Migration triplet decay

The triplet decay rates kd for (65) and (69) are so similar that it is not reasonable to suggest that decay is dominated by unsuccessful migration, but must be due to a radiationless transition. 

Triplet Decay and Exciton Migration. The rate of triplet state decay following a photoexcitation pulse is conveniently followed by monitoring the time dependence of the delayed fluorescence. A limitation of this approach is that absolute triplet concentrations cannot usually be evaluated and so rate constants for processes having a kinetic order greater than unity cannot be determined. 

These results may well be typical of most polymer/dopant systems. Certainly the behavior of these systems with respect to triplet exciton migration seems quite similar in spite of their rather distinctive molecular architecture. Let us now turn to an examination of systems in which chromophore units are regularly arranged by virtue of their being bonded to the backbone of a polymer chain. In this way, it may be possible to assess the effects of chromophore organization on the mechanism of triplet exciton migration and decay. 

There has been very little work completed in reverse micelles. This may be due to the increased complexity of these systems, where probe migration and micelle clustering occurs on time scales similar to triplet decay. In addition, litde work has been carried out in vesicles or liposomes. With the advent of diffuse reflectance laser flash photolysis, the turbid solutions observed for large vesicles no longer represent a stumbling block [196-198]. Thus, we believe that, in the future, much more work will be available in vesicular systems that will be able to complement the studies already completed in micelles. 

Transient absorption spectra from valerophenone in Na-ZSM-5 and Cs-ZSM-5 could be assigned to triplet-triplet transitions no spectra which can be attributed to the hydroxy-1,4-biradicals were detected. Furthermore, the decay of the transient signals could not be fit to either a single or double exponential expression, and samples prepared under apparently identical conditions exhibited half-lives that varied by a factor of 2 [292], All of these spectroscopic observations suggest that the valerophenone molecules reside in a distribution of sites within a zeolite and migration among them under the experimental conditions is slow. Thus, the Norrish II photoproduct ratios must be interpreted in terms of not only the relative populations of alkanophenones at each site type, but also the quantum efficiencies of each and the conformational preferences of the intermediate BR in each environment. 

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 occurrence of non-exponential decays in poly(vinyl alcohol) where there is no possibility of triplet energy migration is an additional proof of the absence of the T-T annihilation mechanism in the phosphorescence decay of benzophenone in polymer matrices. 

The non-exponential decay of benzophenone phosphorescence in PMMA at room temperature was also observed by Fraser et al. (36), under the condition of repeated irradiation of a nitrogen laser pulse, and they proposed a triplet-triplet annihilation mechanism in which the polymer matrix itself participated as an energy acceptor from benzophenone triplet and a medium for the triplet energy migration. 

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