Showing papers on "Radical ion published in 2016"
TL;DR: A deeper understanding of the chemical processes of metal-free ATRP is provided that can aid the design of better catalytic systems and elucidates several important common pathways involved in synthetically useful organic reactions catalyzed by photoredox catalysts.
Abstract: Photoinduced metal-free atom transfer radical polymerization (ATRP) of methyl methacrylate was investigated using several phenothiazine derivatives and other related compounds as photoredox catalysts. The experiments show that all selected catalysts can be involved in the activation step, but not all of them participated efficiently in the deactivation step. The redox properties and the stability of radical cations derived from the catalysts were evaluated by cyclic voltammetry. Laser flash photolysis (LFP) was used to determine the lifetime and activity of photoexcited catalysts. Kinetic analysis of the activation reaction according to dissociative electron-transfer (DET) theory suggests that the activation occurs only with an excited state of catalyst. Density functional theory (DFT) calculations revealed the structures and stabilities of the radical cation intermediates as well as the reaction energy profiles of deactivation pathways with different photoredox catalysts. Both experiments and calculation...
347 citations
TL;DR: Effective bond strength considerations are shown to accurately forecast the feasibility of alkoxy radical generation with a given oxidant/base pair.
Abstract: We report a new photocatalytic protocol for the redox-neutral isomerization of cyclic alcohols to linear ketones via C–C bond scission. Mechanistic studies demonstrate that key alkoxy radical intermediates in this reaction are generated via the direct homolytic activation of alcohol O–H bonds in an unusual intramolecular PCET process, wherein the electron travels to a proximal radical cation in concert with proton transfer to a weak Bronsted base. Effective bond strength considerations are shown to accurately forecast the feasibility of alkoxy radical generation with a given oxidant/base pair.
235 citations
TL;DR: The key finding is that oxygen acts as a renewable energy and electron shuttle following photoexcitation of the Cr(III) catalyst, providing insight into how oxygen can participate as a sustainable reagent in photocatalysis.
Abstract: A combined experimental and theoretical investigation aims to elucidate the necessary roles of oxygen in photoredox catalysis of radical cation based Diels–Alder cycloadditions mediated by the first-row transition metal complex [Cr(Ph2phen)3]3+, where Ph2phen = bathophenanthroline. We employ a diverse array of techniques, including catalysis screening, electrochemistry, time-resolved spectroscopy, and computational analyses of reaction thermodynamics. Our key finding is that oxygen acts as a renewable energy and electron shuttle following photoexcitation of the Cr(III) catalyst. First, oxygen quenches the excited Cr3+* complex; this energy transfer process protects the catalyst from decomposition while preserving a synthetically useful 13 μs excited state and produces singlet oxygen. Second, singlet oxygen returns the reduced catalyst to the Cr(III) ground state, forming superoxide. Third, the superoxide species reduces the Diels–Alder cycloadduct radical cation to the final product and reforms oxygen. We...
124 citations
TL;DR: In this article, the cross-dehydrogenative coupling (CDC) reaction of N-aryltetrahydroisoquinolines (THIQ) was investigated under both aerobic and anaerobic conditions.
Abstract: The cross-dehydrogenative coupling (CDC) reaction of N-aryltetrahydroisoquinolines (THIQ) is one of the most exploited photocatalytic transformation and a test reaction for an exceptional variety of catalysts. However, its mechanism remained unclear concerning involved intermediates, reactive pathways of the amine radical cation and the influence of oxygen and the light source. Therefore, nuclear magnetic resonance (NMR), electron spin resonance (ESR) and synthetic methods were combined to provide a comprehensive picture of the reaction mechanism using Ru(bpy)3Cl2 as a photocatalyst under aerobic and anaerobic conditions. The reaction profiles and involved intermediates were monitored and analyzed by NMR spectroscopy. Several intermediates contributing to product formation were identified, the iminium ion, the hydroperoxide and dimer of THIQ, and a new ring opened intermediate, cleaved at the benzylic C–N bond. Mechanistic evidence is given that under anaerobic conditions preferentially the α-amino radica...
118 citations
TL;DR: The routes of solution-plasma-induced nanocarbon formation from hexane, hexadecane, cyclohexane, and benzene are investigated and two different reaction fields provide a reasonable explanation for the fastest synthesis rate observed in the case of benzene.
