Triplet states excitation transfer from

Electron transfer from borohydride/borate to tbe catalyst and its excitation to its triplet state. Electron transfer from catalyst to the adsorbed 4-nitropbenolate,... 

The most investigated system for energy transfer from the excited triplet states of fullerenes to an acceptor is the formation of singlet oxygen. Both the fullerenes, C6o and C70, in their triplet states efficiently transfer energy to 302 to... 

Intermolecular triplet-state energy transfer is well known in photochemistry. McDowell et al. (334) first demonstrated by ESR the triplet energy transfer from benzophenone to naphthalene in a rigid glass. The excitation transfer is found to be independent of orientation (335a), while the probability of excitation transfer per lifetime is dependent on concentration (335b). 

Continuing work 158) on photoreactions of electron-rich olefins with biacetyl shows that the complexity of product mixtures obtains in these reactions also. Effects of solvent polarity provide further support for the importance of ionic intermediates in these reactions. The reactions of biacetyl with 1,1-diethoxyethylene are proposed to proceed via the triplet state (in contrast to reactions with dioxoles). The reversal of regiospecifity between thermal and photochemical cycloaddition of this olefin with biacetyl is nicely explained by the assumption of excited state electron transfer from olefin to dione to give the corresponding radical ions. 

Photophysical studies of various triaryl-2-pyrazolines in solution and in microcrystalline forms show that in solution triplet states are formed only by excitation transfer from a donor whilst by contrast they are formed directly in the microcrystalline... 

The luminescence originates from a Cp biq LLCT triplet. The complex shares some remarkable features with the photosynthetic reaction center which is characterized by a pair of two bacteriochlorophyll molecules in close proximity but in an oblique orientation ( special pair ). In the primary photochemical step charge separation takes place by excited state electron transfer from this special pair to an acceptor. In the case of [Cp2Zr biq] + the special pair is represented by both Cp ligands. However, in this complex charge separation takes place by a direct optical transition. 

An argument in favor of the intermediacy of triplet states in at least some of these reactions < an be based on the observation that cycloadditions do not appear to occur when vertical excitation transfer from the triplet state of the ketone to the olefin is energetically feasible. It has been found, for example, that ketones with triplet excitation energies sufficiently greater than those of the stilbenes or the 1,3-pentadienes, among several systems, are quenched with unit efficiency by the olefins and that no adducts are formed (Hammond el at., 1964a). The occurrence of Reaction (64) implies that cycloaddition can compete with nonvertical triplet excitation transfer. 

The evidence favoring the intermediacy of triplet states in the photoconversion of santonin (XXV) to lumisantonin (XXVI) is twofold. First, the rearrangement can be induced by means of triplet excitation transfer from benzophcnone to (XXV), and second, the photoconversion is essentially completely quenched when santonin is irradiated in 1,3-pentadiene. If the quenching process can be attributed to transfer of electronic excitation, then a santonin triplet state is required since singlet excitation transfer from santonin to 1,3-pentadiene is not energetically feasible. 

These authors observed phosphorescence from 4,4-diphenylcyclohexadie-none at 77°K (0-0 band at 68.8 kcal per mole) and pointed out that the similarity between the phosphorescence spectrum of the dienone and that of benzophenone indicates that the lowest triplet state of the former compound is n — IT. This evidence suggests that a triplet state of the dienone may be involved in the unsensitized rearrangement but is not sufficient to justify Zimmerman and Swenton s conclusion that such a state is demanded. Interestingly, naphthalene (triplet 0-0 band at 61 kcal per mole) sensitizes rather than quenches Reaction (110). Such sensitization has also been observed by Caspar and Hammond (unpublished results) in a similar system and can be best interpreted as involving singlet excitation transfer from naphthalene to the dienone. 

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