Quenching diffusional

Traditionally, production of metallic glasses requites rapid heat removal from the material (Fig. 2) which normally involves a combination of a cooling process that has a high heat-transfer coefficient at the interface of the Hquid and quenching medium, and a thin cross section in at least one-dimension. Besides rapid cooling, a variety of techniques are available to produce metallic glasses. Processes not dependent on rapid solidification include plastic deformation (38), mechanical alloying (7,8), and diffusional transformations (10). 

Fig. 3 Transient spectra obtained upon the application of a 200-fs laser pulse to a solution of stilbene (S) and chloranil (Q) in dioxane. (a) The fast decay ( 20 ps) of the contact ion-radical pair S+ , Q generated by direct charge-transfer excitation (CT path), (b) The slow growth ( 1.6 ns) of the ion pair S+ Q due to the diffusional quenching of triplet chloranil (A path) as described in Scheme 13. Reproduced with permission from Ref. 55.

Donor/acceptor association and the electron-transfer paradigm form the unifying theme for the C—C bond cleavage of various benzpinacols and diary-lethane-like donors in the presence of different electron acceptors (such as chloranil (CA), dichlorodicyanobenzoquinone (DDQ), tetracyanobenzene (TCNB), triphenylpyrylium (TPP+), methyl viologen, nitrosonium cation, etc.). Scheme 13 reminds us how this is achieved by either CT photolysis of the D/A pair or via diffusional quenching of the excited electron acceptor A by the electron donor D. 

Electron-transfer activation. Both charge-transfer (CT) photolysis as well as the diffusional quenching of photoexcited chloranil with pinacol donors occur via a reactive ion pair as the common intermediate (equation 57). 

However, the short lifetimes ( 50 ps) of the ion-radical pair ArMe"1", CA- owing to rapid back electron transfer ( bet) does not allow other reactions to compete effectively.203 In contrast, the diffusional quenching of the photoex-cited chloranil with methylbenzenes leads to (spectrally) indistinguishable ion-radical pairs with greatly enhanced lifetimes,204 i.e.,... 

Time-resolved spectroscopy establishes that the fluorescence of the excited (singlet) anthracene ( ANT ) is readily quenched by maleic anhydride (MA), which leads to the formation of the ion pair ANT+, MA via diffusional electron transfer (see Fig. 12), i.e.,... 

In eq. 8 are shown the results of a kinetic analysis of the series of reactions in Scheme 1. The analysis is based on the quenching rate constant k, corrected for diffusional effects, which would be measured for the quenching of Ru(bpy>3 + by PQ +. 

In dynamic quenching (or diffusional quenching) the quenching species and the potentially fluorescent molecule react during the lifetime of the excited state of the latter. The efficiency of dynamic quenching depends upon the viscosity of the solution, the lifetime of the excited state (x ) of the luminescent species, and the concentration of the quencher [Q], This is summarized in the Stern-Volmer equation ... 

Changes in fluidity of a medium can thus be monitored via the variations of Jo/J — 1 for quenching, and Ie/Im for excimer formation, because these two quantities are proportional to the diffusional rate constant kj, i.e. proportional to the diffusion coefficient D. Once again, we should not calculate the viscosity value from D by means of the Stokes-Einstein relation (see Section 8.1). 

K = k T. When energy-transfer experiments were used to deduce the x values, the quenching was assumed to proceed with a diffusion-controlled rate, and the transfer was supposed to take place when the acceptor and donor molecules were in contact (no static quenching). The transfer rate coefficients were calculated using the usual diffusional equations [24,25,59] (see Sec. 3.1). 

Indirect evidence for the formation of a complex intermediate in diffusional energy transfer processes may be provided by the measurement of kt from both donor quenching and acceptor sensitization at low temperatures where kQC kf = 1/T° in this case exciplex relaxation should reduce the quencher sensitization efficiency but leave the donor fluorescence quenching constant unchanged. 

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... 

Let us regard a binary A-B system that has been quenched sufficiently fast from the / -phase field into the two phase region (a + / ) (see, for example, Fig. 6-2). If the cooling did not change the state of order by activated atomic jumps, the crystal is now supersaturated with respect to component B. When further diffusional jumping is frozen, some crystals then undergo a diffusionless first-order phase transition, / ->/ , into a different crystal structure. This is called a martensitic transformation and the product of the transformation is martensite. 

Such transformations have been extensively studied in quenched steels, but they can also be found in nonferrous alloys, ceramics, minerals, and polymers. They have been studied mainly for technical reasons, since the transformed material often has useful mechanical properties (hard, stiff, high damping (internal friction), shape memory). Martensitic transformations can occur at rather low temperature ( 100 K) where diffusional jumps of atoms are definitely frozen, but also at much higher temperature. Since they occur without transport of matter, they are not of central interest to solid state kinetics. However, in view of the crystallographic as well as the elastic and even plastic implications, diffusionless transformations may inform us about the principles involved in the structural part of heterogeneous solid state reactions, and for this reason we will discuss them. 

The molecules M and Q can come into contact (within the sphere of action) through their random diffusional motion. The rate constant kD of diffusion controlled encounters is the upper limit for any bimolecular reaction. This must be multiplied by the probability of an encounter leading to reaction (quenching in the present case), and the luminescence quantum yield then follows the Stern-Volmer equation... 

Figure 3.42 Kinetics of dynamic (diffusional), D, and static, S, quenching. In dynamic quenching the excited state lifetime gets shorter with increasing quencher concentration, from to with no quencher to t1 t2 with added quencher. In static quenching excited state lifetime remains unchanged but the initial concentration of excited states is reduced...

Venkatasubban and Schowen34 provided a key refinement in which they specify that the role of a pre-associated catalyst is to quench a reactive intermediate. Since the catalyst does not participate in stabilizing the transition state it has another role and a special name The [catalyst] is a spectator during the bond fission, and the catalysis is of the type we have called spectator catalysis... It is conceivable that the diffusional approach of BH to the tetrahedral adduct is the rate-limiting step. ... 

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