Anthraquinone Radical Anion Research Articles

Anthraquinones as organophotoredox catalysts for atom transfer radical polymerization via sequential absorption of visible light

AbstractAnthraquinones have been used as the organophotoredox catalysts to catalyze metal‐free atom transfer radical polymerization (ATRP) under visible light irradiation. Polymers with different molecular weight and molecular weight distribution were obtained under different reaction conditions. The “living”/controlled nature of the polymerization was confirmed by the kinetic study, light on/off experiment and the block copolymer synthesis. The anthraquinones are found to catalyze ATRP via redox mechanism via sequential absorption of visible light. Evidence supported by control experiments showed that the excited anthraquinone radical anion or semiquinone anion generated by photo‐induced single electron reduction or subsequent protonation and further reduction were responsible for the initiation/activation of the polymerization.

Operando benchtop NMR reveals reaction intermediates and crossover in redox flow batteries

Redox flow batteries (RFBs) provide a promising battery technology for grid-scale energy storage. High-field operando NMR analyses of RFBs have yielded useful insight into their working mechanisms and helped improve battery performance. Nevertheless, the high cost and large footprint of a high-field NMR system limit its implementation by a wider electrochemistry community. Here, we demonstrate an operando NMR study of an anthraquinone/ferrocyanide-based RFB on a low-cost and compact 43 MHz benchtop system. The chemical shifts induced by bulk magnetic susceptibility effects differ remarkably from those obtained in high-field NMR experiments, due to the different orientations of the sample relative to the external magnetic field. We apply Evans method to estimate the concentrations of paramagnetic anthraquinone radical and ferricyanide anions. The degradation of 2,6-dihydroxy-anthraquinone (DHAQ) to 2,6-dihydroxy-anthrone and 2,6-dihydroxy-anthranol has been quantified. We further identified the impurities commonly present in the DHAQ solution to be acetone, methanol and formamide. The crossover of DHAQ and impurity molecules through the sseparation Nafion® membrane was captured and quantified, and a negative correlation between the molecular size and crossover rate was established. We show that a benchtop NMR system has sufficient spectral and temporal resolution and sensitivity for the operando study of RFBs, and anticipate a broad application of operando benchtop NMR methods for studying flow electrochemistry targeted for different applications.

Effects of hydrogen bonding on intramolecular/intermolecular proton-coupled electron transfer using a Ruthenium-anthraquinone dyad in ultrafast time domain

A ruthenium-anthraquinone dyad consisting of the bi-imidazole spacer in which one imidazole proton acts as a hydrogen bonding agent is employed to investigate the effects of H-bonding on both photo-induced electron transfer (PET) and proton-coupled electron transfer process (PCET). The H-bonding of the anthraquinone radical anion (AQ•-) with a bi-imidazole spacer in complex 1 generated by the electron transfer reduction of complex 1 is indicated by the symbolic anodic shift of the reduction potential of anthraquinone. The present investigation has established that the arrangement of an intramolecular H-bond between the attached AQ and the imidazole proton consequences in a notable increase in the rate of intramolecular electron transfer from Ru(II) to AQ. Further, we have also demonstrated the ultrafast electron transfer and PCET dynamics in presence of water. We have successfully detected the transient absorption photoproduct Ru(III) by which we could determine the time constants of each step which is rare in literature. Most importantly, our study demonstrates electron transfer and intra-molecular PCET dynamics in the femtosecond time domain. The results of the steady-state and transient absorption spectroscopic studies suggest that bi-imidazole spacer and water molecules have a significant role in both PET and PCET dynamics as a proton source in the excited state. We have demonstrated PCET reaction in ultrafast time scale using Ru-polypyridyl complex, with significant kinetic isotope effect.

Electrocatalytic Depolymerization of Self-Immolative Poly(Dithiothreitol) Derivatives.

