Site photoselection

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

Of considerable interest is the fact that not only the steady-state anisotropy but also its kinetics depend on the excitation wavelength. In this case another red-edge effect connected with site photoselection may be observed. Dipole-orientational relaxation may occur not only by rotation of the dipoles surrounding the fluorophore but also by rotation of the aromatic group itself. If red-edge excitation results in the photoselection of fluorophores whose energy of interaction with the environment already corresponds to that in the excited state, then the relaxation-associated rotation should not be observed and the rotation that occurs should be completely Brownian in character.(22)... 

Iron pentacarbonyl (Fe(CO)s) may be photooxidized in a similar maimer. Although no fewer than five distinct iron carbonyl intermediates are seen, there is no sign of any photoselectivity in this reaction sequence. It is possible, however, to bmld up reasonable concentrations, at least of the early intermediates, by carefiilly controlling the time of photolysis. Hence, it appears that the first oxidation product is (jj -02)Fe(C0)4, where the O2 ligand occupies a basal site in a square pyramidal framework (33). 

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