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Crossover in non‐aqueous redox flow batteries remains a critical challenge to the cycle stability of these devices. The use of bipolar redox‐active organic materials (ROM) is an emerging strategy for mitigating crossover. Herein we report the first example of a bipolar ROM derived from an isoindoline nitroxide, a ring class which gives a number of advantages over the more commonly employed piperidines, including greater stability and a 200 mV higher oxidation potential. Through facile synthetic transformation, the unsubstituted isoindoline nitroxide was nitrated to give a novel bipolar molecule, 5‐nitro‐1,1,3,3‐tetramethylisoindoline‐2‐yloxyl (NTMIO). This material was investigated electrochemically, revealing a reversible oxidation and quasi‐reversible reduction giving a cell potential of 2.07 V. NTMIO was then assessed as an active material in both a static and flow battery model, where cycling was observed for both oxidative and reductive redox couples for over 70 and 20 cycles respectively. [ABSTRACT FROM AUTHOR]

The radical reactions of dimethylsulfoxide (DMSO) and tetrahydrothiophene-1-oxide (THTO) with reactive oxygen species (ROS) in the presence of a nitroxide radical scavenger have been evaluated both synthetically and in analytical practice. Fenton-mediated generation of oxygen-centred radicals produced several unusual products that reflect the fragmentation and ring-opening radical mechanisms of DMSO and THTO respectively. Addition of pollution-derived ROS to DMSO/THTO nitroxide solutions produced LC-MS detectable amounts of the same products isolated from the larger-scaled Fenton reactions. For air pollution analysis, these results highlight the complexity surrounding DMSO reactivity and fragmentation, and indicate that THTO produces simpler outcomes that should facilitate analysis of the processes involved. [ABSTRACT FROM AUTHOR]

A substrate‐independent and versatile coating platform for (spatially resolved) surface functionalization, based on nitroxide radical coupling (NRC) reactions and the formation of thermo‐labile alkoxyamine functional groups, was introduced. Nitroxide‐decorated poly(glycidyl methacrylate) (PGMA) microspheres, obtained through bioinspired copolymer surface deposition using dopamine and a nitroxide functional dopamine derivative as monomers, were conjugated with small functional groups in a rewritable process. Reversible coding of the nitroxide functional microspheres by NRC and decoding through thermal alkoxyamine fission were monitored and characterized by electron paramagnetic resonance (EPR) spectroscopy and X‐ray photoelectron spectroscopy (XPS). In addition, this nitroxide coating system was exploited in "grafting‐to" polymer surface ligations of poly(methyl methacrylate) (PMMA) and poly(2,2,2‐trifluoroethyl methacrylate) (PTFEMA) in spatially confined areas. Polymer strands terminated with an Irgacure 2959 (2‐hydroxy‐4′‐(2‐hydroxyethoxy)‐2‐methylpropiophenone) photoinitiator were obtained through chain‐transfer polymerization, and subsequently coupled to nitroxide‐immobilized poly(dopamine) (PDA)‐coated silicon substrates by using rapid photoclick NRC reactions. Light‐driven polymer surface coding was visualized by time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) and XPS imaging. Coating with nitroxide: A versatile, bioinspired nitroxide functional coating platform is presented for precise programming of various materials. Nitroxide‐coated polymeric microspheres with rewritable properties were manufactured, exemplified in consecutive nitroxide radical coupling (coding) and thermal alkoxyamine fission (decoding) reaction steps. Photoinduced dual‐polymer‐patterned surfaces were further generated highlighting the broad applicability of our nitroxide coating system. [ABSTRACT FROM AUTHOR]

BODIPY is a highly versatile fluorophore for biological imaging with a tunable fluorescence emission (500-800 nm) that overlaps the optically transparent window for tissue (600-1300 nm). Herein, we describe the synthesis of optically distinct BODIPY-based profluorescent probes bearing meso- and β-substituted isoindoline nitroxides and their corresponding methoxyamine derivatives. These profluorescent nitroxide probes possess strongly suppressed fluorescence, which can be revealed upon reduction or reaction with other radicals. Examination of the pentafluorophenylhydrazine reduction of the prepared probes by using tandem electron paramagnetic resonance (EPR) and fluorescence spectroscopy demonstrated that the asymmetric bis-β-substituted probe 9 ( λem = 603 nm) was reduced the fastest; however, the greatest difference in fluorescence emission between the nitroxide and its reduced hydroxylamine analogue was observed for probe 7 ( λem = 570 nm). The significant difference in fluorescence output between the nitroxides and their corresponding diamagnetic derivatives makes these probes ideal tools for imaging reactive oxygen species in biological systems. [ABSTRACT FROM AUTHOR]

