Acetophenone triplet energy

Because of our obvious prejudices we will describe this study in some detail. The energy levels of the acetophenone and -methylstyrene moieties are ideally situated for intramolecular triplet energy transfer (see Figure 6.12). If these two chromophores are not interacting, then the absorption spectra for these compounds should be the composite of the acetophenone-/ -methylstyrene spectra. Figure 6.13 indicates that this is true for n = 4 (and also 2 and 3) however, it is not correct for n = 1 (Figure 6.14). In the case of n = 1 the increased intensity of the vibronic structure of the n - n ... 

Thus Saltiel has concluded that the small increase in [T]S/[C], in going from benzene to acetone indicates that a mixed mechanism is operative for acetone-sensitized isomerization, that is, both triplet energy transfer and, to a minor extent, Schenck intermediates are involved. When acetophenone or benzophenone is used as a sensitizer the pss is close to the thermodynamic... 

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). 

Fig. 1. Substituent effects on the triplet energy (0-0 band) of substituted acetophenones in methylcyclohexane isopentane (4 1).

Similar decomposition is observed in p-bromoacetophenone, o-bromo-, p-bromo, and p,p -dibromobenzophenone, and p-iodobenzophenone44 but not in the fluoro- and chloro-substituted compounds. This order of reactivity follows the bond dissociation energies for aromatic halides which are about 90 kcal/mole for chlorobenzene, 70 kcal/mole for bromobenzene, and 60 kcal/ mole for iodobenzene. The lowest-lying triplet of p-bromoacetophenone is 71.2 kcal45 while that of the substituted benzophenones is slightly lower since benzophenone itself has a lower triplet energy than acetophenone. p,p Dibromobenzophenone was the least reactive of the compounds that photoeliminated halogen atoms. 

The addition of aliphatic aldehydes and ketones to alkenes is less successful as a preparative procedure for oxetanes. An essential requirement for addition is that the triplet energy of the alkene must be considerably greater than that of the carbonyl. If this condition is not fulfilled, energy transfer to the alkene can occur,279 sensitizing, for example, dimerization of the alkene. This is clearly illustrated 280, 281 for norbornene (264) which on irradiation in the presence of benzophenone (ET 68.5 kcal/mole) forms the adduct 265 photolysis in acetone (ET 75 kcal/mole) affords only norbornene dimers (266 and 267), whereas acetophenone, which has intermediate triplet energy (Et 73.6 kcal/mole) forms both oxetanes and norbornene dimers. 

The [2 + 2] cycloaddition of benzene derivatives with alkenes was also carried out using photosensitization. Maleimide 36 was added to benzene in high yields (Scheme 5.8, reaction 16) [42]. In this case, the sensitizer acetophenone 37 transfers its triplet energy to 36, after which the cycloadduct VII reacts immediately with an... 

In the case of benzophenone, the cycloaddition competes with the isomerization of 103 to cycloheptatriene. Exclusive isomerization was observed with acetophenone and acetone. Carbonyl compounds with triplet energies lower than 69kcal/mol prefer the cycloaddition path. Cyclopent-2-en-l-one is an exception to this rule in spite of its triplet energy of 74 kcal/mol, 2 + 2 cycloadducts were formed rather efficiently. 

Irradiation of simple 1,3-dienes in the presence of a sensitizer leads to dimers, with the product ratio dependent on the triplet energy of the sensitizer. For example, acetophenone 14 converts 1,3-butadiene 3 almost exclusively to the [2+2] adduct 6 (80 20 trans. cis), (Sch. 4) [31], whereas the use of pyrene 15 gives mostly the [4+2] adduct 7 [25]. 

Attempts to sensitize the rearrangement of [79a] to [80a] with xanthone (ET = 74 kcal/mole) under conditions where the sensitizer absorbed essentially all of the incident radiation resulted in no observable chemical change (35). To determine if the cyclobutanone was receiving triplet energy from the sensitizer, the direct and sensitized photochemistry of syn- and anti-2-sec-butylidenecyclobutanones [81] and [82] were investigated (35). Under direct irradiation (313 nm), the isomeric acetals [83] and [84] were produced quantitatively ( = 0.1-0.2) yet under sensitized conditions (e.g., with xanthone, acetophenone, benzophenone, or triphenylene),... 

The excited states involved in the mechanisms of the photochemical transformations of the 2-alkylidenecyclo-butanones were elucidated without special difficulty (vide supra). These a, B-unsaturated ketones undergo only one of the reactions characteristic of cyclobutanones (i.e., ring expansion). In addition, the triplet energy of the enone chromophore is low enough that this excited state may be efficiently and selectively populated by standard carbonyl triplet sensitizers (e.g., acetophenone, xanthone, and benzophenone), thereby demonstrating that ring expansion occurs via the 2-alkylidenecyclobutanone state, while the isomerization process (i.e., [81]J[82]) occurs via T. ... 

Since most simple alkenes have a high triplet energy ( t = 75-78 kcal/ mol), triplet sensitizers have to be chosen accordingly to prevent oxetane formation (see Section 7.4.4), as shown in Scheme 14 for norbornene with acetophenone (E = 75 kcal/mol) and benzophenone ( t = 69 kcal/mol), respectively, as sensitizers (Arnold et al., 1965) ... 

Triplet sensitization of alkenes, such as norbomene (ET = 314kJ mol ), having triplet energies lower than or close to that of a sensitizer, such as acetophenone (ET = 10 kJ mol 1), leads to the [2 + 2] cycloadduct 126 (Scheme 6.55).721 In contrast, oxetane 127 formation (Section 6.3.2) is observed when benzophenone (ET = 288 kJ mol ), being unable to transfer energy efficiently, is used.722... 

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 ... 

Ionol undergoes only trans - cis isomerization on sensitization with sensitizers of low triplet energies such as 1-acetylnaphthalene, while with sensitizers of higher triplet energies such as acetophenone, isomerization occurs in both directions [85]. This is because acetophenone can sensitize both trans and cis isomers however, 2-acetylnaphthaIene sensitizes mostly trans isomers. 

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