Migration of excitation

Migration of excitation away from the site of the original radiation event has been invoked In polymers and has also been observed In biopolymers (30,31). 

Polymers Chain dynamics Local viscosity in polymer environments Molecular orientation in solid polymers Migration of excitation energy along polymer chains... 

Artificial and natural antennae Migration of excitation energy... 

If the recombination is delayed, e.g., by migration of excited electrons, luminescence takes place by a second-order bimolecular reaction. The probability of a luminescent recombination of the excited electron with the holes is then proportional to the product of the concentration of electrons and the concentration of holes. The lower the initial intensity is, and the further the decay has progressed, the slower the decay to the half value is. This hyperbolic decay law is only of limited validity. If the excited electron is momentarily trapped before recombination, very complex interactions can arise. 

The probability for resonance transfer of electronic excitation decreases as the distance between the two molecules increases. If chlorophyll molecules were uniformly distributed in three dimensions in the lamellar membranes of chloroplasts (Fig. 1-10), they would have acenter-to-center spacing of approximately 2 nm, an intermolecular distance over which resonance transfer of excitation can readily occur (resonance transfer is effective up to about 10 nm for chlorophyll). Thus both the spectral properties of chlorophyll and its spacing in the lamellar membranes of chloroplasts are conducive to an efficient migration of excitation from molecule to molecule by resonance transfer. 

The characteristics of fluorescence also provide information on the lifetime of the excited singlet state of chlorophyll in vivo and thus on the time available for migration of excitations. Specifically, approximately 1 to... 

In the antiferromagnetic phase the migration of excitation energy is slowed down, because nearest neighbor Gld (or Tb ) ions are oriented antiparallel, which makes energy transfer by exchange interaction impossible. 

Fluorescence techniques can also be used to study solid surfaces. A fractal approach has been used to interpret rhodamine 6G probed morphology of porous silica surfaces . Migration of excitation agrees with a one step energy transfer mechanism. 

An experimental value of R0 can be obtained by plotting f as a function of the logarithm of the acceptor concentration and fitting the theoretical curve calculated according to Forster s theory to the experimental results. An experimental value larger than the calculated one may be considered as evidence for migration of excitation prior to the transfer. Agreement was obtained between the experimental results and Forster s theory for the system polystyrene—tetraphenylbutadiene [165]. 

A further property that makes transition metal ions tremendously useful in photochemistry is obviously the possibility of complexing several mmio- or polydentate ligands around a number of metal ions and thus to assemble sophisticated structures where electronic and geometric factors drive the photochemical processes. This principle has been largely applied in artificial photosynthesis for optimizing light collection and make migration of excitation toward a specific site preferred. 

The decay of the monomer fluorescence from the copolymer is also nonexponential but convergence was not obtained when analysed using equation (6) with all parameters adjustable. If migration of excitation energy occurs in P2NMA it could contribute to the poor fit obtained by... 

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