We report the use of electrogenerated anthraquinone radical anion (AQ•−) to trigger fast catalytic depolymerization of polymers derived from poly(dithiothreitol) (pDTT)—a self-immolative polymer (SIP) with a backbone of dithiothreitols connected with disulfide bonds and end-capped via disulfide bonds to pyridyl groups. The pDTT derivatives studied include polymers with simple thiohexyl end-caps or modified with AQ or methyl groups by Steglich esterification. All polymers were shown to be depolymerized using catalytic amounts of electrons delivered by AQ•−. For pDTT, as little as 0.2 electrons per polymer chain was needed to achieve complete depolymerization. We hypothesize that the reaction proceeds with AQ•− as an electron carrier (either molecularly or as a pendant group), which transfers an electron to a disulfide bond in the polymer in a dissociative manner, generating a thiyl radical and a thiolate. The rapid and catalytic depolymerization is driven by thiyl radicals attacking other disulfide bonds internally or between pDTT chains in a chain reaction. Electrochemical triggering works as a general method for initiating depolymerization of pDTT derivatives and may likely also be used for depolymerization of other disulfide polymers.

Catalytic electron drives host-guest recognition.

Electron injection is demonstrated to trigger electrocatalytic chain reactions capable of releasing a solvent molecule and forming a redox active guest molecule. One-electron reduction of a hydroxy anthrone derivative (AQH–CH2CN) results in the formation of an anthraquinone radical anion (AQ˙−) and acetonitrile (CH3CN). The resulting fragment of AQ˙− exhibits high stability under mild reducing conditions, and it has enough reducing power to reduce the reactant of AQH–CH2CN. Hence, subsequent electron transfer from AQ˙− to AQH–CH2CN yields the secondary AQ˙− and CH3CN, while the initial AQ˙− is subsequently oxidized to AQ. Overall, the reactants of AQH–CH2CN are completely converted into AQ and CH3CN in sustainable electrocatalytic chain reactions. These electrocatalytic chain reactions are mild and sustainable, successfully achieving catalytic electron-triggered charge-transfer (CT) complex formation. Reactant AQH–CH2CN is non-planar, making it unsuitable for CT interaction with an electron donor host compound (UHAnt2) bearing parallel anthracene tweezers. However, conversion of AQH–CH2CN to planar electron acceptor AQ by the electrocatalytic chain reactions turns on CT interaction, generating a host CT complex with UHAnt2 (AQ ⊂ UHAnt2). Therefore, sustainable electrocatalytic chain reactions can control CT interactions using only a catalytic amount of electrons, ultimately affording a one-electron switch associated with catalytic electron-triggered turn-on molecular recognition.

Multiresponsive Microgels with Phase-Separated Nanodomains and Self-Regulating Properties via Incorporation of Anthraquinone Moieties.

Of the multitude of stimuli-responsive microgels, it is still a challenge to achieve multiple responsivenesses to one single stimulus, which can even revert to the corresponding original state autonomously after stimulus. In this work, we reported a series of anthraquinone functionalized microgels (PNI-xVAQ) with thermosensitivity and redox-actuated self-regulating color, size, and fluorescent properties, which were easily synthesized via surfactant-free emulsion copolymerization (SFEP) with N-isopropylacrylamide (NIPAm) as the monomer, 2-vinylanthraquinone (VAQ) as the comonomer, and N,N'-methylenebis(acrylamide) (BIS) as the cross-linker in an aqueous solution at 70 °C. The hydrophobic interactions of comonomer VAQ also led to the formation of internal phase-separated hydrophobic nanodomains in the obtained PNI-xVAQ microgels. The self-regulating color, size, and fluorescence changes of the PNI-xVAQ microgels were reliant on the nonequilibrium redox process of anthraquinone moieties by the addition of sodium dithionite as the chemical fuel to activate the positive feedback that was the reduction of anthraquinone to transient anthraquinone radical anions, following the slow oxidation of anthraquinone radical anions by autonomous "breathing" oxygen in air as the delayed negative feedback. These autonomous self-regulating properties of the PNI-xVAQ microgel were recyclable to a certain extent by repeated feeding of sodium dithionite.

