Radical anions from biphenyl

The 1,1,4,4-tetraphenylbutane disodium salt was found to eject electrons scavenged by added 1,1-diphenylethylene [59, 60]. In the case of trans-stilbene dianion, the photoejection of an electron was accompanied by isomerization in ds-stilbene radical anion while biphenyl was used as electron scavenger [61]. ds-Stilbene itself was used as scavenger of electron photoejected from aromatic radical anions like sodium perylenide [46], These scavenging experiments are... 

Figure 4-9. Schematic energy level diagrams, (a) The radical anion of biphenyl (PP2 ) (b) the dianion of biphenyl (PP2 ). , electron arrow, electronic transition. The molecular orbital levels are taken from [77],...

Electrochemical reduction of pentatluoronitrobenzene produces an intermediate radical anion that couples at position 4 to form the corresponding biphenyl along with hydroxy derivatives from subsequent nucleophilic substitution meta to the nitio groups [44] (equation 34) Similar reduction of halopyridines such as pen-tafluoropyridine leads mainly to 4,4 bipyridyls [45] (equation 35)... 

More recently, charge-transfer emission was anticipated when solutions of hydrocarbon anion radical salts in dimethoxyethane were mixed with Wurster s blue perchlorate.15 Emission was seen in every instance however, with eight anion radicals derived from 3 to 5 ring-fused aromatic hydrocarbons, the emission was derived from the hydrocarbon rather than the complex. Preliminary studies with smaller hydrocarbons, biphenyl and naphthalene, did show emission in the region (18 kK) where charge transfer was expected. The question as to what pairs of ion radicals will be emissive under what conditions has only begun to be considered. Much opportunity for further experimentation exists in this area. 

Another investigation along this line is the pulse radiolysis study of the electron transfer reactions from aromatic radical anions to styrene this type of reaction is commonly used to initiate anionic polymerization of styrene [35], The electron transfer rates from the unassociated biphenyl radical-anions to styrene derivatives in 2-propanol were found to increase along the... 

The electron transfer from aromatic radical anions to various electron acceptors takes place efficiently in solution. Likewise, when a second solute, pyrene, is added to the MTHF solution of PVB, the electrons transfer from polymer anions to pyrene occurs [50]. The rate constant determined by pulse radiolysis is approximately a third of that of the electron transfer from biphenyl anion to pyrene. 

Fig. 4 The dependence of the rate of thermal ET taking place from a biphenyl radical anion donor to a series of acceptor groups A, as a function of driving force.9...

The study of the photochemistry of aryl carbanions has been restricted to aryllithiums with only a limited number of studies available. Hence, a general picture of their photochemistry is not available at this time. Photolysis of phenyllithium in the presence of aromatic hydrocarbons such as naphthalene, biphenyl, phenylene, etc. in diethyl ether results in electron transfer from the phenyllithium to the aromatic hydrocarbon, with production of the corresponding hydrocarbon radical anion, as observed by ESR spectroscopy [6-8] (Eq. 1). Photolysis of phenyllithium or 2-naphthyllithium alone gave the corresponding biaryl products and metallic lithium [9-10]. For this reaction, it is possible to write a mechanism which does not require electron transfer from the anion [9,10],... 

Fig. 12. Transient photoabsorption spectriun acquisition at ISIR, Osaka University, (a) Two consecutive femtosecond pulses of white light continuiun collected by single scan of CCD camera, (b) Transient absorption spectrum of 100 mM biphenyl in Ar-bubbled tetrahydrofuran solution at 200 ps after the end of pulse. The peaks are attributed to biphenyl radical anion. Each point in near-UV region (ca. from 400 to 500 nm) and visible-NIR region (ca. from 500 to 900 nm) was measured by averaging over 10 and 2 beam-probe shots, respectively. The inset is a magnification in 400-500 nm.

Addition of electron acceptors to acetonitrile confirmed the presence of a transient reducing species through the formation of radical anions of solutes such as biphenyl [21a, 22], pyrene, trans-stilbene, and so forth [21b]. This reducing species has a broad absorption with Amax at 1450 nm, and was shown to exist in the monomeric and dimeric forms (see Eq. 36) from the effect of temperature on the absorbance at - max [21b]. The monomeric form is responsible for the 1450 nm peak, whilst the dimeric form exhibits a weak maximum at 550 nm superimposed on the tail of the monomeric band. The enthalpy change accompanying the reaction of Eq. 36 has been measured to be —34.9 kJ moE [21b] so that CH3CN is the dominant reducing species at room temperature. 

Hoijtink et al. [27] also developed an alternative method of generating anionic species, which was improved by Szwarc et al. [28]. The technique involves potentiometric titration of aromatic compounds with a standard solution of Na-biphenylide. The extremely negative reduction potential of biphenyl assures that most of the common aromatics can be reduced to at least their respective radical anions. The values of the thermodynamic reduction potentials are generally obtained from the potentiometric titration curve. As all experiments are usually carried out in ethereal solutions, such as tetrahydrofuran (THF) or dimethoxyethane, problems of follow-up processes are less severe. Later, Gross and Schindewolf [29] reported on the potentiometric titration of aromatics using solvated electrons in liquid ammonia. 

Figure 2. (A) Dependence on A of the logarithm of the rate constants of reoxidation of the following radical anions (with increasing A ) biphenyl, 1-methylnaphthalene, naphthalene, 2-methylphenanthrene, phenanthrene, / -etrphenyl, and benzonitrile in the presence of chlorobenzene. From Ref. [7]. (B) Rate constants of the homogeneous electron exchange between chlorobenzene and redox catalysis as functions of the half-wave potential difference and of the standard potential difference (at room temperature in the considered solvent =). With increasing AE biphenyl, naphthalene, dibenzothiophene, phenanthrene, / -toluonitrile, m-toluonitrile, /7-terphenyl, and benzonitrile. From Refs 1 and 2.

Although biphenyl may improve formation of R3GeM from R Gej-M, the radical anion of napthalene, [CioH8] Na, cleaves the Ge—Ge bond of PhgGej in DME. This reagent does not cleave Ge—C bonds. 

The one-electron reduction of triphenylamine by sodium or potassium gives the biphenyl radical-anion (Iwaisumi and Isobe, 1965). In contrast, triphenylphosphine and potassium give a radical-anion which is thought to be Ph2P—K the same species is formed from diphenyl-phosphine with an excess of potassium (Britt and Kaiser, 1965). More recently, Cowley and Hnoosh (1966) have found that the reduction of triphenylphosphine oxide with sodium in dimethoxyethane gives the biphenyl radical-anion, whereas reduction with potassium gives, in tetrahydrofuran, PhsPO" and, in dimethoxyethane, a spectrum very similar to that reported by Britt and Kaiser but which is attributed to Ph2P(0)K-. 

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