Triplet-energy migration

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. 

Influence of Chromophore Organization on Triplet Energy Migration in Amorphous Polymer Solids... 

Estimates of the rate and range of triplet energy migration as measured by different techniques vary enormously. For this reason it is worthwhile to simmarize the literature on triplet energy migration in polymers and to point out some assumptions in the different experimental approaches. 

Finally, the triplet states have un character and correspondingly long lifetimes at 77K. As a result the effects of triplet energy migration are expected to be particularly evident. 

Clearly, this is a simplistic view, since there is a considerable variation in 0g among the PS homopolymers. Yet the excimer fluorescence to fluorescence ratio in DMM solutions is very similar from polymer to polymer. A possible alternative, or additional, route to chain scission may involve intramolecular triplet energy migration and transfer processes. It has been shown (J ) that intramolecular triplet migration is facile in both PS and P(S-alt-MMA), but that sequences of two or more MMA-units interrupt the migration process. If an excited triplet trapping site can release its excess energy without bond scission,... 

Triplet-energy migration and transfer processes have been studied to some extent. The efficiency of the decay of the triplet-triplet of excited poly(2-vinylnaphthalene) by an added quencher, piperylene, has been found to decrease with an increase in the molecular weight of the polymer. An increase in the... 

Triplet energy migration can be probed by addition of free quenchers to the polymer, and observing which kinetics, Stern-Volmer or Perrin (see Section II above), pertain. In the case of the quenching of naphthalene phosphorescence or delayed fluorescence in a variety of co-polymers by the dloleflns plperylene and cycloocta-1,3-diene at low temperature where material diffusion... 

The theoretical prediction, for aromatic hydrocarbons (the only ones now available) are as follows . Ionic-exciton states will contribute mainly to neutral excitons of small band width that is, singlet excitons which correspond to weak transitions and triplet excitons. Ionic contributions may broaden the exciton-band widths and may be particularly important in enhancing the rates of triplet-energy migration. A spectroscopic study of anthracene indicates the ionic exciton lies energetically above the first singlet level. No undisputed direct experimental detection of an ionic-exciton level in a one-component organic crystal has yet been made. 

In what follows, we discuss the non-exponential decay of phosphorescence in polymer matrices and its orij along with the effect of a multi-photon process on decay curves. Singlet and triplet energy migrations and excimer formation are important photo-... 

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