Anthraquinones as Photoredox Catalysts for the Reductive Activation of Aryl Halides

Quinones are ubiquitous in nature as structural motifs in natural products and redox mediators in biological electron‐transfer processes. Although their oxidation properties have already been used widely in chemical and photochemical reactions, the applications of quinones in reductive photoredox catalysis are less explored. We report the visible‐light photoreduction of aryl halides (Ar–X; X = Cl, Br, I) by 1,8‐dihydroxyanthraquinone. The resulting aryl radical anions fragment into halide anions and aryl radicals, which react through hydrogen abstraction or C–C bond‐forming reactions. The active photocatalyst is generated from 1,8‐dihydroxyanthraquinone by photoinduced single‐electron reduction to the radical anion or subsequent protonation and further reduction (or vice versa) to the semiquinone anion. Subsequent visible‐light excitation of the anthraquinone radical anion or semiquinone anion converts them into very potent single‐electron donors. A plausible mechanism for the reaction is proposed on the basis of control experiments and spectroscopic investigations.

Pump‐Pump‐Probe Spectroscopy of a Molecular Triad Monitoring Detrimental Processes for Photoinduced Charge Accumulation

Controlling light‐induced accumulation of electrons or holes is desirable in view of multi‐electron redox chemistry, for example for the formation of solar fuels or for photoredox catalysis in general. Excitation with multiple photons is usually required for electron or hole accumulation, and consequently pump‐pump‐probe spectroscopy becomes a valuable spectroscopic tool. In this work, we excited a triarylamine‐Ru(bpy)32+‐anthraquinone triad (bpy = 2,2′‐bipyridine) with two temporally delayed laser pulses of different color and monitored the resulting photoproducts. Absorption of the first photon by the Ru(bpy)32+ photosensitizer generated a triarylamine radical cation and an anthraquinone radical anion by intramolecular electron transfer. Subsequent selective excitation of either one of these two radical ion species then induced rapid reverse electron transfer to yield the triad in its initial (ground) state. This shows in direct manner that after absorption of a first photon and formation of the primary photoproducts, the absorption of a second photon can lead to unproductive electron transfer events that counteract further charge accumulation. In principle, this problem is avoidable by careful excitation wavelength selection in combination with good molecular design.

Electron Transfer Rate Maxima at Large Donor-Acceptor Distances.

Because of their low mass, electrons can transfer rapidly over long (>15 Å) distances, but usually reaction rates decrease with increasing donor-acceptor distance. We report here on electron transfer rate maxima at donor-acceptor separations of 30.6 Å, observed for thermal electron transfer between an anthraquinone radical anion and a triarylamine radical cation in three homologous series of rigid-rod-like donor-photosensitizer-acceptor triads with p-xylene bridges. Our experimental observations can be explained by a weak distance dependence of electronic donor-acceptor coupling combined with a strong increase of the (outer-sphere) reorganization energy with increasing distance, as predicted by electron transfer theory more than 30 years ago. The observed effect has important consequences for light-to-chemical energy conversion.

Photoinduced charge accumulation by metal ion-coupled electron transfer.

