Photochemical unstability

Spin orbit coupling is most effectively increased by using higher Z metals. Heavy atom substituents on the organic ligand have proved of minimal use and may actually make the ligands more photochemically unstable. 

Lastly, photochemically unstable ligands should be avoided. Re(bpy)(CO)3Cl shows a moderately efficient MLCT emission at room temperature (R. M. Ballew, unpublished results from our laboratory). However, the apparently closely related Re(dpk)(CO)3Cl (dpk = 2,2 -dipyridyl ketone) shows a benzophenone like phosphorescence at 77K indicating that the n-n excited state of the ketone in complex is the lowest state of the complex. No luminescence is seen at room temperature, and even at 77K the dpk triplet state is such a powerful hydrogen atom extractor that it removes protons from alcohol glasses as seen by the formation of the intense blue color of the keto free radical. The absence of an MLCT emission is caused by the greater difficulty of reducing dpk relative to bpy, which pushes the MLCT states above the dpk ligand states. 

Another benz-analog of 289, namely 296, has been prepared by ring closure of bisacetylene 295 with subsequent dehalogenation in 10% yield. 296 forms orange-red stable crystals with indefinite melting point a solution in benzene is photochemically unstable and is destroyed in a few minutes." ... 

The technique of spin-trapping radicals has been applied to the measurement of atmospheric hydroxyl by Watanabe et al. (102), although there are no reports of its use for peroxy radicals. The principle involves the reaction of the radical of interest with an organic nitrone immobilized on a filter paper or other substrate. The sample is returned to the laboratory, and the nitrone-radical product is dissolved in a suitable solvent and measured with EPR. The disadvantages of the spin-trapping technique are difficulty in finding suitable organic nitrone compounds and the fact that most of these molecules are photochemically unstable. 

The six-coordinate nickel(II) complex [Ni(N203)3]3[Co(NH3)6]4-3H20 contains the trioxodi-nitrato ion N20 - coordinated as chelate. The complex is thermally and photochemically unstable. It was prepared under N2 at 0°C from NiCl2-6H20, Co(NH3)6Cl3 and Na2N203 in aqueous solution. ... 

The peroxide and azo thermal initiators also are photochemically unstable and have been used as radical sources at well below their normal thermal decomposition temperatures. However, their industrial use as photoinitiators has been limited because their light-absorption characteristics frequently are unsuitable and because of the obvious potential complication owing to their slow thermal decomposition, which leads to poor shelf-life and nonreproducible photoactivity in given formulations (88). Further information on photoinitiators can be found in the literature (92). 

The irradiation of 2,5-dihydrothiophene derivatives in /(-hexane or ether at 254 nm of degassed solutions leads to rapid formation of thermally and photochemically unstable products. 

In one of the earliest reports on ortho photocycloaddition, in which the reaction of benzonitrile with 2-methylbut-2-ene is described, a diradical (triplet) intermediate was proposed [73], The structure of the product corresponds to the most stable of the four possible diradical intermediates. When benzophenone was added as a sensitizer in an attempt to increase the yield of the photoadduct, only 0.05% of ortho adduct was isolated along with 54% of an oxetane formed by the addition of benzophenone to 2-methylbut-2-ene. In the absence of benzophenone, the ortho adduct was isolated in 63% yield. It is, however, thermally as well as photochemically unstable and reverts to starting materials, supposedly also via a biradical. The authors propose that benzophenone catalyzes bond cleavage of the adduct more efficiently than ortho addition and this would account for the low yield of photoadduct in the presence of benzophenone. From these experiments, no conclusion about the identity of the reactive excited state can be drawn. 

The dialkyls and diaryls are colorless liquids or low-melting solids. They react less readily than their zinc and cadmium analogues with air and water, but they are thermally and photochemically unstable, with homolytic Hg—C bond cleavage being the key in their decomposition. They may be kept for several months in the dark at room temperature. All are toxic. 

The isostructural pentacarbonyl metallates Mn(CO)5 and Re(CO)5" form a series of thermally or photochemically unstable charge-transfer salts with N-methylpyridinium cations. For example, A-methylacridinium reacts with penta-carbonylrhenate immediately upon mixing in acetonitrile to form the (A-methyl-9-acridanyl)pentacarbonylrhenium(I) adduct in 90 % yield [126] (Eq. 46). 

Thietanes are photochemically unstable and should be protected from light if they are to be stored for any length of time. Short-lived (hot) biradical intermediates, for example, 98a, appear to be formed and can undergo a variety of reactions, as shown for thietane. " Mercury-sensitized photolysis gives triplet biradicals that are longer lived than the singlet biradicals formed on direct excita-tion. " Some cyclopropane product is produced in the sensitized photolysis. The second excited singlet state of thietane decomposes either to ethylene and thio-formaldehyde via a 1,4-biradical or to elemental sulfur and cyclopropane. ... 

