Site-Photoselection Model

As has been reported, 8 22) specific effects at the red edge in fluorescence spectra and at the blue edge in excitation spectra are observed in systems with delayed relaxation They are also manifested in the 

The analysis of these phenomena requires the use of more complicated models which take into account the fact that at the moment of excitation individual aromatic molecules in the ensemble under study may interact differently with their environment. 7l The existence of a distribution of fluorophores differing in such interactions leads to inhomogeneous broadening of the spectra. Upon excitation by light whose energy is insufficient to excite all the fluorophores in the ensemble, there occurs a selection of those species whose spectral properties differ from the average ones. These properties and their changes with time may characterize the relaxation process/1,24,33,98) 

If excitation is by light of very low energy, then there will be photo- 

Here vmMn and vedge are the frequencies of the fluorescence spectra excited at fixed excitation frequencies at the maximum and at the red edge, respectively, and v a — v dge is the limiting value of the red-edge effect, given by the 

Analogous results for the temperature dependence of tR have been obtained in fluorescence studies of indole and tryptophan in glycerol/24,33) Therefore, the above approach may be considered to be adequate for the description of the dynamics of the model viscous media. 

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Figure 2.8. Energy level diagram (top) and spectra (bottom) illustrating the site-photoselection model of >... 

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