Abstract: Although solution-plasma processing enables room-temperature synthesis of nanocarbons, the underlying mechanisms are not well understood. We investigated the routes of solution-plasma-induced nanocarbon formation from hexane, hexadecane, cyclohexane, and benzene. The synthesis rate from benzene was the highest. However, the nanocarbons from linear molecules were more crystalline than those from ring molecules. Linear molecules decomposed into shorter olefins, whereas ring molecules were reconstructed in the plasma. In the saturated ring molecules, C-H dissociation proceeded, followed by conversion into unsaturated ring molecules. However, unsaturated ring molecules were directly polymerized through cation radicals, such as benzene radical cation, and were converted into two- and three-ring molecules at the plasma-solution interface. The nanocarbons from linear molecules were synthesized in plasma from small molecules such as C2 under heat; the obtained products were the same as those obtained via pyrolysis synthesis. Conversely, the nanocarbons obtained from ring molecules were directly synthesized through an intermediate, such as benzene radical cation, at the interface between plasma and solution, resulting in the same products as those obtained via polymerization. These two different reaction fields provide a reasonable explanation for the fastest synthesis rate observed in the case of benzene.
78 citations
TL;DR: The single resonances observed in all [n]CPP(2+) strongly suggest the complete delocalization of the charges over the CPPs, and the contribution of biradical character was clarified for [10]- and [12] CPP by VT-NMR experiment and theoretical calculation.
Abstract: Radical cations and dications of [n]cyclo-p-phenylenes ([n]CPPs, n = 5, 6, 10, and 12), which are the models of those of linear oligo-p-phenylenes without a terminus, were synthesized as hexafluoroantimonate salts by the one- and two-electron chemical oxidation of CPP by NOSbF6 or SbF5. The radical cations, [n]CPP•+, and dications, [n]CPP2+, exhibited remarkable bathochromic shifts in their UV–vis–NIR absorption bands, suggesting that [n]CPP•+ and larger [n]CPP2+ exhibit longer polyene character than the shorter analogues. The larger bathochromic shift was consistent with the narrower HOMO–SOMO and HOMO–LUMO gaps in larger [n]CPP•+ and [n]CPP2+, respectively. In [n]CPP•+, the spins and charges were equally and fully delocalized over the p-phenylene rings of the CPPs, as noted by ESR. 1H NMR revealed that the hydrogen of [n]CPP2+ shifted to a high magnetic field from the neutral compounds due to the diamagnetic ring current derived from the in-plane aromaticity of [n]CPP2+. The single resonances observed i...
72 citations
TL;DR: A new catalytic method is described to access carbocation intermediates via the mesolytic cleavage of alkoxyamine radical cations through spontaneous scission, enabling catalytic cation generation in the presence of both acid sensitive and easily oxidized nucleophilic partners.
Abstract: A new catalytic method is described to access carbocation intermediates via the mesolytic cleavage of alkoxyamine radical cations. In this process, electron transfer between an excited state oxidant and a TEMPO-derived alkoxyamine substrate gives rise to a radical cation with a remarkably weak C-O bond. Spontaneous scission results in the formation of the stable nitroxyl radical TEMPO(.) as well as a reactive carbocation intermediate that can be intercepted by a wide range of nucleophiles. Notably, this process occurs under neutral conditions and at comparatively mild potentials, enabling catalytic cation generation in the presence of both acid sensitive and easily oxidized nucleophilic partners.
63 citations
TL;DR: A supramolescular strategy to accelerate the Fenton reaction through the construction of supramolecularly activated radical cations, a promising method for promoting radical reactions and may also open up a new route for the catalytic oxidation of organic pollutants for water purification and widen the realm of supramsolecular catalysis.
Abstract: Tuning the activity of radicals is crucial for radical reactions and radical-based materials Herein, we report a supramolecular strategy to accelerate the Fenton reaction through the construction of supramolecularly activated radical cations As a proof of the concept, cucurbit[7]uril (CB[7]) was introduced, through host-guest interactions, onto each side of a derivative of 1,4-diketopyrrolo[3,4-c]pyrrole (DPP), a model dye for Fenton oxidation The DPP radical cation, the key intermediate in the oxidation process, was activated by the electrostatically negative carbonyl groups of CB[7] The activation induced a drastic decrease in the apparent activation energy and greatly increased the reaction rate This facile supramolecular strategy is a promising method for promoting radical reactions It may also open up a new route for the catalytic oxidation of organic pollutants for water purification and widen the realm of supramolecular catalysis
62 citations
TL;DR: Repairs capacities are strongly sequence-dependent, creating DNA regions with different tendencies of self-repair, and this self-healing activity represents the simplest sequence- dependent DNA repair system.
