Benzophenone triplet state

Let us now return to the question of the benzophenone triplet state. The problem was to explain why the benzophenone triplet lifetime is shorter... 

Benzophenone Triplet State Intermediate Insertion with Covalent Bond Formation... 

Backstrom, Steneryr, and Perlmann18 found that in the liquid-phase benzophenone-sensitized decomposition of i-C3H7OH at 25°C, the yields were reduced by adding NO. They interpreted their results in terms of quenching of the benzophenone triplet state by NO. 

The primary intermediates formed from the benzophenone triplet state in the presence of tertiary amines such as dimethylaniline have been observed by picosecond transient absorption spectroscopy [99]. The initial step of the process involves one-electron transfer from amine (dimethylaniline, DMA) to the triplet state... 

Benzophenone triplet states have been characterized by photothermal beam diffraction spectroscopy. This is a calorimetric method which depends upon sensing the temperature dependent refractive index gradient at the edge of the excitation beam and... 

Excited-state Phenomena.—Czarniecki and Breslow provide an experimental test for structure and organization in micelles and vesicles based on inter-molecular hydrogen-atom abstraction by benzophenone triplet-state. This leads to functionalization at a particular site in the chain, which may be identified by degradation and mass spectrometry. With didodecyl phosphate and (97) or (98), the specificity is as shown (Figure 9) for sonicated surfactant, presumed to be in vesicular form. This shows a much altered specificity from the corresponding reaction in micelles. When sonication is carried out in a sodium borate buffer, a technique considered to provide multilamellar layers (opaque dispersion ) then the terminal selectivity is completely lost and C(6) is the most favoured site for attack. 

Table 9 Rate Constants for the Quenching of Benzophenone Triplet States by Monomer...

Figure Bl.16.16 shows an example of RTPM in which the radical species is TEMPO (10), a stable nitroxide radical, while the triplet state is produced by photoexcitation of benzophenone (11) [45].

In weaker acid systems, other reactions involving the triplet state supervene to the exclusion of dimerization. Photolysis of 85 in 3-3% sulfuric acid, 96-5% acetic acid, and 0-2% water gave as products tri-phenylmethane (93), 9-phenylfluorene (94), 6is-9-phenylfluorenyl peroxide (95) and benzophenone (96). When benzene was present, tetra-phenylmethane (97) was also formed in addition to the other products. When the triphenylmethyl cation is irradiated in 3-3% H2SO4, 80 1% HOAc, 16-4% toluene, and 0-2% H2O, the products observed were... 

Hydroxy Benzophenone P-Naphthoxy Energy Transfer Processes Triplet State Intramolecular Excimers" ... 

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... 

We have already discussed one of the earliest photoreactions to be studied, that is, the (4w + 4w) photodimerization of anthracene. That the singlet state was involved in this reaction was conclusively shown in the period 1955-1957. The first reaction in which the triplet state of the molecule was shown to be involved was the photoreduction of benzophenone by Hammond and co-workersa) and Backstrom and co-workers<2) 1959-1961. This was the first in a series of many papers from Hammond s laboratory... 

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). 

Since the phosphorescence emission from (6) (68.8 kcal/mole) is very similar in energy and vibrational structure to benzophenone, and has a short lifetime (0.5 msec), it was proposed that the photorearrangement takes place via the triplet state. A Zimmerman-like mechanism is as follows for the formation of the cyclopropyl ketone (7) from dienone (6) ... 

Dimers (73) and (74) were formed in approximately equal amounts in all cases, although, as in the cases of 2-cyclopentenone and 2-cyclohexenone, the relative amount of (72) (either cis-syn-cis or cis-anti-cis) was found to vary substantially with solvent polarity. As in 2-cyclopentenone, this increase in the rate of head-to-head dimerization was attributed to stabilization of the increase in dipole moment in going to the transition state leading to (72) in polar solvents. It is thought that the solvent effect in this case is not associated with the state of aggregation since a plot of Stem-Volmer plot and complete quenching with 0.2 M piperylene indicate that the reaction proceeds mainly from the triplet manifold. However, the rates of formation of head-to-head and head-to-tail dimers do not show the same relationship when sensitized by benzophenone as in the direct photolysis. This effect, when combined with different intercepts for head-to-head and head-to-tail dimerizations quenched by piperylene in the Stem-Volmer plot, indicates that two distinct excited triplet states are involved with differing efficiencies of population. The nature of these two triplets has not been disclosed. 

In acidic solution the rearrangement of a-santonin to lumisantonin and then ultimately to photosantonic acid is not as efficient as rearrangement to the hydroxy ketone, isophotosantonic lactone (5), shown on page 308. Fisch and Richards(6) found that the photorearrangements of a-santonin could be sensitized with benzophenone or Michler s ketone. Moreover, if the irradiation of a-santonin (3660 A) is carried out in piperylene as solvent, the photoreaction is completely quenched. This suggests that the rearrangements proceed via triplet states. 

Comparison of these experimental results with the calculated charge densities (S0 and Si) at the 2 and 3 positions (Table 11.5) shows that this is the expected result. Except for those compounds discussed below, the failure to observe quenching with triplet quenchers and reaction in the presence of a photosensitizer indicated singlet reactions. Compound (89) was found to also undergo benzophenone-photosensitized substitution, indicating that the triplet state of this compound is also reactive. The reaction, however, was less clean than that observed in the direct photolysis. Similarly, 1,6-dinitro-naphthalene was found to undergo both direct and benzophenone-photosensitized substitution ... 

Molecules with two or more unpaired electrons may be divided into two classes by far the most common examples are molecules where the unpaired electrons are contained in a set of degenerate atomic or molecular orbitals with qualitatively similar spatial distributions, e.g., an octahedral Cr(m) (4A2g) or Ni(n) (3A2g) complex, a ground state triplet molecule like 02, or the excited triplet states of naphthalene or benzophenone. 

Figure 5.33 Benzophenone-4-iodoacetamide reacts with sulfhydryl-containing compounds to give thioether linkages. Subsequent photoactivation of the benzophenone residue gives a highly reactive triplet-state ketone intermediate. The energized electron can insert in active C—H or N—H bonds to give covalent crosslinks.

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