Intramolecular excitation energy migration

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,... 

In order to determine whether energy migration makes a significant contribution to the photophysical behavior of P2VN and PS in dilute miscible blends, it is instructive to calculate the expected exdmer-to-monomer fluorescence quantum yield ratio in the absence of energy migration. To do so, it is first necessary to assume that intermolecular and non-adjacent intramolecular EFS are absent. In addition, the adjacent intramolecular EFS are assumed to be frozen into the aryl vinyl polymer and must be excited by direct absorption of a photon. Since the absorption spectrum of an EFS is no different from that of non-EFS chromophores, then the calculated fraction of rings within EFS is sufficient to determine the fluorescence ratio. 

Electronic energy transfer from an excited state donor to a suitable acceptor is a fundamental process in photochemistry and is of prime importance in artificial photosynthesis. A variety of mechanisms exist by which energy migration and transfer can take placeand such processes have been used to create the so-called antenna effect . In particular, much research has concentrated on designing molecular systems in which an organic host is used to bind a lanthanide cation in such a way that photoexcitation of the host results in intramolecular energy transfer to the bound cation. ... 

If measurements of 4>p and/or Tp are to be used to evaluate intramolecular relaxation rates, the effect of intermolecular processes on these quantities must be assessed. In addition, the eflBciency of excitation energy transfer is of intrinsic interest, especially in connection with solid state photochemical and photophysical processes. If excitation energy is transferred to an identical center, i,e, the same species in precisely the same environment, then all of the relaxation rates k2-ks are unaffected, and no change in measureable quantities (except polarization) is expected. However, if transfer occurs between non-identical centers, then observable changes will occur. Two situations can be distinguished (36) (a) single step donor-acceptor transfer and (b) migration transfer. 

The excited state properties, principally fluorescence, of probe molecules incorporated in the polymer chain can be used to a large extent to study molecular motion, order, and energy migration in polymeric systems. The present review has revealed how fluorescence methodologies can provide an insight into the nature of the intramolecular and intermolecular interactions, which are responsible for thermally initiated variations as well as the formation of microstructures and the morphology of a polymeric medium. 

There are two limiting cases for the effects of polymer folding on energy transfer efficiency. Folding of a polymer before excitation into a conformation in which the sensitizers are held within a hydrophobic pocket improves the efficiency of energy migration. This takes place with a large number of intramolecular hops or when bond interactions intervene between the sensitizer and the ultimate trap [93]. If the polymers are flexible, however, they can also bend after photo-excitation to... 

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