UV-Visible Spectrooelectrochemistry of the Reduction Products of Anthraquinone in Dimethylformamide Solutions: An Advanced Undergraduate Experiment

Abstract

The widespread occurrence of quinones in nature and their importance in biological electron transport has led to extensive studies of their electrochemical behavior (1,2). Much of what is known about the redox behavior of biologically significant quinones is related to the behavior of simple quinones such as 1,4-benzoquinone and 9,10-anthraquinone (AQ). Although the parent quinones have little visible absorption, their reduced forms have distinct colors arising from intense π → π* transitions and this has led to the frequent use of UV-vis spectroscopy in distinguishing quinone reduction products.Successive one-electron reduction of quinones was clearly demonstrated by Wawzoneck et al. (3) in a classic polarographic study of AQ and other quinones in dimethylformamide (DMF) and acetonitrile solvents. They also found that quinone anions are significantly stabilized when associated with alkali metal cations. Furthermore this association in solution and the electrosorption of alkali metal cations reduces electrostatic repulsion at the electrode giving faster electron transfer kinetics (4).Wawonzek et al.(3) also studied the reduction behavior when the solvents contained small quantities of water and of benzoic acid as proton donors. They attributed the reduction behavior of AQ largely to the ready protonation of the dianions. Given and Peover (5), in a further study of AQ in DMF, suggested that the radical anion is protonated by benzoic acid and immediately undergoes further reduction. More recently Wightman et al. (6) carried out a thorough study of AQ reduction reactions in DMF with added benzoic acid. They showed that protonation of the radical anion is followed by disproportionation.The following experiment shows how cyclic voltammetry (7) may be used to study the reduction behavior of AQ in dimethylformamide, a typical laboratory aprotic solvent, and to examine the influence of a proton donor, benzoic acid, on this behavior. Digital simulations of the cyclic voltammograms (8,9) have been used to test protonation reaction mechanisms. The UV-vis spectroelectrochemistry of this system has also been employed to characterize the reduction products of AQ under conditions of different proton availability. This experiment is part of an Otago thirdyear undergraduate course in Chemical and Electrode Kinetics. The experimental data are collected in a four-hour laboratory period. TheoryQuinones (Q) undergo two successive one-electron reductions in aprotic solvents to form stable radical anions (Q៛) and quasistable dianions (Q 2᎑ ).The equilibrium potentials for these two processes (E 1 and E 2 ) are related to the concentrations of the quinone species by(3)The tendency of the radical anion to disproportionateis given by the disproportionation constantBy considering eq 6 and eqs 3 and 4 one obtainsSuccessive electron transfers of quinones in aprotic solvents usually occur at quite separate potentials. Thus E 2° -E 1° is generally large and negative. This results in very low K disp values (eq 7) and there is little tendency f...

Use of multivariate curve resolution analysis in the spectroelectrochemistry of 9,10-anthraquinone reduction in dimethylformamide solution

“…The pure spectra shown in Fig. 1 are relatively similar to those reported by McQuillan and co-workers [28,46]. The existing differences may be due to the coexisting of at least two species at each applied potential.…”

“…The potential of the reference electrode was close to that of the saturated calomel electrode (SCE) in a 0.1 M NaClO 4 + DMF solution. The cell design was similar to that we published previously [46].…”

“…The spectrum of the third component, which is assumed to be AQ 2À , has two peaks at 466 and 610 nm. Previously, McQuillan and co-workers [46] in a comprehensive spectroelectrochemical study on anthraquinone sulphonates (AQS) reported that the maximum in the absorption spectrum of the de-protonated (AQ 2À ) and protonated (AQSH 2 ) forms of the AQSH À species show red and blue shifts relative to those of AQSH À , respectively. Therefore, since in this work the absorbance maxima of the third component are red-shifted relative to those of AQH À , the third component should be AQ 2À .…”

Proton‐Coupled Electron‐Transfer Processes in Ultrafast Time Domain: Evidence for Effects of Hydrogen‐Bond Stabilization on Photoinduced Electron Transfer

“…Interestingly,w ith time, the broad positive absorption band was convertedi nto two separate transient absorption bands peaking at 542 and 630 nm, which can be attributed to AQC À and the cation radicalo ft he Ru complex, respectively.M ore importantly,t he transienta bsorption band attributed to AQC À in acetonitrile/water (2:3, v/v) wass hiftedt ot he red region of the spectra (542 nm) compared with that in pure acetonitrile (530 nm;F igure 3). [35,52] In addition, as the solvent medium is changed from acetonitrilet oa cetonitrile/water (2:3, v/v), the dielectric constanto ft he medium increases from 35.94 to 55.87, [53] which,i nt urn, increases the driving force for intramolecular electront ransfer from the Ru centert ot he AQ center in the RuQ complex. This observation clearly suggests that water plays am ajor role in both ET and PCET reactions in the present investigation.…”

Conjugated Main Chain Azo‐Polymers Based on Polycyclic Aromatic Hydrocarbons

“…At the potential of −0.7 V, there are clear bands near 400 and 500 nm and a weak shoulder at ≈550 nm. The latter is a characteristic band attributed to radical anion . However, the shoulder disappears upon further reduction and the bands near 400 and 500 nm continue to develop, which is an indication of a very short‐lived radical cation.…”