Ketones intersystem crossing

In aliphatic and allylic ketones, the reaction proceeds through both singlet and triplet excited states, but in aryl ketones, intersystem crossing is very fast and triplet state is more effective. For examples, aromatic ketones give major P-cleavage... 

The sensitizer in our experiments is benzophenone (BP) which reacts as shown in Scheme 2. UV light of 300 to 400 nm is absorbed and excites the aromatic ketone group to a singlet state which by intersystem crossing (ISC) reverts to a triplet state, abstracts a... 

When Hammond and co-workers(59) found that the intersystem crossing quantum yield for aromatic ketones was unity (see Chapter 3) it was a short but very important step to realize that these compounds should be ideal triplet sensitizers. Thus one can excite the triplet state of molecules that otherwise would be formed inefficiently, if at all, by intersystem crossing. This idea resulted in a number of papers in the early 1960 s from the Hammond group on this topic. It is not possible in this short section to survey this area, but a few of the early studies are indicated by the following reactions ... 

The yields of cyclopropanes in this case are low in relation to the amount of acetophenone formed. However, similar cyclopropane product ratios are obtained when photolysis is carried out in the presence of Michler s ketone as sensitizer. Thus the carbene intermediate produced in the direct irradiation is thought to be a triplet, as suggested by the nonstereospecificity of its addition. Whether this intermediate arose from singlet diazoacetophenone (via singlet decomposition and intersystem crossing of the singlet carbene) or by decomposition of the triplet molecule was not determined. 

Intersystem crossing in aromatic ketones is very efficient, so that their photochemistry is dominated by the triplet-state processes. The efficient formation of triplet states, the small singlet-triplet energy and the accessible long-wavelength absorption make the aryl ketones excellent triplet sensitisers. 

The first step of the reaction involves the (n, it ) excited state of the carbonyl compound reacting with the ground-state alkene. For aromatic ketones, rapid intersystem crossing from the excited singlet state to the excited triplet state occurs, forming initially a 1,4-biradical and then the oxetane ... 

For aliphatic ketones, although intersystem crossing is fast, the singlet (njjt ) state will react by a stereospecific process with alkenes that have electron-withdrawing groups in their molecules ... 

Many aromatic aldehydes and ketones (e.g. benzophenone, anthrone, 1- and 2-naphthaldehyde) have a low-lying n-n excited state and thus exhibit low fluorescence quantum yields, as explained above. The dominant de-excitation pathway is intersystem crossing (whose efficiency has been found to be close to 1 for benzophenone). 

As has already been mentioned (vide supra, p. 118) the population of the triplet state of a carbene depends on the effectiveness of the intersystem crossing steps kisc and k lse- These rates can be altered by dilution. Another technique which exclusively populates the triplet state of a carbene is sensitization, or energy transfer. A triplet sensitizer is required for this purpose — usually an aromatic ketone. In these ketones the intersystem crossing efficiency is almost 100%. Energy is then transferred from the sensitizer triplet to the diazoalkane, thus populating the triplet state of the latter. 

The photoreactivity of o-methyl acetophenone 11 has been studied exten-sively it is somewhat different from 1 because the singlet excited ketone (Sik) in 11 intersystem crosses to its triplet state in less than quantitative yields, as observed for 1 (Scheme 8). Thus, Sik in 11 decays by both intramolecular H-atom abstraction to form exclusively photoenol Z-13 and intersystem crossing to Tik of 11. Haag et al. estimated that Tik of 11 has a lifetime of 10 ns in benzene and decays by intramolecular H-atom abstraction to form biradical 12. The maximum... 

Photoinduced electron transfer (PET Scheme 6.2) is a mild and versatile method to generate radical ion pairs in solution," exploiting the substantially enhanced oxidizing or reducing power of acceptors or donors upon photoexcitation. The excited state can be quenched by electron transfer (Eq. 7) before (aromatic hydrocarbons) or after intersystem crossing to the triplet state (ketones, quinones). The resulting radical ion pairs have limited lifetimes they readily undergo intersystem ... 

Recently, however, examples of intramolecular triplet sensitization have been described. Alkyl aryl ketones with pendant azido groups in the alkyl moiety were irradiated into the benzoyl chromophore. The excited alkylphenyl ketones undergo intersystem crossing to their triplet states within picoseconds and they can then transfer their triplet energy to the nearby alkyl azido group. This process... 

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