Dimerization donor radical cations

In the BChl g containing heliobacteria Heliobacillus mobilis and Heliobacterium chlorum symmetric dimers for the primary donor radical cation PgJ5 have been found based on EPR and ENDOR data.85 This symmetric dimer is consistent with the homodimeric structure of the RC. The same reason was invoked to explain the high symmetry of the donor radical-cation Pgg5 in green sulfur bacteria, which is made up from a BChl a dimer.86 For a review see reference 87. Note that these RCs belong to the type I RCs. 

The Primary Donor. - The radical-cation P+ In the bRC of purple bacteria and also in PS I the primary electron donors have been identified as (B)Chl dimers and EPR/ENDOR clearly showed that the unpaired electron and the positive charge - is (asymmetrically) distributed in a supermolecular orbital extending over both dimer halves (see sections 2.1,3.1). Dimer formation has the important consequence of charge delocalization and this stabilization of the primary donor radical-cation leads to a decrease of the oxidation potential. A fine tuning of the potential is possible through interactions with the environment, e.g. via H-bonds. 

A Hiickel MO treatment of a simple dimer model is introduced to rationalize the observed spin density asymmetry of primary donor radical cations in terms of differences of molecular orbital energies of the isolated monomeric moieties. Such a model can provide a semiquantitative method to trace spin density asymmetries back to structural and/or environmental asymmetries. 

The structures of the radical cation salts of the TTF derivatives were determined by X-ray crystallographic analyses <1997SM1871>. The (CIO4) and (SbFs) salts of 45 presented, respectively, as monoclinic CZIm space group) and triclinic (FI space group) black single crystals. A coplanar 2-D network characterized by short intermolecular S- N (3.27 A) and S- S (3.58 A) contacts and a column structure formed by a dimer of the donor 45 with... 

It is also practical to invoke a one-electron approximation in the FCD method [41] when one estimates donor and acceptor charges. Thus, one approximates the fragment charges of the radical cations [(G C),(AT)] and [(GC),(A+T)] via the corresponding Mulliken populations of the HOMO and HOMO-1 of the neutral dimer. Then, the charge on fragment / in [(G+C),(AT)] is... 

The radical anion Cw, can also be easily obtained by photoinduced electron transfer from various strong electron donors such as tertiary amines, fer-rocenes, tetrathiafulvalenes, thiophenes, etc. In homogeneous systems back-electron transfer to the reactant pair plays a dominant role resulting in a extremely short lifetime of Qo. In these cases no net formation of Qo is observed. These problems were circumvented by Fukuzumi et al. by using NADH analogues as electron donors [154,155], In these cases selective one-electron reduction of C6o to Qo takes place by the irradiation of C6o with a Xe lamp (X > 540 nm) in a deaerated benzonitrile solution upon the addition of 1-benzyl-1,4-dihydronicoti-namide (BNAH) or the corresponding dimer [(BNA)2] (Scheme 15) [154], The formation of C60 is confirmed by the observation of the absorption band at 1080 nm in the near infrared (NIR) spectrum assigned to the fullerene radical cation. 

There is a long standing interest in the chemistry and the properties of cyclic compounds containing sulfur atom in modern material chemistry due to their redox chemistry. In particular, the focus has been on dithiole derivatives, e.g., dithiafulvenes and tetrathiafulvalenes, since the finding of metallic conductivity and low temperature superconductivity in radical cation salts. The quite low oxidation potentials of 1,4-dithiin compounds have been reported, recently [109]. On the other hand, thioketene dimers (2,4-bis(alkyli-dene)-l,3-dithietane) have been known for more than 100 years and synthesized by various methods [110-115]. The structure of these dimer compounds is similar to that of the redox-active sulfur compounds therefore, the potential electronic property of the thioketene dimer moiety is considerably attractive with the aim of application to a new and better -donor. 

Here a secondary electron transfer between the radical cation and a neutral donor molecule produces a 1,4-cation radical. The acyclic 1,4-cation radical is in equilibrium with a cyclobutane radical cation. Other dimerizations have been described by Farid et al. [73-75], Arnold et al. [76], Pac et al. [77,78], and others [79-81]. 

If the substituents on an aromatic compounds prevent the close approach of the cation radical to its neutral counterpart (due to steric hindrance), n-mer formation is inhibited. Such effects can clearly be seen on comparing 2,3,6,7-tetramethylnaphthalene (TMN) and its hindered analogue OMN. Spectrophotometric studies show that the tetramethylnaphthalene forms the dimeric (TMN)2+ cation radical, as characterized by a very broad absorption band at 1150 nm and the formation constant Kdimer = 490 m 1 at —10 °C. In contrast, dimer formation is not observed with the hindered cation radical OMN+. The fact that relatively few ion-radical dimers are known is associated with the opposing requirements for their formation. On the one hand, the cation radical should be relatively stable, which is usually the case with encumbered donors (for which steric hindrance prevents dose contact between the radicals), while on the other hand they should be able to approach each other in order for electronic interaction to be appreciable. 

Radical ions are created in solution by chemically or electrochemically induced electron transfer to or from a conjugated ir-system. Even if these ions are thermodynamically stable they are only of limited persistence since they are susceptible to reactions with electrophiles and nucleophiles or undergo other processes like dimerization or electron-transfer induced bond cleavage [9, 10]. Pairs of radical anions and radical cations can also be formed by electron transfer between neutral donors and acceptors either in the ground state or upon photochemical excitation [11, 12]. 

Photoinduced electron transfer reactions that occur between neutral electron donor molecules and neutral electron acceptor molecules have several characteristic features (1) a radical cation and a radical anion are produced as a pair, (2) radical ion species are produced under neutral and mild conditions, and (3) the polarity inversion (umpolung) of original electron donor and electron acceptor molecules arises through their conversion into radical ion species. As a result, the radical cation D can interact with another D or a different electron donor molecule D to yield a dimer radical cation Dj or heterodimer radical... 

A paper by Suppan draws attention to electrostatic interaction effects on condensed phase photoinduced electron transfer and the need to take account of the fact that solvent is not in reality a uniform dielectric material. Pressure effects on exciplex formation has been exemplified in the pyrene-p-cyanobenzene system. Ternary electron donor acceptor complexes are formed and in the case of anthracene-tetracyanoethylene gives rise to (DO ) dimer radical cations. Laser flash photolysis shows that perylene in acetonitrile undergoes three distinct electron transfer processes, (i) gives pt + MeCNT, (ii) gives... 

In all of the preceding cases a nonbonding pair of electrons is used to stabilize a sulfur radical cation by forming a 2c, 3e bond. It has also been reported [385] that 7r-electron donors can stabilize sulfur radical cations. Monomeric or dimeric radical cations were generated from thiirane, thietane, thiolane,... 

Good electron donors such as sulfides, phosphines, or arsines can react with N-fluoropyridinium cation by a single-electron transfer (SET) pathway. This conclusion was reached after finding products known to be derived from free-radical processes. For example, it is believed that the SET process is operative in the reaction of sulfides (74) to give pyridyl-substituted sulfides (78) through the intermediary of a radical 75 and a radical cation 76 (Scheme 7). In addition to 78 this reaction produces a dimer of a radical 77 derived from the radical cation (76) and a number of other products known to be formed from 76 or 77 (93JHC329). The... 

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