Electron charge transfer radiationless processes

Charge transfer between electronic states in the electrode and solution are radiationless processes whose standard rate coefficients span over a considerable range of magnitude. The electronic transition occurs between levels of the same energy for, otherwise, radiation would be emitted. 

For systems in which no electron migration or stabilization can occur after excitation, the excited electron returns to its ground state by either luminescence or radiationless transition. Since most charge transfer transitions occur with very high probability, the excited state persists for only about 10 8 sec. Therefore, the secondary process must be extremely fast to compete with spontaneous emission of the excited state. 

Figure 1. JabJonski-type diagram of the lowest energy levels of electron donor-acceptor molecules formally linked by a single bond which show dual fluorescence phenomenon. D-A, (D A), (D+-A ), (D -A ) and (D-A) denote the ground state, the primary excited and charge-transfer (CT) singlet states, and CT and locally excited triplet states, respectively. The arrows correspond to the radiative (absorption, A, fluorescence, F, and phosphorescence, Ph) and the radiationless (internal conversion, IC, and intersystem crossing, ISC) processes.

Processes at electrodes are radiationless. Therefore energy levels at the Fermi level in the metal must be matched with suitable vacant (LUMO) or occupied (HOMO) orbitals in the reactant, depending on the direction of charge transfer, for significant rates of charge transfer to occur (Fig. 1). Normally an applied, or spontaneously generated, potential is required to modify the electron work function to some value eV to achieve this condition of balance (Fig. 1) required for facile electron transfer to take place at the potential V, usually by tunneling. 

The Marcus Inverted Region (MIR) is that part of the function of rate constant versus free energy where a chemical reaction becomes slower as it becomes more exothermic. It has been observed in many thermal electron transfer processes such as neutralization of ion pairs, but not for photoinduced charge separation between neutral molecules. The reasons for this discrepancy have been the object of much controversy in recent years, and the present article gives a critical summary of the theoretical basis of the MIR as well as of the explanations proposed for its absence in photoinduced electron transfer. The role of the solvent receives special attention, notably in view of the possible effects of dielectric saturation in the field of ions. The relationship between the MIR and the theories of radiationless transitions is a topic of current development, although in the Marcus-Hush Model electron transfer is treated as a thermally activated process. 

As far as kinetics is concerned, electron transfer processes can be described in terms of quantum mechanical [7-9] or classical [10-14] models. From a quantum mechanical viewpoint, both photoinduced electron transfer (Eq. 2) and charge recombination (Eq. 4) can be viewed as examples of radiationless transitions between... 

X = 8000 cm" ). It is seen that the charge recombination process leading to the Cr(ni) doublet state is expected to be in the nearly activationless regime, whereas that leading to the ground state is likely to lie deep into the "inverted region" of electron transfer (section 1.3.2). In terms of radiationless transition theory, the excited-state charge recombination is favored by its... 

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