ULTRAVIOLET IRRADIATION Subject

One curious case of the effect of light on electron-transfer equilibrium involves the reduction of ,p-di(t-butyl)stilbcnc with potassium in DME. The reaction leads directly to a diamagnetic dianion a solution of this dianion remains ESR silent unless subjected to ultraviolet irradiation by a Hg/Xe lamp. The anion radical of a,(3-di(t-butyl)stilbene then formed from the dianion by the loss of an electron. The electron reverted within 5-10 min after ultraviolet irradiation was turned off, transforming the anion radical into the dianion (Gerson et al. 1996). This case deserves to be clarified. Maybe the light effect consists simply in singlet-triplet transformation of the dianion, with the formation of some more or less stable biradical state of the dianion, which possesses two unpaired electrons and can even be a paramagnetic one. 

Studies with cells in culture show a cytotoxic effect of vitamin Be. This may be from the formation of cytotoxic products when the vitamin is subjected to ultraviolet irradiation and may not be relevant in vivo (Maeda et al., 2000). 

Figure 3. Ultraviolet spectrum of anthraquinone adsorbed on zeolite subjected to thermal treatment in vacuum at 300°C (1) NaY (2) sample 1 with adsorbed anthraquinone after ultraviolet irradiation (3) CaX (4) HY (5) sample 4 with adsorbed anthraquinone after ultraviolet irradiation...

Methyl nitrate was found to be present in the products of the decomposition of azomcthanc (Cllj-N- N-nij) when the latter substance was subjected to ultraviolet irradiation II6). This was due to an intermediate formation of CTI3O1 radical which reacted with NO to yield the nitrate ester ... 

Kortiim and Braun (118) found that a ground-up mixture of silica-gel or AljOs with solid anthracene subjected to ultraviolet irradiation in open air displays very rapidly a new absorption band at 275 m.fi (in diffusely reflected light) and acquires on further exposure a yellow-brown coloration. In similar mixtures with MgO and KCl the photooxygenation of anthracene is much slower, which indicates a specificity of silica gel. It could be spectrally shown that anthraquinone is a primary product giving on further exposure 1,4-dihydroanthraquinone (quinizarine), which could be eluted, producing a red solution with an absorption maximum close to that observed on silica-alumina, as just mentioned above. 

To determine if either of these answers were correct, the properties of a series of polyurethane networks which contained photo labile disulfide bonds were studied (7). The creep curve of the s imple in the initial crosslink density state (comparable to Vq) was determined in the usual way. Then, after complete recovery, another creep experiment was started but, at time t during this run, the sample was subjected to ultraviolet irradiation which photochemlcally broke some of the network chains. 

For a long time aromatic compounds were believed to be stable when exposed to ultraviolet irradiation. The interest in the photochemistry of arenes only started in the late 1950s when several groups observed both isomerization and addition reactions of benzene and its derivatives. Among the pioneers who were active at that time are the groups of Bryce-Smith and Schenck. Since then the photochemistry of aromatic compounds has become the subject of innumerable papers dealing with their conversion to other aromatic systems (by substitution or isomerization) or even to nonaromatics. 

Analogous decarbonylations have been observed for ketones under ultraviolet irradiation at elevated temperatures. From a preparative point of view the behavior of diphenyl-substituted and a,a-divinyl-substituted ketones is interesting, as they are decarbonylated when their benzene solutions are subjected to ultraviolet irradiation.69 This favorable effect of phenyl sub-stitutents on decarbonylation has been noted also for cyclic ketones for example, under suitable conditions l-phenyl-2-indanone eliminates carbon monoxide and yields 90% of 5,6,ll,12-tetrahydro-5,6-diphenyldibenzo[a,e]-cyclooctene as a mixture of cis- and trans-forms,10 whereas 2-indanone itself is hardly decarbonylated at all in benzene solution. That in other cases decarbonylation is usually more profitable in the gas than in the liquid phase is shown by studies of the photolysis of tetramethyl-l,3-cyclobutanedione which in the gas phase gives 2 equivalents of carbon monoxide and a quantitative yield of 2,3-dimethyl-2-butene.71... 

To study selectively the effect of primary and secondary molecular motions on thrombogenesis, the PNF was subjected to low-dose ultraviolet irradiation the same dose rate as used in the extracorporeal studies. It was anticipated that this low-dose treatment would selectively cross-link the pendant side chains intramolecularly, followed by intramolecular/intermo-lecular cross-linking at higher dose rates. 

Lee SH, Ruckenstein E. Stability of polymeric surfaces subjected to ultraviolet irradiation. J Colloid Interface Sci 1987 117 172-8. 

Sugars and h droxyaldehydes are produced as secondary products when formaldehyde is subjected to ultraviolet irradiation in the presence of water (page 107). 

That the hydrated electron is a separate chemical entity has been demonstrated by the technique of pulse radi l sis This consists of subjecting a sample of pure water to a very short pulse of accelerated electrons. The energetic electrons have the same effect upon water as a beam of y-ray photons. Shortly after the pulse of electrons has interacted with the water, a short flash of radiation (ultraviolet and visible radiation from a discharge tube) is passed through the irradiated water sample at an angle of 90° to the direction of the pulse to detect the absorption spectra... 

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