Benzophenone triplet

Given sufficient time the triplet benzophenone also will decay back to the ground state singlet in the absence of a hydrogen atom donor in a unimolecular process as follows  

The photocycloaddition of triplet benzophenone to norbornene has been originally reported by Scharf and Korte (Sch. 30) [94]. The photoproduct 101 which is formed in high cxo-selectivity could be thermally cleaved to the 5,8-unsaturated ketone 102, an application of the carbonyl-olefm-metathesis (COM) concept [95]. 

In the case of benzophenone, electron donor radicals are formed by the hydrogen abstraction of triplet benzophenone from either monomer or deliberately added substances  

Laser flash photolysis of benzophenone in pure SC CO2 at 33°C and 110 bar resulted in generation of triplet benzophenone within the laser excitation pulse (Figure 

They also obtained a similar CIDEP spectrum by triplet sensitisation with triplet benzophenone [13]. The polarization was found to consist of two net component, one reflecting the polrization of the triplet sensitizer due to the p-type TM, and the other being attributable to the d-type TM. 

It is at least amusing and perhaps even revealing that the dipole moment of triplet benzophenone is that of a C—0 single bond 71>. The measured h values for the n,n triplets of a-diketones are 2—3 orders of magnitude lower than for monoketones 71a>. It is likely that the low Ei values ( 55 kcal/mole) are not compensated for by lowered jr-bond energy. 

Cohen and Guttenplan were the first to recognize that the photoreduction of triplet benzophenone by amines occurs via PET followed by proton transfer to 

Assuming for the moment without proof that the reaction arises from the excited triplet benzophenone (B), one can write the following simplified mechanism  

Wong and Wan (136a) have suggested application of photo-CIDEP to the ISC study of triplet benzophenone. They pointed out that while PMDR methods normally study the dynamics of the ISC is zero field and in solid state, the photo-CIDEP study is potentially useful to estimate the intersystem crossing relative rates at a constant external magnetic field and in the liquid state. 

Hurley JK, Sinai N, Linschitz H. Actinometry in monochromatic flash photolysis the extinction coefficient of triplet benzophenone and quantum yield of triplet zinc tetrap-henylporphyrin. Photochem Photobiol 1983 38 9-14. 

At first, such CIDEP was considered to be due to the polarization transfer from an excited triplet molecule to a radical. The initial spin polarization of triplet benzophenone is emissive, but that of triplet pyruvic acid is absorptive. Similarly, the spin polarization of the radicals was always emissive regardless of the spin polarization of the triplet states [8]. Thus, such emissive signals were attributed to the T-D quenching as shown by Processes (13-8) 

As with arene-amine radical ion pairs, the ion pairs formed between ketones and amines can also suffer a-deprotona-tion. When triplet benzophenone is intercepted by amino acids, the aminium cation radical can be detected at acidic pH, but only the radical formed by aminium deprotonation is detectable in base (178). In the interaction of thioxanthone with trialky lamines, the triplet quenching rate constant correlates with amine oxidation potential, implicating rate determining radical ion pair formation which can also be observed spectroscopically. That the efficiency of electron exchange controls the overall reaction efficiency is consistent with the absence of an appreciable isotope effect when t-butylamine is used as an electron donor (179). 

Sensitization by acetophenone and benzophenone could be demonstrated for the cyclization of 74, R=4—C(CH3)3 to 20, R = 6—C(CH3)s. With the latter sensitizer, enhanced yields of N-hydroxy-2-indoUnone formation have been observed relative to direct irradiation runs. Triplet benzophenone does not act as dehydrogenating agent in the step 19 20, since the pinacolization of benzo- 

A different explanation was identified in the quenching properties of these heterocycles. Thiophene and mono methyl derivatives are efficient quenchers of triplet benzophenone. The Stem-Volmer plot showed a linear relationship [104, 105]. On the contrary, 2,5-dimethylthiophene (a compound able to give the cycloaddition reaction) is not a good quencher of benzophene [106]. N-Benzoylpyrrole also does not act as a quencher of the triplet benzophenone [106]. On the contrary, pyrrole and selenophene are quenchers of the excited benzophenone [106]. In this case, the Stern-Volmer plot is not linear. This situation is commonly encountered when the quencher employed quenches two excited states. It seems reasonable that pyrrole acts as quencher of both triplet benzophenone and the exciplex between triplet benzophenone and pyrrole [106]. 

The yield of trans product (18) is decreased by the presence of a radical scavenger such as 1,1-diphenylethylene and increased by dilution of the reactants with methylene chloride or butane, indicating this product to result from the triplet carbene. A heavy-atom effect on the carbene intermediate was observed by photolysis of a-methylmercuridiazoacetonitrile. With c/s-2-butene as the trapping agent either direct photolysis or triplet benzophenone-sensitized decomposition results in formation of cyclopropanes (19) and (20) in a 1 1 ratio  

It is common for many second-order reactions to exhibit pseudo-first-order behavior under conditions of nLFP. This is due to the fact that, while reactive intermediates are present in micromolar concentrations, (typically 10-50 pA/), the molecules with which they react are present in concentrations several orders of magnitude larger. As a result, the concentration of these reagents remains essentially constant during the decay of the transient species. An example is shown in Figure 18.4, where triplet benzophenone is quenched by melatonin.  

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