Trans-Stilbene radical anion

Emission spectra have been recorded for electron injection into Au and Ag spherical electrodes and hole injection into Au(lll) planar electrodes. These processes were brought about in solutions of acetonitrile containing tetrabutylammonium hexafluoro-phosphate (TBAHP), using the trans-stilbene radical anion as the electron injector and the thianthrene radical cation as hole injector. The spectrum for the hole injection process into planar Au(lll) electrodes has been resolved into the P S-polarised components of the emitted light. A comparison of the spectral distribution of emitted light for the above electron injection process, occurring at both Au and Ag... 

Isomerization. Finally, some investigations of unstable radical anions will be discussed. Radical anions of ds-stilbene rapidly isomerize to radical anions of trans-stilbene therefore, their spectra or relative electron affinities cannot be studied by conventional techniques. Flashing a solution of perylene radical anions mixed with cis-stilbene yields cis-stilbene radical anions, which persist for 200-300 jls before the ejected electrons are recaptured by the perylene formed by the photolysis. The difference spectrum of the photolyzed solution is shown in Figure 12. The concentration of the ds-stilbene radical anions formed can be determined by the degree of bleaching the absorption peak of the perylene radical anions. The absorption spectrum of cis-stilbene radical anions can also be determined in this way (19). Similar experiments, results of which are shown in Figure 13, permit the determination of the absorption spectrum of trans-stilbene radical anions. Because this spectrum has been established by other methods, the reliability of the flash-photolytic method can be determined by comparing the differ-... 

Figure 14. Absorption spectra of cis- and trans-stilbene radical anions determined by flash photolysis. (Reproduced from reference 19. Copyright 1977 American Chemical Society.)...

Radical-anions from cis- and /ran. f-stilbene are distinct species with individual uv- and esr-spectra [7], The cw-form is metastable and slowly converts to the trans-form. The kinetic features of this interconversion have been studied by generating the cw-stilbene radical-anion in hexamethylphosphoramide by photochemi-cally assisted electron transfer from a second radical-anion of less negative... 

Photosensitized electron transfer reactions conducted in the presence of molecular oxygen occasionally yield oxygenated products. The mechanism proposed to account for many of these reactions [145-147] is initiated by electron transfer to the photo-excited acceptor. Subsequently, a secondary electron transfer from the acceptor anion to oxygen forms a superoxide anion, which couples with the donor radical cation. The key step, Eq. (18), is supported by spectroscopic evidence. The absorption [148] and ESR spectra [146] of trans-stilbene radical cation and 9-cyanophenanthrene radical anion have been observed upon optical irradiation and the anion spectrum was found to decay rapidly in the presence of oxygen. 

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... 

Tertiary amines have been reported to undergo addition to singlet trans-stilbene product selectivity appears to be determined by the orientation of deprotonation of the aminium radical intermediate by the stilbene radical anion. Intramolecular addition has been observed in 5-2-(r-methylalkyl)-aniline (220) on irradiation in methanol to give the 25,3/ -2,3-dimethylindoline... 

Under optimum conditions electron transfer can produce excited states efficiently. Triplet fluoranthrene was reported to be formed in nearly quantitative yield from reaction of fluoranthrene radical anion with the 10-phenylphenothia2ine radical cation (171), and an 80% triplet yield was indicated for electrochemiluminescence of fluoranthrene by measuring triplet sensiti2ed isomeri2ation of trans- to i j -stilbene (172). 

Dietz and Peover examined the electrochemical reduction of cis and trans stilbene (114) in DMF containing carbon dioxide, 9>. The first electron transfer to trans-114 affords a planar radical anion (115) which then undergoes rapid reaction with carbon dioxide to produce, ultimately, 2,3-diphenylsuc-cinic acid (116) in... 

Chemically inert triplet quenchers e.g. trans-stilbene, anthracene, or pyrene, suppress the characteristic chemiluminescence of radical-ion recombination. When these quenchers are capable of fluorescence, as are anthracene and pyrene, the energy of the radical-ion recombination reaction is used for the excitation of the quencher fluorescence 15°). Trans-stilbene is a chemically inert 162> triplet quencher which is especially efficient where the energy of the first excited triplet state of a primary product is about 0.2 eV above that of trans-stilbene 163>. This condition is realized, for example, in the energy-deficient chemiluminescent system 10-methyl-phenothiazian radical cation and fluoranthene radical anion 164>. 

Nearly at the same time, the same group reported a study dealing with the electron-transfer initiated oxidation of trans-stilbene (TS) 10a sensitized by the singlet excited states of both DCA and methylene blue (MB+) [124]. The authors proposed that, although the initial electron-transfer step was identical for the two systems, the subsequent steps leading to products (predominantly benzaldehyde) must be different. In fact, dicyanoanthracene radical anion DCAT reduces molecular oxygen to superoxide, whereas reduced methylene blue MB°, owing to its lower redox potential (Ered = —0.25 V vs SCE), doesn t. 

