Electron Transfers in Donor–Acceptor Supramolecular Systems: Highlighting the Dual Donor and Acceptor Role of ZSM‑5 Zeolite

After coadsorption of electron-donor (<i>p</i>-terphenyl, PTP) and electron-acceptor (1,4-dicyanobenzene, DCB) molecules within the channels of silicalite-1 and MZSM-5 (M = Na⁺, H⁺) zeolites, photoinduced or spontaneous electron transfers were investigated. In aluminum-free silicalite-1, the reaction mechanisms after PTP ionization are similar in the presence and in the absence of the acceptor molecule. Photoionization leads to a PTP^•+ radical cation, which recombines directly. In NaZSM-5, <i>p</i>-terphenyl photoexcitation induces PTP^•+ formation evolving to an electron–hole pair through capture of another electron of zeolite. This behavior is observed with and without DCB. However, when DCB is coadsorbed with PTP, recombination decays for PTP^•+ and for the electron–hole pair are significantly slower. Pulsed EPR experiments show strong electron density close to DCB, through a coupling of unpaired electrons with ¹⁴N nuclei. Nevertheless, the electron transfer remains insufficient to allow DCB^•– radical anion formation. High confinement within ZSM-5 and intrinsic strength of zeolite acceptor sites might be put forward to explain the nonformation of the anion. The acceptor properties of DCB and of the zeolite might then be competitive. The zeolite electron acceptor character is even more marked when PTP is coadsorbed with DCB in acidic HZSM-5. Ionization occurs spontaneously, and transient species are stabilized for months. No electronic coupling with nitrogen atoms of DCB could be observed, indicating no partial transfer to the acceptor molecule and electron trapping in acidic zeolite

Influence of Confinement Effect on Electron Transfers Induced by <i>t-</i>Stilbene Sorption in Medium Pore Acidic Zeolites

The mere exposure of <i>trans</i>-stilbene (<i>t</i>-St) to three types of dehydrated medium pore acid zeolites that differ by their pore diameter induces <i>t</i>-St spontaneous ionization in high yield. In situ diffuse reflectance UV–visible, EPR, and Raman spectra recorded over several months highlight the exceptional stability of the charge separated states formed in ferrierite (H-FER), H-MFI, and mordenite (H-MOR). The increase in the pore diameter from H-FER to H-MOR induces different behaviors after radical cation formation. <i>t-</i>St^•+ is stabilized for months in the narrow pores of H-FER, whereas in the larger pore H-MFI, relatively fast electron abstraction (hole transfer) takes place from the zeolite framework to create charge transfer complexes. Pulsed EPR experiments were performed using <i>t</i>-St and marked [D₁₂]<i>t-</i>St and [¹³C₂]<i>t-</i>St molecules to reveal the structural environment of the unpaired electrons through the assignment of the couplings with ¹H, ²H, ¹³C, ²⁷Al, and ²⁹Si nuclei

Gold-Catalyzed Oxidative Transformation of Free Sugars into Biobased Platform Molecules

Due to their easy conversion into high-added value products, sugar lactones and their derivatives are very attractive biobased platform molecules. Yet, conventional transformation of free sugars into such activated compounds is not so handy: a multistep procedure requiring protection/oxidation/lactonization-esterification/deprotection is often necessary. We report herein a procedure allowing one to rapidly and efficiently form lactones/esters directly from free sugars under mild conditions, catalyzed with a small amount (0.36 mol %) of recyclable gold nanocatalyst under oxygen atmosphere. The conditions were optimized using galactose as a model, quantitatively and selectively affording 1,4-galactonolactone in 2 h at room temperature. The procedure was then successfully applied to a variety of hexoses and pentoses leading to excellent conversion (>86%). Due to the equilibrium between lactone regioisomers and ester forms, a mixture of 1,4-lactone, 1,5-lactone and methyl ester can be generally obtained depending on the sugar series. A subsequent reaction of the crudes with benzylamine leads to a total conversion of lactones/esters into corresponding amides, confirming the efficiency of the procedure and paving the way to a one-pot transformation of free sugars into high added value sugar-based derivatives. Based on NMR and ESR analyses, a mechanism of the reaction involving CH3O• radical species seems to be taking shape

Across the Structural Re-Entrant Transition in BaFe₂(PO₄)₂: Influence of the Two-Dimensional Ferromagnetism

