Long-Range Electronic-Energy Transfer

Table 6.6 Long-range electronic-energy transfer between aromatic molecules. Values of 3 are experimental rate constants in benzene, at room temperature (which we shall take as 293 K), obtained from Stem-Volmer plots by Equation (6.15) or (6.16) (Section 6.2.2). The theoretical value calculated for reaction at every encounter (diffusion control) at 293 K is ku =4RT/r) = 1.5 x lO s" . J is the overlap integral, obtained from fluorescence and absorption spectra. See text. Rq is the critical distance experimental values (column 4) are calculated from 3 (cf. Table 6.5) and theoretical values (column 5) are calculated from Equation (6.42). See text. Data from Ref. [38]...

Figure 6.9 Electron movements occurring in long-range Coulombic energy transfer. Note that the electrons initially on D remain on D and electrons initially on A remain on A. This energy transfer does not require physical contact between the donor and acceptor...

K5 rychenko, A. and B. Albinsson (2002). Conformer-dependent electronic coupling for long-range triplet energy transfer in donor-bridge-acceptor porphyrin dimers. Chem. Phys. Lett. 366(3-4), 291-299. 

Kyrychenko A and Albinsson B. Conformer-Dependent Electronic Coupling for Long-Range Triplet Energy Transfer in Donor-Bridge-Acceptor Porphyrin Dimers. Chem. Phys. Lett. 2002 366 291-299. 

Electron mediators successfully used with oxidases include 2,6-dichlorophenolindophol, hexacyanoferrate-(III), tetrathiafulvalene, tetracyano-p-quinodimethane, various quinones and ferrocene derivatices. From Marcus theory it is evident that for long-range electron transfer the reorganization energies of the redox compound have to be low. Additionally, the redox potential of the mediator should be about 0 to 100 mV vs. standard calomel electrode (SCE) for a flavoprotein (formal potential of glucose oxidase is about -450 mV vs SCE) in order to attain rapid vectrial electron transfer from the active site of the enzyme to the oxidized form of the redox species. 

Concept The rates of long-range electron transfer (ET) and excitation energy transfer (EET) processes between a pair of chromo-phores (redox couple) may be strongly facilitated by the presence of an intervening non-conjugated medium, such as saturated hydrocarbon bridges, solvent molecules and n-stacks, e.g.,... 

G. L Closs, M. D. Johnson, J. R. Miller, P. Piotrowiak, A Connection between Intramolecular Long-Range Electron, Hole, and Triplet Energy Transfers , J. Am Chem Soc 1989, 111, 3751-3753. 

In conclusion, the author believes that consideration should be given to the points discussed above and the effects of hydrodynamic repulsion (Chap. 9, Sect. 4) when considering reactions between ions. There are so many factors which may influence such reaction rates, that many experimental studies of ionic reactions may have found agreement with the Debye—Smoluchowski theory (or corrected forms) by cancellation of correction terms. Probable complications due to long-range electron and energy transfer are discussed in Chap. 4. 

Many other papers on long-range electron transfer between two reactive sites of modified proteins were published [270-288] after the above mentioned pioneering works. Most of them dealt with photoinduced electron tunneling from triplet states of closed shell Mg(II) and Zn(II) porphyrins to Fe(III) or Ru(III). In agreement with the prediction of Marcus theory the rate constants for the majority of these intraprotein electron transfer reactions were found to increase as the free energy of reaction decreased. However for one of the reactions disagreement with this theory was observed [285],... 

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