Weak-coupling limit, electron-transfer

The Landau-Zener expression is calculated in a time-dependent semiclassical manner from the diabatic surfaces (those depicted in Fig. 1) exactly because these surfaces, which describe the failure to react, are the appropriate zeroth order description for the long-range electron transfer case. As can be seen, in the very weak coupling limit (small A) the k l factor and hence the electron transfer rate constant become proportional to the absolute square of A ... 

In Equation 6.88, K0 is the equilibrium constant for the formation of the collision complex, (V) 2 is the electronic coupling, and F is the Franck-Condon factor. In contrast to the radiationless relaxation, the energy transfer process cannot be rationalized only in the limit of the strong and weak coupling limits shown in Figure 6.16. 

When the donor and acceptor are sufficiently close, as in an ion pair or in covalently linked complexes, electron transfer can be promoted by the absorption of light. An absorption band corresponding to the light induced electron transfer is usnally called a charge transfer (CT) absorption band . The molecnlar parameters that determine the CT absorptivity, bandwidth, and band shape are the same molecular parameters that determine the magnitude of the electron-transfer rate constant.In the weak-coupling limit, the absorptivity of the CT absorption band is small (much less than 10 cm ) in the strong-... 

Pure charge-transfer exeited states are most rigorously defined in the weakly coupled limit. Simple examples of this limit are found in ion-pair complexes, complexes in whieh there is no covalent linkage between the donor and acceptor. Standard models work very well in this limit, and the IPCT absorption maximum is successfully described by the sum of the reduction potentials and the electron-transfer reorganizational energies of the constituent ionic partners, as in Eqs. 7 and 9 ( DA 0)-... 

One of the basic mechanisms in multichromophoric systems, electronic excitation transfer has been in the past and still is in many studies largely described using Forster theory. As stated by Forster [20], this model is developed for the weak coupling limit as it is based on an equilibrium Fermi Golden Rule... 

The material in this chapter is largely organized around the molecular properties that contribute to electron transfer processes in simple transition metal complexes. To some degree these molecular properties can be classified as functions of either (i) the nuclear coordinates (i.e., properties that depend on the spatial orientation and separation, and the vibrational characteristics) of the electron transfer system or (ii) the electronic coordinates of the system (orbital and spin properties). This partitioning of the physical parameters of the system into nuclear and electronic contributions, based on the Born-Oppenheimer approximation, is not rigorous and even in this approximation the electronic coordinates are a function of the nuclear coordinates. The types of systems that conform to expectation at the weak coupling limit will be discussed after some necessary preliminaries and discussion of formalisms. Applications to more complex, extended systems are mentioned at the end of the chapter. 

Charge-carrier transport is generally discussed in terms of semiclassical Marcus theory [112, 113, 300-303]. The charge-carrier mobility (in the weak coupling limit) is related to the rate of electron transfer, kjj, between two adjacent molecules/ polymer chain segments ... 

In this model there is a quantitative difference between RLT and electron transfer stemming from the aforementioned difference in phonon spectra. RLT is the weak-coupling case S < 1, while for electron transfer in polar media the strong-coupling limit is reached, when S > 1. In particular, in the above example of ST conversion in aromatic hydrocarbon molecules S = 0.5-1.0. 

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