Organic acceptors

Figure C3.2.10.(a) Dependence of electron transfer rate upon reaction free energy for ET between biphenyl radical anions and various organic acceptors. Experiments were perfonned with the donors and acceptors frozen into...

As we began this chapter, we saw that photosynthesis traditionally is equated with the process of COg fixation, that is, the net synthesis of carbohydrate from COg. Indeed, the capacity to perform net accumulation of carbohydrate from COg distinguishes the phototrophic (and autotrophic) organisms from het-erotrophs. Although animals possess enzymes capable of linking COg to organic acceptors, they cannot achieve a net accumulation of organic material by these reactions. For example, fatty acid biosynthesis is primed by covalent attachment of COg to acetyl-CoA to form malonyl-CoA (Chapter 25). Nevertheless, this fixed COg is liberated in the very next reaction, so no net COg incorporation occurs. 

Much effort has also been directed toward mimicking electron transfer on natural photosynthetic systems. Recently, the group of Harada has been able to prepare monoclonal antibodies against metallo porphyrins and show that the biological edifice can control photoinduced electron transfer from the porphyrin to organic acceptor molecules in solution. As it was important to design a biomolecule able to accommodate not only the metalloporphyrin unit but also organic substrates, Harada recently used a hexacoordinated phosphorus... 

Little used organic acceptors for these reactions, also [53,54], The use of a glycal as the radical source together with a functionalized enone as the radical acceptor is remarkable. Enones are swiftly reduced by Cp2TiCT [55] and thus epoxide activation must be considered as even more efficient. The product of the addition constitutes a valuable intermediate en route to derivatives of thyrsiferiol. 

Acceptor guest molecules such as quinone, TCNQ, or toluene can be accommodated in the flexible cavity of face-to-face bismacrocyclic Nin complexes (656). The host-guest interactions decrease the energy gap between the LUMO of the organic acceptors and the HOMO of the complex donors.1664... 

The acceptor strength is quantitatively evaluated by the energetics of the reductive conversion of an organic acceptor (A) to its anion radical (A- ) and it is most readily evaluated in solution (Table 4) by the reversible potential E for one-electron reduction,63 i.e.,... 

Since the values of for many organic acceptors are generally unobtainable (in organic solvents), an alternative measure of the electron-acceptor property is often based on the irreversible cathodic peak potential F (in cyclic voltammetry). Thus for a series of related compounds, Fig. 6 shows that the values of Fred are linearly related to gas-phase electron affinities (EA).70... 

Fig. 7 Correlation of the reduction potentials red (V versus SCE) with the electron affinities EA (eV) of various types of organic acceptors (in Table 4).

The attachment of an electron to an organic acceptor generates an umpolung anion radical that undergoes a variety of rapid unimolecular decompositions such as fragmentation, cyclization, rearrangement, etc., as well as bimolecular reactions with acids, electrophiles, electron acceptors, radicals, etc., as demonstrated by the following examples.135"137... 

The series of organometal donors I-IV are also readily oxidized by the one-equivalent oxidant hexachloroiridate(IV) as well as by the organic acceptor tetracyanoethylene (TCNE) (3). In both... 

Electron attachment to the LUMO of a neutral organic acceptor produces a radical anion [61]. This process can be initiated either chemically using a one-electron reducing agent [62, 63], electrochemically by cathodic reduction [64, 65] or photochemically in the presence of an electron donor in its excited state [12, 66]. 

The concept of molecular orbitals (MOs) helps to explain the electron structure of ion-radicals. When one electron abandons the highest occupied molecular orbital (HOMO), a cation radical is formed. HOMO is a bonding orbital. If one electron is introduced externally, it takes the lowest unoccupied molecular orbital (LUMO), and the molecule becomes an anion-radical. LUMO is an antibonding orbital. Depending on the HOMO or LUMO involved in the redox reaction, organic donors appear as n, a, or n species, whereas organic acceptors can be tt or a species. Sometimes, a combination of these functions takes place. 