Profluorescent nitroxides (PFNs) have emerged as an important class of imaging agents for monitoring intracellular redox status and levels of oxidative stress. However, the fast reduction of nitroxides upon incubation within cells limits the window of opportunity for detection. By increasing the concentration of nitroxides per fluorophore, their reduction to the corresponding hydroxylamines and the subsequent switch-on of fluorescence can be delayed. Herein the preparation of nitroxide-containing polymers of different chain length coupled to a fluorophore is reported and their reduction with pentafluorophenylhydrazine is examined. The fluorescence switch-on kinetics and radical concentrations are monitored by fluorescence and electron paramagnetic resonance spectroscopy and compared to a conventional PFN bearing a single nitroxide moiety. The polymeric PFNs display significant delays in reduction and fluorescence switch-on and higher turn-on ratios than their single-nitroxide counterparts. The results of this study indicate that polymeric PFNs are a promising architecture for future imaging agents. [ABSTRACT FROM AUTHOR]

Novel profluorescent mono- and bis-isoindoline nitroxides linked to napthalimide and perylene diimide structural cores are described. These nitroxide-fluorophore probes display strongly suppressed fluorescence in comparison to their corresponding non-radical diamagnetic methoxyamine derivatives. The perylene-based probe possessing two isoindoline systems tethered through ethynyl linkages was shown to be the most photostable in solution, demonstrating significantly enhanced longevity over the 9,10-bis(phenylethynyl)anthracene fluorophore used in previous profluorescent nitroxide probes. [ABSTRACT FROM AUTHOR]

Cyclic nitroxide radicals represent promising alternatives to the iodine-based redox mediator commonly used in dye-sensitized solar cells (DSSCs). To date DSSCs with nitroxide-based redox mediators have achieved energy conversion efficiencies of just over 5 % but efficiencies of over 15 % might be achievable, given an appropriate mediator. The efficacy of the mediator depends upon two main factors: it must reversibly undergo one-electron oxidation and it must possess an oxidation potential in a range of 0.600-0.850 V (vs. a standard hydrogen electrode (SHE) in acetonitrile at 25 °C). Herein, we have examined the effect that structural modifications have on the value of the oxidation potential of cyclic nitroxides as well as the reversibility of the oxidation process. These included alterations to the N-containing skeleton (pyrrolidine, piperidine, isoindoline, azaphenalene, etc.), as well as the introduction of different substituents (alkyl-, methoxy-, amino-, carboxy-, etc.) to the ring. Standard oxidation potentials were calculated using high-level ab initio methodology that was demonstrated to be very accurate (with a mean absolute deviation from experimental values of only 16 mV). An optimal value of 1.45 for the electrostatic scaling factor for UAKS radii in acetonitrile solution was obtained. Established trends in the values of oxidation potentials were used to guide molecular design of stable nitroxides with desired ${E{{{\circ}\hfill \atop {\rm ox}\hfill}}}$, and a number of compounds were suggested for potential use as enhanced redox mediators in DSSCs. [ABSTRACT FROM AUTHOR]

Abstract: Isoindoline nitroxides are potentially useful probes for viable biological systems, exhibiting low cytotoxicity, moderate rates of biological reduction and favorable Electron Paramagnetic Resonance (EPR) characteristics. We have evaluated the anionic (5-carboxy-1,1,3,3-tetramethylisoindolin-2-yloxyl; CTMIO), cationic (5-(N,N,N-trimethylammonio)-1,1,3,3-tetramethylisoindolin-2-yloxyl iodide, QATMIO) and neutral (1,1,3,3-tetramethylisoindolin-2-yloxyl; TMIO) nitroxides and their isotopically labeled analogs (2H12- and/or 2H12–15N-labeled) as potential EPR oximetry probes. An active ester analogue of CTMIO, designed to localize intracellularly, and the azaphenalene nitroxide 1,1,3,3-tetramethyl-2,3-dihydro-2-azaphenalen-2-yloxyl (TMAO) were also studied. While the EPR spectra of the unlabeled nitroxides exhibit high sensitivity to O2 concentration, deuteration resulted in a loss of superhyperfine features and a subsequent reduction in O2 sensitivity. Labeling the nitroxides with 15N increased the signal intensity and this may be useful in decreasing the detection limits for in vivo measurements. The active ester nitroxide showed approximately 6% intracellular localization and low cytotoxicity. The EPR spectra of TMAO nitroxide indicated an increased rigidity in the nitroxide ring, due to dibenzo-annulation. [Copyright &y& Elsevier]

