N,ir* triplet states

Data obtained from the photoreduction of benzophenone in isopropanol indicates that the quantum yield (for the formation of acetone) is nearly constant using irradiation at several different wavelengths between 366 and 254 mp..16 The quantum yield for phosphorescence emission from the carbonyl n,ir triplet state is independent of exciting wavelength.22 Furthermore, the singlet excitation spectrum for the phosphorescence emission has been found to be very similar (if not identical) to the absorption spectrum.23 There are other examples where irradiation in the region of a given transition has ultimately led to a triplet of a different type.24... 

The rearrangement process of 4,4-dialkylated cydohexenones to isomeric bicyclohexanones (e.g., 1 - -2, Chart 1) has been rationahzed by homolytic fission of the 4,5-bond in the n,ir triplet state (general formula 7S, Chart 11), followed by consecutive bonding between positions 3 and 5 (76 - 77), and 2 and 4 (77 - 78). An obvious route which would account for the observed interconversion of compounds 1 and 2 is seen in the reversal of this process involving homolytic fission of the 2,4-bond in the n,T excited state 79. The stereochemical results of the interconversion 74 78, established for cases where groups R and R ... 

In this mechanism the excitation of the sensitizer by the absorption of light produces the exclted singlet state from which the photochemlcally reactive n-ir triplet state Is formed by Intersystem crossing. This triplet state abstracts a hydrogen atom from the thiol molecule and forms a ketyl radical and a thlyl radical. The ketyl radicals dimerize to plnacols and the thlyl radicals add to the polymer by a chain mechanism. The ketyl radicals may also Initiate and terminate chains either by direct reaction and/or by hydrogen transfer. The preferred ene compounds are those having the allyllc structure because of their Inherent chemical stability (38). 

Several explanations can be offered for the reactivity of the naph-thaldehydes. The reaction could be taking place with (/) the 77,77 triplet (2) the n,ir triplet, which could be formed on the way to, or in some cases, be in thermal equilibrium with, the 77,77 triplet or (5) the h,tt singlet, formed either directly upon absorption or on the way to the triplet. It is not possible, at this time, to definitely rule out any of these possibilities. However, there is evidence indicating that, in the case of naphthaldehyde, the 77,77 triplet can be the reactive state involved, since the reduction is sensitized by triplet-triplet transfer with sensitizers having a triplet energy such that only this state is activated.47... 

When benzophenone is irradiated in either cis- or trans-2-butene solution, the photocycloaddition yields essentially the same mixture of both isomeric oxetanes, 40 and 41, before significant isomerization of the starting olefin occurs.37 This result strongly supports a mechanism involving the diradical 39. Since 39 was undoubtedly formed initially in the triplet state (from the n,ir triplet) spin inversion was necessary before bonding. There is no evidence pertaining to the formation of this intermediate from the n,n singlet. 

In Zimmerman s picture of n,ir triplets of dienones undergoing skeletal rearrangements, the second double bond is essential. However, Chapman s idea of the excited state adapts itself readily to explaining the behavior of the enones. Both authors have now examined the behavior of ketone 35. Chapman finds that irradiation of an optically active sample of 35 yields a product which is almost 100% optically active.415 Zimmerman finds that the reaction can be quenched very efficiently by both naphthalene (ET = 61 kcal) and di-te/7-butylni-troxide, but only weakly by biphenyl (ET 70 kcal).416... 

Intramolecular quenching seeems to require an n,ir triplet, since the lifetime of 8-phenyl-p-methoxypropiophenone in non polar solvents (which has a low lying ir,ir state) is 60 ns, compared to ca. 1 ns for, J (19). 

However, for n,ir triplet aromatic ketones and aldehydes, the increased spin delocalization onto the rings should reduce the zero-field splitting, and the failure of their ESR detection may be attributed mainly to their short lifetimes. Hayashi and coworkers (290) have examined the ESR fine-structure parameters of a number of molecular complexes in their phosphorescent state and have derived the dependence of the fine-structure values upon the degree of charge-transfer character in the lowest triplet state of the molecular complexes. Some of their results are collected in Table 15. 

In contrast to aromatic hydrocarbons, heavy-atom substitution onto carbonyl and heterocyclic molecules appears to have little effect on radiative and nonradiative intercombinational transitions. Wagner< has shown that as determined by the type II photoelimination, aliphatic ketones (n- ir excited states) are not sensitive to external heavy-atom perturbation. As seen previously in our discussion of type II photoelimination, aliphatic ketones undergo this cleavage from both the excited singlet and triplet states (in... 

The most efficient phosphorescence often occurs from the n, tr excited state, which tends to be shorter lived and thus less susceptible to deactivation than a tt, ir triplet state. Also, intersystem crossing is less probable for ST. TT excited states than for n, tt stales because the energy difference between the singlet and triplet states is larger and spin-orbit coupling is less likely. 

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). 

The failure to observe photosubstitution in the presence of a sensitizer in which the latter is the principal absorber, the invariance of product quantum yield with wavelengths shorter than 350 nm (onset of n -> -n absorption), and the observation that chloride and bromide ions (known to catalyze S-+T intersystem crossing) strongly diminish the quantum yields of these reactions, strongly points to the lowest excited ir- n singlet state as the reactive species in these transformations. Excitation into the n->ir absorption band results in little product formation. A triplet state may, however, be involved in the photoamination of nitrobenzene.a41)... 

This raises the question as to why one should observe any photoreduction at all with those compounds in which the lowest triplet clearly is of it -> rr character. The answer to this question lies in the fact that the electronic states are not pure in character. That is, an n - it state has some character obtained from a mixing of other electronic levels. This mixing results from vibronic and spin-orbit coupling/51-52 Likewise a it - w state has a degree of n —> w character obtained by mixing. It is this degree of n—> ir character which is responsible for the observation of photoreduction... 

The bond orders for both n- n and ir - n singlet and triplet states (PPP Cl calculation) indicate (Table 7.1) that the one excited state which is not ft/ bonding is the triplet state. Such bonding is predicted to be... 

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