Abstract: Absorption of UV-radiation in nucleotides initiates a number of photophysical and photochemical processes, which may finally cause DNA damage. One major decay channel of photoexcited DNA leads to reactive charge transfer states. This study shows that these states trigger self-repair of DNA photolesions. The experiments were performed by UV spectroscopy and HPLC on different single and double stranded oligonucleotides containing a cyclobutane pyrimidine dimer (CPD) lesion. In a first experiment we show that photoexcitation of adenine adjacent to a CPD has no influence on this lesion. However, excitation of a guanine (G) adenine (A) sequence leads to reformation of the intact thymine (T) bases. The involvement of two bases for the repair points to a long-living charge transfer state between G and A to be responsible for the repair. The negatively charged A radical anion donates an electron to the CPD, inducing ring splitting and repair. In contrast, a TA sequence, having an inverted charge distribution (T radical anion, A radical cation), is not able to repair the CPD lesion. The investigations show that the presence of an adjacent radical ion is not sufficient for repair. More likely it is the driving power represented by the oxidation potential of the radical ion, which controls the repair. Thus, repair capacities are strongly sequence-dependent, creating DNA regions with different tendencies of self-repair. This self-healing activity represents the simplest sequence-dependent DNA repair system.
61 citations
TL;DR: The quantitative formation of a new M2 L4 molecular capsule from metal ions and dihydrophenazine-based ligands and a stable tetra(radical cation) capsule can be reversibly obtained by chemical as well as electrochemical oxidation.
Abstract: Preparation of molecular nanostructures with polyradical frameworks remains a significant challenge because of the limited synthetic accessibility which is entirely different from that of neutral and ionic ones Herein we report the quantitative formation of a new M2L4 molecular capsule from metal ions and dihydrophenazine-based ligands The capsule has a spherical nanocavity (ca 1 nm in diameter) enclosed by eight redox-active, dihydrophenazine panels Electrochemical oxidation of the capsule leads to the generation of multiple radical cations on the shell framework Moreover, a stable tetra(radical cation) capsule can be reversibly obtained by chemical as well as electrochemical oxidation
60 citations
TL;DR: The first examples of air-stable 20π-electron 5,10,15,20-tetraaryl-5,15-dihydroporphyrins, their 18π- Electron dications, and the 19π-Electron radical cation were prepared through metal-templated annulation of nickel(II) bis(5-arylamino-3-chloro-8-mesityldipyrrin) complexes followed by oxidation.
Abstract: The first examples of air-stable 20π-electron 5,10,15,20-tetraaryl-5,15-diaza-5,15-dihydroporphyrins, their 18π-electron dications, and the 19π-electron radical cation were prepared through metal-templated annulation of nickel(II) bis(5-arylamino-3-chloro-8-mesityldipyrrin) complexes followed by oxidation. The neutral 20π-electron derivatives are antiaromatic and the cationic 18π-electron derivatives are aromatic in terms of the magnetic criterion of aromaticity. The meso N atoms in these diazaporphyrinoids give rise to characteristic redox and optical properties for the compounds that are not typical of isoelectronic 5,10,15,20-tetraarylporphyrins.
TL;DR: A relatively persistent, open-shell aza-thia[7]helicene with cross-conjugated electron-rich π-system is reported, which violates the Aufbau principle on singly occupied molecular orbital (SOMO) energy levels.
Abstract: We report a relatively persistent, open-shell aza-thia[7]helicene with cross-conjugated electron-rich π-system. The singly occupied molecular orbital (SOMO) energy levels of both radical cation and neutral radical of the [7]helicene are below the highest occupied molecular orbital (HOMO) energy levels, thereby violating the Aufbau principle. The aza-thia[7]helicene is prepared from β-hexathiophene by a three-step one-pot reaction, in which the pyrrole ring is constructed by two consecutive C–N bond formations. Chemical oxidation converts the helicene to its radical cation, while in the presence of base (Cs2CO3), the oxidation gives neutral aminyl radical, likely via proton dissociation from the aminium radical cation with a low pKa. Reaction of the aza-thia[7]helicene with NaH provides the corresponding anion, which shows characteristic cyclic voltammetry wave at anodic peak potential Epa ≈ +0.2 V. Chemical oxidation of the anion with ferrocenium hexafluorophosphate at room temperature gives persistent ne...