Abstract: Degradation of polypropylene (PP) during melt processing was studied using a novel profluorescence technique. The profluorescent nitroxide probe, 1,1,3,3-tetramethyldibenzo[e,g]isoindolin-2-yloxyl (TMDBIO) was added to PP during melt processing to act as a sensor for carbon-centred radicals. Trapping of carbon-centred radicals, formed during degradation of PP, led to an increase in fluorescence emission from TMDBIO adducts. Through analysis of viscosity changes during processing cumulative chain scission degradation was estimated. At processing temperatures of 210°C or below, fluorescence emission from TMDBIO adducts could be correlated with cumulative chain scissions when the number of chain scissions was small. At higher temperatures, a correlation was not observed most probably due to radical-trap instability rather than decomposition of the TMDBIO. Thus, TMDBIO may be used as a profluorescent sensor for degradation of PP during melt processing when the processing temperature is 210°C or below. [Copyright &y& Elsevier]

Abstract: In this study, three profluorescent nitroxides 1,1,3,3-tetramethyldibenzo[e,g]isoindolin-2-yloxyl (TMDBIO), 1,1,3,3-tetramethyl-2,3-dihydro-2-azaphenalene-2-yloxyl (TMAO) and 5-[2-(4-methoxycarbonyl-phenyl)-ethenyl]1,1,3,3-tetramethylisoindoline-2-yloxyl (MeCSTMIO) were tested as probes for radical-mediated damage in polypropylene arising from both UV and thermally initiated sources. These nitroxides possess a very low fluorescence quantum yield due to quenching by the nitroxide group; however, when the free-radical moiety is removed by reaction with alkyl radicals (to give an alkoxyamine), strong fluorescence is observed. The results obtained from this profluorescent nitroxide trapping technique compare favourably with other methods of monitoring degradation, provided the appropriate probe is chosen for the conditions of oxidation, signalling an indication of damage well before other techniques show any response. The technique was also applied to the monitoring of crosslinked polyester coating resins. Differentiation in the oxidative stability of the resins was evident after as little as 200min where other monitoring techniques require up to 300h of accelerated degradation. This highlights the sensitivity of this method as well as demonstrating the scope of this technique to assess polymer stability. [Copyright &y& Elsevier]

Abstract: The novel profluorescent nitroxide, 1,1,3,3-tetramethyldibenzo[e,g]isoindolin-2-yloxyl (TMDBIO) was investigated as a probe for the formation of polymer alkyl radicals during the thermo-oxidative degradation of unstabilised polypropylene. TMDBIO possesses a very low fluorescence quantum yield due to quenching by the nitroxide group; however, when the free-radical moiety is removed by reaction with alkyl radicals (to give an alkoxyamine), strong fluorescence is observed. Using spectrofluorimetry, the reaction of the nitroxide with polymer alkyl radicals during oxidation has been monitored. Significantly, the trapping of polymer alkyl radicals during the “induction period” at 120°C is observed, when it is not possible to detect changes in the polymer using either chemiluminescence or infrared spectroscopy. This highlights the sensitivity of this method and represents the direct observation of free-radical generation in polypropylene in the “induction period”. TMDBIO also successfully stabilises polypropylene under thermo-oxidative conditions, which is consistent with its action as a radical trap. At elevated temperatures (150°C), at the end of the “induction period” when the polymer is extensively degraded, the fluorescence decreases, due to secondary oxidation of the TMDBIO. [Copyright &y& Elsevier]

