Semiquinone Radical Research Articles

Long-lasting UV-blocking Mechanism of Lignin: Origin and Stabilization of Semiquinone Radicals.

Semiquinone (SQ) radicals play a critical role in the long-lasting UV-blocking application of lignin, while their origin and stable structure are unclear. Here, the organosolv lignin extracted from poplar (OL-P) is self-assembled into normal micelles (LNM) with more phenolic hydroxyl groups on the surface, and reverse micelles (LRM) with more methoxyl groups on the surface. After 12h UV irradiation, the SQ radical contents in LNM and LRM increase 33% and 78% respectively. The performance of LNM based sunscreen keeps upswinging due to radical stabilization of phenolic hydroxyl groups. LRM based sunscreen experiences a gradual decrease after reaching maximum UV absorbance due to the quick generation and over oxidation of SQ radicals. Density functional theory (DFT) simulations reveal that methoxyl groups in OL-P has bigger bond length and smaller bond dissociation enthalpy than phenolic hydroxyl groups, and are easy to form SQ radicals. The Gibbs free energy (ΔG) needed for SQ-quinone transformation is above 26.10kcalmol.-1 , while that for SQ-hydroquinone transformation is below -66.78kcalmol.-1 . Hydroquinone is the stable structure of SQ radicals. This work discloses the origin and stable structure of SQ radicals in lignin under UV irradiation, and provides an important guidance for its long-lasting UV-blocking application.

Insight into the reaction mechanism of photocatalytic production of solketal

In this work, the behavior of a commercial TiO2 (Evonik P25) and synthesized CeO2 and Bi2WO6 photocatalysts in the photoacetalization of glycerol with acetone to obtain solketal under simulated sunlight was studied. The solids were characterized by X-ray diffraction (XRD), N2 adsorption desorption isotherms, Raman spectroscopy and UV–Vis spectroscopy techniques. The influence of the oxygen content in the reaction atmosphere (1–40% O2) was analyzed. It was concluded that oxygen is essential for the reaction taking place, suggesting the participation of the superoxide radical as an intermediate in the transfer of electrons. To confirm this point, p-benzoquinone, a known superoxide radical scavenger, was used to evaluate the role of O2. However, two opposite effects were observed for p-benzoquinone depending on the amount added to the reaction medium. Thus, for small amounts of p-benzoquinone a negative effect on the yield of solketal was observed, in agreement to its superoxide radical scavenger character. On the contrary, at high concentrations, p-benzoquinone enhanced the solketal yield obtained in the process. A mechanism is proposed by which the semiquinone radical formed acts as an intermediate capable of facilitating the transfer of electrons in the process, thereby improving the overall process. The catalyst showing the lowest catalytic activity was CeO2, whereas Bi2WO6 had a higher reaction rate and activity than TiO2 (P25), with both reaching approximately 78% conversion under optimum conditions. These results are promising because the conversion achieved is comparable to that obtained by acid catalysis, using sunlight as energy source.

Mechanism of Self-Oxidative Copolymerization and its Application with Polydopamine-pyrrole Nano-copolymers.

Currently, the copolymer of dopamine (DA) and pyrrole (PY) via chemical and electrochemical oxidation usually requires additional oxidants, and lacks flexibility in regulating the size and morphology, thereby limiting the broad applications of DA-PY copolymer in biomedicine. Herein, the semiquinone radicals produced by the self-oxidation of DA is ingeniously utilized as the oxidant to initiate the following copolymerization with PY, and a series of quinone-rich polydopamine-pyrrole copolymers (PDAm-nPY) with significantly enhanced absorption in near-infrared (NIR) region without any additional oxidant assistance is obtained. Moreover, the morphology and size of PDAm-nPY can be regulated by changing the concentration of DA and PY, thereby optimizing nanoscale PDA0.05-0.15PY particles (≈ 150nm) with excellent NIR absorption and surface modification activity are successfully synthesized. Such PDA0.05-0.15PY particles show effective photoacoustic (PA) imaging and photothermal therapy (PTT) against 4T1 tumors in vivo. Furthermore, other catechol derivatives canalso copolymerize with PY under the same conditions. This work by fully utilizing the semiquinone radical active intermediates produced through the self-oxidation of DA reduces the dependence on external oxidants in the synthesis of composite materials and predigests the preparation procedure, which provides a novel, simple, and green strategy for the synthesis of other newly catechol-based functional copolymers.