TL;DR: It is reported that monolayers of diamondoids effectively confer dramatically enhanced field emission properties to metal surfaces, suggesting a new paradigm for low-work-function coatings based on the design of nanoparticles with stable radical cations.
Abstract: Air-stable monolayers of diamondoids can rival cesium's work-function-lowering ability and can dramatically increase field emission current through a radical cation mechanism. Electron emission is critical for a host of modern fabrication and analysis applications including mass spectrometry, electron imaging and nanopatterning. Here, we report that monolayers of diamondoids effectively confer dramatically enhanced field emission properties to metal surfaces. We attribute the improved emission to a significant reduction of the work function rather than a geometric enhancement. This effect depends on the particular diamondoid isomer, with [121]tetramantane-2-thiol reducing gold's work function from ∼5.1 eV to 1.60 ± 0.3 eV, corresponding to an increase in current by a factor of over 13,000. This reduction in work function is the largest reported for any organic species and also the largest for any air-stable compound1,2,3. This effect was not observed for sp3-hybridized alkanes, nor for smaller diamondoid molecules. The magnitude of the enhancement, molecule specificity and elimination of gold metal rearrangement precludes geometric factors as the dominant contribution. Instead, we attribute this effect to the stable radical cation of diamondoids. Our computed enhancement due to a positively charged radical cation was in agreement with the measured work functions to within ±0.3 eV, suggesting a new paradigm for low-work-function coatings based on the design of nanoparticles with stable radical cations.
TL;DR: Direct time-resolved absorption spectroscopy on two [Ru(bpy)3](2+)-tryptophan (bpy = 2,2'-bipyridine) analogue complexes shows that also tryptophan oxidation with water as a proton acceptor can occur via a concerted pathway, provided that the oxidant has weak enough driving force.
Abstract: Proton-coupled electron transfer (PCET) is a fundamental reaction step of many chemical and biological processes. Well-defined biomimetic systems are promising tools for investigating the PCET mechanisms relevant to natural proteins. Of particular interest is the possibility to distinguish between stepwise and concerted transfer of the electron and proton, and how PCET is controlled by a proton acceptor such as water. Thus, many tyrosine and phenolic derivatives have been shown to undergo either stepwise or concerted PCET, where the latter process is defined by simultaneous tunneling of the electron and proton from the same transition state. For tryptophan instead, it is theoretically predicted that a concerted pathway can never compete with the stepwise electron-first mechanism (ETPT) when neat water is the primary proton acceptor. The argument is based on the radical pK(a) (∼4.5) that is much higher than that for water (pK(a)(H3O(+)) = 0), which thermodynamically disfavors a concerted proton transfer to H2O. This is in contrast to the very acidic radical cation of tyrosine (pK(a) ∼ -2). However, in this study we show, by direct time-resolved absorption spectroscopy on two [Ru(bpy)3](2+)-tryptophan (bpy = 2,2'-bipyridine) analogue complexes, that also tryptophan oxidation with water as a proton acceptor can occur via a concerted pathway, provided that the oxidant has weak enough driving force. This rivals the theoretical predictions and suggests that our current understanding of PCET reactions in water is incomplete.
TL;DR: Organo-SOMO activation expanded the scope of enamine/iminium based organocatalytic transformations and it enables new reaction partners to be combined efficiently, providing diversely substituted chiral carbonyl compounds.
Abstract: Enamines, formed from the corresponding carbonyl compounds and appropriate chiral amine catalysts, can be oxidized to radical cation species. These radical cations can be intercepted by a range of SOMO-philic reagents, such as alkenes, arenes and some heteroatom based reagents. This methodology affords diversely substituted chiral carbonyl compounds. Organo-SOMO activation expanded the scope of enamine/iminium based organocatalytic transformations and it enables new reaction partners to be combined efficiently.
TL;DR: Theoretical calculations show thatdeprotonation of the cation radical induces only weak spectral changes, in line with the spectra of the adenyl radical cation and the deprotonated radical trapped in low temperature glasses.
