Photoelectron spectra studies of organic free radicals. The nitroxide radical

The photoelectron spectra of the nitroxide radicals, di-tert-butylnitroxide (DTBN) and 2,2,6,6-tetramethyl-piperidine-N-oxyl, have been studied and molecular orbital calculations made. The adiabatic first ionization potentials were found to be 6.77 and 6.73 eV for these two nitroxide radicals respectively. Four vertical ionization potentials which are common to each nitroxide radical were attributed to ionization of the odd electron in the NO anti-bonding π orbital, oxygen lone pair electrons and NO bonding π electrons. Doublet splitting of the lone pair electron peak with different peak intensities can be quantitatively understood in terms of triplet and singlet states of the photoionized nitroxide cation.     

Synthesis of block copolymers consisting of liquid crystalline and amorphous segments by living/controlled radical polymerization

Living polymerization is most often observed in systems where the growing species are ions. In such systems the chain ends do not react to each other due to elestrostactic repulsion, but only to monomers allowing, this way, the control in structure of the formed polymer. Free radicals, which are the growing species in the radical polymerization, easely undergo combination and prevent a living radical polymerization. Thus, a great challenge to polymer science was in meeting a system that offered to the radical polymerization a radical stabillization alike in ionic polymerizations. At the same time, the radicals should undergo rapid propagation and should not be able to initiate new chains, in a controlled reaction. Some succesfull techniques of living/controlled radical polymerization, such as stable free radical polymerization (SFRP), mediated by nitroxide, INIFERTER and atom transfer polymerization (ATRP) will be overviewed here, as well as their application to the synthesis of liquid crystalline polymers.     

Impact of open-shell loading on mass transport and doping in conjugated radical polymers

Radical-containing polymers are an evolving class of redox-active macromolecules that have received great interest; however, most reports regarding radical polymers have focused on materials with nonconjugated backbones because their application drivers did not require this conjugation. Conversely, there has been a recent rise in the development of radical polymers for next-generation applications where imparting conjugation to the backbone of the radical polymer could be of significant benefit. To this end, we designed and synthesized a series of 3,4-propylenedioxythiophene (ProDOT)-based polymers bearing nitroxide radical pendent groups via direct arylation polymerization. Specifically, we present four radical polymers with open-shell loadings ranging from 24% to 82% of the total number of repeat units per polymer chain. The impact of open-shell loading on the electrochemical behaviors of these polymers in different electrolytes was then established using cyclic voltammetry, spectroelectrochemical analyses, and electrochemical quartz-crystal microbalance with dissipation monitoring. We demonstrate that incorporating the open-shell moieties in the ProDOT-based polymers lowers the oxidation onset potential of the conjugated backbone and increases the solvent and ion uptake significantly. Thus, this effort provides a clear picture of the mass transfer and doping mechanism of the ProDOT-based radical polymers to aid in guiding their future design.     

Quantifying internal charge transfer and mixed ion-electron transfer in conjugated radical polymers

Macromolecular radicals are receiving growing interest as functional materials in energy storage devices and in electronics. With the need for enhanced conductivity, researchers have turned to macromolecular radicals bearing conjugated backbones, but results thus far have yielded conjugated radical polymers that are inferior in comparison to their non-conjugated partners. The emerging explanation is that the radical unit and the conjugated backbone (both being redox active) transfer electrons between each other, essentially “quenching” conductivity or capacity. Here, the internal charge transfer process is quantified using a polythiophene loaded with 0, 25, or 100% nitroxide radicals (2,2,6,6-tetramethyl-1-piperidinyloxy [TEMPO]). Importantly, deconvolution of the cyclic voltammograms shows mixed faradaic and non-faradaic contributions that contribute to the internal charge transfer process. Further, mixed ion-electron transfer is determined for the 100% TEMPO-loaded conjugated radical polymer, from which it is estimated that one triflate anion and one propylene carbone molecule are exchanged for every electron. Although these findings indicate the reason behind their poor conductivity and capacity, they point to how these materials might be used as voltage regulators in the future.

Conjugated radical polymers can exhibit internal electron transfer depending on the radical loading.     