The red-shift of the C=0 and Si=0 stretching vibration of 100 and 49 cm" , respectively, the calculated bond energy of complex 10 of more than 20 kcal/mol (RHF/6-31G(d,p)), and the calculated non-bonding Si-O distance of 1.98 A, being only slightly longer than the normal Si-0 bond distance, indicate the unusual electronic properties of 10. Evidently dioxasiletane 9 is formed as a thermally or photochemically unstable intermediate which rapidly decomposes to the silanone-formaldehyde complex. 

The allenic esters (169) yield the the cyclobuteneones (170) on irradiation. In these examples this is the major product and is accompanied by 1,1,4,4-tetraphenylbuta-l,3-diene. The cyclobutenones are photochemically unstable and C2in be completely destroyed by irradiation for 4-6 h. When the phenyl ester (171) was irradiated no cyclization occurred and the only compound obtained was the photo-Fries product (172). One interpretation of the cyclization process is that the ester undergoes Fission of the carbonyl-ester O bond yielding a radical pjur. Cyclization and recombination would afford the product. However, attempts to trap intermediates such as (173) were unsuccessful. ... 

Catecholamines are thermodynamically and photochemically unstable compounds that yield aminochromes and melanins on photooxidation (283-285) (Scheme 6). Thus, irradiation (254 nm) of oxygen-saturated dilute solutions of adrenaline, isoprenaline, and noradrenaline produced the corresponding aminochromes in 65, 56, and 35% yield, respectively (285). Longer irradiation produced melanins, thus providing evidence for the photolabile character of aminochrome (284). Studies of the action spectrum confirmed the excited state of the catecholamine as the primary... 

Although very large hyperpolarizabilities have been obtained with long and unprotected polyene bridges, these materials are chemically and photochemically unstable or it is impossible to prepare them in adequate yield and purity, and therefore, they are not suitable for practical applications. On the other hand, chromophores based on fused ring systems, such as naphthalene benzimidazoles [38] or phthalocyanines [39,40] possess good nonlinearity and thermal stabilities over 350°C, but they are poorly soluble in spin-casting solvents and they cannot be effectively poled. 

Some dyes are photochemically unstable. When a molecule absorbs a photon, the molecule enters an excited state. It may lose that energy by converting it to heat, re-emitting it as luminescence, or causing a chemical reaction. The reaction could disrupt the bond between the dye and polymer and thus allow the dye to be lost from the sensor. The reaction could also convert the dye to a pH-insensitive form. These inactivating reactions may be facilitated by the presence of oxygen and be more prominent at elevated Po2- At least one oxygen sensor was based on a photochemical reaction that allowed at least 250 measurements before its performance became unacceptable [14]. 

Quinoxaline 1-oxides are photochemically unstable. Irradiation of quinoxaline 1-oxide itself in dilute aqueous solution gives quinoxalin-2-one. Irradiation of both the 1- and 4-oxides of 2-phenylquinoxaline gives a common product, the unstable benzoxadiazepine 39 in each case some 2-phenylquinoxaline is also formed. Irradiation of the 4-oxide yields a third product, 3-phenylquinoxalin-2-one. A benzoxadiazepine is also obtained on irradiation of 2,3-diphenylquinoxaline 1-oxide. " ... 

Pure PVC is thermally and photochemically unstable and has a tendency to lose hydrogen chloride when heated, hence a stabiliser (based on tin, lead, or other heavy metals) is commonly used. Pure PVC is also brittle and needs a plasticiser to soften and flexibilise the system. 

Groenewege, 1958). The alkyltitanium trichlorides are thermally and photochemically unstable, but can be isolated under certain conditions (Beermann and Bestian, 1959 Bawn and Gladstone, 1959). Rapid decomposition usually occurs schematically according to Eq. (34). Although the fate of the alkyl group has fre-... 

It is important to understand that alkyl azides may be thermally or photochemically unstable, and sometimes explosive, so great care is always exercised when this chemistry is used. If one exercises the proper caution, however, azide substitution reactions are a reasonable method for making primary amines from primary and secondary alkyl halides. 

The effect of the accumulation of hydroperoxides in the polymer certainly needs further investigation since the hydroperoxides are thermally and photochemically unstable. Upon dissociation, they may restart the chain oxidation. At present, it is not yet clear whether this has a measurable influence on the mechanical properties of the photopolymer the oxidation process may cause chain scission as well as further crosslinking. 

An alkyne may also be used in place of the diene component. With the cyclic sulfone complex 11.174a, this leads to an interesting synthesis of cyclooctatetraenes as the initial cycloadduct 111.76 is photochemically unstable and readily loses SO2 (Scheme 11.59). The adduct may be isolated if uranium glass filters are employed to exclude the high-energy wavelengths. Subsequent photolysis with the more transparent vycor filters causes chelotropic elimination of sulfur dioxide to give the tetraene 11.177. 

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