Abstract: Adenyl radicals generated in DNA single and double strands, (dA)20 and (dA)20·(dT)20, by one- and two-photon ionization by 266 nm laser pulses decay at 600 nm with half-times of 1.0 ± 0.1 and 4 ± 1 ms, respectively. Though ionization initially forms the cation radical, the radicals detected for (dA)20 are quantitatively identified as N6-deprotonated adenyl radicals by their absorption spectrum, which is computed quantum mechanically employing TD-DFT. Theoretical calculations show that deprotonation of the cation radical induces only weak spectral changes, in line with the spectra of the adenyl radical cation and the deprotonated radical trapped in low temperature glasses.
TL;DR: Control of the structural type in metallic enantiopure and racemic radical cation salts is achieved through hydrogen bonding interactions between the chiral donor DM-EDT-TTF and the XF6 anions, determined by the anion size and theChiral information.
TL;DR: The results suggest that one molecule of arbutin reduced two ABTS radical cation molecules to ABTS and then cleaved the third ABTSradical cation molecule to generate two products, anArbutin-ABTS fragment adduct and 3-ethyl-6-sulfonate benzothiazolone.
Abstract: Arbutin, a glucoside of hydroquinone, has shown strong 2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) radical cation-scavenging activity, especially in reaction stoichiometry. This study investigated the reaction mechanism of arbutin against ABTS radical cation that caused high stoichiometry of arbutin in an ABTS radical cation-scavenging assay. HPLC analysis of the reaction mixture of arbutin and ABTS radical cation indicated the existence of two reaction products. The two reaction products were purified and identified to be a covalent adduct of arbutin with an ABTS degradation fragment and 3-ethyl-6-sulfonate benzothiazolone. A time-course study of the radical-scavenging reactions of arbutin and the two reaction products suggested that one molecule of arbutin scavenges three ABTS radical cation molecules to generate an arbutin–ABTS fragment adduct as a final reaction product. The results suggest that one molecule of arbutin reduced two ABTS radical cation molecules to ABTS and then cleaved...
TL;DR: The compounds forming a stable radical cation by 3-D homoconjugation produce a uniform amorphous film and show high short-circuit current, high fill factor, and hence high power-conversion efficiency when used as a hole-transporting layer of an organic-inorganic hybrid lead perovskite solar cell.
Abstract: Stabilization of the radical cationic state of a donor molecule by 3-D homoconjugation was probed using a substituted carbon-bridged oligophenylenevinylene backbone (COPV, or 5,5-diarylindeno[2,1-a]indenes). For molecules bearing electron-donating groups as the 5,5-aryl moieties, a one-electron oxidation of the COPV backbone results in delocalization of the cationic charge over the whole molecule with a small reorganization energy. The compounds forming a stable radical cation by 3-D homoconjugation produce a uniform amorphous film and show high short-circuit current, high fill factor, and hence high power-conversion efficiency when used as a hole-transporting layer of an organic–inorganic hybrid lead perovskite solar cell. This material thus shows a performance and stability in air comparable to those obtained with the benchmark material, spiro-MeOTAD.
TL;DR: The redox chemistry and electrochromic response of a few newly synthesized thienosquaraines are presented and these properties are compared to those of the commercial 2,4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl]squaraine.
Abstract: Thienosquaraines are an interesting class of electroactive dyes that are useful for applications in organic electronics. Herein, the redox chemistry and electrochromic response of a few newly synthesized thienosquaraines are presented. These properties are compared to those of the commercial 2,4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl]squaraine. The stability of the radical ions formed in electrochemical processes strongly affects these properties, as shown by cyclic voltammetry, in situ spectroelectrochemistry, and quantum chemical calculations. Furthermore, all of the dyes show aggregation tendency resulting in panchromatic absorption covering the whole UV/Vis spectral range.
TL;DR: This approach represents a mild and straightforward assembly of the naphthalene skeleton using a visible light photocatalytic cascade strategy.
Abstract: We report a radical cation [4+2] annulation of arylalkenes to afford naphthalene derivatives using an organic photosensitizer (9-mesityl-10-methylacridinium perchlorate) under visible light photocatalysis. In the presence of oxygen (in the air), the oxidative dimerization/intramolecular [4+2] cycloaddition of two alkene molecules provides 3,4-dihydronaphthalen-1(2H)-ones in good to high yields. Under a nitrogen atmosphere, (dihydro)naphthalenes were attained in moderate to excellent yields by using Selectfluor as the oxidant. The transformation proceeds via a tandem dimeric electrophilic addition/Friedel–Crafts cyclization/radical coupling/elimination sequence. This approach represents a mild and straightforward assembly of the naphthalene skeleton using a visible light photocatalytic cascade strategy.
