Radiationless Transfer Theories

The lifetime of the MLCT excited state of [Ru(bipy)3] has been measured in several solvents and at different temperatures. Among the- conclusions drawn are that k, is only slightly solvent-dependent, that k , agrees quantitatively with predictions of energy gap law for radiationless transitions, and that the solvent dependence of kinetic parameters for MLCT —> d, dctransitions can be considered in the context of electron-transfer theory. These results may have implications for the use of [Ru(bipy)3] as sensitizer. The solvent dependence of the MLCT transitions of [Ru(bipy)3], [Os(bipy)3], [Os(bipy)2(py)2p, and [Os(bipy)2 l,2-(Ph2P)2CsH4 f has been interpreted in... 

While a partly classical inner-sphere contribution to the activation energy in cases where the electrostatic continuum approximation applies has been assumed by Marcus for a redox-type (i.e., non-atom-transfer) process, no successful efforts have been made to describe the instantaneous radiationless electron transfer mechanisms for heavy nuclei (see, however. Ref. 78). Overall, it seems reasonable to conclude that the instantaneous transfer theory is an unlikely hypothesis for combined atom and electron transfer. A more reasonable approach is dependent on the fact that the lifetime of an activated molecule (i.e. the proton tightly bound to its neighboring water molecules, e.g., in 11304" ) is very long compared with the transition time for one single-electron passage. 

Scholes GD and Ghiggino KP. Rate expressions for excitation transfer 1. Radiationless transition theory perspective. J. Chem. Phys. 1994b 101 1251-1261. 

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... 

Using the data obtained by Meyer and co-workers for the ligand-to-ligand charge-transfer absorption bands of the complex represented in Figure 15.18 gives = 45 cm (= 0.53 kJ morO- This value of the electronic coupling is within the limits required by radiationless transition theories. 

Juzeliunas G and Andrews D L 1999 Unified theory of radiative and radiationless energy transfer Resonance Energy Transfer ed D L Andrews and A A Demidov (New York Wiley) pp 65-107... 

Though theories have been proposed (32-35) to explain this phenomenon, the mechanism of fluorescence is still not yet fully understood. Jankow and Willis (36) proposed a mechanism which involves a direct excitation of the molecule or an impurity to an excited state, followed by internal conversion and then reversion back to the original state with emission of light. This mechanism can be explained as follows A molecule in the lowest vibrational level of the ground state A is transferred to a certain vibrational level in the excited state D. The molecule tends to cascade into the lowest vibrational level of state D by collisions with other excited molecules. It passes from state D to state C and then to state B by radiationless transi-... 

Instead of the quantity given by Eq. (15), the quantity given by Eq. (10) was treated as the activation energy of the process in the earlier papers on the quantum mechanical theory of electron transfer reactions. This difference between the results of the quantum mechanical theory of radiationless transitions and those obtained by the methods of nonequilibrium thermodynamics has also been noted in Ref. 9. The results of the quantum mechanical theory were obtained in the harmonic oscillator model, and Eqs. (9) and (10) are valid only if the vibrations of the oscillators are classical and their frequencies are unchanged in the course of the electron transition (i.e., (o k = w[). It might seem that, in this case, the energy of the transition and the free energy of the transition are equal to each other. However, we have to remember that for the solvent, the oscillators are the effective ones and the parameters of the system Hamiltonian related to the dielectric properties of the medium depend on the temperature. Therefore, the problem of the relationship between the results obtained by the two methods mentioned above deserves to be discussed. 

These three factors are included in the expression derived by Robinson and Frosch from time-dependent perturbation theory, for the nonradiative energy transfer or radiationless transition probability kNR per unit time,... 

Radiationless excitation transfer occurs only when (D + A) initial state is in or near resonance with (D +A ) final state and there is a suitable donor-acceptor interaction between them. The rate of transfer, kt> a. is given by the time-dependent perturbation theory,... 

Electron transfer from the excited states of Fe(II) to the H30 f cation in aqueous solutions of H2S04 which results in the formation of Fe(III) and of H atoms has been studied by Korolev and Bazhin [36, 37]. The quantum yield of the formation of Fe(III) in 5.5 M H2S04 at 77 K has been found to be only two times smaller than at room temperature. Photo-oxidation of Fe(II) is also observed at 4.2 K. The actual very weak dependence of the efficiency of Fe(II) photo-oxidation on temperature points to the tunneling mechanism of this process [36, 37]. Bazhin and Korolev [38], have made a detailed theoretical analysis in terms of the theory of radiationless transitions of the mechanism of electron transfer from the excited ions Fe(II) to H30 1 in solutions. In this work a simple way is suggested for an a priori estimation of the maximum possible distance, RmSiX, of tunneling between a donor and an acceptor in solid matrices. This method is based on taking into account the dependence... 

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