Benzophenone, photoreduction energy transfer

TTie photoreduction can be quenched by known triplet quenchers. The effecti e quenchers are those which have T] states less than 69kcal/moI above S,. Quenchers with higher triplet energies are ineffective because the benzophenone n-n triplet is then not sufficiently energetic to effect energy transfer. 

Attempts to sensitize the rearrangement with benzophenone, propiophenone, and chlorobenzene failed, as indicated in Table 8.1. Although the reaction could not be sensitized, triplet energy transfer was taking place inasmuch as compound (1) quenched the photoreduction of benzophenone without the formation of any new products (Table 8.2). 

At 0.1M acceptor, for example, the rate of energy transfer is 19.2 times the rate of abstraction. However, this means that about 5% of the benzophenone triplet will still abstract from the solvent to form ketyl radicals. A compound which is itself a poor hydrogen abstractor may show a greatly enhanced quantum yield of photoreduction under these conditions. 

Photoreduction of the herbicide paraquat dichloride in aqueous propan-2-ol is more efficient in the presence of a sensitizer such as benzophenone than on direct irradiation.84 Hyde and Ledwith84 propose that the paraquat cation radical is formed by electron transfer from ketyl radicals, in turn produced during the conventional photoreduction of the sensitizer ketone. The suggested mechanism is given in reactions (23)—(25), where PQ2+ is the paraquat dication. The reduction process therefore involves chemical sensitization, rather than electronic energy transfer. 

Using a simple energy-transfer experiment, one can show that the photoreduction of benzophenone proceeds via the Tj excited state of benzophenone rather than the Sj excited state. If naphthalene is added to the reaction, the photoreduction is stopped because the excitation energy of the benzophenone triplet is transferred to naphthalene. The naphthalene is said to have quenched the reaction. This occurs in the following way. 

Space by a dipole-dipole coupling mechanism, but triplet energy transfer requires the two molecules involved in the transfer to collide. In the usual organic medium, about 10 collisions occur per second. Thus, if a triplet state has a lifetime longer than 10 second, and if an acceptor molecule B, which has a lower triplet energy than that of A, is available, energy transfer can be expected. If the triplet A undergoes a reaction (such as photoreduction) at a rate lower than the rate of collisions in the solution, and if an acceptor molecule is added to the solution, the reaction can be quenched. The acceptor molecule, which is called a quencher, deactivates, or "quenches," the triplet before it has a chance to react. Naphthalene has the ability to quench benzophenone triplets in this way and to stop the photoreduction. 

To obtain more information on this point, let us examine the data given in Table 3.6<42-47> for some substituted benzophenones. The data in Table 3.6 indicate that benzophenone derivatives having lowest triplet states of n->TT character undergo very efficient photoreduction in isopropyl alcohol. Those derivatives having a lowest it- -it triplet, on the other hand, are only poorly photoreduced, while those having lowest triplets of the charge-transfer type are the least reactive toward photoreduction. In additon, in some cases photoreduction is more efficient in the nonpolar solvent cyclohexane than in isopropanol. This arises from the solvent effect on the transition energies for -> , ir- , and CT transitions discussed in Chapter 1 (see also Table 3.7). 

In Section 3.1 it was shown that the photoreduction of benzophenone can be quenched by addition of small amounts of triplet quenchers such as oxygen or ferric dipivaloylmethide.<60) In fact this was presented as evidence that the benzophenone triplet was involved in the photoreduction. This reaction can also be quenched by naphthalene. In the presence of naphthalene, light is still absorbed by benzophenone and thus benzophenone triplets are produced. However, photoreduction products are decreased. On examining this reaction with flash photolysis, triplet-triplet absorptions were observed but these absorptions corresponded to those of the naphthalene triplet. Thus the triplet excitation energy originally present in the benzophenone triplet must have been transferred to naphthalene and since little of the photoreduction product was observed, this transfer must have been fast in relation... 

Photolytic decomposition of peroxides is not v y efBcient in crosslinking. An enhancement effect on the extent of photocrosslinking of polyolefins in the presence of peroxides is displayed by aromatic hydrocarbons such as naphthalene. These transfer the exdtation energy absorbed to a peroxide. This procedure, however, does not represent an important improvement when compared with that refored to earlier, namely the photoreduction of pdyethylene with aromatic ketones and quinones [84. From aromatic ketones and quinones, particulariy bena>phenone [32], chlorinated benzophenones, benzoyl-l-( dohexanol [82], a, -dimethot - hen acdr henone, 2,4,6-trimethyl benzoyl phenyl phosphinic ethyl ester [85], anthrone [86], anthraqui-none [87], naphthoquinone, benzoquinone, and their d vatives have all been examined. 

Photosensitizers can act in two ways. For example, benzophenone is raised to the triplet state by ultraviolet light. The triplet state then transfers its energy to monomers. A photoreduction then occurs in the presence of amines and sulfur compounds, possibly under the intermediate formation of exiplexes ... 

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