TL;DR: The photooxidation of the nitrogen atom to a radical cation induces a positive polarization on the chlorine atom, which results in a higher reactivity in electrophilic aromatic chlorination reactions as discussed by the authors.
TL;DR: Results suggest o-hydroxyphenyl + O2 and decarbonylation of o-benzoquinone serve as plausible OH and CO sources in combustion.
Abstract: Gas-phase product detection studies of o-hydroxyphenyl radical and O2 are reported at 373, 500, and 600 K, at 4 Torr (533.3 Pa), using VUV time-resolved synchrotron photoionisation mass spectrometry. The dominant products are assigned as o-benzoquinone (C6H4O2, m/z 108) and cyclopentadienone (C5H4O, m/z 80). It is concluded that cyclopentadienone forms as a secondary product from prompt decomposition of o-benzoquinone (and dissociative ionization of o-benzoquinone may contribute to the m/z 80 signal at photon energies ≳9.8 eV). Ion-trap reactions of the distonic o-hydroxyphenyl analogue, the 5-ammonium-2-hydroxyphenyl radical cation, with O2 are also reported and concur with the assignment of o-benzoquinone as the dominant product. The ion-trap study also provides support for a mechanism where cyclopentadienone is produced by decarbonylation of o-benzoquinone. Kinetic studies compare oxidation of the ammonium-tagged o-hydroxyphenyl and o-methylphenyl radical cations along with trimethylammonium-tagged analogues. Reaction efficiencies are found to be ca. 5% for both charge-tagged o-hydroxyphenyl and o-methylphenyl radicals irrespective of the charged substituent. G3X-K quantum chemical calculations are deployed to rationalise experimental results for o-hydroxyphenyl + O2 and its charge-tagged counterpart. The prevailing reaction mechanism, after O2 addition, involves a facile 1,5-H shift in the peroxyl radical and subsequent elimination of OH to yield o-benzoquinone that is reminiscent of the Waddington mechanism for β-hydroxyperoxyl radicals. These results suggest o-hydroxyphenyl + O2 and decarbonylation of o-benzoquinone serve as plausible OH and CO sources in combustion.
TL;DR: The efficiency of charge exchange in both DA-APPI andDA-APCI was shown to depend heavily on the solvent flow rate, with best efficiency seen at lowest flow rates studied and at increasing flow rate the abundance of chlorobenzene M+.
TL;DR: The work provides a novel boron-centered radical intermediate, connecting anionic and neutral boryl radicals, and is shown to be affected by the NK interaction.
TL;DR: In this article, pulse radiolysis techniques were applied to unravel the OH and Cl 2 − /Br 2 − induced oxidation mechanism, and a higher sulfur radical cation concentration was obtained by applying Cl 2−/Br 2− compared to OH, on account of a unique stabilized intermediate formed during the OH induced oxidation.
Abstract: Advanced oxidation processes are about to be implemented for the elimination of the residual antibacterial activity of wastewater matrices that has a stimulating effect on the evolution of antibiotic resistance. These techniques apply OH which by scavenging Cl − /Br − yields Cl 2 − /Br 2 − in case of brackish or saline waters. The one-electron oxidation mechanism is peculiar in case of penicillins since a competition occurs between the aromatic and thioether moieties for the free radical attack. This competition determines the efficiency of antibacterial inactivation due to the vicinity of the sulfur to the β-lactam pharmacophore. In order to unravel the OH and Cl 2 − /Br 2 − induced oxidation mechanism, pulse radiolysis techniques were applied. As a result, reactive intermediates typical of sulfur oxidation were observed. Interestingly, a higher sulfur radical cation concentration was obtained by applying Cl 2 − /Br 2 − compared to OH, on account of a unique stabilized intermediate that formed during the OH induced oxidation. The reported mechanism anticipates that scavenging of OH by Cl − /Br − can even have a positive effect on advanced oxidation of organic sulfides.
TL;DR: An investigation of the redox-active tris[4-(pyridin-4-yl)phenyl]amine (NPy3) ligand in the solution state and upon its incorporation into the solid-state metal-organic framework (MOF) revealed that the radical generated was highly delocalized throughout the entire ligand backbone.