The dermacozines and light: a novel phenazine semiquinone radical based photocatalytic system from the deepest oceanic trench of the Earth.

Dermacozines, the secondary metabolites of the Mariana Trench sediment bacterium Dermacoccus abyssi MT1.1T, were studied using cyclic voltammetry (CV), electron paramagnetic resonance (EPR), furthermore literature and own experimental UV-Vis spectroscopic data. With those measurements, we determined experimentally the positions of the HOMO, which shifts towards more positive potentials, and the constant LUMO on the standard hydrogen electrode scale, while the HOMO-LUMO gap gets deeper, respectively. The HOMO energies of dermacozines experimentally were proven to be water oxidising. EPR spectroscopy demonstrated the formation of semiquinone radicals in the case of dermacozines E and O upon irradiation with visible light corresponding to the absorption maxima (AM) of the chromophores. Our findings suggest that the dermacozines may assist the strain by maintaining redox homeostasis through its respiratory chain.

Interaction of magnesium ions with semiquinone radicals of tiron used as an indicator of reactive oxygen species

Electron paramagnetic resonance spectroscopy (EPR) and quantum chemical calculations based on density functional theory were used to demonstrate that the earlier observed changes in the EPR spectra of Tiron semiquinone radical dissolved in sea water solution occur due to interaction of Mg2+ ions with Tiron radical. This interaction is caused by electrostatic attraction between Mg2+ ions and Tiron radicals, which bears great charges of opposite sign (+2 and -3), on the one hand, and due to the ability of Mg2+ ion to bind to bidentate oxygen-containing ligands efficiently, on the other hand. The formation of tight contact ion pairs leads to electron and spin density redistribution in the Tiron radical, as can been seen by the observed changes in the EPR spectra of the radical.

Colorimetric sensor and visual enzyme sheets for sensitive detection of dimethoate residue in vegetables based on laccase-like activity of coral-like silver citrate

The overutilization of dimethoate may lead to its deposition in both the environment and crops, which potentially induces neurological, respiratory, and hepatic abnormalities upon absorption by organisms. Colorimetry can accurately determine dimethoate, but there are still many challenges in the rapid, convenient, and point-of-care detection. Herein, based on the inhibitory effect of dimethoate on the laccase activity of silver citrate (AgCit), a smartphone-assisted colorimetric sensor and rapid detection enzyme sheets were constructed. It was demonstrated that dimethoate could adsorb on the surface of AgCit and inhibit the production of semiquinone radicals in the reaction system. Then, the AgCit laccase activity was inhibited, causing the colorimetry sensor and the enzyme sheets turned light red or even colorless. The constructed sensor has higher sensitivity and the limit of detection for dimethoate is 1.0 μg⋅L−1. Moreover, the developed enzyme sheets have good selectivity and specificity, and display the detection results within 5 min. Thus, the colorimetric sensor and the constructed enzyme sheets are expected to be used for large-scale preliminary detection of dimethoate residues in actual agricultural products.

