Stilbenes radical cations

Of course, a close stmctural relationship between radical cations and parent molecules is not likely to hold generally, but it is a fair approximation for alternant hydrocarbons. Deviations have been noted some stilbene radical cations have higher-lying excited states without precedent in the PE spectrum of the parent for radical cations of cross-conjugated systems (e.g., 1) already the first excited state is without such precedent. These states have been called non-Koop-manns states. Alkenes also feature major differences between parent and radical cation electronic structures. 

The 9,10-dicyanoanthracene sensitized irradiation of c/i-stilbene results in nearly quantitative isomerization (>98%) to the trans isomer with quantum yields greater than unity. Therefore, the isomerization was formulated as a free radical cation chain mechanism with two key features (1) rearrangement of the c/i-stilbene radical cation and (2) electron transfer from the unreacted cis-olefin to the rearranged (trans-) radical cation. 

Spectroscopic evidence for the transient formation of the trans-stilbene radical cation could be obtained when colloidal TiOj suspended in an acetonitrile solution containing trans-stilbene (a species which should also be exothermically oxidized by a TiO valence band hole) was excited with a laser pulse The observed transient was identical in spectroscopic features and in lifetime with an authentic sample of the stilbene cation radical generated in the same medium via pulse radiolytic techniques. That the surface influences the subsequent chemistry of this species can be seen in the distribution of products observed under steady state illumination, Eq. (4) 2 . ... 

The behavior of stilbene radical cations in the semiconductor catalysis is in keeping with the result of photoisomerization of other olefins like 6-methylstyrene sensitized by electron acceptors like chloranil in polar solvents (48). The semiconductor photocata-lyzed isomerization of strained cyclobutanes to strained dienes (isomerization of quadricyclene to norbomadiene and similar reactions of complex cage compounds (49)) is related to the olefin isomerization discussed above. 

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. 

Among the electron transfer induced reactions of cyclobutane systems, cycloreversions are the most prominent. These reactions are the reverse of the cycloadditions discussed in Sect. 4.1. The reactivity of the corresponding radical cations depends on their substitution pattern. We have mentioned the fast two-bond cycloreversion of quadicyclane radical cation as well as the ready ring closure of a tetracyclic system (3, Sect. 4.1). A related fragmentation of cis-, trans-, cis-1,2,3,4-tetraphenylcyclobutane (84) can be induced by pulse radiolysis of 1,2-dichloro-ethane solutions. This reaction produces the known spectrum of trans-stilbene radical cation (85) without a detectable intermediate and with a high degree of... 

Although Ceo is easily reduced, it is very difficult to oxidize [46, 53, 54, 72], The only definitive electrochemical oxidation of Cgo occurs at a potential of -F 1.76 V vs SCE in benzonitrile, and is irreversible [54]. The radical cation was reported to be produced by y-irradiation at 77 K in a glass, and to absorb near 980 nm [65, 66]. Attempts to generate the radical cation (Ceo) by electron transfer to singlet excited dicyanoanthracene, which has a reduction potential near + 2.0 V [73] were unsuccessful. This method has been used to generate, for example, trans-stilbene radical cation [73, 74]. The ion pair probably does not... 

Photoexcitation of aromatics in zeolites also led to cation radicals, as reported by lu and Thomas for pyrene and anthracene [128]. Ultraviolet photoexcitation of trani-stilbene in NaX formed the rran.y-stilbene radical cation (475 nm) and the zeolite-entrapped electron shows up as Na4 + (500 nm) [129]. Other organic species that form radical cations in zeolites upon high-energy UV excitation include 4-vinylanisole, tra .y-anethole, several styrenes and di(4-methoxyphenyl)ethylenes [130]. 

The photoisomerization of cis-and trans-stilbenes has been studied in ionic liquids and evidence is presented that the mechanism depends on the particular ionic liquid used. In basic N-butylpyridinium chloride/AlCb the photoequilibration involves stilbene radical cations, whereas in basic l-ethyl-3-methyl-imidazolium chloride/AlCb the process occurs via the standard singlet state photoisomerization mechanism. It has been found that the tetra-O-acetylribo-flavin (126) sensitized dehydrogenation of substituted benzyl alcohols, giving the... 

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]. 

V) but not in the presence of anisole = 1.72 V) or biphenyl (Egjj = 1.82 V). Direct evidence for an electron transfer process was obtained by irradiating the tra/is-retinyl cation in the presence of tro/u-stilbene. Transient absorption spectroscopy showed bleaching of the cation signal and enhanced absorption at 470 nm, corresponding to formation of the stilbene radical cation. 

F re 14. Computed potential energy surfaces of isomerization of stilbene radical cations [147]. 

As to acceleration of the isomerization, addition of salts such as lithium perchlorate in acetonitrile solution and tetrabutylammonium tetrafluoro-borate in dichloromethane in DCA-sensitized irradiation of cis-stilbene enhanced the isomerization. A salt effect is probably responsible for retardation of recombination between the resulting stilbene radical cations and counteranions (DCA--, etc.) [143, 146]. 

Stilbene radical cations are well known from matrix studies and have been generated by laser flash photolysis techniques. The parent trans and cis radical cations have strong absorption maxima at 482 and 308 nm in acetonitrile solution with weaker bands between 700-800 nm. A number of substituted stilbene radical cations have also been studied and both electron withdrawing and donating substituents result in bathochromic shifts (Table I). The c s-stilbene radical cations... 

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