These values are not appropriate for reducing the majority of the organic acceptors. Of course, there are some cases of single electron transfer from OH ion to an organic acceptor. All these cases, however, refer to substrates with very strong electron affinity, such as tetracyanoethylene or dinitrobenzene (Blumenfel d et al. 1970). Quinones, ketones, and other substrates have less affinity. In the ground (unexcited) states, they are unable to capture an electron from the OH ion (Sawyer and Roberts 1988). 

In some cases, cation-radicals are formed from neutral organic molecules by the action of neutral organic acceptors such as tetracyanoethylene, tetranitrofluorenone, quinones, and free radicals—aroxyls, nitroxyls, and hydrazyls. 

It is worth mentioning that there are some solvents that combine good solvency power with coordinating properties. The most salient example is 1,2-dimethoxyethane as a solvent in the reduction of organic acceptors by alkali metals. The acceptors transform into anion-radicals, and alkali metals into their cations. These cations are bound in chelates with 1,2-dimethoxyethane, and the one-electron reduction of the acceptors becomes more energy advantageous. 

Most organic acceptors used in the preparation of organic conducting materials belong to a class of polycyano compounds. The reasons are quite obvious if the unique properties of the cyano group are taken into account. It is a very strong acceptor and has a very small size with minimal steric strain. (Note the rod-shaped form of C N group.)... 

One-electron transfer processes are rather common in the reactions of Group I and II organometallic reagents with substrates, including alkyl halides, R —X, and unsaturated organic acceptors, A (113) ... 

Another interesting characteristic of the trinuclear [Au3(CH3N = COR)3] (R = Me, Et) complexes is that they can act as electron donors with organic acceptor molecules such as nitro-9-fluorenones [46]. Thus, they form adducts with 2,4,7-trinitro-9-fluorenone (deep yellow and red R=Me, Et, respectively), 2,4,5,7-tetranitro-9-fluorenone (red R = Me), 2,7-dinitro-9-fluorenone (red R = Et). The solid state structures of the complexes formed with [Au3(CH3N = COMe)3] and 2,4,7-trinitro-9-fluorenone or 2,4,5,7-tetranitro-9-fluorenone consist of planar gold(I) trimers interleaved with the nitro-9-fluorenones to form columns in the crystal. The distances between the faces of both portions indicate that interactions occur between the gold atoms, rich in electronic density, and the nitroaromatic portion of the electron acceptor. The difference between them stems from the greater complexity of the... 

III. Photopolymerization Sensitized by Interaction between Monomer and Organic Acceptor or Donor... 

However, this commonly accepted theory is incomplete and applies with much difficulty to systems involving nonvolatile substances. The most relevant example is metals. For a heterogeneous system, only the mechanical effects of sonic waves govern the sonochemical processes. Such an effect as agitation, or cleaning of a solid surface, has a mechanical nature. Thus, ultrasound transforms potassium into its dispersed form. This transformation accelerates electron transfer from the metal to the organic acceptor see Chapter 2. Of course, ultrasonic waves interact with the metal by their cavitational effects. 

Thus, the exoelectrons that result from the vibromilling of aluminum initiate the reaction between aluminum and organic acceptors. 

The recent approaches toward the functional macromolecules in the dithia-fulvene-based systems have been highlighted. The conjugated poly-donor molecules have been synthesized to combine the processability with the significant electron-donating properties of the dithiafulvene systems. Some of them formed CT complexes with organic acceptors, such as TCNQ, achiev-... 

Activation of elemental metals by mechanical methods (Sec. 3.4) in the presence of organic acceptor molecules in solid phase, described in detail [14], allows us also to obtain metal-polyradical complexes [206,216-219]. The formed radical-pair species have some unusual properties [217,218]. Compared with triplet radical pairs generated photochemically with the same donor-acceptor composition, these mechanically induced species appear to be much more stable [216]. As an example... 

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