Photoenols lifetimes

Another common hydrogen transfer reaction of carbonyl triplet is the photoenolization of the c-methylbenzoyl chromophore, illustrated in reaction 3 for the syn conformer of c-methylaceto-phenone (j+). Reaction 3 can act as a very efficient energy sink, and a number of properties of this group led us to believe that this process could be used to reduce photodegradation i.e. the excellent absorption characteristics of the chromophore, the short triplet lifetime and the fact that the disappearance of the carbonyl triplet does not take place at the expense of the formation of another excited state. 

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... 

The transient absorption formed from laser flash photolysis of 15 was not quenched in oxygen-saturated solutions, presumably because the photoenolization to form 16 takes place from a singlet excited state of 15. Furthermore, Schworer and Wirz did not observe any transient absorption that can be attributed to the formation of the triplet excited state of 15 from direct irradiation. In contrast. Hurley and Testa used energy transfer to estimate that the triplet excited state of 15 is formed in 67% yield,whereas Takezaki et al. have measured the yields for forming the triplet excited state of 15 to be slightly higher or above 80%. They estimated the lifetime of the lowest triplet excited state of 15 to be 350 ps in ethanol. 

Laser flash photolysis of 46 showed results similar to those obtained for 45. The lifetimes and yields of Z and E photoenols from 46 are comparable to those obtained for 56. Similarly, laser flash photolysis of 47 reveals that the major reactivity pattern of 47 is intramolecular H-atom abstraction to form Z-58 and E-58 even though no products were observed that can be attributed to the formation of photoenol 58. Laser flash photolysis of 47 in methanol showed formation of biradical 57 ( max 330 nm, r = 22ns), which was efficiently quenched with oxygen (Scheme 32). Biradical 57 intersystem crosses to form Z-58 and E-58, which have maximum absorption at 400 nm. Enols Z-58 to E-58 were formed in the approximate ratio of 1 4. Enol Z-58 had a lifetime of 6.5)0,s in methanol, but its lifetime in dichloro-methane was only 110 ns. The measured lifetime of E-58 in methanol was 162)0,s, while it was 44 ms in 2-propanol. Thus, E-58 is considerably shorter-lived than E-56. Furthermore, E-58 is also shorter-lived than the analogous E-59 (Scheme 33), which cannot decay by intramolecular lactonization and has a lifetime of 3.6 ms in methanol. Thus, we proposed that E-58 undergoes solvent-assisted reketonization that is facilitated by the intramolecular H-atom bonding, as shown in Scheme 34. 

It is well known that o-hydroxyphenyl ketones phosphoresce very weakly and do not undergo photoreduction363,368 or photoelimination,366 presumably because of rapid enolization in the excited state. Lamola has performed quenching studies which indicate lifetimes of o-hydroxybenzophenone triplets of less than 10"8 sec. However, these lifetimes, as well as phosphorescence efficiencies, are markedly enhanced by the addition of alcoholic solvents,383 presumably because hydrogen bonding slows down photoenolization. 

The singlet excited state (V) was excluded as the intermediate imder-going isomerization due to the long lifetime of the first transient. The photoenol (VI) which absorbs in the visible, reverts back to the ketone in the dark. Such behavior is a common phenomenon in photochemistry. In general, systems which assume color upon irradiation, but revert to colorless in the dark, are said to be phototropic (for a review on phototropic systems see Dessauer and Paris, 1963). 

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