Benzophenones quenching

Direct evidence for triplet-triplet transfer has been provided by sensitizing both the isomerization of cis- or ira/w-olefins,28 38 79,80 and the dimerization of some cyclic olefins81-83 with carbonyl compounds. Furthermore, the phosphorescence of some carbonyl compounds can be quenched by olefins (for example, acetone with 2-pentene30 and phenyl-cyclopropyl ketone with 2-methyl-2-butene37). On the other hand, the phosphorescence of benzophenone is not quenched by 2-methyl-2-butene37 nor is the photoreduction of benzophenone quenched efficiently by cyclohexene (Table II). 

E.N. Step, J. Turro, M.E. Gande, and P.P. Klemchuck, Flash photolysis and time-resolved electron spin resonance studies triplet benzophenone quenching by hindered amine light stbilizers (HALS). A comparison of HALS amines and aminoethers and hydrogen atom donors, J. Photochem. Photobiool. A Chem. 1993, 74, 203-210. 

TTie photoreduction can be quenched by known triplet quenchers. The effecti e quenchers are those which have T] states less than 69kcal/moI above S,. Quenchers with higher triplet energies are ineffective because the benzophenone n-n triplet is then not sufficiently energetic to effect energy transfer. 

Chloromethylphetiyl sulfoxide, metalated with butyllithium and then quenched with a symmet-rical ketone, e.g., cyclohexanone, acetone, or benzophenone, gave only one diastereomeric species. ... 

The pinacol formation reaction follows a radical mechanism. Benzopinacol, benzophenone and the mixed pinacol are formed jointly with many radical species [72, 74]. In the course of the reaction, first a high-energy excited state is generated with the aid of photons. Thereafter, this excited-state species reacts with a solvent molecule 2-propanol to give two respective radicals. The 2-propanol radical reacts with one molecule of benzophenone (in the ground state, without photon aid) to lengthen the radical chain. By combination of radicals, adducts are formed, including the desired product benzopinacol. Chain termination reactions quench the radicals by other paths. 

In Section 3.1 it was shown that the photoreduction of benzophenone can be quenched by addition of small amounts of triplet quenchers such as oxygen or ferric dipivaloylmethide.<60) In fact this was presented as evidence that the benzophenone triplet was involved in the photoreduction. This reaction can also be quenched by naphthalene. In the presence of naphthalene, light is still absorbed by benzophenone and thus benzophenone triplets are produced. However, photoreduction products are decreased. On examining this reaction with flash photolysis, triplet-triplet absorptions were observed but these absorptions corresponded to those of the naphthalene triplet. Thus the triplet excitation energy originally present in the benzophenone triplet must have been transferred to naphthalene and since little of the photoreduction product was observed, this transfer must have been fast in relation... 

Attempts to sensitize the rearrangement with benzophenone, propiophenone, and chlorobenzene failed, as indicated in Table 8.1. Although the reaction could not be sensitized, triplet energy transfer was taking place inasmuch as compound (1) quenched the photoreduction of benzophenone without the formation of any new products (Table 8.2). 

As with 2-cyclopentenone, the ratio (70) (71) varies with the molar concentration of the enone, the head-to-head dimer (71) becoming increasingly important at higher concentrations/133 This reaction is efficiently sensitized by acetophenone, benzophenone, thioxanthone, and naphthalene. The same enone concentration effect was observed in the sensitized photo-dimerization as in the direct photolysis. Similarly, quenching of the dimerization by piperylene was not accompanied by a change in dimer ratio. Systematic... 

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. 

Let us now discuss some of the characteristics of this quenching with mercaptans and disulfides. Interestingly, both sulfur derivatives are equally effective in inhibiting the photoreduction and are in fact interconverted during the reaction. The same equilibrium mixture of mercaptan and disulfide is obtained regardless of which was initially added to the reaction mixture. Furthermore, there appears to be no appreciable consumption of the sulfur compounds/64 When benzophenone is irradiated in the presence of isopropanol (OD) and mercaptan, isopropanol containing two deuterium atoms is isolated,... 

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 ... 

The quenching of benzophenone phosphorescence has been used by Mar and Winnik (1981) as a photochemical probe of hydrocarbon chains in solution. The bimolecular reaction for quenching the triplet state of 4-methoxy-carbonylbenzophenone [24] by 1-pentene occurs at rates which are below the diffusion limit by two to three orders of magnitude. Consequently, the intramolecular quenching reactions of to-alkenyl esters of benzophenone-4-carbo-xylic acid [25] occurs under conformational control. In [25] the point of... 

Fig. 23 Entropy effects on intramolecular reactions of polymethylene chains. Plot of 9AS (e.u.) against number of single bonds for (O) nucleophilic substitutions at saturated carbon ( ) electron-exchange reactions (A) quenching of benzophenone phosphorescence. The straight line has intercept +30 e.u. and slope —4.0 e.u. per rotor. The right-hand ordinate reports the purely entropic EM s calculated as exp(0AS /J )...

The photoreduction of nitrobenzene using p5o ex filtered light from a medium pressure mercury arc was studied in petroleum, toluene, ether, 2-propanol, tert-butyl alcohol, diethylamine, triethylamine, aqueous solutions of 2-propanol and diethylamine and also in aqueous t-butylalcohol containing sodium boro-hydiide 3 >. Varying amounts of aniline, azo- and azoxybenzene were obtained. In the presence of a fourty-fold excess of benzophenone, a six-fold increase in the rate of aniline formation in ethereal solution was observed, and aniline formation was completely suppressed by addition of biacetyl or octafluomaphthalene Since unreacted nitrobenzene could be recovered in these experiments, it is demonstrated that the triplet state of nitrobenzene was quenched. 

The photohydrolysis of 2-fluoro-4-nitroanisole to 2-methoxy-5-nitrophenole is sensitized by benzophenone and completely quenched by sodium sorbate The excited state multiplicity in photoaminations has also been studied. Photolysis of mNA in liquid ammonia yields m-nitroaniline. If the amination is carried out in a large excess of benzophenone, 2-methoxy-4-nitroaniline is formed instead and thus an excited singlet state as reacting species is envisaged in the unsensitized photoamination loo.ioi). it may well be that uptake of the nucleophile present in high concentration successfully competes with intersystem crossing. 

No, because the molecule carries a built in quencher around with it. Each benzophenone triplet formed will be quenched at a rate much faster than hydrogen abstraction from solvent. 

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