An oligotriarylamine (OTA) unit, a Ru(bpy)3(2+) photosensitizer moiety (Ru), and an anthraquinone (AQ) entity were combined to a molecular dyad (Ru-OTA) and a molecular triad (AQ-Ru-OTA). Pulsed laser excitation at 532 nm led to the formation of charge-separated states of the type Ru(-)-OTA(+) and AQ(-)-Ru-OTA(+) with lifetimes of ≤10 ns and 2.4 μs, respectively, in de-aerated CH3CN at 25 °C. Upon addition of Sc(OTf)3, very long-lived photoproducts were observed. Under steady-state irradiation conditions using a flux of (6.74 ± 0.21) × 10(15) photons per second at 450 nm, the formation of twofold oxidized oligotriarylamine (OTA(2+)) was detected in aerated CH3CN containing 0.02 M Sc(3+), as demonstrated unambiguously by comparison with UV-Vis absorption spectra obtained in the course of chemical oxidation with Cu(2+). Photodriven charge accumulation on the OTA unit of Ru-OTA and AQ-Ru-OTA is possible due to the lowering of the O2 reduction potential caused by the interaction of superoxide with the strong Lewis acid Sc(3+). The presence of the anthraquinone unit in AQ-Ru-OTA accelerates the rate-determining reaction step for charge accumulation by a factor of 10 compared to the Ru-OTA dyad. This is attributed to the formation of Sc(3+)-stabilized anthraquinone radical anion intermediates in the triad. Possible mechanistic pathways leading to charge accumulation are discussed. Photodriven charge accumulation is of key importance for solar fuels because their production will have to rely on multi-electron chemistry rather than single-electron reaction steps. Our study is the first to demonstrate that metal ion-coupled electron transfer (MCET) can be exploited to accumulate charges on a given molecular unit using visible light as an energy input. The approach of using a combination of intra- and intermolecular electron transfer reactions which are enabled by MCET is conceptually novel, and the fundamental insights gained from our study are relevant in the greater context of solar energy conversion.

Interspecific differences in radial oxygen loss from the roots of three Typha species

A comparison was made of the radial oxygen loss (ROL) from the roots of three Typha species, Typha latifolia L., Typha orientalis Presl and Typha angustifolia L., which resemble each other in morphology. ROLs were evaluated in the laboratory for seedlings of T. orientalis and T. angustifolia in order to compare them with the ROL value for T. latifolia obtained in our previous study. Measurements were conducted using the highly oxygen-sensitive anthraquinone radical anion as an oxygen indicator, which enabled us to simulate the natural conditions in which the oxygen released from the root is immediately consumed by the soil. Among the three Typha species, the ROL was the highest in T. angustifolia, followed by T. latifolia and T. orientalis. Illumination significantly enhances the ROL of T. orientalis, and this effect was also observed for T. latifolia in our previous study, whereas it did not affect the ROL of T. angustifolia. These results indicate that ROL differs significantly between species, even among members of the same genus that are similar in morphology.

A Method to Estimate Practical Radial Oxygen Loss of Wetland Plant Roots

The estimation of practical radial oxygen loss (ROL) of wetland plant roots was attempted in this study. We have devised a new method to measure ROL of wetland plant roots. The whole root system was bathed in an anoxic nutrient solution. Oxygen released from the root was removed immediately by introducing oxygen-free nitrogen gas (O2 < 4 nmol L−1) to mimic natural habitats where released oxygen is consumed rapidly due to chemical and biological oxidation processes. Oxygen removed from the root-bathing chamber was simultaneously detected colorimetrically by use of the highly oxygen-sensitive anthraquinone radical anion (AQ·−) in a cell outside the root-bathing chamber, which decolorized by a rapid reaction with oxygen. An emergent macrophyte Typha latifolia L. was incubated, and its ROL was measured by both the new method and one of the conventional methods, the closed chamber/electrode method, by which the ROL of Typha latifolia L. had not yet been measured. The new method succeeded in detecting the ROL, whereas the conventional method was not able to detect oxygen, due to the level being below the detection limit of the oxygen electrode. The oxygen supply via the seedlings of Typha latifolia L. was ca. 10 times higher compared with control measurements without plant. Light illumination significantly enhanced the ROL of Typha latifolia L. (0.33 nmol O2 g−1 root dry weight s−1 under light and 0.18 nmol O2 g−1 root dry weight s−1 in the dark). Theses values fall between those previously reported by the closed chamber/titanium citrate method and the open chamber/electrode method.

Solvent dependence of intramolecular electron transfer in a helical oligoproline assembly.