The utility of profluorescent nitroxides (PFNs) as sensitive probes to detect early stage photodegradation in a cyclic olefin copolymer, TOPAS®, during both natural aging and accelerated aging under laboratory conditions is reported. PFN additives in TOPAS® capture radicals to form fluorescent adducts as the material degrades. The levels of fluorescence detectable from the polymer reflect the degree of free-radical degradation in the material, with the PFN probes delivering enhanced sensitivity over traditional analytical methods for the detection of photodegradation of TOPAS®. The probes are able to highlight polymer degradation occurring within the oxidation "induction" period, where little change can be observed using infrared spectroscopy; however, their efficacy does not extend far beyond this period. The effective probe lifetime however can be significantly extended through the use of common additives such as the UV absorber (Tinuvin P) and a hindered amine stabiliser analogue (1,1,3,3-tetramethylisoindol-2-yloxyl, TMIO). • Profluorescent nitroxides (PFNs) have been shown to be sensitive probes for early stage photodegradation.. • The PFN probes deliver enhanced sensitivity over traditional analytical methods such as infrared spectroscopy. • The PFN probes may provide key early indications of (in)stability of materials during natural exposure. [ABSTRACT FROM AUTHOR]

Abstract: The profluorescent nitroxide, 1,1,3,3-tetramethyldibenzo[e,g]isoindolin-2-yloxyl (TMDBIO) was investigated as a probe for the radical-mediated degradation of stabilised polypropylene. TMDBIO has been previously shown to be a sensitive probe for free-radical degradation during the thermo-oxidation of unstabilised polypropylene. Here we report on the effect that adding hindered phenol or phosphite stabilisers to polypropylene has on the free-radical sensing ability of TMDBIO during thermo-oxidation. In addition, novel dual-functional, hindered phenol containing profluorescent nitroxides, 5-[2-(4-hydroxy-3,5-di-tert-butylphenyl)ethenyl]-1,1,3,3-tetramethylisoindolin-2-yloxyl (HSTMIO) and its derivatives were investigated as probes for the radical-mediated degradation of polypropylene. These dual-functional probes were shown to be efficient stabilisers for polypropylene during thermo-oxidation at 150°C in oxygen and sensors of thermo-oxidation during its early stages, in the so-called “induction period”. [Copyright &y& Elsevier]

We pioneer a versatile surface modification strategy based on mussel-inspired oxidative catecholamine polymerization for the design of nitroxide-containing thin polymer films. A 3,4-dihydroxy-l-phenylalanine (l-DOPA) monomer equipped with a 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-derived oxidation-labile hydroxylamine functional group is employed as a universal coating agent to generate polymer scaffolds with persistent radical character. Various types of materials including silicon, titanium, ceramic alumina, and inert poly(tetrafluoroethylene) (PTFE) were successfully coated with poly(DOPA-TEMPO) thin films in a one-step dip-coating procedure under aerobic, slightly alkaline (pH 8.5) conditions. Steadily growing polymer films (∼1.1 nm h-1) were monitored by ellipsometry, and their thicknesses were critically compared with those obtained from atomic force microscopic cross-sectional profiles. The heterogeneous composition of surface-adherent nitroxide scaffolds examined by X-ray photoelectron spectroscopy was correlated to that examined by in-solution polymer analysis via high-resolution electrospray ionization mass spectrometry, revealing oligomeric structures with up to six repeating units, mainly composed of covalently linked dihydroxyindole along the polymer backbone. Critically, the reversible redox-active character of the nitroxide-containing polymer scaffolds was investigated by cyclic voltammetric measurements, revealing a convenient and facile access route to electrochemically active nitroxide polymer coatings with potential application in electronic devices such as organic radical batteries. [ABSTRACT FROM AUTHOR]

The current study introduces a novel synthetic avenue for the preparation of profluorescent nitroxides via nitrile imine-mediated tetrazole-ene cycloaddition (NITEC). The photoinduced cycloaddition was performed under metal-free, mild conditions allowing the preparation of a library of the nitroxide functionalized pyrazolines and corresponding methoxyamines. High reaction rates and full conversion were observed, with the presence of the nitroxide having no significant impact on the cycloaddition performance. The formed products were investigated with respect to their photophysical properties in order to quantify their "switch on/off" behavior. The fluorescence quenching performance is strongly dependent on the distance between the chromophore and the free radical spin as demonstrated theoretically and experimentally. Highest levels of fluorescence quenching were achieved for pyrazolines with the nitroxide directly fused to the chromophore. Importantly, the pyrazoline profluorescent nitroxides were shown to efficiently act as sensors for redox/radical processes. [ABSTRACT FROM AUTHOR]