Triplet Processes

Two more recent studies of the nature of the sulfur-carbon bond cleavage in sulfoxide photolyses have been conducted, using ESR5 and CIDNP6 detection methods, respectively. In the latter case there is some evidence for a triplet process being involved in the photocleavage of aryl methyl sulfoxides. 

In both cases discussed above determinations should be made at several different flash intensities and the results extrapolated to 7 = 0 to eliminate errors due to bimolecular triplet processes arising from the high triplet populations. 

The products formed upon direct irradiation of 1 include 9-phenanthrol and fluorene. The former is believed to be formed in a triplet process. This is supported by the capacity to quench phenanthrol formation with tram-1,3-pentadiene and to circumvent the rearrangement to 292 (R = H) by utilization of sensitizers such as benzophenone or triphenylene. In an attempt to confirm that the chemically significant excited state in the conversion of 1 to 292... 

Their kinetic and product analysis is consistent with the working hypothesis indicated previously for insertions the singlet process is stereospecific, while the triplet process is at best only selective. Here, as in the reaction of sulfur atoms with alkenes, (217), some progress has been made in separating the problem of the reacting species and their reaction rates from the observed SS in the product (Gunning and Strausz, 1966). In general, however, interconversion rates of triplet cisoid and transoid diradicals are uncertain, and deductions from overall stereoselectivity are also uncertain. 

Although bond cleavage and photoionization are common processes occurring from upper excited states, the energy stored in these states can also facilitate the formation of new bonds. For example, multiphoton excitation of perylene and pyrene in cyclohexane leads to hydrogen abstraction from the solvent and subsequent radical coupling products [81]. Two-color irradiation of anthracene (47) in the presence of acrylonitrile leads to a four-fold increase in the amount of cycloadduct 101 as compared to that obtained under one-laser irradiation [52] (Scheme 7). That this is an upper triplet process was indicated by the triplet depletion... 

In analogy to hydrocarbon chemistry, the triplet process presumably proceeds stepwise and it could occur either with or without waiting for a spin inversion (equation 45). 

It has previously been thought that 1,3-acyl shifts occur uniquely by singlet mechanisms, the triplet processes giving 1,2-shifts several examples of triplet... 

The energy transfer is a triplet-triplet process (p. 197) and because ketones provide high yields of the triplet state, compounds such as acetone, benzophenone, and diacetyl have been used in the study of these reactions. Sensitized reactions often result in a higher reaction quantum yield than observed in direct photolysis. The reaction quantum yield for the direct photolysis of ethyl pyruvate at 313 nm in benzene... 

Unlike the situation in the singlet excited state, cts-fraws-isomeriza-tion is very effective from the triplet state. 9,18,20,48) When the carbon-carbon double bond is not geometrically constrained (f. e. the bond is part of an acyclic system or not in a small ring) m-fra s-isomerization can become so efficient that it is (almost) the only triplet process observed and the other triplet processes do not compete. This effective process has been called the /rcc rotor effect for. y-unsaturated ketones in view of the similarity to the concept proposed earlier for flexible di-it-methane systems. 89) Flexible double bonds thus provide an efficient pathway for dissipation of the energy of the triplet state. The free rotor effect is a chemical pathway of intersystem crossing. The effect is nicely borne out... 

The methylenebicyclo[2,2,lJpentane (135) is produced in the photolysis of either trans- or cis-7-phenyl-2,5,5-trimethylhepta-2,3,6-triene (135a). This [2 + 2] product has been shown to be the result of a triplet process. Singlet processes give rise to di-ic-methane rearrangement products. 

Such processes have been the focus of many studies, especially in Uquid phases. The triplet-triplet process is interesting as it allows excitation of the triplet state of the molecule A that would otherwise be inaccessible (for instance due to a poor intersystem crossing Si Ti). This then may increase the yield of a reaction but also initiate specific reactions. If the energy required to excite the initial state D (prior to its transition to the triplet state) is lower than the excitation energy of A, then photosensitized reactions of A become possible at longer wavelengths [3]. 

I think for two reasons. Gas chromatography had come in, so it was possible to analyze expeditiously complex mixtures of rather small molecules, and George Hammond and his students showed quite clearly that the photoreduction of benzophenone was a triplet process which could be studied conveniently by competitive kinetics (22). Triplet states are essentially biradicals and much of photochemistry turned out to be free radical chemistry. I ve only had a mild hand in this area Morton Gibian and I showed the close similarity in reactions of benzophenone triplets and t-butoxy radicals ( 3) > of my former students,... 

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