Fluorescence quenching, electron-transfer

In addition, it is worth noting that the quenching process can also work with a fluorescent electron acceptor. Irradiation of the acceptor results in the creation of its excited state, and in this scenario it is energetically favorable for an electron from 

This relationship also conveys one of the reasons why the sensing of explosives can be so effective with this method. Most explosives, especially TNT and other nitroaromatic compounds, are highly electron deficient and have favorable reduction potentials. For instance, the reduction potential of TNT and DNT are only —0.7 and—1.0 V (versus SCE) respectively, quite favorable when compared with other electron acceptors, such as 1,4-dicyanobenzene (—1.7 V versus SCE). This means that if the sensory material emits hght, for example, at 460 nm, the oxidation 

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The mechanism for the photoreaction between 133 and cyclohexene can be summarized as in Scheme 8. The initiating electron transfer fluorescence quenching of 133 by cyclohexene resulted in the formation of an w-amino radical-radical cation pair 136. Proton transfer from the 2-position of the cyclohexene radical cation to the nitrogen atom of the a-amino radical leads to another radical cation-radical pair 137. Recombination of 137 at the radical site affords the adduct 134, while nucleophilic attack at the cation radical of 136 leads to another radical pair 138 which is the precursor for the adduct 135. 

Figure 1. Schematic representation of electron transfer sensitization. 1 photo-oxidation of sensitizer 2 forward electron transfer (fluorescence quenching) 3 back electron transfer 4 product formation...

FIGURE 15. Electron-transfer fluorescence quenching. (Reprinted from Ref. 90.)... 

In a study devoted to the fluoresceinamine fluorescence it was shown that this molecule exhibits no fluorescence until the amine s electron lone pair is made unavailable for electron transfer. Thus, protonation of the ammonium ion restores the fluorescence of the molecule in the deprotonated form, an intramolecular electron transfer fluorescence quenching occurs, the resorufin moiety of the molecule behaving here as an acceptor [157],... 

The key step in these DCA-sensitized photooxygenations of enol ethers, involves a diffusion-controlled rate electron-transfer fluorescence quenching of the singlet excited sensitizer by the donors (Eox in the range 1.24-1.30 V vs SCE) with the generation of the radical ion pairs. Seemingly, formation of the ultimate products 59-63 could be easily rationalized on the basis of the classical Foote mechanism... 

Kosower and co-workers have found the photoinduced, barrierless charge separation processes of substituted polyaromatics to be controlled by solvent relaxation behavior over a large temperature range in alcohol solvents. Heitele and Michel-Beyerle reported on the complex solvent- and temperature-dependent electron transfer fluorescence quenching in some covalently linked aromatic donor-acceptor compounds in viscous solvents. These authors have attempted a critical comparison between current theoretical models and their experimental results, and the limitations of current theoretical models are discussed. 

Fig. 3.9 Electron-transfer fluorescence quenching in conjugated polymers (Reprinted with permission from Toal and Trogler (2006). Copyright 2006 Royal Society of Chemistry)...

PEDI fluorescence. The intramolecular CT excited state formed in the PI chains is preferably non-fluorescent as described in the previous section. Similar multistep electron transfer was observed in a porphyrin connected to two quinones and in several porphyrin-quinone-carotenoid systems [105-108]. The probability of exciplex formation (electron transfer fluorescence quenching) in polar solvents can be estimated from the free energy changes, AGet, given by the Rehm-Weller equation [109,110] ... 

Characterizing the Behavior and Properties of an Excited Electronic State Electron-Transfer Mediated Quenching of Fluorescence 24... 

Forward Electron Transfer as Studied by Fluorescence Quenching 69... 

This makes us conclude that the process of quenching is associated with an electron transfer. The efficiency of phosphorescence quenching by acceptors follows, as well, the growth of electron affinity of the latter. Phosphorescence quenching constants are two orders of magnitude lower than fluorescence quenching constants. This indi-... 

Elegant evidence that free electrons can be transferred from an organic donor to a diazonium ion was found by Becker et al. (1975, 1977a see also Becker, 1978). These authors observed that diazonium salts quench the fluorescence of pyrene (and other arenes) at a rate k = 2.5 x 1010 m-1 s-1. The pyrene radical cation and the aryldiazenyl radical would appear to be the likely products of electron transfer. However, pyrene is a weak nucleophile the concentration of its covalent product with the diazonium ion is estimated to lie below 0.019o at equilibrium. If electron transfer were to proceed via this proposed intermediate present in such a low concentration, then the measured rate constant could not be so large. Nevertheless, dynamic fluorescence quenching in the excited state of the electron donor-acceptor complex preferred at equilibrium would fit the facts. Evidence supporting a diffusion-controlled electron transfer (k = 1.8 x 1010 to 2.5 X 1010 s-1) was provided by pulse radiolysis. 

In two other studies, it was observed that C60 in LB films can quench the fluorescence of pyrene [293] and of 16-(9-anthroyloxy)palmitic acid [294] by photoinduced electron transfer. In these studies, both C60 and the electron-donating fluorophore were incorporated into a tricosanoic acid LB film in different ratios. 

The fluorescence of a range of polycyclic aromatic hydrocarbons is found to be quenched in the presence of alkyltriphenyl-phosphonium salts via electron-transfer from the hydrocarbon to... 

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] ... 

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