The helical oligoproline assembly CH3-CO-Pro-Pro-Pro-Pra(Ptzpn)-Pro-Pro-Pra(RuIIb2m2+ -Pro-Pro-Pra(Anq)-Pro-Pro-Pro-NH2, having a spatially ordered array of functional sites protruding from the proline backbone, has been prepared. The 13-residue assembly formed a linear array containing a phenothiazine electron donor, a tris(bipyridine)ruthenium(II) chromophore, and an anthraquinone electron acceptor with the proline II secondary structure as shown by circular dichroism measurements. Following RuII --> b2m metal-to-ligand charge-transfer (MLCT) excitation at 457 nm, electron-transfer quenching occurs, ultimately to give a redox-separated (RS) state containing a phenothiazine (PTZ) radical cation at the Pra(Ptzpn) site and an anthraquinone (ANQ) radical anion at the Pra(Anq) site. The redox-separated state was formed with 33-96% efficiency depending on the solvent, and the transient stored energy varied from -1.46 to -1.71 eV at 22 +/- 2 degrees C. The dominant quenching mechanism is PTZ reductive quenching of the initial RuIII(b2m*-) MLCT excited state which is followed by m*- --> ANQ electron transfer to give the RS state. Back electron transfer is highly exergonic and occurs in the inverted region. The rate constant for back electron transfer is solvent dependent and varies from 5.2 x 10(6) to 7.7 x 10(6) s-1 at 22 +/- 2 degrees C. It is concluded that back electron transfer occurs by direct ANQ*- --> PTZ*+ electron transfer. Based on independently evaluated kinetic parameters, the electron-transfer matrix element is HDA approximately 0.13 cm-1.

Long-distance charge transport in duplex DNA: the phonon-assisted polaron-like hopping mechanism.

An anthraquinone-linked duplex DNA oligomer containing 60 base pairs was synthesized by PCR. The strand complementary to the quinone-containing strand has four isolated GG steps, which serve as traps for a migrating radical cation. Irradiation of the quinone leads to electron transfer from the DNA to the quinone forming the anthraquinone radical anion and a base radical cation. The radical cation migrates through the DNA, causing reaction at GG steps revealed as strand breaks. The efficiency of strand cleavage falls off exponentially with distance from the quinone (slope = -0.02 A(-1)). This finding necessitates reinterpretation of mechanisms proposed for radical cation migration in DNA. We propose that radical cations form self-trapped polarons that migrate by thermally activated hopping.

Mediated Electrocatalysis with Polyanthraquinone-Functionalized Monolayer-Protected Clusters

This paper describes the reduction of 1,1-dinitrocyclohexane by electrogenerated anthraquinone radical anions that are ligated to monolayer-protected gold clusters. Each cluster bears multiple (ca. 18) anthraquinone units. The clusters are prepared by place-exchange of a fraction of the cluster's original octanethiolate ligands with 1-(1,3-dithiapropyl)anthracene-9,10-dione. The electrocatalytic reduction currents are compared to those observed for anthraquinone monomer at an equivalent concentration. The electrocatalytic efficiency is larger for cluster-bound anthraquinone. The results are analyzed by digital simulation and by a discrete mathematical approach so as to determine the rate constant for electron transfer between the substrate and anthraquinone radical anion (cluster-bound and free). The rate constant is slightly larger for the free anthraquinone monomer. The enhancement in catalytic efficiency for cluster-bound anthraquinone arises from the smaller diffusion coefficient of cluster-bound anthraquinone, relative to that of the monomer, resulting in spatial compression of the reaction zone next to the electrode.

Electroactive Three-Dimensional Monolayers: Anthraquinone ω-Functionalized Alkanethiolate-Stabilized Gold Clusters

This paper describes the synthesis and electrochemical reactivity of octanethiolate monolayer-protected Au cluster compounds (MPCs) in which some of the original octanethiolate monolayer has been substituted by place-exchange reactions with 1-(1,3-dithiapropyl)anthracene-9,10-dione. The mixed monolayer MPC can contain as many as 25 anthraquinone sites/cluster, which are all electroactive in rapid, successive one-electron reactions. The voltammetric behavior includes mass-transport-controlled charging of the electrical double layers of the Au cores, which are, in effect, diffusing nanoelectrodes. Mediated electrocatalytic reduction of a gem-dinitrocyclohexane by clusters bearing reduced anthraquinone sites yields electrocatalytic currents larger than those generated using freely diffusing anthraquinone radical anion monomer as mediator.

