Electron transfer donor-acceptor pairing

Further improvements can be achieved by replacing the oxygen with a non-physiological (synthetic) electron acceptor, which is able to shuttle electrons from the flavin redox center of the enzyme to the surface of the working electrode. Glucose oxidase (and other oxidoreductase enzymes) do not directly transfer electrons to conventional electrodes because their redox center is surroimded by a thick protein layer. This insulating shell introduces a spatial separation of the electron donor-acceptor pair, and hence an intrinsic barrier to direct electron transfer, in accordance with the distance dependence of the electron transfer rate (11) ... 

The ability to switch a molecular unit on and off is a key component of an efficient molecular device, since it allows modulation of the physical response of such a device by external physical or chemical triggers. A molecular device, based on a trinuclear metal complex, shown in Figure 59, functions as an electroswitchable-photoinduced-electron-transfer (ESPET) device.616 Electrochemical switching of the redox state of a spacer intervening between a donor-acceptor pair can dictate the type of the observable charge separation and the lifetime of the resulting ion pair.616... 

Chow TJ, Chiu NR, Chen HC et al (2003) Photoinduced electron transfer reaction tuned by donor-acceptor pairs via rigid linear spacer heptacyclo[6.6.0.02, 6.03, 13.04, 11.05, 9.010, 14]tetradecane. Tetrahedron 59 5719-5730... 

The wide diversity of the foregoing reactions with electron-poor acceptors (which include cationic and neutral electrophiles as well as strong and weak one-electron oxidants) points to enol silyl ethers as electron donors in general. Indeed, we will show how the electron-transfer paradigm can be applied to the various reactions of enol silyl ethers listed above in which the donor/acceptor pair leads to a variety of reactive intermediates including cation radicals, anion radicals, radicals, etc. that govern the product distribution. Moreover, the modulation of ion-pair (cation radical and anion radical) dynamics by solvent and added salt allows control of the competing pathways to achieve the desired selectivity (see below). 

Various enol silyl ethers and quinones lead to the vividly colored [D, A] complexes described above and the electron-transfer activation within such a donor/acceptor pair can be achieved either via photoexcitation of charge-transfer absorption band (as described in the nitration of ESE with TNM) or via selective photoirradiation of either the separate donor or acceptor.41 (The difference arising in the ion-pair dynamics from varied modes of photoactivation of donor/acceptor pairs will be discussed in detail in a later section.) Thus, actinic irradiation with /.exc > 380 nm of a solution of chloranil and the prototypical cyclohexanone ESE leads to a mixture of cyclohexenone and/or an adduct depending on the reaction conditions summarized in Scheme 5. 

The electronic spectrum of the complex consists of a combination of the spectra of the parent compounds plus one or more higher wavelength transitions, responsible for the colour. Charge transfer is promoted by a low ionization energy of the donor and high electron affinity of the acceptor. A potential barrier to charge transfer of Va = Id — Ea is predicted. The width of the barrier is related to the intermolecular distance. Since the same colour develops in the crystal and in solution a single donor-acceptor pair should be adequate to model the interaction. A simple potential box with the shape... 

The photolysis of donor-acceptor systems shows a reaction pattern of unique synthetic value. Direct irradiation of the donor-acceptor pairs, such as arene-amine, leads by intramolecular electron transfer, to amine radical cations and arene radical anions. The generated radical cation and radical anion intermediates undergo cyclization reactions providing efficient synthetic routes to N-heterocycles with a variety of ring sizes. 

Excited molecular complexes of the donor-acceptor type are called excimers if formed from identical molecules and exiplexes if originated from different molecules. From the theory, it is concluded that photochemical influence will more readily accelerate electron transfer in a weak donor-acceptor pair than in a strong pair (Juillard and Chanon 1983). An organic molecule in an electron-excited state is a more active oxidant or stronger reducer than the same molecule in a ground state. 

The electrical contact of redox proteins is one of the most fundamental concepts of bioelectronics. Redox proteins usually lack direct electrical communication with electrodes. This can be explained by the Marcus theory16 that formulates the electron transfer (ET) rate, ket, between a donor-acceptor pair (Eq. 12.1), where d0 and d are the van der Waals and actual distances separating the donor-acceptor pair, respectively, and AG° and X correspond to the free energy change and the reorganization enery accompanying the electron transfer process, respectively. 

