Coupling, between donor and acceptor

Electronic absorption spectroscopy charge transfer transitions, 19 71 d-d transitions, 19 70, 71 flavocytochrome b, 36 269-271 intraligand transitions, 19 71-80 of organometallics, 19 69-80 Electronic coupling, between donor and acceptor wave functions, 41 278 Electronic nuclear double resonance spectroscopy, molybdenum center probes, 40 13... 

Rate constant for charge transfer between donor and acceptor Effective coupling between donor and acceptor states Matrix element of Hamiltonian between diabatic donor and acceptor states... 

The following terms in the expression for the coupling between donor and acceptor is obtained by taking the higher terms in the eq.(37). The term D BD 1 gives for example ... 

The partitioning technique also nicely shows why a bridge, consisting of a molecule or a solvent, is a better mediator than empty space. In empty space the direct matrix element Hda is the only coupling between donor and acceptor. This matrix element decreases with P 4, which effectively forbids ET distances larger than 4-5 A. 

When donor-acceptor pairs lack interactions with an intervening medium, e.g. solvent molecules, the electron transfer mechanism is supposed to occur through space. Considering that the electron density of molecular orbitals falls off exponentially, a similar postulate may be formulated for the electronic coupling between donor and acceptor, fljfg... 

Rotations in the monomer 9b, dimer 9c and trimer 9d do not seem to influence the electronic structure and the coupling between donor and acceptor. The rotational barriers comply with these found in the < xTTF-oPPV -C60 triads (0.34 kcal/mol). 

Figure 2.1(a) above illustrates the potential energy surface for a diabatic electron transfer process. In a diabatic (or non-adiabatic) reaction, the electronic coupling between donor and acceptor is weak and, consequently, the probability of crossover between the product and reactant surfaces will be small, i.e. for diabatic electron transfer /cei, the electronic transmission factor, is transition state appears as a sharp cusp and the system must cross over the transition state onto a new potential energy surface in order for electron transfer to occur. Longdistance electron transfers tend to be diabatic because of the reduced coupling between donor and acceptor components this is discussed in more detail below in Section 2.2.2. 

The conceptual framework that is used to understand thermal and photoinduced electron transfer is illustrated by a classical potential energy-configuration diagram. The simpliest case for electron transfer in which the electron is coupled between donor and acceptor by a single oscillator having the same frequency in both the initial and the final states is illustrated in Figs, la and lb. 

For weak coupling cases, Hush showed that the intensity of an intervalence transition was related to the extent of coupling between donor and acceptor. The derivation (11) begins by considering the theoretical expression for oscillator strength,... 

The simplest model consists of two centres, one donor (D) and one acceptor (A), separated by a distance I and contains two electrons. Here we consider this simple system to illustrate some general relations between charge transfer, transition intensities and linear as well as non-linear optical polarizabilities. We will show below that the electro-optic parameters and the molecular polarizabilities may be described in terms of a single parameter, c, that is a measure of the extent of coupling between donor and acceptor. Conceptually, this approach is related to early computations on the behaviour of inorganic intervalence complexes (Robin and Day, 1967 Denning, 1995), Mulliken s model for molecular CT complexes (Mulliken and Pearson, 1969) and a two-form/two-state analysis of push-pull molecules (Blanchard-Desce and Barzoukas, 1998). 

The truly metallic intervalence complexes often suffer from low extinction coefficients despite the low energy gap of the transition, the extent of the electronic coupling between donor and acceptor over the framework chosen remains comparatively small. 

The value of p reflects the efficacy of the medium in coupling the donor and the acceptor. Since electronic coupling between donors and acceptors is often treated in terms of a superexchange model, the decay of electronic coupling is sometimes reported per bond rather than per unit distance. In the bond-coupling model, the exponent in Eq. 5 becomes [- P (A - l)/2]. 

Traditional transition state theory does not hold for longdistance electron transfer reactions. Due to weak electronic coupling between donor and acceptor moieties, formation of a transition state does not necessarily lead to... 

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