Selective electron transfer quenching

Selective Electron Transfer Quenching of Radical Anions in the Excited State... 

Much use has been made of micellar systems in the study of photophysical processes, such as in excited-state quenching by energy transfer or electron transfer (see Refs. 214-218 for examples). In the latter case, ions are involved, and their selective exclusion from the Stem and electrical double layer of charged micelles (see Ref. 219) can have dramatic effects, and ones of potential imfKntance in solar energy conversion systems. 

The observation of selective fluorescence quenching by G C base pairs is consistent with the energetics of electron transfer (charge separation) from the bases to the singlet excited state Sa, which can be estimated using Weller s equation [26] ... 

Appropriate combinations of boronic acid and fluorophores lead to a remarkable class of fluorescent sensors of saccharides (Shinkai et ah, 1997, 2000, 2001). The concept of PET (photoinduced electron transfer) sensors (see Section 10.2.2.5 and Figure 10.7) has been introduced successfully as follows a boronic acid moiety is combined intramolecularly with an aminomethylfluorophore consequently, PET from the amine to the fluorophore causes fluorescence quenching of the latter. In the presence of a bound saccharide, the interaction between boronic acid and amine is intensified, which inhibits the PET process (Figure 10.42). S-l is an outstanding example of a selective sensor for glucose based on this concept (see Box 10.4). 

The possibilities of the two kinds of back electron transfer can be diminished to one by selecting the reaction components. When the excited state of P is quenched oxidatively by T, the only possible back electron transfer is from (T,)red, to (P)M. On the contrary, when P is quenched reductively by T2, the back electron transfer to be considered is only from (P)rcd to (T2)ox. In either case, the back electron transfer can be prevented by a molecular design based on reaction dynamics. For the... 

The Q-P-P-Q tetrad 43 has been synthesized by Sessler and coworkers [69, 98]. As expected, it demonstrates strong quenching of the porphyrin fluorescence, and this presumably occurs by electron transfer. It is probable that selective metallation of such a tetrad would produce a species which would behave as a triad in a manner analogous to 17 and 18, but it is not obvious that such a molecule would have any significant advantages over these systems. As is the case with a number of the other complex systems, 43 may in principle demonstrate stereoisomerism when rotations about the linkage bonds are slow on the time scales of electron and energy transfer. 

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