Electron transfer. See

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. 

FIG. 6 SHG intensity as a function of time (ON) with and (OFF) without illumination of the interface by a probe ETV pulse. The increased SHG intensity during illumination arises from the production of the species I at the interface by photoinduced electron transfer, see text for more details. (From Ref. 103, copyright American Chemical Society.)... 

We now turn to an example of nonadiabatic chemistry where the nonadiabatic process starts on the ground state, and is followed by an excursion upward onto the excited state electron transfer (see references 2-5). 

Figure 12. CID of precursor ion Cu(DMSO)3+, which shows that this ion undergoes charge reduction by electron transfer (see equation 22 which leads to Cu(DMSO)2 and DMSO+. From Blades, A. T. Jayaweera, P. Ikonomou, M. G. Kebarle, P. J. Chem. Phys. 1990, 92, 5900, with permission.

Time-resolved spectroscopy establishes that the fluorescence of the excited (singlet) anthracene ( ANT ) is readily quenched by maleic anhydride (MA), which leads to the formation of the ion pair ANT+, MA via diffusional electron transfer (see Fig. 12), i.e.,... 

Insofar as the intermediate B obeys the steady-state approximation, as is usually the case in practice, there are two limiting situations as to the nature of the rate-limiting step according to the value of the parameter A e/lc = k-rCp/kc, which measures the competition between the followup reaction and the backward electron transfer (see Section 6.2.7). 

Most fluorescent PET molecular sensors, including pH indicators of this type, consist of a fluorophore linked to an amine moiety via a methylene spacer. Photo-induced electron transfer (see Chapter 4, Section 4.3), which takes place from amino groups to aromatic hydrocarbons, causes fluorescence quenching of the latter. When the amino group is protonated (or strongly interacts with a cation), electron transfer is hindered and a very large enhancement of fluorescence is observed. 

Fe(CN)g] , Fig. 16. However, experiments with the two Ru-modified derivatives clearly indicate that rate constants for electron transfer from His59 are small. Residues 42-45 are more distant from the Cu and are considered less likely lead-in groups. Moreover, attachment of cytochrome c(II) at 42-45 by the car-bodiimide method does not lead to a productive electron transfer (see below) [138]. Therefore Tyr83 (Fig. 17) becomes a prime focus as a lead-in group for electron transfer. 

The very fast change relates to direct reduction of the Fe(III) center by the radical. The amount of absorbance change of this compared to the slow change can be understood if the hydrophobic nature of the heme site is considered. The rate of the slow change is similar for all systems since it involves a (common) intramolecular electron transfer. See (5.8.4). 

Advances in the chemistry of [M(CN)5L]" complexes, for M = Fe, Ru, and Os, have been reviewed.There has been rather little activity in the preparation of novel complexes, but considerable activity in studying the properties, especially solvatochromism and various aspects of kinetics of substitution, of known complexes. However there has been an attempted preparation of [Fe(CN)5(Ci2H25NH2)], in the hope of generating micelles or lyotropic liquid crystals. This preparation appeared to yield [Fe(CN)4(H20)(Ci2H25NH2)], whose alkali metal salts gave a hexagonal mesophase in water, but were also readily hydrolyzed to [Fe(CN)4(H20)2] . Heterobinuclear complexes of the form [(NC)5FeL ML 5] " " have been much studied, especially in relation to intramolecular electron transfer (see Section 5.4.2.2.5). 

These include 14-crown-4 ethers (30a-c), their aza analogs and other crown compounds. The phenoxide can provide a convenient counterion for the hthium cation. Otiier variants may involve fluorescence and photoinduced electron transfer. See text in Section III.A.3. 

The kinetic values are consistent with Fdx-to-CYP51 electron transfer... (see... 

You may recall from the discussion of electron transfer (see Table 4-4) that a stable configuration precisely filled an r-type subshell and a />-type subshell. Only five elements have atoms with their valence y>-subshells filled these are the inert gases in the far right column of the periodic table. Their lack of chemical reactivity is explained by their stable electron configurations. 

A systematic investigation of this type of electronic interaction would probably give valuable results, particularly if it could be made with respect to the filling of the electron bands of the adsorbent. It may be expected that adsorbed alkali atoms lower 4>, especially in the case of transition metals with incomplete electron levels, but for metals with filled d levels, a smaller effect can be expected. An exception may result from preadsorption with O atoms, which create new vacancies in the bands owing to electron transfer (see section IV,2c). 

Ey2 reversible half-wave potential, k, vxl> experimental standard rate constant, a transfer coefficient, (outer-Helmholtz plane, L ron standard rate constant after correction for the double-layer effect (see 4)), lcex rate constant for the homogeneous self-exchange electron-transfer (see 4) in Chapter 9) obtained with a HMDE in DMF-0.5 M BU4NCIO4 at 22 2°C, except the last two obtained with a DME in DMF-0.1 M Bu4NI at 30°C. 

The solvent coordinate z(f) is dimensionless it is zero when n(t) is equal to Ht and it is unity when n(t) is equal to nc. This representation of the solvent coordinate leads to a compact form for some of the key equations of solvation and electron transfer (see Section 11I.B) in solution. 

Owing to a relatively high (compared with molecules in the ground electron state) probability of electron tunneling for excited molecules, this process, at sufficiently short distances between the excited molecules and the particles of electron acceptors, can compete with the ordinary over-barrier electron transfer (see the scheme in Fig. 9). In practice this effect manifests itself in the transition, as the concentration of acceptor rises, from the usual... 

Of much significance is the realization of long-lived photo generated tautomeric states and long-range proton transfer (LRpT) processes. The latter could lead to proton transfer charge separated states to be put in parallel with the extensively studied charge separation by photoinduced electron transfer (see Section 8.2.3). A number of systems present photochromism on the basis of photoinduced proton transfer [8.229]. 

In the above sections, nothing was said about the type of reaction between M and Q. This is because the Stem-Volmer equation is model independent, as explained above and also because eqs. (20)-(22) are for a diffusion-controlled reaction. Some information can be obtained regarding an electron transfer from various quenchers of similar chemical structures towards M. In this case, one may derive a relationship between ksv (as obtained from eq. (17)) and the ionization potential of these inhibitors. This is the Rehm-Weller equation, which is schematically depicted in fig. 4. In this plot, the plateau value corresponds to fcdin. For a general overview of problems related to electron transfers, see Pouliquen and Wintgens (1988) (in French). 

Electron transfer was interpreted in Ref. [54] in terms of the nonradiative decay process [20-24,44]. For an up-to-date review of theoretical works on electron transfer see the relevant chapter in this volume (R. A. Marcus — Recent developments in fundamental concepts of PET in biological systems). 

Photoexcitation of a deaerated PhCN solution of Acr+-Mes by a nanosecond laser light flash at 430 nm results in the formation of Acr -Mes+ with a quantum yield close to unity (98 %) via photoinduced electron transfer from the mesitylene moiety to the singlet excited state of the acridinium ion moiety ( Acr -Mes) [54]. The decay of Acr -Mes+ obeyed second- rather than first-order kinetics at ambient temperature as observed in the case of Fc+-ZnP-H2P-C60 , when the bimo-lecular back electron transfer predominates owing to the slow intramolecular back electron transfer (see above) [50]. In contrast, the decay of Acr -Mes+ obeys first-order kinetics in PhCN at high temperatures (e.g. 373 K). This indicates that the rate of the intramolecular back electron transfer of Acr -Mes4 becomes much faster than the rate of the intermolecular back electron transfer at higher tempera-... 

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