Electrochemistry and in-situ spectroelectrochemistry properties of N-, S-substituted-1,4-naphthoquinone compounds (NQ’s)

Quinones are ubiquitous in nature and form one of the largest classes of antitumor agents approved for clinical use. Naphthoquinones and their derivatives are important in many fields such as pharmaceutical, medical, and environmental applications. It is also known that such compounds show strong biological activity against various bacteria, viruses, fungi, and cancer cells. They are known to be efficient in inhibiting cancer cell growth. Under physiological conditions, they can undergo non-enzymatic one-electron reduction to give the moderately toxic species of semiquinone radical-anion. Thus, the electrochemical study of quinones might provide a basic knowledge of semi-quinone radical formation in both in vivo and in vitro under different media.In this study, firstly, novel N-, S- and/or halo(Br/Cl)-substituted-1,4-naphthoquinone derivatives (2a-d, 3e, 4, 6a-f, 7, 8 and 9) were synthesized. The structures of all compounds were elucidated by using spectroscopic methods [FT-IR, NMR (1H/13C), MS, and microanalysis]. Electrochemical characterization indicated that all compounds underwent two well-resolved reduction reactions assigned to NQ/NQ.− and NQ.−/NQ2− processes respectively. Additionally, an all-defined irreversible oxidation wave was also recorded for all compounds. The substituents on the NQ ring altered the peak position and reversibility of the electron transfer reactions. Redox processes considerably influenced the spectra of the compounds which showed different colors and spectra for different electrogenerated anionic and cationic forms of the compounds.

Signal amplification strategy by chitosan-catechol hydrogel modified paper electrode for electrochemical detection of trace arsenite

Arsenic contamination is widespread in water bodies and poses a significant risk to human health, thus developing a rapid and inexpensive in situ method for the detection of arsenic is urgently needed. In this study, a chitosan-catechol carbon paper electrode was fabricated by electrodeposition of chitosan on a conductive carbon paper and subsequential electro-oxidative grafting of catechol. This chitosan-catechol modified electrode demonstrated good performance in detecting trace amounts of arsenic, which is mainly due to the significantly amplification of oxidation signals generated by ferrocene (Fc) on chitosan-catechol hydrogel. The interaction of As(III) with quinone and semiquinone radicals results in the decrease of electronic transmission nodes. The modified electrode exhibited a good detection capability of low concentration As(III) with a detection limit 0.82 ppb by differential pulse voltammetry (DPV). In addition, the chitosan-catechol modified sensor has the merits of high reproducibility, durability and strong resistance to ionic interference. Further successful detection of As(III) in actual rice and soil samples suggests its good potential in the field of in situ detection.

Redox-active polyphenol nanoparticles deprive endogenous glutathione of electrons for ROS generation and tumor chemodynamic therapy

Chemodynamic therapy (CDT) based on generating reactive oxygen species (ROS) is promising for cancer treatment. However, the intrinsic H2O2 is deficient for CDT, and glutathione (GSH) eliminates ROS to protect tumor cells from ROS cytotoxicity. Herein, we propose a strategy to switch the electron flow direction of GSH for O2 reduction and ROS generation rather than ROS clearance by using P(DA-Fc) nanoparticles, which are polymerized from ferrocenecarboxylic acid (Fc) coupled dopamine. P(DA-Fc) NPs with phenol-quinone conversion ability mimic NOX enzyme to deprive electrons from GSH to reduce O2 for H2O2 generation; the following •OH release can be triggered by Fc. Semiquinone radicals in P(DA-Fc) are significantly enhanced after GSH treatment, further demonstrated with strong single-electron reduction ability by calculation. In vitro and in vivo experiments indicate that P(DA-Fc) can consume intrinsic GSH to produce endogenous ROS; ROS generation strongly depends on GSH/pH level and eventually causes tumor cell death. Our work makes the first attempt to reverse the function of GSH from ROS scavenger to ROS producer, explores new roles of PDA-based nanomaterials in CDT beyond photothermal reagents and drug carriers, and provides a new strategy to improve the efficiency of CDT. Statement of SignificanceP(DA-Fc) nanoparticles performing tumor microenvironment response capacity and tumor reductive power utilize ability were fabricated for CDT tumor suppression. After endocytosis by tumor cells, P(DA-Fc) deprived GSH of electrons for H2O2 and •OH release, mimicking the intrinsic ROS production conducted by NADPH, further inducing tumor cell necrosis and apoptosis. Our work makes the first attempt to reverse the function of GSH from ROS scavenger to producer, explores new functions of PDA-based nanomaterials in CDT beyond photothermal reagents and drug carriers, and provides a new strategy to improve CDT efficiency.

