Oxygen, triplet state

Typical singlet lifetimes are measured in nanoseconds while triplet lifetimes of organic molecules in rigid solutions are usually measured in milliseconds or even seconds. In liquid media where drfifiision is rapid the triplet states are usually quenched, often by tire nearly iibiqitoiis molecular oxygen. Because of that, phosphorescence is seldom observed in liquid solutions. In the spectroscopy of molecules the tenn fluorescence is now usually used to refer to emission from an excited singlet state and phosphorescence to emission from a triplet state, regardless of the actual lifetimes. 

A principal appHcation for photomedicine is the photodynamic treatment of cancer. Photochemical and clinical aspects of this topic have been reviewed (10,11). Direct irradiation of tumors coupled with adininistration of a sensitizer is used to effect necrosis of the malignancy. In this process, an excited state sensitizer interacts with dissolved in vivo to effect conversion of the oxygen from its triplet ground state to an excited singlet state, which is highly cytotoxic. In principle, excited sensitizers in either the singlet or the triplet state can effect this conversion of molecular oxygen (8). In... 

Catalytic oxidation of isobutyraldehyde with air at 30—50°C gives isobutyric acid [79-31-2] ia 95% yield (5). Certain enzymes, such as horseradish peroxidase, cataly2e the reaction of isobutyraldehyde with molecular oxygen to form triplet-state acetone and formic acid with simultaneous chemiluminescence (6). 

Such an orbital phase picture in Fig. 14 is also applicable to rationalize the relative S-T gaps of hetero diradicals 19 and 20. hi comparison with their parent system, 1,3-dimethylenecyclobutadiene (DMCBD, 10), the introduction of oxygen atoms does destabilize the triplet state. The calculated energy gap between singlet and triplet states, AE deaeases in the order 10 (18.2 kcal moF ) > 19 (7.7 kcal moF ) > 20 (-20.7 kcal moF ) [64]. These results supported the orbital phase predictions. 

On the other hand. Type II process competes efficiently with the electron-transfer pathway in aerobic environments where the concentration of ground triplet state molecular oxygen is relatively high ( 0.27 mM), and singlet molecular oxygen (1O2) is the most abimdant ROS generated under these conditions, with a quantum yield 0.48 (Valle et al., 2011), eqn. 8. It is also possible an electron-transfer reaction from 3RF to 02 to form anion superoxide, but this reaction occurs with very low efficiency <0.1% (Lu et al., 2000). 

As can be seen in these reactions, carotenoids may protect photosynthetic bacteria at various levels by quenching the singlet-excited state of O2 or the tiiplet-excited state of chlorophyll. The ground states of oxygen would be 3O2 and for CHL the triplet state. The carotenoids may be the preferred substrates for oxidation or may act in quenching reactive species. ... 

The dioxygen molecule exists in two forms a triplet or ground state in which it is a stable biradical and a singlet or excited state in which it is not a radical. Reactions of carotenoids with singlet oxygen have already been presented in this chapter and we now focus on the reactions of carotenoids and oxygen in the ground or triplet state. 

So far, the solid state type I reaction has been reliable only when followed by the irreversible loss of CO to yield alkyl-alkyl radical species (RP-B or BR-B) in a net de-carbonylation process. The type 11 reaction relies on the presence of a y-hydrogen that can be transferred to the carbonyl oxygen to generate the 1,4-hydroxy-biradical (BR C). The type-1 and type-11 reactions are generally favored in the excited triplet state and they often compete with each other and with other excited state decay pathways. While the radical species generated in these reactions generate complex product mixtures in solution, they tend to be highly selective in the crystalline state. 

The photocoloring reaction for spiroindolinobenzopyrans with a nitro group proceeds mainly via the formation of the excited triplet state of the molecule. The reaction proceeds partly from the triplet state [(SP )3] of the spiropyran to the triplet state (X)3 of the cis-cisoid isomer which subsequently transforms into the CF and partly from (SP )3 to the CF. This process from (X)3 to the colored form is accelerated by the presence of atmospheric oxygen (Scheme 6).2,28 For the photocoloring reaction, the participation of singlet or triplet state depends not only on the substituent but also on the nature of the heterocyclic component. 

Since the first two processes are spin-forbidden, it can clearly be seen that in the absence of triplet quenchers (e.g., oxygen) the triplet will be long lived. Consequently the experimental determination of the lifetime of triplet states... 

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