Anthracene radical cation

In complex organic molecules calculations of the geometry of excited states and hence predictions of chemiluminescent reactions are very difficult however, as is well known, in polycyclic aromatic hydrocarbons there are relatively small differences in the configurations of the ground state and the excited state. Moreover, the chemiluminescence produced by the reaction of aromatic hydrocarbon radical anions and radical cations is due to simple one-electron transfer reactions, especially in cases where both radical ions are derived from the same aromatic hydrocarbon, as in the reaction between 9.10-diphenyl anthracene radical cation and anion. More complex are radical ion chemiluminescence reactions involving radical ions of different parent compounds, such as the couple naphthalene radical anion/Wurster s blue (see Section VIII. B.). 

The dehydrodimerization reaction involving aromatic radical-cations is fast only when electron donating substituents are present in the benzene ring. These substituents stabilise the a-intermediate. Benzene, naphthalene and anthracene radical-cations form a a-sandwich complex with the substrate but lack the ability to stabilise the a-intermediate so that radical-cation substrate reactions are not observed. The energy level diagram of Scheme 6.4 illustrates the influence of electron donating substituents in stabilising the Wheland type a-intermediate. 

In 2004, a report appeared that 9,10-dimethylanthracene-9,10-endoperoxide (48) arose from an electron-transfer photooxidation of 9,10-dimethylanthracene (38) with the use of the sensitizer Acr+-Mes in 02-saturated CH3CN at 0 °C [32]. Subsequent coupling of the anthracene radical cation and the superoxide ion generated endo-peroxide 48. Endoperoxide 48 was detected during the initial stage of the photooxidation, but was not isolated over time, the reaction yielded 10-hydroxanthrone, anthraquinone, and H202. 

In another example, Yildirim et al. photochemically generated anthracene radical cations in the presence of TEMPO [29]. TEMPO immediately trapped the radical to form the TEMPO-anthracene cation, which was subsequently used to initiate cationic polymerization of cyclohexene oxide (CHOX). The resulting alkoxyamine-functional polycyclohexene oxide was used to macroinitiate styrene polymerization, resulting in the formation of S-6/-CHOX (Scheme 8.9). 

A weak base such as H2O can accelerate the rate of electron transfer disproportionation of anthracene radical cation in MeCN by the interaction between anthracene dication produced in the electron transfer and H2O [361]. The four-electron oxidation of 9-benzylanthracene (PhCH2An) with more than 4 equiv. of... 

As an example, photochemical excitation of donor-acceptor complexes may be considered. Irradiaiion into the CT band of the anthracene-tetracyano-ethylene complex leads directly to the radical ion pair, the components of which are identifiable from their UV-visible spectra. The transient absorptions decay in 60 ps after excitation, as the radical ion pairs undergo rapid back electron transfer to afford the original donor-acceptor complex (Hilinski et al., 1984). With tetranitromethane as acceptor, however, an addition product is obtained in both high quantum and chemical yield. This is due to the fact that the tetranitromethane radical anion undergoes spontaneous fragmentation lo a NO, radical and a trinitromethyl anion, which is not able to reduce the anthracene radical cation (Masnovi et al., 1985) ... 

The rapid reaction of the anthracene radical cation with MeCN is nicely illustrated by the observation of an irreversible, two-electron oxidation peak in the voltammogram at + 1.2 V versus SCE, whereas in CH2Cl2-(CF3C0)20 the peak corresponds to a reversible one-electron oxidation process. 

Benchmark ab initio quantum dynamical studies are carried out for the prototypical naphthalene and anthracene radical cations of the PAH family aiming to understand the vibronic interactions and ultrafast decay of their low-lying electronic states. The broadening of vibronic bands and ultrafast internal conversion through conical intersections in the Da — — D2 electronic states of these species is... 

Vibronic Coupling in Naphthalene and Anthracene Radical Cations Implications in the Interstellar Chemistry... 

Naphthalene and anthracene radical cations are the two simplest members in the family of the PAH radical cations. Investigation of the photophysics and photochemistry of the latter are of major concern in contemporary chemical dynamics. The radical cations of PAHs are of fundamental importance in the chemistry of the interstellar space, environmental, biological processes and combustion [103-106]. Radical cations of PAHs are most abundant in the interstellar and extragalactic environments [41]. They absorb strong UV radiation emitted by the young stars and get electronically excited. Examination of the fate of electronically excited PAH radical cations invited critical measurements of their optical spectroscopy in the laboratory in recent years [42 4]. Attempt is made to understand the important issues like, (1) photostability and lack of fluorescence emission and (2) the origin of the enigmatic... 

Acknowledgements This study is supported, in part, by a grant from the DST, New Delhi (Grant No. DST/SF-04/2006). The authors thank CMSD, University of Hyderabad for the computational facilities. VSR thanks CSIR, New Delhi for a senior research fellowship. The authors thank S. Ghanta for his help in obtaining the results on the anthracene radical cation. 

Greater time resolution can be gained by coupling pulse electrolysis to stopped-flow apparatus [44]. This technique has been used in the study of anthracene radical cation reactions with nucleophiles described later [45]. Electrochemical techniques are considered rather slow for studying these reactions the radical cations are much more conveniently generated and studied by laser flash photolysis. 

In the presence of 1-5% of iron trichloride or TBPA under conventional conditions, 60-70% yields of the adduct are formed even at -50 C, in times as short as 15 min.43 Therefore, the conclusion is that, imder appropriate conditions, the anthracene radical cation is most probably on the reaction pathway, in contrast with the case of maleic anhydride cycloaddition. A coincidence seems then to exist between the possibility of a mechanism involving the diene radical cation and the presence of a sonochemical effect. 

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