Is Hydrogen Peroxide Generated in Infusions of Medicinal Herbs?

Hydrogen peroxide is an important intra- and inter-cellular messenger. It has been reported that beverages such as tea and coffee generate H2O2. The aim of this study was to examine whether H2O2 is formed in infusions of medicinal herbs and to explain the mechanism of its formation. Infusions of sixteen herbs were studied and the presence of H2O2 was found in all cases. The highest concentrations of H2O2, exceeding 50 µM, were found in the infusions of leaves of Betula and Polygonum, as well as of the inflorescence of Tilia. Considerable amounts of diverse phenolic compounds were detected in the infusions. Two successive one-electron oxidations of phenolics are the main source of H2O2 in these infusions; as expected, the generation of semiquinone and superoxide radicals was confirmed. The infusions of medicinal herbs were also able to scavenge H2O2. The herbal infusions were cytotoxic to human ovary cancer SKOV-3 and PEO1 cells; this cytotoxicity was compromised by catalase added to the growth medium, demonstrating the contribution of H2O2 to the cytotoxic action of herbal extracts.

Encapsulation Alleviates the Auto-browning of Epigallocatechin-3-gallate in Aqueous Solutions through Regulating Molecular Self-Aggregation Behavior.

Catechins are widely recognized for superb antioxidant capability, but their application as food antioxidants is hindered by susceptibility to auto-browning under high-moisture conditions. Here, we proposed a strategy of ordered encapsulation with cyclodextrin-based metal-organic frameworks (CD-MOFs) to alleviate the auto-browning phenomenon of catechins while preserving their antioxidant capability and demonstrated the feasibility of this strategy via selecting epigallocatechin-3-gallate (EGCG) as a model. Even in aqueous solutions, EGCG@CD-MOFs still possessed delayed browning, in contrast with pristine EGCG, characterized by suppressed efficiencies on the generation of oxidative dimers (theasinensin A) and semiquinone radicals. Mechanism insights revealed that ordered encapsulation brought dual regulations on the self-aggregation behavior of EGCG: EGCG@CD-MOFs exhibited a gradual structural collapse from the framework toward irregular aggregates as O-K bonds broke progressively, which restricted molecular mobility of EGCG, and EGCG molecular conformations became constrained by the structure of EGCG@CD-MOFs as well as rich intermolecular forces, even after structural collapse.

The photocatalytic mineralization of phenolic wastewater via self-generation and -activation of H2O2 technology

Achieving efficient photocatalytic degradation and mineralization of phenolic pollutants under mild conditions is the key to solving the environmental crisis. Herein, a novel self-generation and -activation of H2O2 technology is achieved on graphitic carbon nitride (GCN)/anthraquinone-based covalent organic frameworks (TD-COF) heterojunction photocatalysts (CNTD-X). Some of the photogenerated electrons (e-) that migrate directionally to the surface of TD-COF could generate H2O2via oxygen reduction reaction (ORR) route, while the e- gathered around anthraquinone can reduce it to stable semi-quinone radicals, which can further release hydroxyl radical (·OH) through electron transfer with H2O2. The photogenerated holes (h+) gathered on the surface of GCN would provide important raw materials for ORR reaction through oxidation of water, and participate in the mineralization process of pollutants. With the ring opening of·OH and the oxidation of h+, this heterojunction system can achieve efficient phenolic pollutants removal. The optimal sample of CNTD-0.02 possesses an incredible phenol removal (99.4%) within 12 min under visible light irradiation (λ > 420 nm), and total organic carbon (TOC) analysis shows 81% phenol is converted into CO2 and H2O. The photo-degradation rate of CNTD-0.02 is about 204.2 and 26.8 times that of GCN and TD-COF, respectively. This novel photocatalytic strategy over CNTD-X could also be applied in other phenolic pollutants removal, and our work paves a new way for environmental remediation in clean and efficient technology.

