Coulombic mechanisms

Trivial mechanism Coulombic mechanism E ectron exchange... 

The Coulombic mechanism is a relatively long-range process in as much as energy transfer can be significant even at distances of the order of lOnm. 

In order for rapid and efficient energy transfer to occur by the Coulombic mechanism ... 

The Coulombic mechanism can only occur where spin multiplicity is conserved since it is only in such transitions that large transition dipoles... 

A detailed theory of energy transfer by the Coulombic mechanism was developed by Forster, so the process is often referred to as Forster resonance energy transfer (FRET). According to the Forster theory, the probability of Coulombic energy transfer falls off inversely with the sixth power of the distance between the donor and the acceptor. For... 

The Coulombic mechanism would require that both 3D — 3D and 1A —> 3A were allowed transitions, which clearly they are not as both are spin-forbidden processes. Thus, triplet-triplet energy transfer by the long-range Coulombic mechanism is forbidden. 

For allowed transitions on D and A the Coulombic interaction is predominant, even at short distances. For forbidden transitions on D and A (e.g. in the case of transfer between triplet states (3D + 3A —> 1D + 3A ), in which the transitions Ti —> S0 in D and So —> Ti in A are forbidden), the Coulombic interaction is negligible and the exchange mechanism is found, but is operative only at short distances (< 10 A) because it requires overlap of the molecular orbitals. In contrast, the Coulombic mechanism can still be effective at large distances (up to 80-100 A). 

The Forster mechanism is also known as the coulombic mechanism or dipole-induced dipole interaction. It was first observed by Forster.14,15 Here the emission band of one molecule (donor) overlaps with the absorption band of another molecule (acceptor). In this case, a rapid energy transfer may occur without a photon emission. This mechanism involves the migration of energy by the resonant coupling of electrical dipoles from an excited molecule (donor) to an acceptor molecule. Based on the nature of interactions present between the donor and the acceptor, this process can occur over a long distances (30—100 A). The mechanism of the energy transfer by this mechanism is illustrated in Figure 11. 

Purely organic light-harvesting dendrimers have been prepared by Moore and coworkers [42], In these systems (Figure 17), the light absorbed by the peripheral chromophores (phenylacetylene monodendrons) is transferred to the core (per-ylene) via a coulombic mechanism. The efficiency of the process depends on the... 

Energy transfer according to the Coulomb mechanism is, in the Fdrster approximation (Forster, 1946, 1948), based on a dipole-dipole interaction that can be significant even at large donor-acceptor distances and that has no obvious bridge dependence. There is, however, a bulk solvent dependence on the refractive index (Eq. 8) that is assumed to take into... 

Introduction of reasonable numerical values into this equation leads to the expectation that k idip) can be much larger than the diffusion rate, so that energy transfer by dipole-dipole interaction can be significant even at distances considerably larger than molecular diameters. The Coulombic mechanism, in fact, does not involve formation of an encounter complex and its selection rules are the same as those for the corresponding electric dipole transitions in the isolated molecular partners. In particular, no change in spin in either partner is allowed. Thus processes (12) and (13) are fast. 

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