Patterned nitroxide polymer brushes for thin-film cathodes in organic radical batteries

Nitroxide polymer brushes were covalently patterned on flexible conducting substrates via surface-initiated atom transfer radical polymerization and microcontact printing. As a cathode of organic radical batteries, the nitroxide polymer brushes prevent the nitroxide polymer from dissolving into electrolyte solvents, which improves the cycle-life performance of batteries.     

Reactions of psoralen radical cations with biological substrates

The reactions of several psoralen and coumarin radical cations with biological substrates such as nucleotides, amino acids and alkenes that serve as models for unsaturated fatty acids have been examined. The radical cations were generated by laser photoionization of the parent psoralen or coumarin in aqueous buffer in most cases. Easily oxidized substrates such as tyrosine, tryptophan and guanosine monophosphate react with the 8-methoxypsoralen and several methoxy-substituted coumarin radical cations with rate constants in excess of 2 x 10(9) M-1 s-1. In each case reaction occurs via electron transfer, as demonstrated by the observation of quencher-derived radical cations or radicals by transient absorption spectroscopy. For other substrates such as histidine, methionine and adenosine monophosphate the measured rate constants are significantly slower and vary with the oxidation potential of both the parent psoralen or coumarin and the quencher, again indicative of electron transfer reactivity. Most of the alkenes studied also react with the psoralen or coumarin radical cations via electron transfer, although there is some evidence for addition for linoleic acid. Product studies carried out using both lamp and laser irradiation in the presence of deoxyguanosine as a radical cation trap lead to the formation of characteristic base-derived Type-I (electron transfer) products. This lends support to our previous hypothesis that photoionization occurs via a monophotonic process and is thus relevant to conditions used in clinical phototherapeutic applications of psoralens. The results demonstrate the relevance of electron transfer chemistry to the use of psoralens and related compounds as photoactivated drugs.     

Controlled/living heterogeneous radical polymerization in supercritical carbon dioxide

Supercritical carbon dioxide (scCO₂) is an inexpensive and environmentally friendly medium for radical polymerizations. ScCO₂ is suited for heterogeneous controlled/living radical polymerizations (CLRPs), since the monomer, initiator, and control reagents (nitroxide, etc.) are soluble, but the polymer formed is insoluble beyond a critical degree of polymerization (J_crit). The precipitated polymer can continue growing in (only) the particle phase giving living polymer of controlled well‐defined microstructure. The addition of a colloidal stabilizer gives a dispersion polymerization with well‐defined colloidal particles being formed. In recent years, nitroxide‐mediated polymerization (NMP), atom transfer radical polymerization (ATRP), and reversible addition fragmentation chain transfer (RAFT) polymerization have all been conducted as heterogeneous polymerizations in scCO₂. This Highlight reviews this recent body of work, and describes the unique characteristics of scCO₂ that allows composite particle formation of unique morphology to be achieved. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3711–3728, 2009     

Photo-stabilising action of ortho-hydroxy aromatic compounds in polypropylene film: UV absorption versus radical scavenging

The relative importance of uv absorption and radical scavenging in the photo-stabilisation of polypropylene film by various concentrations of 4-methoxy and 4-n-octoxy substituted 2-hydroxybenzophenones and 2(2′-hydroxy-3′-t-butyl-5′-methyl phenyl)-5-chlorobenzotriazole is examined using infra-red and second-order derivative uv absorption spectroscopy and hydroperoxide analysis. Under both photolysis with polychromatic light wavelengths greater than 300 nm and photo-sensitised oxidation with monochromatic light of mainly 365 nm, the additives were considerably more effective photo-stabilisers when present in the polymer than when used as screens. Only under exposure to far uv light of 254 nm wavelength at 0·5 and 1·0% w/w of additives was any evidence of screening important. At 0·01% w/w concentration both the ortho-hydroxybenzophenones sensitised the photo-oxidation of the polymer and this correlated with higher concentration levels of hydroperoxide in the polymer films, confirming the pro-oxidant behaviour of these additives. Enhanced photo-protection by long n-alkyl groups in the 4-position of ortho-hydroxybenzophenones under photo-sensitised oxidation is associated with the lower mobility of the n-alkyl radical and cage effects in the polymer encouraging radical recombination. These conclusions are confirmed by solution photolysis experiments in the absence and presence of cumene hydroperoxide. The results clearly show that whilst reversible proton transfer may be important for protecting the additive itself, a mechanism involving uv absorption is of little or no importance in photo-stabilisation.     