Utilizing coordination chemistry through formation of a CuII-quinalizarin complex to manipulate cell biology: An in vitro, in silico approach

Quinalizarin, an analogue of anthracycline anticancer agents, is an anticancer agent itself. A CuII complex was prepared and characterized by elemental analysis, UV-Vis & IR spectroscopy, mass spectrometry, EPR and DFT. The intention behind the preparation of the complex was to increase cellular uptake, compare its binding with DNA against that of quinalizarin, modulation of semiquinone formation, realization of human DNA topoisomerase I & human DNA topoisomerase II inhibition and observation of anticancer activity. While the first two attributes of complex formation lead to increased efficacy, decrease in semiquinone generation could results in a compromise with efficacy. Inhibition of human DNA topoisomerase makes up this envisaged compromise in free radical activity since the complex shows remarkable ability to disrupt activities of human DNA topoisomerase I and II. The complex unlike quinalizarin, does not catalyze flow of electrons from NADH to O2 to the extent known for quinalizarin. Hence, decrease in semiquinone or superoxide radical anion could make modified quinalizarin [as CuII complex] less efficient in free radical pathway. However, it would be less cardiotoxic and that would be advantageous to qualify it as a better anticancer agent. Although binding to calf thymus DNA was comparable to quinalizarin, it was weaker than anthracyclines. Low cost of quinalizarin could justify consideration as a substitute for anthracyclines but the study revealed IC50 of quinalizarin/CuII-quinalizarin was much higher than anthracyclines or their complexes. Even then, there is a possibility that CuII-quinalizarin could be an improved and less costly form of quinalizarin as anticancer agent.

Degradation of Sulfamethoxazole by Manganese(IV) Oxide in the Presence of Humic Acid: Role of Stabilized Semiquinone Radicals.

In this work, we demonstrate for the first time the abatement of sulfamethoxazole (SMX) induced by stabilized ortho-semiquinone radicals (o-SQ•-) in the MnO2-mediated system in the presence of humic acid. To evaluate the performance of different MnO2/mediator systems, 16 mediators are examined for their effects on MnO2 reactions with SMX. The key role of the bidentate Mn(II)-o-SQ• complex and MnO2 surface in stabilizing SQ•- is revealed. To illustrate the formation of the Mn(II)-o-SQ• complex, electron spin resonance, cyclic voltammetry, and mass spectra were used. To demonstrate the presence of o-SQ• on the MnO2 surface, EDTA was used to quench Mn(II)-o-SQ•. The high stability of o-SQ•- on the MnO2 surface is attributed to the higher potential of o-SQ•- (0.9643 V) than the MnO2 surface (0.8598 V) at pH 7.0. The SMX removal rate constant by different stabilized o-SQ• at pH 7.0 ranges from 0.0098 to 0.2252 min-1. The favorable model is the rate constant ln (kobs,7.0) = 6.002EHOMO(o-Qred) + 33.744(ELUMO(o-Q) - EHOMO(o-Qred)) - 32.800, whose parameters represent the generation and reactivity of o-SQ•, respectively. Moreover, aniline and cystine are competitive substrates for SMX in coupling o-SQ•-. Due to the abundance of humic constituents in aquatic environments, this finding sheds light on the low-oxidant-demand, low-carbon, and highly selective removal of sulfonamide antibiotics.

Sulfurization alters phenol-formaldehyde resin microplastics redox property and their efficiency in mediating arsenite oxidation

