Exchange energy transfer

3 Concepts of Photoinduced Electron and Energy Transfer Processes 

Exchange energy transfer from the lowest spin forbidden excited state is expected for singlet-triplet intersystem-crossing (ISC) reactions presented further in the following parts of this thesis. 

Interestingly, in a homogeneous series of dyads where triplet energy transfer (Eq. 3.12), electron transfer (Eq. 3.13), and hole transfer (Eq. 3.14) processes 

Jortner J, Ratner M (eds) (1997) Molecular electronics. Blackwell, London 

Sachs SB, Dudek SP, Hsung RP, Sita LR, Smalley JF, Newton MD, Feldberg SW, Chidsey CED (1997) J Am Chem Soc 10 563 

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SHORT-RANGE ELECTRON-EXCHANGE ENERGY TRANSFER... 

Dexter s formulation of exchange energy transfer (very weak coupling) In contrast to the inverse sixth power dependence on distance for the dipole-dipole mechanism, an exponential dependence is to be expected from the exchange mechanism. The rate constant for transfer can be written as... 

A number of cyano-bridged complexes are included here even though they strictly do not fall in the general family-type defined for the section. The syntheses and photophysical properties of [(NC)(bpy)2Ru(/r-NC)Cr(CN)5] and [(NC)5Cr(Ai-CI Ru(bpy)2(M-NC)Cr(CN)5] have been described. Absorption of visible light by the Ru(bpy)2 unit results in phosphorescence from the Cr(CN)g luminophore, and the results evidence fast intramolecular exchange energy transfer from the MLCT state of the Ru(bpy)2 chromophore to the doublet state of the Cr -based unit. Time-resolved resonance Raman and transient UV-vis absorption spectroscopies have been employed to investigate the MLCT excited states of [(NC)(bpy)2Ru(//-CN)Ru (bpy)2(CN)], [(NC)(bpy)2Ru(//-CN)Ru(phen)2(CN)]+, [(NC)(phen)2Ru(//-CN)Ru (bpy)2(CN)]+, [(NC)(bpy)2... 

The Schenck Mechanism. Two mechanisms were originally proposed for photosensitized olefin reactions. One suggested that sensitization occurred by exchange energy transfer from an excited donor (S) to yield ground state donor and excited state acceptor.24,136... 

Little attention has so far been paid to studying exchange energy-transfer processes in media so viscous that a steady-state is no longer established. Butler and Pilling [200] specifically sought experimental evidence for time-dependent rate coefficients of the form of eqn. (98). They chose to study triplet phenanthrene in methanol—water mixtures and used cupric chloride as the acceptor since it is readily soluble and a very efficient quencher of triplet phenanthrene. To observe even the t 1/2 dependence of the time-dependent rate coefficient, concentrations [A] > 10-2 are required that is with Re 1 nm and [A] > 10 mmol... 

Collisional Energy Transfer (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).

Fig. 3.7 Pictorial representation of resonance (coulombic) and exchange energy transfer mechanisms...

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...

For reasons similar to those discussed above for ET processes, the rate constant of exchange energy transfer is expected to be sensitive to the nature of the bridge. 

When an acceptor molecule is in the vicinity of an excited donor the transfer of energy may take place by one of the following prooessest a.Resonance excitation transfer,b.Exchange energy transfer. The theory of such energy processes is well developed (34,35) ... 

Wu, Z.Z., Nash, J., and Morrison, H., 1992, Photoepimerization of 3-a(dimethylphenylsilyl-oxy)-5a-androstane-6,17-dione and its 3fS isomer through bond exchange energy transfer, J. Am. Chem. Soc. 114, 6640-6648. 

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