Unusual Internal Electron Transfer in Conjugated Radical Polymers

Nitroxide‐containing organic radical polymers (ORPs) have captured attention for their high power and fast redox kinetics. Yet a major challenge is the polymer's aliphatic backbone, resulting in a low electronic conductivity. Recent attempts that replace the aliphatic backbone with a conjugated one have not met with success. The reason for this is not understood until now. We examine a family of polythiophenes bearing nitroxide radical groups, showing that while both species are electrochemically active, there exists an internal electron transfer mechanism that interferes with stabilization of the polymer's fully oxidized form. This finding directs the future design of conjugated radical polymers in energy storage and electronics, where careful attention to the redox potential of the backbone relative to the organic radical species is needed.     

Photosensitized alkoxyamines as bicomponent radical photoinitiators

The photopolymerization ability of photosensitized alkoxyamines has been investigated. These compounds behave as interesting two‐component photoinitiators. Laser flash photolysis, electron spin resonance, and density functional theory allow to support the interactions encountered between the photosensitizer (benzophenone and isopropylthioxanthone) and the alkoxyamines (C? O bond breaking and hydrogen transfer) and the side reactions of the nitroxide radical with photosensitizer (electron transfer). © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2910–2915, 2010     

Genesis of the CSIRO polymer group and the discovery and significance of nitroxide‐mediated living radical polymerization

The background to the formation of the Commonwealth Scientific and Industrial Research Organization (CSIRO) polymer group is discussed. In particular, the challenges of working with high‐conversion polymerization, as found in commercial systems, and the need to explain variations in polymer properties led to important advances in the theory of radical polymerization and control over both the initiation and termination steps. Studies on the fate of the macromonomer, formed in termination by disproportionation, led to an early form of addition/fragmentation now known as reversible addition–fragmentation chain transfer, whereas detailed studies on initiation pathways using nitroxide trapping led to nitroxide‐mediated living radical polymerization. These studies contributed to the renaissance in free‐radical polymerization studies. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 5748–5764, 2005     

Competitive consecutive electron transfer in determination of ionization potentials: ketene derivatives

Kinetics of competitive consecutive electron transfer was used to determine ionization potentials of transient species. Kinetics of two-stage electron transfer reactions in aprotic solvent was studied using 355 nm laser flash photolysis. The concentrations of transients produced by the laser flash photolysis were monitored by their light absorption. Triplet-excited tetrachloro-p-benzo-quinone (p-chloranil) generated by a 355 nm laser flash oxidized diethyl ketene, diphenyl ketene, or phenyl ethyl ketene to form radical cations. The ketene radical cations, in turn, oxidized tertiary amine, forming ground state ketene and ammonium radical cation. The kinetics of the disappearance of ketene radical cations (and/or appearance of ammonium radical cations) due to consecutive, competitive electron transfer to ketene and p-chloranil radical cations was monitored. By monitoring kinetics in the presence of tertiary amines with different oxidation potentials, it was established that in acetonitrile the oxidation potential of diethyl ketene was 5.4 eV; for phenyl ethyl ketene, it was approximately 4.8 eV; and for diphenyl ketene, it was 4.6 eV. The results were in agreement with the oxidation potentials of ketenes computed using published data.     

Catalytic oxidation of cellulose with a novel amphiphilic nitroxide block copolymer as a recoverable catalyst

A novel amphiphilic block copolymer of poly(ethylene glycol)-b-poly(2,2,6,6-tetramethylpiperidinyloxy-4-yl-methacrylate) was prepared through activators regenerated by electron transfer atom transfer radical polymerization of 2,2,6,6-tetramethylpiperidine methacrylate monomer, followed by oxidizing it with 3-chloroperoxybenzoic acid. This nitroxide block copolymer was used as a recoverable catalyst instead of free 2,2,6,6-tetramethylpiperidine-1-oxyl for selective catalytic oxidation of cellulose. According to its amphiphilic property, a mixture of acetonitrile and water was used as the reaction medium. The resulting carboxyl content of oxidized cellulose reached 1.07 mmol/g, equivalent to 73.2% of free TEMPO, was satisfactory. Furthermore, the block copolymer was easy to recycle and the activity did not decrease to a noticeable level after 4 cycles.     

