Acetophenone triplet state energy

While direct photolysis of (46) gave (48), the sensitized photolysis of (46) with acetophenone gave only the cyclopropane product (>93%). These authors favor a stepwise oxa-di-w-methane reaction mechanism (8.69) inasmuch as a concerted reaction should produce the product without a change in multiplicity. That is, the product would have to be formed in the triplet state and not enough energy is available for the product to be formed in an excited state ... 

It is worth mentioning that triplet-triplet energy transfer can be used to populate the triplet state of molecules in which intersystem crossing is unlikely (e.g. the triplet state of thymine in frozen solutions can be populated by energy transfer from acetophenone). 

For related reasons, and because their excited-state energies are lower than for dialkyl ketones, diaryl ketones and simple alkyl aryl ketones do not fragment on irradiation in solution, even at higher temperatures. This leads to a photostability that is one factor contributing to the successful employment of ketones such as benzophe-none tPh-CO) or acetophenone (PhCOMe) as triplet sensitizers. a-Cleavage for ketones in solution at room temperature is promoted if structural factors cause the bond adjacent to the carbonyl group to be somewhat weaker than normal. Hence t-alkyl ketones give decar-borylation products readily (4.5), as do benzyl ketones (4.6 and benzoin derivatives (4.7). 

Photoisomerization of alkenes via the triplet excited state is known to be possible by triplet sensitization, usually efficient in conjugated C=C bonds that fulfill the requirement of possessing triplet excited energies below those of the typical triplet sensitizers such as acetone, acetophenone, benzophenone, etc. (Table 2). Sensitization with the opposite order of triplet excited energies is possible in cases with strong electronic or strong... 

Possible examples of the importance of the coincidence of transition-state and excited-state geometries and also the importance of the location of the essential reaction or localization of reaction energy are the chemiluminescent rearrangements of Dewar benzene [ 1 ] and Dewar acetophenone [2] to benzene and acetophenone, respectively (Lechtken et al., 1973 Turro et al., 1974c Turro et al., 1975). The efficiency of excited state production though quite low is approximately the same for both systems. Yet the triplet state of... 

Carbonyl groups are most significant in triplet state photochemistry. Spectroscopic studies include time resolved ESR studies of enols of -hydroxybenzaldehyde 2 energy transfer from acetophenones to 9, 1 0-d ibr omoa n t hr a ce ne (S.,) which involves higher triplet states of the donor283, two photon excitation of substituted triplets of acetophenone , transient resonance Raman spectra of deuterated benzophenones , and diffuse reflectance of... 

The direct irradiation at 254 nm of the allenes (67) in hexane gives the molecule in its triplet state. Bridging within this state affords the intermediate biradical (68). Bond formation then yields the major products (69) and (70). Minor products (71) and (72) are also formed. Irradiation at X. > 280 nm fails to yield products. However, acetophenone sensitization is effective. The authors suggest that in this instance the vinyl moiety is the chromophore which is excited and that intramolecular energy transfer is involved. The yields obtained from the reactions are shown in Table 1. 

Acetaldehyde, 119-20, 380-82 Acetone, chemical titration. 428 oxetane formation, 318, 427 singlet and triplet states, 382. 428 Acetophenone. 407. 467 Acetylene, 203. 348 cycloaddition, 415, 423 excited state geometry. 45-46 2-Acetylnaphthalene, 398 6-Acetyloxycyclohexadienones. 463 Acidity, excited states, 48-52 Acrolein, 34, 382-83, 433 Acrylonitrile, 328, 414-15, 417 Activation energy, 382-83, 400 Acyl radical. 3S2-S5, 380-82, 460 Adiabatic. See Potential energy surface ... 

Coincident with the decrease in the quantum yield of photoreduction with lowest ir- -it and CT triplet states, there is an increase in the triplet lifetime. This behavior is also observed in the photoreduction of substituted acetophenones, the data for which are presented in Table 3.8. ° In this case the quantum yield for photoreduction in isopropanol drops by a factor of 1000 from that of acetophenone itself to that m-methoxyacetophenone although there is only a minor difference in their triplet energies. These facts again indicate that a factor related to the electronic structure of the lowest triplet state must be responsible for the change in reactivity with substitution. We note in Table 3.8 that the lowest triplet of acetophenone is an n->n statd while that of m-methoxyacetophenone is A qualitative idea of the differences in electronic structure in these three types of triplet states can be obtained from the following exaggerated structures ... 

The role of the triplet state in the cis-trans isomerization of stilbenes effected by photosensitizers, such as acetophenone, benzophenone, or anthraquinone, which have large So Ti excitation energies, was first revealed in Ref. [65]. Theoretical considerations and experimental data on intermolecular triplet-triplet energy transfer leading to the sensitized stilbene photoisomerization are described in Section 4.2.2. It was shown that data on positional dependence of the heavy-atom effect on the cis-trans photoisomerization of bromostilbenes were consistent with the fact that, in contrast to the para position, the meta position is near a node in the highest occupied and the lowest unoccupied MO of stilbene [66]. According to [67], internal and external heavy-atom effects induce phosphorescence in frans-stilbene... 

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