Triplet-state reactions

For a triplet state reaction the quantum yield is not only dependent upon the relative rates of the process and other processes leading to deactivation of the triplet state, but also to the efficiency of population of the triplet state (d>isc) ... 

It was shown that the patterns of the relative rates of reaction of DPC and DPC with alcohols are essentially identical and followed the relative acidity of the alcohols (MeOH > /-PrOH > f-BuOH) and showed a kinetic deuterium isotope effect on reaction with the OH bond (Table 9.13). These results indicate that DPC attacks the O—H bond rather than the C—H bond of the alcohol. If the C—H bonds of the alcohol were attacked by DPC, as in the ground triplet state reaction, then one would expect 2-propanol would react faster than methanol. Lack of any dis-cernable quenching of DPC by diethyl ether and THF indicates that DPC ... 

We focused here on triplet-state reactions occurring after photoexcitation. We have not treated the ISC event rigorously, and believe, subject to the evidences provided above, that the ISC event itself is of minor interest in these systems. However, there exist systems where the ISC itself plays an important role. Recent advances in method developments have provided a tool for treating intersystem crossing events. Worthwhile to mention is the work of Gonzalez (SHARC) [71] and Thiel and coworkers [72] in treating internal conversion (IC) and ISC events on the same footing. 

When an alkyl or aryl ketone, or an aryl aldehyde, reacts with an alkyl-substituted ethylene, or with an electron-rich alkene such as a vinyl ether, the mechanism involves attack by the (n,n triplet state of the ketone on ground-state alkene to generate a 1,4-biradical that subsequently cyclizes. The orientation of addition is in keeping with this proposal, since the major product is formed by way of the more stable of the possible biradicals, as seen for benzophenone and 2-melhylpropene (4.64). As would be expected for a triplet-state reaction, the stereoselectivity is low, and benzophenone gives the same mixture of stereoisomers when it reacts with either trans or... 

In the case of dianthrylethanone 34, the course of the photochemical isomerization was found to be concentration-dependent (cf. Table 6). Triplet state reaction products are the 47t + 2n cyclomers 35 and 36, which are formed mainly (80%) by irradiation of 34 at low concentration [51], The predominant (98%) formation of the singlet state product, i.e., 47t + 47t cycloadduct 37, is observed when dianthrylethanone 34 at high initial concentrations is... 

Wagner and Sakamoto reported the first example of the intramolecular (2 + 2) photocycloaddition of simple alkenes to the excited triplet state of naphthalene [290], Cyclizations of 266 and 267 are entirely triplet-state reactions The formation of cycloadducts is sensitized by Michler s ketone and is quenched by conju-... 

Photochemical reactions in the solid state may also go to limited conversion when the product does not compete for incident light but may act as an efficient energy quencher. This is likely to be of importance in triplet state reactions when the product may act as an energy trap. Although this is a factor that may also limit the conversion of photochemical reactions in solution, because of efficient energy delocalization by dipole-dipole and exchange mechanisms it is likely to be more important in the crystalline solid state. 

Consider a photoreactive molecule M. If the triplet state of M is involved in any photochemical reaction, then addition of triplet acceptors to the reaction solution should quench triplet state reactions by the diffusion controlled process... 

Reaction 1 represents formation of the geminate reaction products [the chlorophyll r-cation radical ( Chl+) and quinone radical anion ( Q )] within the bilayer by electron-transfer quenching of the photoexcited chlorophyll triplet state reaction... 

A sensitized photolysis is one in which the light is absorbed by one compound, such as acetone or benzophenone, which then interacts with the substrate (acrylonitrile in this case) in such a way that the latter is promoted to an excited state and the former reverts to the ground state. The sensitizers generally used fulfil this function from their triplet states, thus, almost all sensitized photolyses are triplet-state reactions. 

The Norrish Type II reaction of aliphatic and aromatic ketones in isotropic solvents has been studied in considerable detail (26,43), and several aspects of the reaction depend on the conformational mobility of the excited ketone or the 1,4-biradical intermediates formed by y-hydrogen abstraction. In the case of aromatic ketones for example, the triplet lifetime can provide an indication of the facility with which the proper geometry for hydrogen abstraction can be obtained (29,43), the distribution of fragmentation O-cleavage) and cyclization products obtained depends on the conformations available to the triplet 1,4-biradical intermediate and their relative kinetic behavior prior to intersystem crossing (27-30,43-47), and the total quantum yield for the reaction is a function of both of the above factors. For practical reasons, product ratios are usually the easiest aspect of the reaction to monitor, and this is the approach that has been used most commonly in studies of Norrish II reactivity in ordered media (27-30,45). The pertinent features of the triplet state reaction arc illustrated in Scheme 1 (30). 

Perfluorobenzene 76 when irradiated in the gas phase (35 mm) at 25° produced the Dewar benzene 77 5 -58). The triplet state reaction of 76 gives only higher molecular weight materials. 

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