Quenching of triplet state

Figure 8.3 Quenching of triplet state of a substrate, 3S, by generating JOs by exchange mechanism.

Quenching of triplet states of aromatic hydrocarbons and carbonyl compounds by inorganic anions (I-, Br , NOj, Cl-) Quenching of excited aromatic molecules by aromatic hydrocarbons, nitriles, methoxy- and amino-aromatics Quenching of excited aromatic molecules by methoxy and amino-aromatics Quenching of excited cyanoanthracenes, by aromatic hydrocarbons, methoxy-aromatics and sulfides... 

Quenching of triplet states of aromatic hydrocarbons by quinones Benzene ( 22 0)... 

A comparison of the experimental data on quenching of triplet states of dyes in solutions in the absence and in the presence of DNA permits estimation of the steric complexation effect on the quenching process and conclusions about the structure of the dye-DNA complexes formed. In the case of dye K4, we may conclude that complexation with the biopolymer has relatively weak effect on the kq value. This is probably due to the fact that the quenching process for K4 occurs in the kinetic regime (kc k a, see reaction (2)), and diffusion of the quencher to dye molecules boimd to DNA exerts no substantial effect on kq (another assumed reason for this phenomenon could be partial decomposition of the dye(T)-DNA complex and the presence of free triplet dye molecules in the solution however, the experiments on quenching of the K4 triplet state by iodide ion considered above reject this possibility). 

Circular dichroism provides an additional spectroscopic tool for characterization of excited states 35). Considerable interest has also been extended to esr-spectra of anion radicals of a-diketones 36). Circular polarization of the phosphorescence of camphorquinone has been determined, 51). Biacetyl has been the subject of a CIDNP study 152), of fluorescence quenching by a variety of substrates 153), and of steric effects in quenching of triplet states of alkylbenzenes 154). 

The use of dienes such as piperylene in the quenching of triplet states of carbonyl compounds is well known. However, in many photochemical studies, attempts to quench the carbonyl Ti state are often complicated by the formation of oxetane photoproducts.Since the photosensitized dimerization of diene triplets is well known, synthetic applications utilizing dienes in the Patemo-BUchi photocycload dition are rather limited. However, the diene addend not only allows for rapid assemblage of functionality but remains a continuing challenge in terms of experimental efficiency and generality. 

Gamer, A., Wilkinson, F., Quenching of Triplet States by Molecular Oxygen and Role of Charge transfer Interactions, Chem. Phys. Lett. 1977, 45, 432 435. 

Phosphorescence is associated with formally spin-forbidden transitions such as T,—> Sq. It occurs rarely in fluid solution due to efficient quenching of triplet states by oxygen. Phosphorescence is usually observed from molecules that are frozen in glassy solvents or adsorbed on a solid substrate such as filter paper—so-called room-temperature phosphorescence. 

The oxygen quenching of triplet state molecules has been the subject of numerous experimental and theoretical studies [Refs. 60, 81, 326, 331, 332,379,486,654]. The processes expected to be involved in the quenching of triplet state molecules are... 

Table 4 Rate constants for the quenching of triplet states by ferrocene in ethanol solution 828...

Some previous reports of uphill quenching of triplet states by ferrocene may have arisen from assignment of too high a value for the energy of Tx ferrocene... 

Since the rate constants for energy transfer are on the order of 10 -10 2/sec., our calculations easily account for the experimental observation that the O2 quenching of triplet states is diffusion controlled. (If M and O2 only form collision complexes, the rate constant for dissociation would be on the order of 10 /sec., much smaller than lO -lO /ggc.) The above results answer the three questions we posed earlier. We now... 

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). 

The calculation of the rate of quenching of triplet states by O2 was essentially an evaluation of the magnitude of interaction between two different states of the complex. Determination of potential energy curves for the M-O2 interaction, requires an evaluation of the total energy of the complex as a function of R, the intermolecular distance. 

OXYGEN QUENCHING OF TRIPLET STATES IN ISOLATED PHOTOSYSTEM 2 REACTION CENTRES A MECHANISM FOR PHOTODAMAGE. 

Oxygen Quenching of Triplet States in Isolated Photosystem 2 Reaction Centres A... 

Equation (9) is included because it probably accounts for the efficient quenching of triplet states by oxygen or nitric oxide. 

---