Photo-induced excited molecule fluorescence

Photophysical studies on 9(3) and 10(5) revealed that extremely rapid photo-induced ET took place in both systems, from the locally excited methoxybenzene donor to the DCV acceptor.59,60 Particularly noteworthy was the observation of CT fluorescence in 10(5) which was the first documented example of exciplex-like emission from a rigid D-B-A system with a donor-acceptor separation exceeding three C-C bonds. These data clearly point to TB-mediated ET processes in these molecules. 

The quantum yield provides information about the effectiveness of a certain photo-induced process. The quantum yield of loss of starting material or product formation will provide valuable information about structure-activity relationships. Fluorescence and phosphorescence quantum yields will indicate the fraction of molecules likely to be found in the excited singlet and triplet state. The quantum yield is a useful parameter to predict the importance of a certain reaction for example, an isolated degradation product can have a long phosphorescence lifetime and should therefore be considered as a possible sensitizer. If the quantum yield of formation of this product is very low, however, it is less likely to be formed in a biologically active concentration and may therefore play a minor role in phototoxicity reactions. 

Figure 5 In a covalently linked too-component system F1 M, an eT process from the donor subunit M to the nearby photo-excited fragment FI induces a drastic rearrangement of solvent molecules around the Fl M ion pair (upper part of the figure). Freezing of the solution (e, g. at liquid nitrogen temperature) immobilizes solvent molecules, thus preventing the occurrence of the eT process and allowing the FI subunit to fluoresce (lower part of the figure).

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