Relationship between Electron and Electronic Excitation Transfer

Relationship between Electron and Electronic Excitation Transfer... 

Piotrowiak P (2001) Relationship between electron and electronic excitation transfer. In Balzani V (ed) Electron transfer in chemistry, vol 1. Wiley-VCH, Weinheim... 

The relationship between electron transfer in the normal and inverted regions is illustrated in Figure 3 for the case of quenching of Ru(bpy) + by a nitroaromatic quencher. Excitation... 

It should be emphasized that the linearity of the plot in Figure 3 is merely a consequence of the rates of both charge transfer and triplet excitation transfer being exponentially distance dependent. It is the slope of the resulting line that contains the information about the relationship between the respective electronic coupling elements. In Figure 3, the slope is, within experimental error, equal to one, confirming that, indeed, 0 = + 0. ... 

The relationship between driving force and proton transfer has been much more elusive despite considerable evidence that the vast photo synthetic electron transfer machinery mainly exists to set up a charge gradient to drive proton transfer. This is due to a combination of two factors. First, there is a vast reservoir of readily available materials with which to examine electron transfer. Second, the relationship between rates and driving force for electron transfer, based upon the excitation energies and the relevant redox potentials (the Rehm-Weller equation [1]) is reasonably straightforward. 

Chemical reactions can be studied at the single-molecule level by measuring the fluorescence lifetime of an excited state that can undergo reaction in competition with fluorescence. Reactions involving electron transfer (section C3.2) are among the most accessible via such teclmiques, and are particularly attractive candidates for study as a means of testing relationships between charge-transfer optical spectra and electron-transfer rates. If the physical parameters that detennine the reaction probability, such as overlap between the donor and acceptor orbitals. 

Photosensitization of diaryliodonium salts by anthracene occurs by a photoredox reaction in which an electron is transferred from an excited singlet or triplet state of the anthracene to the diaryliodonium initiator.13"15,17 The lifetimes of the anthracene singlet and triplet states are on the order of nanoseconds and microseconds respectively, and the bimolecular electron transfer reactions between the anthracene and the initiator are limited by the rate of diffusion of reactants, which in turn depends upon the system viscosity. In this contribution, we have studied the effects of viscosity on the rate of the photosensitization reaction of diaryliodonium salts by anthracene. Using steady-state fluorescence spectroscopy, we have characterized the photosensitization rate in propanol/glycerol solutions of varying viscosities. The results were analyzed using numerical solutions of the photophysical kinetic equations in conjunction with the mathematical relationships provided by the Smoluchowski16 theory for the rate constants of the diffusion-controlled bimolecular reactions. 

Fig. 18 Potential energy diagram qualitatively illustrating the relationship between the charge-transfer excitation energy and the thermal barrier to electron transfer in...

To alleviate the apparent problem, Rehm and Weller proposed an empirical relationship for electron transfer between excited donor/acceptor pairs (7), eq. 7. 

By means of time-resolved fluorescence studies we were able to determine the C60 fluorescence deactivation rates, as 2.1 x 1010 s-1 in 9a, 6.6 x 109 s 1 in 9b and 1.3 x 109 s-1 in 9c. Importantly, the indulging trend resembles the relationship between the quantum yields of the conjugates (9a-d) and reference (1). In short, an intensified excited-state deactivation emerges with decreasing bridge length. However, no measurable decay rates were found for the trimer 9d. Conclusively, the indirect or direct population of Cgo possibly leads to an exothermic electron-transfer reaction, resulting in the radical-ion-pair state ... 

Fig. 3. Relationship between the electron transfer pathways for deactivation of the excited [2, 2, 3] complex and the optical charge transfer transitions.

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