Triplet state of oxygen

In the theory outlined above we assumed a weak collision complex between the triplet state of the organic molecule and the triplet state of oxygen, and we were able to account for the rate of quenching of triplet states by O2 on this basis. Further investigation into the nature of this interaction indicates that it is not necessarily weak (7). 

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 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. 

The reaction mechanism for the aerobic oxidation of the pz to seco-pz can be attributed to a formal 2 + 2 cycloaddition of singlet oxygen to one of the pyrrole rings, followed by cleavage (retro 2 + 2) of the dioxetane intermediate to produce the corresponding seco-pz (160). This mechanism is shown in Scheme 29 for an unsymmetrical bis(dimethylamino)pz. Further photophysical studies show that the full reaction mechanism of the photoperoxidation involves attack on the reactant by singlet oxygen that has been sensitized by the triplet state of the product, 159. As a consequence, the kinetics of the process is shown to be autocatalytic where the reactant is removed at a rate that increases with the amount of product formed. 

Using laser flash photolysis with a frequency-quadrupled neodynium laser, Stevens and al 161b) measured the lifetime of the triplet state of fluoro- and pentafluoro-benzene in the gas phase along with the energy transfer efficiencies to cis-2-butane and oxygen. The triplet transient absorption decay was found to be predominantly first order with a... 

The reaction of atoms, radicals or excited triplet states of some molecules with silicon hydrides is the most important way for generating silyl radicals [1,2]. Indeed, Reaction (1.1) in solution has been used for different applications. Usually radicals X are centred at carbon, nitrogen, oxygen, or sulfur atoms... 

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]. 

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