Triplet intramolecular

Fig. 20. Molecular orbital description for the electron transfer reactions within ECL processes of intramolecular donor-acceptor systems A-D to give (A) excited singlet intramolecular charge-transfer state (B) excited triplet intramolecular charge-transfer state (C) locally excited triplet and (D) ground state. Adapted from [138],...

The bicyclic product is formed by coupling of the two radical sites, while the alkene results from an intramolecular hydrogen-atom transfer. These reactions can be sensitized by aromatic ketones and quenched by typical triplet quenchers and are therefore believed to proceed via triplet excited states. 

A large red shift observed in polar solvents was indicative of the intramolecular charge transfer character of the triplet state. The change of dipole moment accompanying the transition Tj - Tn, as well as rate constants for electron and proton transfer reactions involving the T state of a-nitronaphthalene, were determined. The lower reactivity in polar solvents was attributed to a reduced n-n and increased charge transfer character of the triplet state... 

To summarize, the properties of triplet and singlet diradicals are closely related to the effectiveness of through-bond and through-space interactions, which are governed by the orbital phase continuity/discontinuity properties. In the next two sections, we will utilize this simple model to predict the spin preference and intramolecular reactivity for a broad range of diradicals. 

Hydroxy Benzophenone P-Naphthoxy Energy Transfer Processes Triplet State Intramolecular Excimers" ... 

Singlet Carbene C-H Insertions Although [1,2]-H shifts are formally carbene C-H insertions, these rearrangements have different orbital symmetry aspects than those of intramolecular insertions. As described above, overwhelming evidence exists that triplet carbenes undergo abstraction-recombination reactions to... 

The intramolecular triplet energy transfer studies described so far generally have used phosphorescence emission measurements to follow the... 

A number of intramolecular singlet and triplet energy transfer studies have been reported by Morrison and co-workers<90-92) and an extensive review of this general area has been published recently.f93)... 

Filipescu and co-workers(84-88) have also examined intramolecular singlet and triplet energy transfer using fluorescence and phosphorescence as a gauge for energy transfer. Lamola<87) has repeated one of the studies... 

Because of our obvious prejudices we will describe this study in some detail. The energy levels of the acetophenone and -methylstyrene moieties are ideally situated for intramolecular triplet energy transfer (see Figure 6.12). If these two chromophores are not interacting, then the absorption spectra for these compounds should be the composite of the acetophenone-/ -methylstyrene spectra. Figure 6.13 indicates that this is true for n = 4 (and also 2 and 3) however, it is not correct for n = 1 (Figure 6.14). In the case of n = 1 the increased intensity of the vibronic structure of the n - n ... 

Table 6.7. Rates and Rate Ratios for Intramolecular Triplet Energy Transfer in Compounds (7), n = 2,3,4...

The triplet state of carbonyl chromophores frequently shows a high reactivity in hydrogen abstraction reactions (l ). These processes can take place intermolecularly (photoreduction) ( l) or intramolecularly, for example in the Norrish Type II process, reaction 1 (.2,3.). 

Tung CH, Zhang LP, Li Y et al (1997) Intramolecular long-distance electron transfer and triplet energy transfer. Photophysical and photochemical studies on a norbomadiene-steroid-benzidine system. J Am Chem Soc 119 5348-5354... 

The a,( -unsaturated aldehyde 452 is generated from the unstable spiro-oxetane 451, and hydrogen abstraction from the aldehydic C-H bond by 3449 gave a triplet radical pair 453 and 454. Intersystem crossing and radical recombination followed by intramolecular nucleophilic attack of the hydroxyl group toward the ketene functionality furnish the diastereomeric products 54 and 55 (Scheme 102) <20000L2583>. 

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