Novel hyperbranched polymers synthesized via A3 + B(B′) approach by radical addition‐coupling polymerization

In this study, a novel application of radical addition‐coupling polymerization (RACP) for synthesis of hyperbranched polymers is reported. By Cu/PMDETA‐mediated RACP of 2‐methyl‐2‐nitrosopropane with trimethylolpropane tris(2‐bromopropionate) or a bromo‐ended 3‐arm PS macromonomer, two types of hyperbranched polymers with high degree of polymerization are synthesized under mild conditions, respectively. The chemical structures of the hyperbranched polymers are carefully characterized. By selective degradations of the ester groups and weak bonds of NO? C in the polymers, high degree of alternative connection of the two monomers in the synthesized polymers have been identified. Based on the experimental results, mechanism of formation of the hyperbranched polymer is proposed, which includes formation of carbon radicals from the tribromo monomer through single electron transfer, its capture by 2‐methyl‐2‐nitrosopropane that results in nitroxide radical, and cross‐coupling reaction of the nitroxide radical with other carbon radicals. Hyperbranched polymer can be formed in a step‐growth mode after multiple steps of such reactions. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 904–913     

Degradable multisegmented polymers synthesized by consecutive radical addition‐coupling reaction of α,ω‐macrobiradicals and nitroso compound

A consecutive radical addition‐coupling reaction induced by spin‐trapping agent is applied to produce degradable multisegmented polymer using α,ω‐dibromo polymer as a precursor. The macroradical generated by single electron transfer process catalyzed by Cu/PMDETA from α,ω‐dibromo polymer can be efficiently captured by 2‐methyl‐2‐nitrosopropane (MNP), which results in nitroxide radical. The in situ formed nitroxide radical immediately undergoes cross‐coupling reaction with polymeric radical, generating block polymer bridged with alkoxyamine moiety. The consecutive radical addition‐coupling reaction generates multisegmented polymer via step‐growth mechanism. Different multisegmented polymers have been prepared from α,ω‐dibromo‐PS, PtBA, and PtBA‐PS‐PtBA. The block number of multisegmented polymers can be tailored by varying the feed ratio of α,ω‐dibromo precursor to MNP. The multisegmented polymer can be degraded in the presence of hydrogen atom donor or air, and the molecular weight distribution transformed back into shape of its original precursor as it is conjugated by alkoxyamine moieties. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

The flash-quench technique in protein-DNA electron transfer: reduction of the guanine radical by ferrocytochrome c

Electron transfer from a protein to oxidatively damaged DNA, specifically from ferrocytochrome c to the guanine radical, was examined using the flash-quench technique. Ru(phen)2dppz2+ (dppz = dipyridophenazine) was employed as the photosensitive intercalator, and ferricytochrome c (Fe3+ cyt c), as the oxidative quencher. Using transient absorption and time-resolved luminescence spectroscopies, we examined the electron-transfer reactions following photoexcitation of the ruthenium complex in the presence of poly(dA-dT) or poly(dG-dC). The luminescence-quenching titrations of excited Ru(phen)2dppz2+ by Fe3+ cyt c are nearly identical for the two DNA polymers. However, the spectral characteristics of the long-lived transient produced by the quenching depend strongly upon the DNA. For poly(dA-dT), the transient has a spectrum consistent with formation of a [Ru(phen)2dppz3+, Fe2+ cyt c] intermediate, indicating that the system regenerates itself via electron transfer from the protein to the Ru(III) metallointercalator for this polymer. For poly(dG-dC), however, the transient has the characteristics expected for an intermediate of Fe2+ cyt c and the neutral guanine radical. The characteristics of the transient formed with the GC polymer are consistent with rapid oxidation of guanine by the Ru(III) complex, followed by slow electron transfer from Fe2+ cyt c to the guanine radical. These experiments show that electron holes on DNA can be repaired by protein and demonstrate how the flash-quench technique can be used generally in studying electron transfer from proteins to guanine radicals in duplex DNA.     

