Quencher migration

Case 4 Only the quencher migrates. This is the most common situation with excited singlet state probes. The quenching efficiency will be determined by the rate of access of the quencher to the supramolecular strucmre ( g+[H]) and the efficiency of quenching within the supramolecular system The association to the supramolecular complex is a bimolecular process, whereas the quenching efficiency within the supramolecular structure is viewed as a uni-molecular process. This picture is concepmally analogous to the formation of an encounter complex in solution before reaction, where the volume of the encounter complex is defined by the supramolecular structure. Thus, an overall effective quenching rate constant [ q(eff)] can be defined which takes into account the association process and the intrinsic reactivity ... 

We want to emphasize that Fig. 1 is a minimum mechanistic scheme that describes the dynamic processes involved in probe and/or quencher mobility. Every supramolecular system can present additional complexities and has to be analyzed individually. Some examples of increased complexity are the possibility of probe and/or quencher migration by collision of aggregates [70-73],... 

The values for the association (kg+) and dissociation (kg ) rate constants of the quenchers can be determined when the conditions discussed in Section II are met. Most of the studies in micelles have been based on the model that leads to Eq. (8), where rate constants are recovered from a four-parameter fit of the fluorescence decay in the presence of quencher. The assumptions of this basic model have been discussed in Section II. This model and inclusion of additional processes, such as probe and quencher migration, have been employed for over... 

Thus, chelates are able to dissipate the radiation harmlessly as infrared radiations or heat through resonating structures. Carlsson and Wiles [22] have confirmed in their studies that the quencher slowly migrate through the solid polymer destroying the hydroperoxide group. 

Even couples of lanthanide ions show this quenching process. The Ce(III) and Eu(III) ions, for example, quench each other s luminescence [127]. Here a MMCT state with Ce(IV)-Eu(II) character is responsible. In solid [Ce <= 2.2.1] cryptate there occurs energy migration over the cryptate species. Also here [Eu c 2.2.l] acts as a quencher [128]. The quenching action is restricted to short distances (about 12 A [129]). 

Photostabilizers, regardless of their mechanism of action, have been added as low molecular weight materials at some point in processing. Subsequently, these stabilizers are often lost in further processing due to their volatility or else later migrate to the surface and evaporate. One method which avoids this modifies the polymer to include the quencher as an additional monomer in the polymerization. This paper will describe some recent efforts in our laboratory to pursue this latter approach in the stabilization of poly(ethylene terephthalate). 

The quenching rate of non-excimeric arylalkyl methacrylate polymers in solution is greater than would be expected in the absence of energy migration. The increase is not totally due to retention of the quencher within the polymer coil, because the quenching rate of the same chromophore dispersed at low concentration in a copolymer is not as large as for the homopolymer. 

The diffusion equation for the motion of all the m quenchers and the fluorophor considers the rate of change of the density n to be due to migration of the fluorophor and each quencher out of their respective volume elements or to quenching of the excited fluorophor. When the flux of a quencher or fluorophor is in the same direction as the concentration gradient and hydrodynamic effects are unimportant, the diffusion tensor is diagonal and eqn. (211) becomes... 

Unless contained in a highly ordered system, excited-state molecules formed by light absorption must migrate through diffusion to the sites of quenchers where electron-transfer can take place. Diffusion in a liquid is a relatively slow process and so excited-state lifetimes must be sufficiently long to allow this primary photochemical process to take place. 

The kinetic complexity seen in oriented micelles persists in inverse micelles. The distribution of electron transfer quenchers within the water pool follows Poisson statistics and enables the kinetic data to describe migration rates to and from the aqueous subphase [65]. These orientation effects also make possible topological control of non-electron transfer photoreactions occurring within AOT micelles [66]. 

Interestingly enough [Tb C bpy.bpy.bpy] only shows efficient Tb emission on bpy excitation below 100 K (107). At room temperature backtransfer occurs. There is thermal equilibrium between the bpy triplet state and the Tb D4 state. Due to the rates involved, nonra-diative decay from the triplet level prevails (219,220). This is outlined in Fig. 48. In the solid state the same situation prevails, but the nonra-diative rate is now ascribed to energy migration over the bpy molecules to quenchers (107). 

The preceding analysis and review of the literature indicate a need for additional types of experiments to study triplet mobility in polymers. One experiment which has been particularly useful in studies of singlet energy migration in polymers, involves determination of the quenching of donor emission by a known mole fraction of a copolymerized luminescent quencher (26). We have extended this approach to the study of triplet states. The polymers chosen for study are homopolymers of isomeric acetonaphthyl methacrylates (aceto-NMA, 1). A related monomer, 2,4-diaceto-l-naphthyl... 

We determined photophysical properties for our copolymers in dilute solution, and found no effects (2) upon either total emission spectra (Figure 2) or emission lifetimes. However, because the mole % quencher in our copolymers is quite small, this result is completely consistent with the interpretation of Itagaki, et al (2)- All we can say from our results is that energy migration is not extensive in dilute solution. 

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