Triplet energy transfer, photosensitizers

Cundall has done extensive work on benzene231,237 and acetone243 sensitized isomerizations of the 2-butenes, and in every case reported a photostationary or radiostationary trans/cis ratio of 1.27-1.37. Sato, however, has measured a value of unity for the benzene photosensitized isomerization.510 With higher homologs, from 2-pentene to 2-octene, benzene-sensitized isomerizations yield trans/cis ratios of 1.0,238 while acetone-sensitized isomerization of the 2-pentenes in solution yields a ratio of 1.65.244 At present no explanation is possible for the differences between 2-butene and 2-pentene. Until much more information is gathered relating to rates of triplet energy transfer as functions of olefin structure, sensitizer, and medium, the natural decay ratios of each olefin s common triplet cannot be deduced from photostationary trans/cis ratios. 

Estimates of the triplet energy level of a molecule may be made by observing whether it can accept or transfer energy to several other molecules. Hammond and co-workers42 have shown that ds-trans interconversions of piperylene, 2-pentene, and 1 2 dichloroethylene may be affected by many photosensitizers. The stationary states of the sensitized ds-trans ratios of piperylene with various donors were found to form a coherent pattern if triplet energy transfer was assumed as the key step in the photochemical reaction. From these results they were able to infer the presence and the energies of the triplet states of acetone, phenanthrenequinone, and fluorenone, for which phosphorescence data are not available. The triplet levels were estimated as >70, 65, and 62 kcal./mole, respectively. 

The transformation of monuron photosensitized by fulvic acids (X = 365 nm) was shown to yield the same products as direct photolysis or sensitization by hydroquinone [82]. It was concluded that in this case, sensitization was mediated by triplet energy transfer in contrast, the presence of a humic acid induced an H atom transfer and the products were different [82]. The fulvic acid example shows that long-wavelength sensitization may in specific... 

A final comment on Table 4 concerns the reaction shown in entry 8. Because the di-TT-methane photorearrangement of benzonorbomadiene derivatives requires triplet energy sensitization, we could not use typical, passive amines such as (/ )-( + )-l-phenylethylamine as chiral auxiliaries. We therefore prepared an optically pure amine to which a sensitizing benzophenone moiety was tethered, namely, the 4-benzoylphenyl ester of l-valine [25]. Photolysis of the salt of this amine at wavelengths where only the benzophenone chromophore absorbs led to the photoproduct in 91% ee at 100% conversion, a gratifying vindication of the concept. Optically active photosensitizers have been used in solution with limited success [33], but this represents the first example of simultaneous triplet-triplet energy transfer and asymmetric induction in the crystalline state. 

Triplet-triplet energy transfer is most important in photochemical reactions. It is utilized to specifically excite the triplet state of the reactant. This process is referred to as photosensitization and the donor D is called a triplet sensitizer Sens. For efficient triplet sensitization the sensitizer must absorb substantially in the region of interest, its intersystem crossing effi-... 

The photosensitized crosslinking of amorphous polymeric films containing dimerizable cinnamate moieties show quantum yields of dimerization of about 0.04 (4). Sensitization of such films by ketocoumarins proceeds via triplet-triplet energy transfer (2) (see Scheme V). The irradiations were performed at 405 nm at a wavelength... 

Photosensitization of iodonium and sulfonium salts may occur by electron transfer [60,69,71,90-95], triplet energy transfer [22,70,82], and photochemical sensitization (photosensitization by photochemical formation of a... 

The photochemical configurational isomerization of previtamin D (5) into tachysterol (6) (and back) differs from the photochemical constitutional isomerization of provitamin D (1) or lumisterol (2) into previtamin D (5) (and back see Scheme 3) not only in the reaction type, but also in which of the jr jT-spin isomers is involved. Unlike the constitutional isomerization, the configurational isomerization can proceed not only under direct light excitation (singlet mechanism), but also by triplet-triplet energy transfer (triplet mechanism), with the aid of photosensitizers. 

The role of the triplet state in the cis-trans isomerization of stilbenes effected by photosensitizers, such as acetophenone, benzophenone, or anthraquinone, which have large So Ti excitation energies, was first revealed in Ref. [65]. Theoretical considerations and experimental data on intermolecular triplet-triplet energy transfer leading to the sensitized stilbene photoisomerization are described in Section 4.2.2. It was shown that data on positional dependence of the heavy-atom effect on the cis-trans photoisomerization of bromostilbenes were consistent with the fact that, in contrast to the para position, the meta position is near a node in the highest occupied and the lowest unoccupied MO of stilbene [66]. According to [67], internal and external heavy-atom effects induce phosphorescence in frans-stilbene... 

Various derivatives of thioxanthone are often used as photosensitizers with substituted morpholino ketones as energy aeeeptors. Aetually, the two materials by themselves are effieient initiators. It was shown that when eombinations of thioxanthone derivatives with morpholino ketones are irradiated with light, two types of reaetions ean take plaee. If the triplet-triplet energy transfer oeeurs when the energy level of the thioxanthone is higher than that of the morpholine ketone, eleavage of the morpholino ketone is the result ... 

All these pendant groups require the use of proper photosensitizers. Compounds with earbonyl groups appear to be most aetive. The sensitization proeess was attributed to a likely triplet energy transfer to the thiadiazole. The photolytie deeomposition of 1,2,3-thiadiazoles was pietured to oeeur as... 

To determine the photophysical and photochemical properties of unsubstituted phthalocyanines (MPc, M = Mg, Al, ZrL, SiClj, V=0, 2H) in their aggregates su-pramolecular systems have been developed on the basis of protein (albumin), micelles, biocompatible hydrophihc polymers, and nanoscale silica. The ability of MPc in supramolecular systems to form aggregates of different types depends on the nature of the central metal ion and microenvironment. The dependence of the type of MPc photoactivity method on aggregation was shown. H-aggregates exhibit selective photoactivity in electron transfer to Oj to form ROS (triplet-triplet energy transfer and fluorescence are impossible). Monomers and J-aggregates of MPc fluoresce and may also participate in the triplet-triplet energy transfer with the formation of O. Spectral properties of J-correspond to the requirements of the photosensitizer. 

Light absorption by the photosensitizer Intersystem crossing producing the triplet state Triplet-triplet energy transfer to the added PAH Deactivation of the triplet state - here by a photophysical process such as phosphorescence ... 

The Triplet-Triplet Energy Transfer in Photosensitization Process... 

Compared to benzylic C-H bond, selective functionalization of aliphatic C(sp )-H bond is much more difficult. Using anthraquinone (AQN) [83] or TBADT [84] as photosensitizers, the challenging monofluorination of unactivated C(sp )-H bonds was achieved, rendering excellent regioselectivity (Scheme 3.30). The authors rationahzed that triplet-triplet energy transfer of AQN to Selectfluor generates cationic A-radical, which could abstract one hydrogen atom from the electron-rich C (sp )-H bond. 

Sauvaigo, S., Douki, T, Odin, E et al. 2001. Analysis of fluoroquinolonemedi-ated photosensitization of 2 -deoxyguanosine, calf thymus and cellular DNA determination of Type-1, Type-II and triplet-triplet energy transfer mechanism contribution. > 73 230-37. 

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