Benzophenone triplet

The second light-requiring step ultimately leading to the tetraphenyl methane was shown to occur by energy transfer from triplet benzophenone to the fuchsone, resulting in triplet fuchsone. The mechanisms of energy transfer will be discussed in a later chapter. 

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 ... 

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

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-... 

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. ... 

A study has been made of the relative efficiencies with which various transition metal chelates quench triplet benzophenone.194 The chelates vary widely in efficiency, and no generalizations can be drawn except that in some cases triplet energy transfer to a coupled metal-ligand triplet energy level probably accounts for at least part of the quenching. Rare earth ions can quench excited triplets by energy transfer, since, as discussed earlier, sensitized fluorescence of the metal ion results. 

The exact values for the rate constants for hydrogen abstraction by triplet benzophenone are not yet entirely certain. Three groups338-338 have reported a value of 108M-1 sec-1 for abstraction from 2-propanol in concentrated 2-propanol, while the combination of the data of three other groups333,338 339 for dilute benzene solutions yields a value of only 105M-1 sec-1. This discrepancy could well reflect a solvent effect such as that found in studies of the reactivity of alkoxy radicals.340 However, the hundredfold difference between the reported rates for attack of triplet benzophenone on toluene338,338 undoubtedly reflects experimental problems, because both values were measured in aromatic solvents. 

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). 

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 ... 

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... 

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 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]. 

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. 

The dependence of kp on the type of a-hydrogen is not known, nor can variations in ke be predicted easily. Mataga has studied amine quenching of triplet benzophenone by flash spectroscopy. N,N-dialkylanilines are the only amines which actually yield radical ions, and then only in polar solvents 157). He suggests two competing decay modes of the exciplex. Monoalkylanilines and tertiary aliphatic amines in any solvent and dialkylanilines in nonpolar solvents yield only radicals, presumably from the exciplex. Even though the oxidation potentials of tertiary aliphatic amines are so low that they quench triplet ketones at rates... 

There is really no solid evidence regarding the relative reactivities of n,n and n,7t triplets in CT quenching. The rate constants for tertiary amine quenching of triplet naphthyl ketones are in the 10s—106 M 1 sec-1 range, as compared to values greater than 10 M-1 sec-1 for quenching of triplet benzophenone, acetophenone, etc. A good deal of this difference must reflect the low triplet excitation... 

The rates at which several substituted benzenes quench triplet benzophenone have been measured 17 8). No single linear free energy relationship can be derived. For alkoxybenzenes, alkylbenzenes, benzene itself, and benzotrifluoride as quenchers, one finds a linear plot of log vs. IP with a slope similar to that found for the plot of all substituted benzenes and triplet a-trifluoroacetophenone. A given aromatic such as benzene quenches the fluorinated ketone triplet, which has an E - -E(A /A) value of only 16 kcal, some 50 times faster than it quenches triplet acetophenone or benzophenone 132>. This rate difference reflects only 20% of the full 12 kcal difference in thermodynamic redox potentials. However, the halobenzenes and benzonitrile quench triplet benzophenone faster than does benzene 178>. It seems likely that with these electron poor benzene derivatives, some alternate chemical reaction becomes dominant. Although a reverse CT process has been suggested, with the triplet ketone as donor, it is perhaps more likely that some sort of radical addition occurs with conjugating substituents on... 

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

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

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. 

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. 

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)... 

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. 

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