Study of radical pairs generated by photoreduction of anthraquinone-2,6-disulfonic acid with thymine by Fourier transform electron paramagnetic resonance

Using laser photolysis at 308 nm and FT–EPR, the triplet sensitized electron transfer from thymine to 9,10-anthraquinone-2,6-disulfonate in aqueous solution was studied. The anthraquinone radical anion and the deprotonated thymine-1-yl radical are spin-polarized by the CIDEP triplet mechanism and radical pair mechanism. The structure of the anthraquinone radical anion is strongly influenced by the pH of the solution. In weak acidic solution the radical anion dominates whereas the p K of the radical protonation was determined to be 3.2. The deprotonated thymine-1-yl radical shows two different radical pair polarization patterns which are distinguished by the phase of the polarization. This unusual behavior can be attributed to two different states in the primary radical pair.

Investigation of the Excited-State Dynamics of Radical Ions in the Condensed Phase Using the Picosecond Transient Grating Technique

A study of the dynamics of ground-state recovery of the perylene radical cation (Pe•+), of perylene radical anion (Pe•-), and of anthraquinone radical anion (AQ•-) is reported. In boric acid glass, the excited-state lifetime of Pe•+ is 35 ± 3 ps, while in concentrated sulfuric acid, it is smaller than 15 ps, the time resolution of the experimental setup. The excited-state lifetime of Pe•+, Pe•-, and AQ•- generated by photoinduced intermolecular electron-transfer reaction in MeCN is shorter than 15 ps. In the case of Pe•-, the uncomplete ground-state recovery is ascribed to the occurrence of electron photoejection. The free ion yield in the intermolecular electron-transfer reaction between 9,10-dicyanoanthracene (DCA) and two electron acceptors was measured in a two-pulse experiment, where the second pulse excited the ensuing DCA•-. This excitation has no influence on the magnitude of the free ion yield, indicating a short excited-state lifetime of DCA•-* relative to the time scale of back electron transfer and ionic dissociation. A red emission, ascribed to the fluorescence of protonated Pe, was detected in boric acid glass and sulfuric acid. No fluorescence that could be clearly ascribed to Pe•+* could be observed.

Photochemical energy conversion in a helical oligoproline assembly.

A general method is described for constructing a helical oligoproline assembly having a spatially ordered array of functional sites protruding from a proline-II helix. Three different redox-active carboxylic acids were coupled to the side chain of cis-4-amino-L-proline. These redox modules were incorporated through solid-phase peptide synthesis into a 13-residue helical oligoproline assembly bearing in linear array a phenothiazine electron donor, a tris(bipyridine)ruthenium(II) chromophore, and an anthraquinone electron acceptor. Upon transient 460-nm irradiation in acetonitrile, this peptide triad formed with 53% efficiency an excited state containing a phenothiazine radical cation and an anthraquinone radical anion. This light-induced redox-separated state had a lifetime of 175 ns and stored 1.65 eV of energy.

In situ electrochemical ESR studies of reactive radicals: the reductions of bromo-anthraquinone and methyl viologen

The application of channel electrodes for quantitative in situ electrochemical ESR studies of unstable radicals is discussed with reference to two systems. First, the reduction of 1-bromo-anthraquinone in acetonitrile solution in the presence of laser light of wavelength 488 nm is shown to lead to the loss of bromide and the formation of the anthraquinone radical anion via a photo-ECE process. The kinetics of this reaction as determined by channel electrode ESR studies are found to be in good agreement with those measured by independent methods. Second, the one-electron reduction of the methyl viologen dication in aqueous solution is shown to lead to the formation of viologen radicals which are known to dimerise. However, it is shown that since this reaction is rapid and reversible, measurements of the ESR signal as a function of electrolyte flow rate and current are essentially blind to this reaction. Lower estimates for the dimerisation rate constant are deduced.