Electron transfer quenching mechanism

The phosphorescence of Pt2 (I OsKHq in aqueous solution is quenched by l,l-bis(2-sulfoethyl)-4,4f-bipyridinium inner salt (BSEP). Transient absorption attributable to BSEP ( nax 610 nn) is observed in flash kinetic spectroscopic studies of aqueous solutions containing Pt2( Os Hq and BSEP, thereby establishing an electron transfer quenching mechanism ... 

It should also be noted that it is difficult to demonstrate conclusively an electron transfer quenching mechanism. In several cases, both energy and electron transfer are allowed, and both processes may lead to the same final products. A classical example is the quenching of the ( CT)Ru-(dipy)a excited state by Fe ions (Figure 4). (The validity of the orbital (29) and spin (30) labels of the excited states of metal chelates of... 

Figure 20. Photosensitized oxidative regeneration of NAD(P) cofactors (a) by reductive electron transfer quenching mechanism (b) by an oxidative electron transfer quenching mechanism (b) by an oxidative electron transfer quenching mechanism followed by dark oxidation of NADPH.

The (electron-transfer) quenching mechanism can be both dynamic and static, which makes the /(//plots complicated. 

Application of the energy gap law to the energy conversion mechanism in Scheme 1 leads to a notable conclusion with regard to the efficiency for the appearance of separated redox products following electron transfer quenching. From the scheme, the separation efficiency, sep> is given by eq. 18. Diffusion apart of the... 

Photochemical addition of ammonia and primary amines to aryl olefins (equation 42) can be effected by irradiation in the presence of an electron acceptor such as dicyanoben-zene (DCNB)103-106. The proposed mechanism for the sensitised addition to the stilbene system is shown in Scheme 7. Electron transfer quenching of DCNB by t-S (or vice versa) yields the t-S cation radical (t-S)+ Nucleophilic addition of ammonia or the primary amine to (t-S)+ followed by proton and electron transfer steps yields the adduct and regenerates the electron transfer sensitizer. The reaction is a variation of the electron-transfer sensitized addition of nucleophiles to terminal arylolefins107,108. 

Electron transfer can be accomplished by quenching of a micelle trapped chromophore by ions capable of ion pairing with the micelle surface. For example, excited N-methylphenothiazine in sodium dodecylsulfate (SDS) micelles can exchange electrons with Cu(II). The photogenerated Cu(I) is rapidly displaced by Cu(II) from the aqueous phase so that intramicellar recombination is averted, Fig. 5 (266). Similarly, the quantum yield for formation of the pyrene radical cation via electron transfer to Cu(II) increases with micellar complexation from 0.25 at 0.05 M SDS to 0.60 at 0.8 M SDS (267). The electron transfer quenching of triplet thionine by aniline is also accelerated in reverse micelles by this mechanism (268). 

Protons are relatively simple targets for sensor molecules and do not require engineered receptors, however, achievement of selective interactions with other chemical species requires much more elaborate receptors. In the most cases cations are bound via electrostatic or coordinative interactions within the receptors alkali metal cations, which are rather poor central ions and form only very weak coordination bonds, are usually bound within crown ethers, azacrown macrocycles, cryptands, podands, and related types of receptor moieties with oxygen and nitrogen donor atoms [8], Most of the common cation sensors are based on the photoinduced electron transfer (PET) mechanism, so the receptor moiety must have its redox potential (HOMO energy) adjusted to quench luminescence of the fluorophore (Figure 16.3). 

Key steps of the mechanism are electron transfer quenching of Eq. (11), which can occur in either the singlet or triplet state, fragmentation of the anion radical as in Eq. (12), and various steps which lead to dye, especially the disproportionation step of Eq. (16), which is facilitated by the stability of triarylmethane radicals. The reaction steps of Eqs. (15,16) represent the process thought to contribute most of the observed product The cation radical DH+- was observed experimentally by low-temperature ESR as well as by flash photolysis. The product D+ (13) is blue and absorbs at 590 nm. 

The reader may desire an explanation of the low values of y derived by Barton and coworkers [30, 131, 137-140] from fluorescence quenching data for systems in which the dynamics of electron transfer have not been directly measured. In most cases, the absolute efficiency (quantum yield) of the quenching processes studied in these systems is rather low, and thus they may represent long-range electron transfer by mechanisms other than superexchange, such as to those described by Felts et al. [125], Davis and colleagues [126], and Okada et al. [127]. However, the author considers that it is highly unlikely that such processes occur with rate constants > 10 s . In view of the complex nature of these systems, the author is loath to offer a detailed interpretation, and refers the reader to commentaries by others who have been directly involved in this research [13, 15]. 

Sensitized photooxidation of sulfur-containing amino acids in neutral aqueous solution occurs via electron transfer from the sulfur to the triplet state of the sensitizer (the mechanism can be adapted to other dye-sensitized photoinitiated polymerization). Electron transfer quenching in the system is followed mainly by diffusion of the sulfur-centered radical cation and benzophenone radical anion. Laser... 

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