.5) or block or graft copolymers (Section 7.6). In these cases the need to maximize the fraction of chains that contain the reactive or other desired functionality is obvious. However, there are also well-documented cases where weak links formed by initiation, termination, or abnormal propagation processes impair the thermal or photochemical stability of polymers. 

There are additional factors that may reduce functionality which are specific to the various polymerization processes and the particular chemistries used for end group transformation. These are mentioned in the following sections. This section also details methods for removing dormant chain ends from polymers formed by NMP, ATRP and RAFT. This is sometimes necessary since the dormant chain-end often constitutes a weak link that can lead to impaired thermal or photochemical stability (Sections 8.2.1 and 8.2.2). Block copolymers, which may be considered as a form of end-functional polymer, and the use of end-functional polymers in the synthesis of block copolymers are considered in Section 9.8. The use of end functional polymers in forming star and graft polymers is dealt with in Sections 9.9.2 and 9.10.3 respectively. 

In all cases the formation of carbenes was accompanied by partial photoisomerization of the precursor diazirines [53] into the corresponding diazo compounds [55] which possess a higher photochemical stability than the diazirines. 

Tpnnesen, H.H, Masson, M., and Loftsson, T., Studies on curcumin and curcuminoids. XXVII. Cyclodextrin complexation solubility, chemical an photochemical stability, Int. J. Pharm., 244, 127, 2002. 

Anthraquinone leuco dyes are widely known as vat dyes.10 Vat dyes possess extensively conjugated aromatic systems containing two or more carbonyl groups, e.g., anthraquinone, indigoid chromophores. The colored form of vat dyes are insoluble in water. The dyes are applied by a process whereby the dye is converted to the reduced form (leuco dye) which is soluble in water and can penetrate into a cellulosic fiber. On exposure to the atmosphere the leuco form is oxidized to the original quinoid form which then precipitates as an aggregate. Vat dyes generally have excellent chemical and photochemical stability. 

The discovery of polyhedral boranes and polyhedral heteroboranes, which contain at least one atom other than in the cage, initiated a new era in boron chemistry.1-4 Most commonly, of the three commercially available isomeric dicarba-closo-dodecaborane carboranes(l,2-, 1,7-, and 1,12-), the 1,2-isomer 1 has been used for functionalization and connection to organic molecules. The highly delocalized three-dimensional cage bonding that characterizes these carboranes provides extensive thermal and kinetic stabilization as well as photochemical stability in the ultraviolet and visible regions. The unusual icosahedral geometry of these species provides precise directional control of all exopolyhedral bonds. 

The quantum yields are 0.15-0.21 in ethanol and 0.01-0.02 in an aqueous medium, but in micelles, the quantum yields are five to tenfold increased. The aggregation of these dyes was studied in [53]. The amphiphilic squaraines 4 combine favorable photophysical properties and good solubility in aqueous media and in addition interact efficiently with micelles, and therefore have the potential to be used as NIR fluorescent sensors. However, our own investigations show that aniline-based squaraines lack chemical and photochemical stability when compared to oxo-squaraines with heterocyclic end-groups. 

Humphry-Baker R, Gratzel M, Steiger R (1980) Drastic fluorescence enhancement and photochemical stabilization of cyanine dyes through micellar systems. J Am Chem Soc 102(2) 847-848... 

In general, coordination complexes and metallized dyes exhibit higher photochemical stability than purely organic molecules so it is clear that coordination chemistry has a major role to play in the future development of ODS media. 

Photochemical stability The layer of AgCI must be remade often... 

Remarkably enough, 47 is a blue dye with a high photoluminescence quantum yield and, nevertheless, a high photochemical stability. Figure 7 shows the spectra of the different rylenetetracarboxdii-mide oligomers. 

The thermal and photochemical stability of both organic dyes and nanocrystals are influenced by an extremely broad variety of conditions that need to be considered excitation wavelength and intensity, matrix or microenvironment, label concentration, and, in the case of nanoparticles, surface chemistry. Therefore, the individual study of the stability of a chromophore under the conditions required can usually not be avoided. 

High thermal, chemical, and photochemical stability, which simplifies sterilization and extends sensor lifetime. 

The second major category of IR fluorescent dyes are the oxazines, as exemplified by the laser dye Oxazine 1 and Nile Blue (Figure 6.16). Probably the major advantage of the oxazines is their photochemical stability, which is typically much better than that of the cyanines or rhodamines.(52) They are synthesized by condensation of the appropriatep-nitrosoaniline derivatives with the corresponding phenol.<45) These dyes... 

The attachment of a distinct number of fluorescent dyes at the periphery of a stiff three-dimensional nanoparticle is of fundamental interest to study interactions between single chromophores in close vicinity to each other. We chose as a dye one of the rylene series, perylenemonoimide, for the decoration of our polyphenylene dendrimers due to their outstanding properties. Rylene dyes show an exceptional chemical and photochemical stability and therefore are well suited for single molecule spectroscopy (SMS) [661. 

Polycarbonates can be included in this category. In fact, the bisphenol A-based polycarbonates are one of the most deeply studied classes of polymers in relation to its photochemical stability. To date, it is well established that the photochemical processes occurring in these polymers are wavelength dependent [226-230]. When photolyzed with > 300 nm, several radical and oxidative reactions take place, whereas with < 300 nm, the PFR as shown in Scheme 76 becomes important. Analogous dual photochemistry has been shown recently for trimethylclohexane-polycarbonate [231]. 

Residual action on plant surfaces. Many crop protection agents exert their effects by residual action following application to plant surfaces. In addition to photochemical stability. 

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). ... 

Fused bridge prevents isomerisation and provides thermal and photochemical stability I------------------------------1... 

The photochemical rearrangement of enamides of carboxylic acids 20,113-116 is analogous to the photochemical rearrangement of enol esters (Section VII), except that it proceeds more efficiently and in much higher yield. It seems that the latter can be attributed to the high photochemical stability of the photoproducts. The )3-iminocarbonyl compound 187 formed immediately after the... 

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