Singlet oxygen quenching excited triplet state

Oxygen also efficiently quenches excited triplet states of other molecules ( iA ) and, in accepting triplet energy, is itself promoted to an excited singlet state. Notice that the total spin orientation is conserved ... 

Foote, Wexler, and Ando 92) showed that, in the oxidation of unsaturated hydrocarbons with oxygen in the anglet state (which may be produced by electrodeless discharge 93) or by reaction of HgOs with sodium hypochlorite), the same products are formed as in the photosensitized oxidation. Kawaoka, Khan and Kearns 94 reviewed the data reported in the literature, which support the formation of singlet Og by quenching excited triplet state molecules. 

The overall process of ()2 quenching simply converts the excess energy of singlet oxygen to heat via the carotenoid [CAR] lowest excited triplet state [3CAR],... 

Moreover, carotenoids may quench electronically excited states and scavenge free radicals formed in the retina, and therefore protect biomolecules from oxidative damage. Due to the low energy level of the first excited triplet state ( Car), carotenoids (Car) can act as efficient acceptors of triplet state energy from photosensitizers (S) (Equation 15.1), such as all-tra .s-retinal, the photosensitizers of lipofuscin (Rozanowska et al., 1998), or singlet oxygen C02) (Equation 15.2) (Cantrell et al., 2003) ... 

A few examples to render tetrapyrrolic compounds less phototoxic can be found in the hterature. In one approach, carotenoid structures were employed for the synthesis of some carotenoporphyrin derivatives [92-94]. Figure 8 shows two stuctures by way of example. Due to similar photophysical properties of the two structural components, the excited triplet state of the porphyrin is quenched by the carotenoid moiety, thus inhibiting the formation of singlet oxygen, while its fluorescence capabilities are still preserved. Biodistribution studies revealed enhanced uptake into tumour tissue [39,93,95]. However, microscopy studies have shown that such compounds are associated with connective tissues in the tumors rather than with cancerous cells indicating low specificities for mahgnant transformation [96]. 

Most photodecarbonylation reactions of cyclic ketones, especially in the vapor phase, have been postulated to proceed from various vibrational levels of excited singlet states.321 However, the elimination reaction leading to unsaturated aldehydes has now been shown to occur largely via excited triplet states. In solution, where the lowest vibrational levels of the excited states are rapidly reached, to-alkenals are the major products observed in both photolysis and radiolysis of cyclopentanone and cyclohexanone. The reaction is quenched by oxygen and dienes,322-324 as well as by the alkenal produced in the reaction.325 The reaction is also sensitized by benzene triplets.322,323 With cyclopentanone, quenching by 1M piperylene occurs some 20 times as fast... 

The transient absorption spectra show that besides the ring-closure product (C), a new transient species was found that could be quenched by oxygen. The authors assigned the new species to an excited triplet state. The time-resolved transient absorption spectra indicated that when the time delay was increased, the excited triplet state was decreased and the ring closure product was increased slightly. The decay of the excited triplet state can be fit to first-order kinetics. The lifetime ( ) of the transient is 0.8 ps in cyclohexane. The authors noted that the photocyclization process proceeds mainly via the excited singlet state, but the excited triplet state is... 

It is apparent that more than one oxygen activation pathway exists. The excited triplet state of tin porphyrins is known to be quenched in the presence of 2, suggesting a possible direct mechanism of O2 activation by the photosensitizer. We have examined reactions of both singlet O2 and superoxide anion under our reaction conditions. Chemically-produced superoxide (K02/18-crown-6) is not reactive under our experimental conditions. On the other hand, singlet oxygen, produced by irradiation of free base protoporphyrin (H2ProtoP), is reactive in the presence of tertiary amines and gives about the same hexanol to hexanone ratio (2.7, see Table 1) as is observed in the presence of the SnP photosensitizer. 

A crucial aspect of the photoprotective role of Cars is their ability to efficiently quench chlorophyll triplet states, thereby preventing the formation of excited, singlet oxygen by triplet-triplet energy transfer from its ground state ... 

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