Quenching rate mechanism

It should be noted that this expression is a general one that can be used for any photochemical reaction that can be quenched. It is commonly called the Stern-Volmer equation. This equation predicts that if the proposed mechanism is correct, the data, when plotted as 4>a0/4>a vs. [Q], should be linear with an intercept equal to unity and a slope equal to kqr. Linear plots were indeed observed out to large d>°/d> values. Assuming a value of 5 x 10 M 1 sec-1 for the quenching rate constant,(7) the data presented in Table 4.1 were obtained. 

Reactions ofO S) Atoms. In spite of ihc higher electronic energy of O(. S ) than of O( /.)) the quenching rate constant of the former atoms by many gases is in general much smaller than that of the latter atoms. Table IV 3 shows some results obtained mainly by Welge ct al. (48, 349, 646, 949, 1035). A mechanism to explain a large difference in physical... 

Tocopherols have been reported to act as free-radical scavengers (Terao et al., 1980) but have also been shown to quench CF via a charge-transfer quenching mechanism (Yamauchi and Matsushita, 1977 Burton and Ingold, 1981). The ratio of 02 quenching rates of a-, y- and 8-tocopherols were found to be 100 69 38. Each tocopherol molecule can deactivate about 120 molecules of 02 before it is destroyed (Zweig and Henderson, 1975). 

Paramagnetic NO and 02 molecules and reactive CO molecules efficiently quench the PL in the order NO > 02 > CO, whereas N20 only weakly affects the PL intensity. This work allowed confirmation of some important steps of the mechanism proposed earlier for the photocatalytic reduction of NO by carbon monoxide on Mo/Si02 (Subbotina et al., 1999). In particular, the NO photoreduction kinetics was consistently described by Subbotina et al. (1999) assuming first that the deactivation rate constant is much smaller than the quenching rate constant and second that the NO molecules efficiently quench the (Mo5+=0-) excited triplet state without chemical interaction (i.e., "physical quenching"), in contrast to the "chemical quenching" by CO molecules to yield C02 molecules. The PL data demonstrate the correctness of those assumptions. 

The ratio of the quenching rate constants for NO and CO calculated from the computer best fits of the experimental pressure dependence is in agreement with earlier data indicating the kinetics and mechanism of the photocatalytic reduction of NO by carbon monoxide on Mo6+/Si02 photocatalysts reported earlier by Shelimov coworkers (Lisachenko et al., 2002 Subbotina et al., 1999,2001). 

The anionic and cationic ends of the NC s TICT state were found to be quenched by electron donors and acceptors respectively. The quenching rate constants were analyzed with the help of the Marcus model and found consistent with the expected value for electron transfer mechanism. This was confirmed by photochemical reactions running through a radical cation or a radical anion intermediate. The isomerization of quadricyclane to norborna-diene was used as a check for the reactivity of the radical-cation end of the NC s... 

Unlike quenching by the mechanism of acceleration of intersystem crossing, the dependence of the quenching rate constant on the solvent polarity is characteristic of the quenching process by the charge transfer mechanism the quenching rate constant increases with growing polarity [19]. 

Information extracted from a meticulous library search, however, invalidates the radical mechanism.In a recent short communication researchers report that the quenching rate of the fluorescence of compound I with electron-rich alkenes is extremely fast, close to the diffusion control limit. Conversely, fluorescence quenching by methanol is 100 times slower. In the present case, this means that photoexcited I would be expected to react much faster with isopropylene than with methanol to form the methoxyl radical required by our hypothetical sequence. 

Also in the case of Cu.lO", where a [2]catenand-type moiety is present, a two-step protonation reaction occurs, but in this case the luminescence of such moiety is quenched by the Cu-based one [66]. Unfortunately, it is not possible to determine the quenching rate and mechanism for a variety of reasons, including (i) the instability of the protonated forms under laser irradiation in CH2CI2 [56] (ii) the lack of electrochemical potentials of the protonated forms and (iii) the weakness of the MLCT emission band of the Cu-based moiety, which is partially masked by the tail of the emission of the protonated subunit, albeit quenched. However, the energy transfer mechanism is thermodynamically allowed and probably active. 

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