Triplet-quenching studies

When a photochemical reaction is quenched by a compound that is known to be a triplet quencher, the chemically-active state is very likely 

The inhibition of product formation by a triplet quencher readily allows a triplet mechanism to be identified where the absence of inhibition by a triplet quencher indicates a singlet mechanism. 

Suitable triplet quenchers should have the following properties  

Conjugated dienes, such as penta-1,3-diene H2C=CH-CH=CH3, are excellent triplet quenchers. 

In addition, penta-1,3-diene and (1) will almost certainly have different absorption characteristics, due to the different chromophoric groups in the two compounds. 

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The photodecarbonylation of a series of dibenzyl ketones was studied by Robbins and Eastman/63 The results of this study are presented in Table 4.5. The data in Table 4.5 indicate that the presence of a p-methyl or a p-methoxy group has little effect on the quantum yield for this reaction. p-Cyano groups, on the other hand, essentially totally eliminated the decarbonylation. Since the reaction could also be quenched (inefficiently) by benzonitrile or biphenyl, it was concluded that the decarbonylation occurs from a short-lived triplet state. The effect of the p-cyano groups then could result from internal triplet quenching. 

The product mixture contained 81% of the c/j-3-p-methoxyphenyl-f/my-2-phenyl derivative. Again a reactive triplet state was involved, as evidenced by sensitization and quenching studies (kr = 1.5 x 1010 sec-1). 

With aromatic carbonyls, oxetane formation appears to arise from the carbonyl triplet state, as evidenced by quenching studies. For example, benzaldehyde irradiated in the presence of cyclohexene yields products indicative of hydrogen abstraction reactions and an oxetane ... 

In addition, it was observed that the sensitized photolysis produced the same distribution of products with the same efficiency (fingerprint characteristic of the triplet state). From quenching studies the specific rate constant for the rearrangement could be obtained. Phenyl migration rearrangement is of intermediate efficiency, interposed between the more efficient and less efficient type A processes (Table 7.4). The type of mechanism proposed for this transformation is as follows ... 

Evidence that eliminates the triplet mechanism as the mode for the cis-trans isomerization of stilbene upon direct photolysis has been provided by azulene quenching studies.(48) Using the experimentally determined decay ratio a/(l — a) and the triplet mechanism, it is possible to calculate what the effect of azulene is upon the pss. The predicted and observed azulene effects on the direct photoisomerization are shown in Figure 9.6. The failure of the triplet mechanism in predicting the very small changes observed in the pss provides a crucial test that is the basis for rejecting the triplet mechanism. 

Quenching studies on the photolysis of 14 (R = C(CH3)3) using 1,3-pentadiene reveal that even at 2.5 mole pentadiene not all excited states are quenched >, thus at present a triplet state as reacting excited state cannot be regarded as proven for the direct irradiations although it seems likely in view of the high triplet yield reported for nitrobenzene. 

Whereas a straight Stem-Volmer plot could not be obtained in quenching studies with octafluomaphthalene, the orders of magnitude of the rate constants of triplet decay ka) and of reaction with olefin ( r) could still be estimated Ad = 6 X 10 s i and ki =2 X 10 1 mole is i >). In view of this rapid decay, a high concentration of olefin is evidently necessary for effective addition. 

In most of the cases studied bimolecular kinetics are followed, the rate constants of product formation depending on the rate of light absorption and on nucleophile concentration. Triplet lifetimes (as determined from quenching studies) also depend on nucleophile concentration. This means that the excited state is quenched by the nucleophile, accompanied by either product formation or reversal to starting material. In view of the inherently different triplet lifetimes of different substrates, it is highly desirable to rely on rate constants rather than... 

Again, a hnear relationship of and [CNS] i shows a bimolecular reaction between the excited triplet state of 1 and the nucleophile to take place. The triplet lifetime of la is 4.7 X 10 s in water and 1.2xl0 8s in aqueous 10 2M solutions of potassium cyanide as determined from quenching studies The nitro group in 7 a is likewise replaced photochemically by methoxide and cyanate ions. 

