Radiationless transfer of electronic

Ermolaev, V. L., Bodunov, E. N., Sveshnikova, E. B., and Shakhverdov, T. A. (1977) Radiationless Transfer of Electronic Excitation Energy, Nauka. Leningrad. 

III. Transfer of Electronic Excitation Energy A. Radiationless Energy Transfer... 

The molecular ion will be of low symmetry and have an odd electron. It will have as many low-lying excited electronic states as necessary to form essentially a continuum. Radiationless transitions then will result in transfer of electronic energy into vibrational energy at times comparable to the periods of nuclear vibrations. 

All these results indicate that, with titanium-silicon binary oxides having low titanium contents the Ti ions are enriched near the surface regions, separated from each other, and present as tetrahedral species in the Si02 carrier matrices. In such species, the radiationless transfer of photon energy absorbed by Ti02 is suppressed because of the low coordination of the ions. As a result, the formation of the (electron-hole) ion pairs, i.e., the excited state of (Ti +- 0 ) complexes, is facihtated (200, 201). 

The transfer of electronic excitation energy from one molecule to another is another phenomenon that is related to the concentration of potentially luminescent molecules. Energy transfer occurs quite frequently in nature, either by direct collision or even over distances as great as 50 A or more by a radiationless mechanism by which the excitation energy is transmitted from the molecules that are originally excited (donors) to the recipient molecules (acceptors). The efficiency with which an excited donor will transfer its excitation energy to an acceptor molecule (rather than fluoresce) is a function of the lifetime of the excited... 

Although the number of EFS traps will be an important factor governing the observed fluorescence, a second factor of equal or sometimes greater importance is the phenomenon of electronic excitation transport (EET) [51, 58-61]. This involves the radiationless transfer of excitation energy, in the singlet state for compounds in this work, from one aromatic chromophore to another. This process may be viewed as a random walk with the rate of transfer between randomly oriented absorption and emission dipoles being given at each step by... 

Radiationless transitions such as internal conversion or intersystem crossing lead to the conversion of electronic energy to vibrational energy which is transferred randomly to the environment. Another important type of deexcitation involves the direct transfer of electronic excitation selectively to individual molecules present in relatively small concentrations in the environment (for summary of literature see Kasha, 1963). Of particular interest to photochemists are excitation transfer processes which involve a donor molecule in either its lowest singlet or lowest triplet excited state and an acceptor molecule in its ground state (usually singlet). Because certain multiplicity selection rules must be obeyed, only the processes shown in Eqs. (7)-(9) are allowed, where D and A stand for donor and acceptor respectively. 

Knowledge of photoiaduced electroa-transfer dyaamics is important to technological appUcations. The quantum efficiency, ( ), ie, the number of chemical events per number of photons absorbed of the desired electron-transfer photoreaction, reflects the competition between rate of the electron-transfer process, eg, from Z7, and the radiative and radiationless decay of the excited state, reflected ia the lifetime, T, of ZA ia abseace ofM. Thus,... 

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. 

In the quantum mechanical formulation of electron transfer (Atkins, 1984 Closs et al, 1986) as a radiationless transition, the rate of ET is described as the product of the electronic coupling term J2 and the Frank-Condon factor FC, which is weighted with the Boltzmann population of the vibrational energy levels. But Marcus and Sutin (1985) have pointed out that, in the high-temperature limit, this treatment yields the semiclassical expression (9). 

In Figure 5.2(a), both electronic states have similar geometries, shown by the nested curves with their minima being coincident. Their electronic energy separation is large, with the v = 0 vibrational level of the initial electronic state being close to the v = 7 vibrational level of the final electronic state. There is very little overlap between the isoenergetic /2 functions and so the rate of radiationless transfer will be slow. 

Kuzmin MG, Soboleva IV, Dolotova EV (2007) The behavior of exciplex decay processes and interplay of radiationless transition and preliminary reorganization mechanisms of electron transfer in loose and tight pairs of reactants. J Phys Chem A 111 206... 

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