Triplet excited state of ketones

S0, Sj, Tt ground, singlet and triplet excited state of ketone AH amine M monomer KH ketyl radical... 

It is well known that amines quench the triplet excited state of ketones, with or without resulting photoreduction of the ketone. Quenching may occur by com-plexation, electron transfer, or hydrogen transfer. For example, Scheme 5... 

Synthetic metal chelates have been known for a long time as efficient quenchers of singlet and triplet excited states of ketones and aromatic compounds (274). In 1967, Briggs and McKellar (275) found a good correlation between the triplet quenching efficiency of a series of Nl(II) chelates and their effectiveness as light stabilizers in polypropylene. They... 

Photochemical irradiation of a,-unsaturated ketones in the presence of Mc3SiOP(OR)2 (R = Me, Et) gave phosphonosilylation products of 1,4-conjugate additions which are hydrolysed to the phosphono ketones (165). The envisaged initiation step is electron transfer to the triplet excited state of the enone to generate a radical ion pair. 

Remarkably, one study [19] reported that ODPM rearrangement is also possible from the singlet excited state, but this is a matter of debate. For the triplet excited state of these (3,y-unsaturated ketones, it was clearly pointed out that it is nn in origin based on low temperature phosphorescence measurements, theoretical and mechanistic studies [20-23]. Furtheron, these phosphorescene and mechanistic studies indicated that the 37i7i excited state is approx. 8-12kcal/mol lower to the corresponding 3nn excited state [25]. 

The kinetics of combination of radical pairs formed by quenching of the triplet excited state of B by 2,4,6-trimethylphenol (HR) in films of rigid and plasticized polyvinylchloride (PVC) were studied by the method of laser flash photolysis by Levin et al. At high concentrations of the phenol quencher ( 15 wt%), most of the triplet excited state of the ketone is quenched and the formation and disappearance of the ketyl and phenoxyl radicals can be followed from changes in their characteristic transient absorptions at 540 and 390 nm, respectively. 

Addition Reactions - The allyl ketone (9) is the product formed on photohydration of the diyne (10). The photohydration step arises from the triplet excited state of (10). The polarization within the excited state is the key to the outcome of the reaction and is such that the protonation occurs at Cl. Shim has published a short review dealing with the hydration of such systems. 

Cantrell102 suggests that the triplet excited state of the acetylthiophen may be predominantly in character, rather than mr. This proposal is backed up by the observation that the long-lived phosphorescence at 77 is mr in character. Similar products are isolated from the reaction of the same ketone with isobutene. 

The photochemical 1,3-acyl migration which converts certain jSy-unsaturated ketones into isomeric enones has been investigated for a series of compounds, including cholest-4-en-7-one (651) and its 6,6-dimethyl derivative (652). Photoequilibration affords the isomers (653), considered to arise via n — tt triplet states. The reactions are influenced by conformational and conjugative features which effect the stability of an intermediate radical pair of the type (654). Other workers have studied the phosphorescence spectra and lifetimes of triplet excited states of a number of j y-unsaturated ketones, including some... 

A variety of photochemical reactions have been found for both the singlet and triplet excited states of p,Y-unsaturated ketones. It has been the molecular rearrangements, however, which appear to be the unique character of this grouping, rearrangements which are controlled by the singlet or triplet nature of the excited state. 

Similar effects on the conformational mobility of a solute were examined by Leigh and co-workers who used transient spectroscopic methods to probe the effect of liquid crystalline control on the rate of decay of excited state triplet ketones of the general structure 3 [354, 355]. In isotropic solution the rate of decay of the triplet excited state of these ketones is orders of magnitude greater than that of the analogous ketone without the )3-phenyl ring, due to efficient... 

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