Radiationless transfer

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. 

With large rigid aromatic molecules there is virtually no change in molecular geometry on excitation. The overlap of tj/2 functions in Figure 5.2(a) shows that in such cases the rate of internal conversion from Si to S0 will be very slow. Thus, in such cases, fluorescence is able to compete favourably with radiationless transfer and hence these rigid systems tend to fluoresce strongly. 

M. Kupryszewska, I. Gryczynski, and A. Kawski, Intramolecular donor-acceptor separations in methionine- and leucine-enkephalin estimated by long-range radiationless transfer of singlet excitation energy, Photochem. Photobiol. 36, 499-502 (1982). 

The only recent example of Forster transfer of photochemical importance is the demonstration by Saltiel163 that the ability of azulene to increase the photostationary transjcis ratio in direct photoisomerization of the stilbenes is due entirely to radiationless transfer of excitation from traw.y-stilbene singlets to azulene. As expected for Forster transfer, this azulene effect did not depend upon solvent viscosity. The experimental value of R0, the critical radius of transfer in Forster s formula,181 was 18 A, in good agreement with the value calculated from the overlap of stilbene emission and azulene absorption. 

This expression, it may be recalled, is similar to the one obtained for intramolecular radiationless transfer rate for internal conversion and intersystem crossing (Section 5.2.1). For intermolecular cases... 

Emission is said to come from the lowest levels of the Y and Z states, which probably have longer lives than the upper levels. Thus, a molecule in the Yo state at any temperature undergoes one of two main reactions that occur in a constant ratio, i.e., emission or radiationless transfer to the Z0 state. The Zo state similarly gives out light or, possibly, is deactivated. 

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

Measurement of r as a function of [M] and extrapolation to [M] = 0 then yields a value for A since A can also be calculated, via B, from extinction coefficients in absorption, the measured value may afford a check on the calculated one. In general, when [M] 0, the observed lifetime is shorter than the natural radiative lifetime (1M). Where intramolecular loss processes (radiationless transfer) occur, and are described kinetically by a first-order rate coefficient kt, then... 

Figure 4.2. Energy levels involved in the sensitized fluorescence of potassium and rubidium, induced in collisions between excited potassium and ground-state rubidium atoms. The s coefficients refer to optical excitation, r 1 to radiative decay, and Z to radiationless transfer of energy by inelastic collisions.

Methods based on radiationless transfer between chromophores in polymers can give information on local conformation and structure in blends but they are unable to give direct values of interaction parameters... 

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). 

Fig.1 Schematic of the FRET process. Excitation of the fluorescent donor is followed by radiationless transfer of energy to the acceptor chromophore, returning the donor to the ground state. FRET is then usually, although not always, manifested by subsequent fluorescence of the acceptor...

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