Triplet energy of sensitizers

Triplet Energies of Sensitizers of 2,5-Dimethylfuran Photooxygenation in Benzene-Methanol and Methanol Solutions at Room Temperature... 

Re-examination of the benzene-sensitized isomerization of alkenes revealed that the photostationary EjZ ratio critically depends on the structure of alkenes and the triplet energy of sensitizers [69]. For instance, thermal E-Z equilibrations of 2-octene 19c and 3,4-dimethyl-2-pentene 20 give the thermodynamic EjZ ratios of 3.3 and 3.5, respectively, while the photostationary E Z ratios upon benzene sensitization are 1.1 and 1.6, respectively (Schs. 9 and 10). For practical runs, the use of /7-xylene or phenol, rather than benzene, in ether is recommended, as the yellowing of the irradiated solution, which retards the photoreaction, is substantially decreased. 

Triple bond, bending, 205 Triplet, 12, 14, 180 impure, 28. 220-21, 227 Triplet energy of sensitizer, 372, 407 Triplet function, 222 Triplet state, aromatics, 76 calculation, 55 ethylene, 64... 

Z- and -2-Butenes in liquid neopentane undergo ci5—/ra/is-isomerization and it is competitive with their photochemical [2+2]-cycloadditions [3]. When the alkene solution is sufficiently diluted with an inert solvent, the isomerization becomes the dominant reaction. The triplet energy of sensitizer PhH is higher ( 84 kcal/mol) than that for 2-butene ( 80 kcal/mol) and hence triplet energy transfer is... 

As the triplet energy of the sensitizer becomes less than that necessary to excite the trans-diene triplet, energy transfer to the cis triplet becomes increasingly important and the product derived from this state (vinylcyclohexene) increases proportionally. This explanation is summarized in Figure 10.1.(11>14)... 

The product distribution in this reaction was found to be dependent upon the triplet energy of the sensitizer. Variation of the relative amounts of cyclobutanes (32) and (33) to cyclohexene (34) with sensitizer triplet energy... 

Triplet energy transfer measurements from porphyrin and phthalocyanine sensitizers give the triplet energies of six (Z)-A4,A4-diethyl-2-(alkyl, aryl)-A1-(3-phenyl-77/-pyrazolo[5,l-r-][l,2,4]triazol-7-ylidene)benzene-l,4-diamine azomethine dyes 154 with adsorption maxima in ethanol at 546-633 nm to lie in the range of 115-88 kJmoF1 <2003PPS563> (Figure 24). 

Triplet sensitized LFP studies of simple alkyl azides and systematic product analyses in solution under these conditions have not been reported, although the triplet energies of alkyl azides are 75-80 kcal/mol" and the quantum yields for azide disappearance with appropriate triplet sensitizers approach unity. 

Fig. 3. Photostationary states obtained in the photosensitized isomerization of stilbene as a function of triplet energy of the sensitizer. (From Wagner and Hammond.1)...

Daubendiek et al.55 have extended this technique to demonstrate electronic energy transfer at the gas-solid interface. They irradiated photosensitizers, in polycrystalline form, in the presence of m-piperylene vapor and found that the extent of isomerization was comparable to values obtained for the same sensitizer in a solution of piperylene. Their results indicated an energy difference of about 1 kcal between triplet states of sensitizers in solution and on the surface of organic crystals. 

Ligands L33 and L34 are both good sensitizers of europium, whilst the former also efficiently sensitizes terbium emission (29). The quantum yield of [EuL34]3+ in aqueous solution, 0.247, is among the highest reported for any europium complex. The lower triplet energy of L34 results in back transfer of energy from the terbium excited state to... 

The dienes, specially 1,3-pentadiene (piperylene) and hexadiene, quench the triplets of suitable sensitizers by energy transfer with unit efficiency. Hence, they are used widely in mechanistic studies of photochemical reactions, either to count the triplets or to establish the triplet energy of a sensitizer whose Et is not determinable from spectroscopic data (chemical spectroscopy). 

It is only very recently that much work has been reported on sensitized cis-trans isomerizations of nonconjugated olefins, probably because the spectroscopic triplet energy of ethylene seemed so much... 

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