Fig. 12. Perrin quenching radii, R, [33J vs. variations of the free energy, - AG°, of electron transfer from the excited donor molecule to the acceptor molecule for donor-acceptor pairs in vitreous /nms-l,5-decalindiol. 1, Rubrene + A/ AT-diethylamline (DEA) 2, rubrene + N,N,-Ar,Ar-tetramethyl-p-phenylenediamine (TMPD) 3, rubrene + tetrakis(dimethylaminoethy-lene) 4, tetracene + DEA 5, tetracene + TMPD 6, 9,10-dinaphthylanthracene + DEA 7, 9,10-dinaphthylanthracene + TMPD 8, perylene + DEA 9, perylene + TMPD 10, 9-methylanthracene + TMPD 11, 9,10-diphenylanthracene + TMPD 12, coronene + TMPD 13, benzo[ Ai jperylene + TMPD 14, fluoranthene + DEA 15, acridine + DEA.

It has been noted by Potasek [105] that electron tunneling in the donor-acceptor pair D-A may lead to the appearance of a charge transfer band in the absorption spectrum of this pair. The author obtained the following formula describing the dependence of the extinction coefficient, , of this band on the energy, E, of the absorbed light quantum... 

The results reported in refs. 105 and 108 show that, in principle, it is possible to use the data on the absorption spectra of a donor-acceptor pair for estimating the distances of electron tunneling stimulated by light. It should be emphasized that, in this case, the illumination is performed in the band of the tunneling charge transfer from the donor to the acceptor without exciting the electron transitions within the donor and the acceptor molecules themselves. 

Irradiation of the Q band causes alu (it) - eg (n ) electron excitation in M(Pc), and the excited species catalyzes an electron transfer reaction between a donor-acceptor pair such as triethanol-amine-methylviologen. There are two possible pathways and the course of electron transfer is determined by the redox potentials of the components. These processes are quenched by intersystem... 

The thermodynamic barrier encountered in charge separation in the forward electron transfer (A + D A + D" ") between a donor-acceptor pair can be overcome easily with the activation afforded by ultraviolet excitation (50-120 kcal/mole). The challenge confronted in elaborating this area of chemistry therefore lies in controlling the rate of the deactivating back reaction (A7 + D" - A + D). If the importance of the reverse electron transfer can be diminished, observable selective chemistry can ensue. 

To alleviate the apparent problem, Rehm and Weller proposed an empirical relationship for electron transfer between excited donor/acceptor pairs (7), eq. 7. 

At very long separations, for example, transfer to the biphenyl cation radical over 34 A in 1C)2 s, electronic interactions seemed to be propagated by ion states of the solvent (25), although quantum mechanical tunnelling may be important when diffusion is blocked by steric factors or by immobilization of the reagents (26). Perhaps most convincing are Miller and Closs demonstration of intramolecular electron transfer between donor-acceptor pairs separated by a rigid steroid spacer (27, 28). In 1, for example,... 

A number of covalently bound porphyrin-quinone systems have recently been synthesized as models for carefully spaced donor-acceptor systems. These are designed to test the possibility of controlling electron transfer rates by spatial separation of donor-acceptor pairs. Among these exceedingly clever studies (30) are molecules designed to separate the donor and acceptor by rigid insulating molecules, for example, 2 (31),... 

By far the most common approach to the inhibition of back electron transfer after photoexcitation of organic donor-acceptor pairs involves the rapid consumption of either the photooxidized or photoreduced species by chemical reaction. 

DNA mediated photoelectron transfer reactions have been demonstrated60 . Binding to DNA assists the electron transfer between the metal-centered donor-acceptor pairs. The increase in rate in the presence of DNA illustrates that reactions at a macromolecular surface may be faster than those in bulk homogeneous phase. These systems can provide models for the diffusion of molecules bound on biological macromolecular surfaces, for protein diffusion along DNA helices, and in considering the effect of medium, orientation and diffusion on electron transfer on macromolecular surfaces. 

For a 7t system to function as an electron acceptor, it must have unfilled orbitals available to accept electrons. In the case of olefins or dienes those are n antibonding molecular orbitals. Thus interaction of the HOMO of one n system with file LUMO of a second n system produces a donor-acceptor pair (HOMO donating to LUMO) enabling electrons to be transferred from one n system to another with resulting bond formation. 

Photo-induced electron transfer (PET) involves an electron transfer within an electron donor-acceptor pair. The situation is represented in Figure 14. 

Thus, e.g. as in the typical investigation in Ref. [110], long-range intramolecular electron transfer has been detected for rigid bifunctional steroid molecules dissolved in THF. The structural formulae of these compounds are represented in Fig. 16. The donor acceptor pairs I-V presented in this figure are separated by various numbers, n, of similar chemical bonds, the so-called n-bond system. The anion-radicals of molecules I-V were obtained via reactions of these molecules... 

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