Electrochemistry of organometallic compounds: I. Cyclic voltammetry of organocobaloximes in aqueous acid solutions

This paper presents a systematic investigation on effects of the nature of the organic axial ligand on the primary electrochemical oxidation steps of organoaquobis(dimethylglyoximato)cobalt(III). Evidence is presented to support a one electron reversible process, yielding a cobalt(III) compound attached to the organic radical. Studies of p-substituted benzyl and phenyl derivatives support further the proposed process. The following step is a pseudo-first order irreversible dissociation of the oxidized species, yielding the trans-Co(DH)₂(H₂O)⁺ and the organic radical that can be further oxidized at the electrode. Linear free energy correlations obtained between E_1/2 and Taft or Hammett parameters, depending on the nature of the organic substituent in axial position, strongly favor that Co-alkyl(aryl) bonding electrons are involved in the electron transfer.     

Synthesis of H‐shaped A3BA3 copolymer by methyl‐2‐nitrosopropane induced single electron transfer nitroxide radical coupling

Amphiphilic H‐shaped [poly(ethylene oxide)]₃‐polystyrene‐[poly(ethylene oxide)]₃(PEO₃‐PS‐PEO₃) copolymer was synthesized by 2‐methyl‐2‐nitrosopropane (MNP) induced single electron transfer nitroxide radical coupling (SETNRC) using PEO₃‐(PS‐Br) as a single precursor. First, the A₃B star‐shaped precursor PEO₃‐(PS‐Br) was synthesized by atom transfer radical polymerization (ATRP) using three‐arm star‐shaped PEO₃‐Br as macro‐initiator. Then, in the presence of Cu(I)Br/Me₆TREN, the bromide group at PS end was sequentially transferred into carbon‐centered radical by single electron transfer and then nitroxide radical by reacting with MNP in mixed solvents of dimethyl sulfoxide (DMSO)/tetrahydrofuran (THF), and in situ generated nitroxide radical could again capture another carbon‐centered radical by fast SETNRC to form target PEO₃‐PS‐PEO₃ copolymer. The MNP induced SETNRC could reach to a high efficiency of 90% within 60 min. After the product PEO₃‐PS‐PEO₃ was cleaved by ascorbic acid, the SEC results showed that there was about 30% fraction of product formed by single electron transfer radical coupling (SETRC) between carbon‐centered radicals. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Oxidation of benzyl alcohol with Fe(III) using polymers containing the nitroxyl radical structure as a mediator

Some polymers containing the nitroxyl radical structure were prepared and applied to the electron transfer agents from Fe(III) to benzyl alcohol. Benzyl alcohol was also oxidized by Fe(III) in two-phase or tri-phase systems using hydrophobic or hydrophilic polymers, which contain 2,2,6,6-tetramethylpiperidine-1-oxyl moiety, as an electron transfer catalyst. Especially, the hydrophilic polymers accelerated the oxidation of benzyl alcohol.     

Spectroscopic detection, reactivity, and acid-base behavior of ring-dimethoxylated phenylethanoic acid radical cations and radical zwitterions in aqueous solution

A product and time-resolved kinetic study of the one-electron oxidation of ring-dimethoxylated phenylethanoic acids has been carried out at different pH values. Oxidation leads to the formation of aromatic radical cations or radical zwitterions depending on pH, and pK(a) values for the corresponding acid-base equilibria have been measured. The radical cations undergo decarboxylation with first-order rate constants (k(dec)) ranging from <10(2) to 5.6 x 10(4) s(-1) depending on radical cation stability. A significant increase in k(dec) (between 10 and 40 times) is observed on going from the radical cations to the corresponding radical zwitterions. The results are discussed in terms of the ease of intramolecular side chain to ring electron transfer required for decarboxylation, in both the radical cations and radical zwitterions.