Microplastics weathering by various types of oxidants in the oxic environment and their interaction with environmental contaminants have drawn numerous scientific attention. However, the environmental fate of microplastics under a reducing environment has been largely unresolved. Herein, the change of physicochemical and redox properties of microplastics during the weathering under a sulfate-reducing environment and the interaction with arsenite were addressed. The sulfurization of phenol-formaldehyde resin microplastics under a sulfate-reducing environment generated smooth and porous particles with the induction of organic S species. Multiple spectroscopic results demonstrated thioether and thiophene groups formed by the substitute removal of O-containing functional groups. Moreover, the sulfurization process induced the reduction of carbonyl groups and oxidation of phenolic hydroxyl groups and resulted in the formation of semiquinone radicals. The O-containing functional groups contributed to microplastics redox property and As(III) oxidation while S-containing functional groups showed no obvious effect. The sulfurized microplastics had lower efficiency in mediating arsenite oxidation than the unsulfurized counterparts due to the decreased electron donating capacity. Producing hydrogen peroxides by electron-donating phenol groups and semiquinone radicals and the direct semiquinone radicals oxidation could mediate arsenite oxidation. The findings of this study help us understand the fate of microplastics in redox fluctuation interfaces.

Insight into synthesis and catalytic performance of mesoporous electroactive biochar for aqueous sulfide adsorptive oxidation

Pristine biochar (PBC) is flourishing in adsorptive oxidation for pollutants remediation as an alternative to carbon adsorbents derived from coal/petroleum carbon sources, multistep, and hash synthesis. However, it is widely overlooked for aqueous S2- remediation because the rational design of biochar with proper conductive and porous structure is challenging. Cobalt or iron ion was impregnated in biomass to produce biochar with different porous and conductive structures and explore S2- adsorptive oxidation mechanism. Finding suggests cobalt and iron ions facilitate biomass decomposition to remove volatile matter, which transforms them into elemental cobalt (Co0) or magnetite (Fe3O4) on biochars (BC-Co) or (BC-Fe) and improves the porous structure. BC-Co benefitted from superior mesopores to adsorb S2- 2.4 and 35 times faster than PBC and BC-Fe, achieving 811 ± 146 mg-S2-/g maximum capacity. BC-Fe removed S2- far less than PBC because Fe3O4 on its surface blocked S2- diffusion channel to the active site. It is postulated that S2- was removed from water via external and internal diffusion and adsorption and oxidation at active sites. S2- diffuses from bulk liquid to film near the biochar pore and into the pores via pore and surface diffusion to the active site, where it is adsorbed and oxidized to elemental sulfur or sulfate by quinone, semiquinone, or hydroxyl radicals at the wall of biochars. This study has documented fundamental information for developing mesoporous electroactive biochar for contaminant removal.

Photoenhanced Radical Formation in Aqueous Mixtures of Levoglucosan and Benzoquinone: Implications to Photochemical Aging of Biomass-Burning Organic Aerosols.

The photochemical aging of biomass-burning organic aerosols (BBOAs) by exposure to sunlight changes the chemical composition over its atmospheric lifetime, affecting the toxicological and climate-relevant properties of BBOA particles. This study used electron paramagnetic resonance (EPR) spectroscopy with a spin-trapping agent, 5-tert-butoxycarbonyl-5-methyl-1-pyrroline-N-oxide (BMPO), high-resolution mass spectrometry, and kinetic modeling to study the photosensitized formation of reactive oxygen species (ROS) and free radicals in mixtures of benzoquinone and levoglucosan, known BBOA tracer molecules. EPR analysis of irradiated benzoquinone solutions showed dominant formation of hydroxyl radicals (•OH), which are known products of reaction of triplet-state benzoquinone with water, also yielding semiquinone radicals. In addition, hydrogen radicals (H•) were also observed, which were not detected in previous studies. They were most likely generated by photochemical decomposition of semiquinone radicals. The irradiation of mixtures of benzoquinone and levoglucosan led to substantial formation of carbon- and oxygen-centered organic radicals, which became prominent in mixtures with a higher fraction of levoglucosan. High-resolution mass spectrometry permitted direct observation of BMPO-radical adducts and demonstrated the formation of •OH, semiquinone radicals, and organic radicals derived from oxidation of benzoquinone and levoglucosan. Mass spectrometry also detected superoxide radical adducts (BMPO-OOH) that did not appear in the EPR spectra. Kinetic modeling of the processes in the irradiated mixtures successfully reproduced the time evolution of the observed formation of the BMPO adducts of •OH and H• observed with EPR. The model was then applied to describe photochemical processes that would occur in mixtures of benzoquinone and levoglucosan in the absence of BMPO, predicting the generation of HO2• due to the reaction of H• with dissolved oxygen. These results imply that photoirradiation of aerosols containing photosensitizers induces ROS formation and secondary radical chemistry to drive photochemical aging of BBOA in the atmosphere.

