Electron exchange energy transfer

SHORT-RANGE ELECTRON-EXCHANGE ENERGY TRANSFER... 

Figure 11.5 Diagrammatic explanation of the coulombic and electron exchange energy transfer mechanisms (A and B are chromophore components and L is a bridging moiety or ligand).

Figure 2.14 Schematic representations of the mechanisms of photoinduced (a) electron transfer, (b) Dexter (electron-exchange) energy transfer, and (c) Fdrster (dipole-dipole) energy transfer mechanism processes in the supramolecular dyad A-L-B spheres represent electrons, while curved arrows indicate the directions of transfer...

The first demonstration that /3-carotene could inhibit photosensitized oxidation and was, therefore, an efficient quencher of O was reported by Foote and Denny (1968). Wilkinson and Ho (1978) showed that quenching by electron exchange energy transfer to produce the carotenoid triplet state ( CAR) is the principal mechanism of carotenoid photoprotection against O ... 

As such, tlie efficiency of electron exchange energy transfer falls off steeply as the separation between D and A increases. 

In electron exchange energy transfer, two electrons are simultaneously transferred between an excited state donor, D, and ground state acceptor, A (see Chap. 1). Orbital overlap is required, and, since electron density falls off rapidly with distance, electron exchange is restricted to intermolecular distances of <8 A. The spin conservation rule is obeyed and this mechanism is possible for both singlet-singlet and triplet-triplet energy transfer. 

Electron exchange energy transfer, which occurs when an excited donor molecule (D ) and an acceptor molecule A) are close enough (10-15 A) that they may be considered to be in molecular contact, i.e. their centres are separated by the sum of their molecular radii. Their electron clouds may overlap each other and an electron on D may also appear on A. 

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