Fluorescence quenching studies can establish the rate constant at which a certain substrate interacts with the excited carbene, but they cannot provide any independent mechanistic information. Absorption studies are somewhat more informative in that the primary product of reaction can sometimes be detected directly. In the reaction of di-p-tolylcarbene with CCI4, the radical, (MeC6H4)2CCl, obviously formed as a result of abstraction of Cl atom from the substrate, is detected. Its formation can be monitored to give a rate constant of 1.1 x 10 M s for the excited state, which should be compared with a rate of 2 x 10 M s for ground-state triplet DPC. ... 

Alternatively, since r is the reciprocal of the sum of rate constants for all the processes undergone by the excited state, the reaction rate constant may be estimated if the others (e.g. for phosphorescence and intersystem crossing in the case of a triplet) are known from other studies. The most convenient way, however, of measuring t for a reaction that can be quenched is to carry out a quantitative quenching study at different quencher concentrations. In the most straightforward systems the results can be fitted to a straight-line plot expressed as < 1.16, where is the quantum yield in the absence of quencher, < > is the measured quantum yield at quencher concentration [Q], and is the rate constant for quenching. 

The problem of accurately determining rates of quenching is important not only for understanding energy transfer but also for estimating rates of physical and chemical reactions of excited triplet species. Quenching studies of the Stern-Volmer type184 yield values of kQrT, where rT is the lifetime of the triplet species and kq is the rate constant with which some compound quenches it. Since quantum-yield and product-yield measurements allow rT to be factored into rate constants for individual reactions, absolute values of these reaction rate constants can be determined provided that the absolute value of... 

It is well known that o-hydroxyphenyl ketones phosphoresce very weakly and do not undergo photoreduction363,368 or photoelimination,366 presumably because of rapid enolization in the excited state. Lamola has performed quenching studies which indicate lifetimes of o-hydroxybenzophenone triplets of less than 10"8 sec. However, these lifetimes, as well as phosphorescence efficiencies, are markedly enhanced by the addition of alcoholic solvents,383 presumably because hydrogen bonding slows down photoenolization. 

Pyruvic acid undergoes photoreduction from its triplet state, and quenching studies indicate that its triplet is several times more reactive than that of benzophenone.100... 

Another physical study which used flash photolysis relates directly on the MB/amine system. Kayser and Young (36) examined a more extensive series of amines, both aromatic and aliphatic, than Steiner (33). Their results are shown in Table 3. Excellent correlation was obtained between the amine ionization potential and the rate constant for MB quenching the slope of the logarithmic plot was -1.75 eV-1- This value is relatively small compared to some oxidizing excited states (e.g. hydrocarbons, -17 eV l (37)), but it is similar to the value observed for ketone triplet quenching by amines (-1.5 eV l (38)), and does indicate that the quenching interaction becomes more facile as... 

The reason for the difference in stereochemistry is that, as quenching studies show, the reactive state of the aromatic carbonyls (which undergo ISC rapidly) is T1 but in aliphatic carbonyls it is The intermediate formed from acetaldehyde and m-2-butene, then, is the singlet biradical 34, whereas the intermediate from benzaldehyde and w-2-butene is the triplet biradical 35. That the stereo-... 

Luminescence quenching of the zinc-substituted cytochrome c (Zn-cyt c) excited state by cytochrome b5 (cyt b5) has been investigated97. The most striking result of the quenching study is that the triplet emission decay rate of Zn-cyt c (3k = 102 s ) is remarkably accelerated in Zn-cyt c/cyt b5 (3k = 5 x 105 s-1) by about a factor of... 

The temperature dependence of the rate constant of electron transfer over large distance from the first triplet state of Zn porphyrin to Rum(NH3)5 covalently attached to histidine-33 in Zn-substituted cytc was studied in Ref. [318]. A temperature independent triplet quenching process with the rate constant 3.6 s-1, was observed at 10-100 K and tentatively attributed to electron transfer facilitated by nuclear tunneling. 

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