Anthraquinone Radical Anion Research Articles

Kinetics of Photoinduced Electron Transfer Reactions of Some Anthraquinone Radical Anions with Various Inorganic Ions: Comparison with Marcus Cross‐Relation

AbstractThe reactions of 2‐, 2,6‐ and 2,7‐anthraquinone (di) sulphonate radical ions with the paramagnetic ions Fe(CN)36, Cu2+, IrCl2‐6 and Fe3+ have been studied at different pH‐values at room temperature, in order to investigate the role of spin states on electron transfer. With 2‐anthraquinone sulphonate the rate constants are in the range 2.6 ‐ 7 × 108 mol−1 dm3 s−1, and for the other two anthraquinone derivatives the rate constants span over two orders of magnitude from 107 to 109 mol−1 dm3 s−1. The experimental rate constants have been compared with the predictions of the Marcus Cross‐Relation, taking also into account the contribution of the diffusion pathway in our calculations.The predicted values are systematically larger than the experimental values. This systematic deviation is justified in the present work as due to the spin‐forbidden character of the reactions studied. The need for suited corrections in the current electron transfer theories has been stressed.

Solvent Effects on the Formation and Decay of Ionic Intermediates for the Photoreduction of Anthraquinone by Triethylamine in Ethanol, Toluene, and Acetonitrile at Room Temperature

Abstract In toluene and ethanol containing triethylamine (TEA), a reaction of the lowest excited triplet state [3AQ(T1)] of anthraquinone (AQ) with ground-state TEA gave rise to the formation of an exciplex [3(AQ–TEA)*] which converted into a contact ion pair between the AQ radical anion (AQ−·) and the TEA radical cation (TEA+·). By an intramolecular proton transfer, this contact ion pair yielded anthrasemiquinone radical followed by the formation of 9,10-anthracenediol (AQH2); in ethanol, the monoanion (AQH−) of AQH2 was also produced. In ethanol/TEA, moreover, not only the decay rate constant of 3(AQ–TEA)* but also the quantum yield for the photoreduction of AQ were affected by the change of TEA concentration. This was interpreted in terms of the existence of a rapid interconversion between 3(AQ–TEA)* and atriplex [3(AQ–TEA2)*] of 3AQ(T1) with ground-state TEA. In contrast to the exciplex formation in ethanol and toluene, free AQ−· (and TEA+·) and 3(AQ–TEA)* [or a contact (or solvent-separated) ion pair between AQ−· and TEA+·] were produced in acetonitrile via the second and lowest excited triplet states of AQ, respectively: In the submicrosecond time regime, 3(AQ–TEA)* or the contact (or solvent-separated) ion pair dissociated into the radical ions (AQ−· and TEA+·) followed by their second-order reaction yielding finally AQH2 and AQH−.

Pulse radiolysis of 9,10-anthraquinone in methanol

The reactions of anthraquinone with intermediates in methanol (e - s, CH 2O -, CH 2OH) were investigated using γ-radiolysis and pulse radiolysis method. The anthraquinone radical anions are reactive towards O 2 ( ca3x10 8mol -1dm 3s -1) and can disproportionate giving corresponding dianion.

Photoelectrochemical electron spin resonance. Part 4.—The ‘photo-ECE’ reaction and the reduction of 1-halogenoanthraquinones

The reduction of 1-bromoanthraquinone in acetonitrile solvent in the presence of tetrabutylammonium perchlorate, at irradiated (565 nm) gold electrodes is shown to produce the radical anion of anthraquinone. A combination of photoelectrochemical hydrodynamic voltammetry, in situ electrochemical ESR and preparative experiments are used to deduce that the mechanism is a ‘photo-ECE’ reaction, in which the first electron transfer produces the radical anion of 1-bromoanthraquinone. On irradiation, this anion dissociates. Bromide ions and anthraquinone are formed and the latter undergoes a further one-electron reduction at the electrode resulting in the detection of significant photocurrents. The H atom abstracted in the formation of the anthraquinone is shown to arise from the tetrabutylammonium cations and not from the solvent as previously suggested in analogous processes. The related species, the 1-chloroanthraquinone radical anion, is shown to be stable towards irradiation on the electrochemical timescale, whereas 1-iodoanthraquinone is found to undergo reduction at gold in the absence of light in an analogous mechanistic manner to 1-bromoanthraquinone in the light.

An investigation of anthraquinone-catalysed alkaline pulping via component modeling and electron spin resonance experiments

The radical anion of anthraquinone (AQ•−) has been generated by the reduction of anthraquinone (AQ) with sugars in aqueous alkaline solution. This appears to be the first time that the radical has been observed (esr spectrometry) in an entirely aqueous medium. Alkaline methanol solutions of AQ, when irradiated with light, yielded colourless solutions also exhibiting strong esr signals characteristic of. AQ•−.The ability of various carbohydrate and lignin preparations to reduce AQ, or to oxidize the reduced forms of AQ, was also investigated, both by direct observation and by electrode potential measurements. In very concentrated alkali, AQ was reduced by glucose to a green solid thought to be a quinhydrone.

The Effect of Water on the Reduction Potentials of Some Aromatic Compounds in the DMF-Water System

Abstract From the viewpoint of the interaction between the oxidized or the reduced form of a depolarizer and solvents, the solvent effect on the half-wave potentials was studied. Another factor affecting the E1⁄2, i.e., pH or ion-pair formation, was also examined. The correction for the liquid junction potential was carried out by the use of the ferrocene standard. The reduction products, radical anions or dianions, were found to be stabilized by the hydrogen-bonding with water where the hydrogen-bond interaction of the dianion was stronger than that of the monoanion. Aromatic hydrocarbon radical anions, whose charges are dispersed over the molecule, were less susceptible to this solvent transfer than were the anthraquinone radical anion and the dianion, whose charges are strongly localized.

The Protonation of Aromatic Hydrocarbon Radical Anions. I. A Comparison of Methods and a Study of the Mechanism

Abstract The protonation mechanism of the aromatic hydrocarbon radical anions, such as biphenyl, naphthalene, phenanthrene, anthracene, 1,2-benzanthracene, and pyrene, in dimethylformamide (DMF) and water mixtures was studied. Three methods of measuring the concentration of the radical anions, i.e. polarography and ESR and UV absorption spectroscopy, were compared with each other by making simultaneous measurements of the same anthraquinone radical anion solution. A good linear relation of the polarographic anodic diffusion current to the UV absorbance was found, while the ESR signal intensity had linear relations with neither of the others in the system which contained an excessive number of parent molecules, 2×10−3m. The change in the visible absorption spectra of the aromatic hydrocarbon radical anions as a function of the time suggested that all the aromatic hydrocarbon radical anions decay by a first-order reaction. As a result, the radical anions were considered to decay through the following sequence: R\ewdot+H2O\oversetk→RH·+OH−, RH·+R\ewdot\ightleftharpoonsRH−+R, and RH−+H2O\oversetk′→RH2+OH−.