Abstract: An investigation of the redox-active tris[4-(pyridin-4-yl)phenyl]amine (NPy3) ligand in the solution state and upon its incorporation into the solid-state metal–organic framework (MOF) [Zn(NPy3)(NO2)2·xMeOH·xDMF]n (MeOH = methanol and DMF = N,N-dimethylformamide) was conducted using in situ UV/vis/near-IR, electron paramagentic resonance (EPR), and fluorescence spectroelectrochemical experiments. Through this multifaceted approach, the properties of the ligand and framework were elucidated and quantified as a function of the redox state of the triarylamine core, which can undergo a one-electron oxidation to its radical cation. The use of pulsed EPR experiments revealed that the radical generated was highly delocalized throughout the entire ligand backbone. This combination of techniques provides comprehensive insight into electronic delocalization in a framework system and demonstrates the utility of in situ spectroelectrochemical methods in assessing electroactive MOFs.
TL;DR: Results show that RP intersystem crossing rates can be strongly influenced by stable radicals nearby strongly coupled RP systems, making it possible to use a third spin to control RP lifetimes down to a picosecond time scale.
Abstract: Photoinduced electron transfer reactions in organic donor–acceptor systems leading to long-lived radical ion pairs (RPs) have attracted broad interest for their potential applications in fields as diverse as solar energy conversion and spintronics. We present the photophysics and spin dynamics of an electron donor − electron acceptor − stable radical system consisting of a meta-phenylenediamine (mPD) donor covalently linked to a 4-aminonaphthalene-1,8-dicarboximide (ANI) electron-accepting chromophore as well as an α,γ-bisdiphenylene-β-phenylallyl (BDPA) stable radical. Selective photoexcitation of ANI produces the BDPA–mPD+•–ANI–• triradical in which the mPD+•–ANI–• RP spins are strongly exchange coupled. The presence of BDPA is found to greatly increase the RP intersystem crossing rate from the initially photogenerated BDPA–1(mPD+•–ANI–•) to BDPA–3(mPD+•–ANI–•), resulting in accelerated RP recombination via the triplet channel to produce BDPA–mPD–3*ANI as compared to a reference molecule lacking the BDP...
TL;DR: Two requirements for attaining long-lived radical cations of N-acylated Aa derivatives are revealed: keeping the reduction potentials for oxidizing the Aa residues under about 1.4 V vs SCE and adding an electron-donating group para to the N-terminal amide of the aromatic ring, which prevents the electron spin density of the radical cation from extending over the C- terminal amide.
Abstract: Oligoamides composed of anthranilic acid derivatives present a promising choice for mediating long-range charge transfer and controlling its directionality. Hole hopping, modulated by the anthranilamide (Aa) permanent dipoles, provides a plausible means for such rectified long-range charge transduction. All aliphatic and most aromatic amides, however, decompose upon oxidation, rendering them unacceptable for hole-hopping pathways. We, therefore, employ electrochemical and computational analysis to examine how to suppress oxidative degradation and stabilize the radical cations of N-acylated Aa derivatives. Our findings reveal two requirements for attaining long-lived radical cations of these aromatic amides: (1) keeping the reduction potentials for oxidizing the Aa residues under about 1.4 V vs SCE and (2) adding an electron-donating group para to the N-terminal amide of the aromatic ring, which prevents the electron spin density of the radical cation from extending over the C-terminal amide. These finding...
TL;DR: In this article, the rate constant of the reduction of oxidized porphyrin dye is enhanced by attaching non-conjugated carbazole triphenylamine moiety using iodine/triiodide and tris(2,2′-bispyridinium)cobalt II/III electrolytes.
Abstract: Reduction kinetics of oxidized dyes absorbed on semiconductor surfaces and immersed in redox active electrolytes has been mainly modeled based on the free energy difference between the oxidation potential of the dye and the redox potential of the electrolyte. Only a few mechanisms have been demonstrated to enhance the kinetics by other means. In this work, the rate constant of the reduction of oxidized porphyrin dye is enhanced by attaching non-conjugated carbazole triphenylamine moiety using iodine/triiodide and tris(2,2′-bispyridinium)cobalt II/III electrolytes. These results are obtained using transient absorption spectroscopy by selectively probing the regeneration kinetics at the porphyrin radical cation and the carbazole triphenylamine radical cation absorption wavelengths. The enhancement in the reduction kinetics is not attributed to changes in the driving force, but to the more exposed dye cation radical orbitals of the dichromophoric dye. The results are important for the development of high efficiency photo-electrochemical devices with minimalized energy loss at electron transfer interfaces.