Quenching by electron transfer

In an electron transfer reaction two neutral molecules, for example, will form an ion pair if one of the neutrals is electronically excited it will be deactivated, and the overall process may appear as a quenching if the ion pair recombines to return to the neutral ground state partners  

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In compound (4) (Figure 12.15) the fluorescence of the anthracene group is quenched by electron transfer from both the crown ether unit and the amino group. The electron transfer processes from the crown ether and the amino group can be prevented by adding Na+ and H+, respectively. 

Ru(bpy)2(CN)2] to Nd3+ can be assumed to be equal to the efficiency of the quenching of the [Ru(bpy)2(CN)2] emission (ca. 90%) because quenching by electron transfer can be ruled out in view of the Nd3+ redox properties. No evidence of energy transfer in the adduct from the naphthyl-localized 7) excited state of the dendrimer to the lowest 3MLCT state of [Ru(bpy)2(CN)2] has been found since no change in the Tx lifetime at 77 K has been observed. 

Class C Fluorophores that undergo no photoinduced proton transfer but only photoinduced electron transfer. The fluorescence quantum yield of these fluorophores is very low when they are in the non-protonated form because of internal quenching by electron transfer. Protonation (which suppresses electron transfer) induces a very large enhancement of fluorescence (see Section 10.2.2.5). The bandshapes of the excitation and fluorescence spectra are independent of pH. 

Photoinduced electron transfer (PET Scheme 6.2) is a mild and versatile method to generate radical ion pairs in solution," exploiting the substantially enhanced oxidizing or reducing power of acceptors or donors upon photoexcitation. The excited state can be quenched by electron transfer (Eq. 7) before (aromatic hydrocarbons) or after intersystem crossing to the triplet state (ketones, quinones). The resulting radical ion pairs have limited lifetimes they readily undergo intersystem ... 

Fluorescent redox switches based on compounds with electron acceptors and fluorophores have been also reported. For instance, by making use of the quinone/ hydroquinone redox couple a redox-responsive fluorescence switch can be established with molecule 19 containing a ruthenium tris(bpy) (bpy = 2,2 -bipyridine) complex.29 Within molecule 19, the excited state of the ruthenium center, that is, the triplet metal-to-ligand charge transfer (MLCT) state, is effectively quenched by electron transfer to the quinone group. When the quinone is reduced to the hydroquinone either chemically or electrochemically, luminescence is emitted from the ruthenium center in molecule 19. Similarly, molecule 20, a ruthenium (II) complex withhydroquinone-functionalized 2,2 6, 2"-terpyridine (tpy) and (4 -phenylethynyl-2,2 6, 2"- terpyridine) as ligands, also works as a redox fluorescence switch.30... 

Figure 3.40 Example of the Perrin plot of static quenching. Luminescence of a metal complex [Ru (bpy)32+] in rigid glycerol in the presence of increasing concentrations of methylviologen (quenching by electron transfer)...

As shown in Scheme 16 the triplet-excited state 3C o is quenched by electron transfer from [(BNA)2] generating the radical ion pair [(BNA)2+ and Qo] in competition with the decay to the ground-state. However, back-electron transfer is reduced by the fast cleavage of the C-C bond in the dimer [(BNA)2+], Finally, a second electron transfer from BNA" occurs leading to two molecules of C60. The... 

Interestingly, the emission of [Pt(tpy)(OH)]+ is quenched in water, but is restored upon intercalation with double-stranded nucleic acids [poly(dA-dT)]2 and [poly(dI-dC)]2 [73]. Although intercalation can also occur at G-C-rich sequences - and indeed it does so more strongly than for A-T - emission is then quenched by electron-transfer from guanine, the most readily oxidized of the nucleobases. The initial intercalative interactions are probably followed by covalent platination of the DNA, most likely by purine-N7 displacing the OH ligand, reminiscent of cisplatin. The absorption... 

According to the reaction scheme (3.257) of the irreversible quenching by electron transfer, the electron donor has three electronic states involved in the interaction with light and electron acceptor, while the latter has only two states participating in the charge transfer ... 

At fast bimolecular recombination of RIPs to the triplet products, the fluorescence quenching by electron transfer becomes irreversible. 

I, but functionalized with covalently attached acrylate groups. In unexposed regions, the fluorescence of the chromophore is largely quenched by electron transfer to the acrylates, whereas in exposed regions where bits are written, the fluorescence is turned on on by polymerization of the acrylates [164, 178]. Several alternative 2 PA memory schemes, based both on conventional 1PA chromophores and on newer 2PA chromophores, have also been reported. 

Figure 4.4 Excitation quenching by electron transfer between excited 1AE> molecule and a quencher in its singlet ground state, 1Q...

The recent flash photolysis study has revealed, that the [Femedta]- excited state is quenched by electron transfer processes by both electron donors and electron acceptors (see Figure 9.12). The latter cause oxidation of the EDTA ligand and the acceptor reduction, whereas the Fem centre does not change its oxidation state. One of the acceptors is molecular oxygen, which results in direct EDTA oxidation at the expense of 02 reduction [21, 22] ... 

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