Migration of electron excitation

The long lifetime of lower excited states favors the efficient transfer of electron excitation energy from molecules of the solvent to molecules of the solute, provided the emission spectrum of the donor overlaps the absorption spectrum of the acceptor. The mechanism of the nonradiative transfer of electron excitation energy was found by Forster and Dexter39-40. The distance over which the energy is transferred may be as large as 20-30 A. The migration of electron excitation also occurs in... 

Haan SW, Zwanzig R (1978) Forster migration of electronic excitation between randomly distributed molecules. J Chem Phys 68(4) 1879-1883. doi 10.1063/1.435913... 

Thus, the data on the kinetics of decay of et in the course of photobleach-ing in matrices containing acceptor additives are well described by a model implying a migration of electrons by a jumpwise mechanism via excitation to the conduction band from traps located far from acceptor particles to those located close to them with the subsequent capture of electrons by these particles via a tunnel mechanism. 

Electronic energy migration (or Hopping) The movement of electronic excitation energy from one molecular entity to another of the same species, or from one part of a molecular entity to another of the same kind (e.g. excitation migration between the chromophores of an aromatic polymer). The migration can happen via radiative or radiationless processes. 

This second molecule thereby becomes excited, indicated by S2(W)W-), and the molecule that absorbed the photon becomes deexcited and is returned to its ground state. Such transfer of electronic excitation from molecule to molecule underlies the energy migration among the pigments involved in photosynthesis (see Chapter 5, Sections 5.3 and 5.4). We will assume that Equation 4.8 represents a first-order reaction, as it does for the excitation exchanges between chlorophyll molecules in vivo (in certain cases, Eq. 4.8 can represent a second-order reaction, i.e., dS /dt then equals fc S j). 

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. 

Shafirovich et al. [71] have also investigated the migration of electrons photo-injected into DNA by two-photon excitation (335 nm) of pyrene derivatives covalently bound to ss and ds DNA. They find that in ds DNA the photoinjected electrons migrate to the acceptor methyl viologen (MV + Figure 6) which is boimd to the DNA within the c. 7-ns time resolution of the laser apparatus. From the dependence of the MV+ yield upon the MV + concentration, and the assumption of a random distribution of the pyrene donor and acceptor, they conclude that... 

There are two types of ener transfer from a point in a lattice to a luminescent center. For semi-conducting phosphors like ZnS, migration of electrons in the conduction band, or holes in the valence band, conve3rs the excitation energy to localized luminescent centers. Excltons (bound electron-hole pairs) is another mechanism. For insulators (which are generally oxygen dominated compositions), the excitation energy can be transferred from an excited point in the lattice, or from an excited luminescent center to other unexcited centers in the lattice by Quantum Mechanical resonance. 

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