Anthracenes, photooxygenation

FIGURE 1.14 Photooxygenation of (a) 7,12-diniethylbenz[a]anthracene, (b) benzo[a]pyrene, and (c) photo-cylization of cw-stilbene. 

The type II direct photooxygenation of anthracene and its derivatives has been studied quantitatively by Bowen,2 Livingston,3 and their co-workers. Quantum yields of fluorescence, dimerizations, and... 

As in the former cases, k2 was calculated from the integrated extinction coefficients,149 k3 + kt was derived from fluorescence quantum yields,149 while k3 and k4 were separately estimated from the maximum quantum yields of photooxygenations at high oxygen concentrations.150 Flash spectroscopy techniques were used in order to determine k5 and k7, while kB was obtained from the Stern-Volmer quenching constant of oxygen.149 The ratio ke/kg was determined from the variation of AOz with the concentration of the anthracene.71 When photodimerization occurred, k13l(kia + k13) was calculated from the maximum yield of... 

Carbon disulfide quenches the fluorescence of anthracene quite efficiently,145,149 but seems to have little effect on its triplet lifetime.147 Diphenylanthracene in benzene fluoresces with a quantum yield of 0.8 and shows a high sensitivity to the oxygen concentration in photooxygenation reactions. With about 1 vol% of CS2 present, AC>2 is practically independent of [02] (> 10"5 mole/liter). In jjoth cases, where carbon disulfide was either used as solvent or was added to an otherwise strongly fluorescent solution, the quantum yields of photooxygenation followed... 

A remarkable effect of small quantities of different compounds on the rate of the direct photooxygenation of anthracene in carbon disulfide has been reported (Table IX).3... 

Effect of Various Compounds, in Concentrations of 3 Mole 7 , on the Rate of Direct Photooxygenation of Anthracene in Carbon Disulfide (from ref. 3)... 

However, even in the latter case (e.g., reaction of 1Oa + anthracene -> anthracene-endoperoxide), there seems to be no direct path from 1X to the stable endoperoxide, since all kinetic results have revealed the participation of two molecules of anthracene in the photooxygenation reaction. This also indicates that the excess energy, contained in 1X after transition from the F state, is used as kinetic energy with which 1S0 and 1Oa fly apart. 

Cyclopentadienes, 1,3-cyclohexadienes, 1,3-cycloheptadienes, as well as furan and aklyl-substituted furans, have been investigated as substrates of photosensitized oxygenation reactions, while aromatic compounds such as anthracenes and tetracenes as well as aryl-substituted carbo-and heterocyclic pentadienes were studied in direct and indirect (photosensitized) photooxygenation reactions. 

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. 

In comparison with the polymer bound sensitizers, the i micelle does not emit excimer emission. Furthermore, LCV is incorporated in micelle so that the loss of molecular motion in molecular aggregate state is not a serious drawback. However, side reaction of anthracene presumably photooxygenation at the 9, 10 position of anthracene is accelerated in micellar system. Results are summarized in Table 3. 

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