Triplet quenching, solvents

The simple triplet-triplet quenching mechanism requires that at low rates of light absorption the intensity of delayed fluorescence should decay exponentially with a lifetime equal to one-half of that of the triplet in the same solution. Exponential decay of delayed fluorescence was, in fact, found with anthracene, naphthalene, and pyrene, but with these compounds the intensity of triplet-singlet emission in fluid solution was too weak to permit measurement of its lifetime. Preliminary measurements with ethanolic phenanthrene solutions at various temperatures indicated that the lifetime of delayed flourescence was at least approximately equal to one-half of the lifetime of the triplet-singlet emission.38 More recent measurements suggest that this rule is not obeyed under all conditions. In some solutions more rapid rates of decay of delayed fluorescence have been observed.64 Sufficient data have not been accumulated to advance a specific mechanism but it is suspected that the effect may be due to the formation of ionic species as a result of the interaction of the energetic phenanthrene triplets, and the subsequent reaction of the ions with the solvent and/or each other to produce excited singlet mole-... 

The diffusion-controlled mechanism of triplet-triplet quenching (whether it be collisional or interaction at a distance) could still be applied to interpret these results, qualitatively at least, if one additional factor is taken into account. The additional factor is the extremely slow rate of diffusion in rigid medium which makes necessary the Consideration of nonsteady-state effects. Immediately after illumination is shut off, those triplet molecules situated close together will diffuse and interact more rapidly than others situated at greater distances. The more favorably situated pairs of triplets will thus be depleted more rapidly and the overall rate of interaction will be greater at shorter times than later when steady-state conditions will ultimately be approached. In fluid solvents at room temperature the steady state is reached after about 10-7 sec. In very highly viscous media, however, much longer times are required and this could explain the non-exponential decay observed with phenanthrene in EPA at 77°K. 

A representative plot is shown in Figure 1.15 this is known as a Stern-Volmer plot, and 0.16) as a Stern-Volmer equation. This method for obtaining reaction rate constants is again a comparative one, since there is competition between the primary reaction step and the quenching process. A value for the quenching rate constant needs to be known, but in many cases this is independent of the substrate and quencher because triplet quenching is controlled by diffusional collision of the two species. So for a particular solvent at a given temperature K, values are available in the literature as an... 

Table I shows that in either dioxane or acetonitrile the quantum yield for degradation of I, is unaffected by the presence of 0.1 M of triplet quencher, either sorbic acid, naphthalene or cyclohexadiene. In ethanol, triplet quenchers reduce < >d from 0.34 to 0.14. Quantum yields for intersystem crossing, as determined by a laser opto-acoustic technique ( ), were 0.36 in ethanol and 0.59 in dioxane. These results agree with our earlier report (3), and indicate that significant reactivity occurs from St of I in protic solvents, and that reaction occurs exclusively from Sx in aprotic solvents. While triplet quenching experiments cannot rigorously exclude participation by short-lived higher triplet states, Palm et al (9) have obtained conclusive evidence from CIDNP experiments for singlet-state participation in a series of aryloxy-acetophenones. Note that the triplet state of I is formed in aprotic solvents, and that in deaerated solutions at room temperature it decays by first-order kinetics with a lifetime of 200 ns (3). Remarkably, despite having lifetimes about 100 times longer than other, differently-substituted, aryloxyacetophenones (the longer lifetimes may...

The two time-resolved spectroscopic methods CIDNP [26,27] and ESR [28-30] as well as classical triplet quenching experiments all indicate that a-GAV decomposes through both triplet and singlet states in protic solvents and exclusively through the singlet state in aprotic solvents. 

A variety of other ketones have been shown to photosensitize iodonium and sulfonium salts from triplet excited states by the electron transfer mechanism [102,103], Rates of electron transfer from triplet 3-(2-isoxazolinyl)-phenyl ketone, XI, to a series of onium salts in aq. acetonitrile have been shown to scale with the free energy change according to the Weller equation (Eq. (28)) [105], In the same study, the rates of triplet quenching of a series of ketones by diphenyliodonium cation in the same solvent were evaluated the results were not tractable in terms of usual linear free energy... 

The termination reaction of free radical polymerization is a typical example of an intermacromolecular diffusion controlled reaction.3 Photophysical studies carried out in the 1980 s demonstrated for the first time that the reaction is solvent- and molecular weight-dependent. The experiments involved triplet quenching of probes attached to polymer chain ends. A benzil group was linked to the end of one PS sample (PS-B) and an anthryl group was linked to the end of a second PS sample (PS-A). The quenching rate coefficient kq of the benzil phosphorescence by anthryl groups is given by Eq. (3.26), where r0 is the lifetime of benzil phosphorescence in the absence of anthryl and ris the benzil phosphorescence lifetime in the presence of anthryl in concentration [A],... 

The effect of the heavy-atom substituents, bromine and iodine, on the electron-donor aniline in the electron-transfer reaction with thiopyronine triplet has been investigated by flash spectroscopy in solvents of different viscosity and polarity. Triplet quenching and radical yields are presented in Table 30. The results are analysed in terms of decay constants of an intermediate triplet exciplex. The influence of an external heavy-atom effect on the phosphorescence spectra of quinoxaline and 2,3-dichloroquinoxaline has also been reported. The influence of chloride ion on the decay rate of Methylene Blue triplet in 0.01 M acid in the... 

Kunze, A., Muller, U, Titles, K., Fouasier, J.P., and Morlet-Savary, F., Triplet quenching by onium salts in polar and nonpolar solvents,/. Photochem. Photobiol. A Chem., 110, 115, 1997. 

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