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. 

Time-resolved Raman spectroscopy also provides valuable mechanistic information on quenching reactions of molecules such as trans-stilbene [123]. The fluorescence of trans-stilbene can be quenched by tertiary amines. TR spectra measured 60 ns after excitation contained peaks that could be assigned to the transient anion radical of trans-stilbene [123]. 

Irradiation of the contact charge transfer complex formed between trans-stilbenes and oxygen molecules in a zeolite NaY matrix at 313 nm leads to generation of the corresponding benzaldehydes in an electron-transfer process from which stilbene cation radicals and superoxide anion radicals arise. By contrast, excitation at 254 nm induces isomerisation and phenanthrene production, but without formation of any oxygenation products. 

Radical anions of frans-stilbene and stiff stilbenes, generated by y-radi-ation in MTHF at — 196°C, exhibit maxima at 500-560nm, e = (1 — 2) x 104M 1cm 1 [509], A similar spectrum and e50o = 6.1 x 104M 1 cm 1 was recorded by pulse radiolysis at room temperature [510]. The anion and cation radicals of cis-stilbene lead efficiently to the trans isomer, while an analogous conversion to the cis isomer from the radical anion and radical cation of frans-stilbene was not found [491]. The cis-stilbene radical cation isomerizes to the more stable frans-stilbene radical cation [511]. The radical anion of cis-4-nitrostilbene has been observed by EPR [512], In the cases of 4-nitro-,4,4-dinitrostilbene and 4,4 -NMS the electron transfer occurs in the triplet state, for example, from DABCO toward the 3t state [513]. 

The contributions of radical ions DH + and neutral radicals D to the product polarizations were also separated by variable-field stationary CIDNP [94a], time-resolved CIDNP, and SNP [94b]. From an evaluation of the CIDNP memory effect, the rate of in-cage proton transfer from the radical cation of triethylamine to the radical anion of trans-stilbene was estimated to be 3 x 107s-1 [94F],... 

A solution of perylene radical anions containing a 1 1 mixture of cis- and trans-stilbenes is flash photolyzed. The ejected electrons are captured by... 

Upon pulse radiolysis of traws-stilbene (t-St) solutions in THE, the radical anion of trans-stilbene was demonstrated to be formed by the reaction of electrons with St (reaction with the rate constant ks = (1.16 0.03) x 10 dm /(mol s)) [86]. The transient absorption spectrum observed with Xmax 500 and 720 nm was attributed to the unassociated radical anion St" . This species reacted with the countercation of THE formed upon radiolysis and with radiolytically generated radicals. Addition of sodium tetrahydridoaluminate (NAH) resulted in the radical anion being associated with Na as a contact ion pair. In the presence of the lithium salt, formation of solvent-separated ion pairs has been detected. 

A further interesting point concerns unusually small differences between the reduction potentials of mono- and dianions of some molecules containing olefinic double bonds. Although in trans-stilbene the formation of the dianion occurs approximately 500 mV negatively to the radical anion formation [35, 120], for tetraphenylethylene the standard potentials for the R/R couple and the... 

Lund and coworkers [131] pioneered the use of aromatic anion radicals as mediators in a study of the catalytic reduction of bromobenzene by the electrogenerated anion radical of chrysene. Other early investigations involved the catalytic reduction of 1-bromo- and 1-chlorobutane by the anion radicals of trans-stilbene and anthracene [132], of 1-chlorohexane and 6-chloro-l-hexene by the naphthalene anion radical [133], and of 1-chlorooctane by the phenanthrene anion radical [134]. Simonet and coworkers [135] pointed out that a catalytically formed alkyl radical can react with an aromatic anion radical to form an alkylated aromatic hydrocarbon. Additional, comparatively recent work has centered on electron transfer between aromatic anion radicals and l,2-dichloro-l,2-diphenylethane [136], on reductive coupling of tert-butyl bromide with azobenzene, quinoxaline, and anthracene [137], and on the reactions of aromatic anion radicals with substituted benzyl chlorides [138], with... 

An analogous cation radical chain process has been proposed for cis to trans isomerization of N-methyl-4-(6-stryl)-pyridinium ions via electron-transfer sensitization by Ru(bpy)-j2+ and metalloporphyrins (145). Quantum yields for isomerization are substantially higher in aqueous anionic micelles versus homogeneous solution due to the higher concentration of cis-styrylpyridinium ions. A radical cation chain mechanism may also account for previous reports of selective cis to trans sensitized photoisomerization of stilbene (25,26). 

Upon one-electron reduction followed by one-electron oxidation, cis isomers of 2-and 4-nitro stilbenes turn into the neutral nitrostilbene molecules, but in the trans forms. Upon oxidation of the naked anion radical, the neutral trans forms are the only products (cis —> trans conversion degrees were 100%). In case of the coordination complexes, the trans isomers are formed only up to 40% (Todres 1992). Scheme 3-39 describes these transformations. 

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