Electrochemical oxidation of gallic acid: A reexamination of the reaction mechanism in aqueous medium

A clear understanding of redox mechanism of antioxidants, like gallic acid (GA) is important to decipher their radical scavenging functioning in the biochemical environment of the body. Herein, electrooxidation process of GA is reexamined in an aqueous medium through a combination of cyclic voltammetric and spectroelectrochemical studies on gold electrode in wide pH and concentration ranges. These studies reveal redox process of GA as an irreversible two electrons process, resulting in the formation of ortho-quinone, where the second electron transfer is the rate limiting step. The redox process of GA at different pH is presented in the form of a bicubic scheme and the associated thermodynamic parameters are analyzed using Laviron's theory. It allows for the first time the determination of the acidity constant of the corresponding ortho-quinone (pKa = 5.33). The number of exchanged electrons and protons depends on the pH conditions, such that (2e−, 2H+) are exchanged at pH < 4.24 and pH > 5.33, while (2e−, 1H+) are exchanged at 4.24 ≤ pH ≤ 5.33. A careful analysis using the theoretical treatment proposed by Amatore and Saveant for an ECE-DISP mechanism demonstrates the existence of a DISP2 mechanism leading to the formation of ortho-quinone through disproportionation of the semiquinone radicals, made possible thanks to their stabilization.

Polydopamine as a Visible‐Light Photosensitiser for Photoinitiated Polymerisation

AbstractPolydopamine (PDA) is a synthetic model for melanin and has a wide range of opto‐electronic properties that underpin its utility in applied and biological settings, from broadband light absorbance to possessing stable free radical species. Here, we show that PDA free radicals are photo‐responsive under visible light irradiation, enabling PDA to serve as a photo‐redox catalyst. Steady‐state and transient electron spin resonance spectroscopy reveals a reversible amplification in semiquinone radical population within PDA under visible light. This photo‐response modifies the redox potential of PDA and supports sensitisation of exogenous species via photoinduced electron transfer (PET). We demonstrate the utility of this discovery by employing PDA nanoparticles to photosensitise a common diaryliodonium photoinitiator and initiate free‐radical polymerisation (FRP) of vinylic monomers. In situ 1H nuclear magnetic resonance spectroscopy reveals an interplay between PDA‐driven photosensitising and radical quenching during FRP under blue, green, and red light. This work provides crucial insights into the photoactive free radical properties of melanin‐like materials and reveals a promising new application for polydopamine as a photosensitiser.

Stacks of Equidistant 5,6-Dichloro-2,3-dicyanosemiquinone (DDQ) Radicals under Variable-Temperature and High-Pressure Conditions

The effects of temperature (80–400 K) and pressure (up to 4.67 GPa) variations on stacks of equidistant semiquinone radicals were studied by single-crystal X-ray diffraction using a salt of 2-amino-5-methyl-N-methylpyridinium (MMA) and 5,6-dichloro-2,3-dicyanosemiquinone radical anion (DDQ). A temperature-driven phase transformation occurs in the temperature range 150–240 K. The nature of stacking changes upon cooling: equidistant radicals in the room-temperature phase are re-arranged into two-electron/multicenter (2 e/mc) bonded dimers of radicals at low temperature. The phase transition proceeds via a pair-exchange dynamic mechanism. By application of pressure, the interplanar separation in the stacks is reduced by 0.37 Å, from 3.25 Å at ambient pressure to 2.88 Å at 4.67(3) GPa. This may indicate that the covalent component of the interaction considerably increases, consistent with previously performed density functional theory (DFT) calculations.