BaFe₂(PO₄)₂ was recently prepared by hydrothermal synthesis and identified as the first two-dimensional (2D) Ising ferromagnetic oxide, in which honeycomb layers made up of edge-sharing FeO₆ octahedra containing high-spin Fe²⁺ ions (<i>S</i> = 2) are isolated by PO₄ groups and Ba²⁺ cations. BaFe₂(PO₄)₂ has a trigonal <i>R</i>-3 structure at room temperature but adopts a triclinic <i>P</i>-1 structure below 140 K due to the Jahn–Teller (JT) instability arising from the (t_2g)⁴(e_g)² configuration. The triclinic crystal structure was refined to find significantly distorted Fe²⁺O₆ octahedra in the honeycomb layers while the distortion amplitude <i>Q</i>_JT was estimated to 0.019 Å. The JT stabilization energy is estimated to be ∼7 meV per formula unit by DFT calculations. Below ∼70 K, very close to the ferromagnetic transition temperature <i>T</i>_c = 65.5 K, the structure of BaFe₂(PO₄)₂ returns to a trigonal <i>R</i>-3 structure in the presence of significant ferromagnetic domains. This rare re-entrant structural transition is accompanied by a discontinuous change in the quadrupolar splitting of Fe²⁺, as determined by Mössbauer spectroscopy. EPR measurements show the presence of magnetic domains well above <i>T</i>_c , as expected for a ferromagnetic 2D Ising system, and support that the magnetism of BaFe₂(PO₄)₂ is uniaxial (<i>g</i>_⊥ = 0)

Salen Complexes as Fire Protective Agents for Thermoplastic Polyurethane: Deep Electron Paramagnetic Resonance Spectroscopy Investigation

The contribution of copper complexes of <i>salen</i>-based Schiff bases <i>N</i>,<i>N</i>′-bis­(salicylidene)­ethylenediamine (C1), <i>N</i>,<i>N</i>′-bis­(4-hydroxysalicylidene)­ethylenediamine (C2), and <i>N</i>,<i>N</i>′-bis­(5-hydroxysalicylidene)­ethylenediamine (C3) to the flame retardancy of thermoplastic polyurethane (TPU) is investigated in the context of minimizing the inherent flammability of TPU. Thermal and fire properties of TPU are evaluated. It is observed that fire performances vary depending upon the substitution of the salen framework. Cone calorimetry [mass loss calorimetry (MLC)] results show that, in TPU at 10 wt % loading, C2 and C3 reduce the peak of heat release rate by 46 and 50%, respectively. At high temperature, these copper complexes undergo polycondensation leading to resorcinol-type resin in the condensed phase and thus acting as intumescence reinforcing agents. C3 in TPU is particularly interesting because it delays significantly the time to ignition (MLC experiment). In addition, pyrolysis combustion flow calorimetry shows reduction in the heat release rate curve, suggesting its involvement in gas-phase action. Structural changes of copper complexes and radical formation during thermal treatment as well as their influence on fire retardancy of TPU in the condensed phase are investigated by spectroscopic studies supported by microscopic and powder diffraction studies. Electron paramagnetic resonance (EPR) spectroscopy was fully used to follow the redox changes of Cu­(II) ions as well as radical formation of copper complexes/TPU formulations in their degradation pathways. Pulsed EPR technique of hyperfine sublevel correlation spectroscopy reveals evolution of the local surrounding of copper and radicals with a strong contribution of nitrogen fragments in the degradation products. Further, the spin state of radicals was investigated by the two-dimensional technique of phase-inverted echo-amplitude detected nutation experiment. Two different radicals were detected, that is, one monocarbon radical and an oxygen biradical. Thus, the EPR study permits to deeply investigate the mode of action of copper salen complexes in TPU

Crystalline Molecular Assemblies of Complexes Showing Eightfold Coordinated Niobium(IV) Dodecahedral Geometry in the Pyridine-Dicarboxylic Acid System

The reactivity of 2,3-pyridine-dicarboxylic (known as quinolinic or H2qui) acid and 2,5-pyridine-dicarboxylic (known as isocinchomeronic or H2icc) acid has been investigated as a complexing agent toward the niobium(IV) tetrachloride precursor (NbCl4·2THF) in different organic solvent mixtures. It resulted in the isolation of four crystalline assemblies of mononuclear coordination complexes 1–4 [Nb(HL)4·solvent], where HL is the monoprotonated quinolinate (Hqui) ligand (complexes 1–3) or the monoprotonated isocinchomeronate ligand (complex 4). For each complex, the discrete niobium(IV) center is eightfold coordinated to four oxygen atoms from the deprotonated carboxylate arm and four nitrogen atoms from the pyridine part of the dicarboxyl ligand with a dodecahedral environment [NbO4N4]. The remaining carboxyl arm (either in 3 or in 5 position) remained under its protonated form, leading to neutral [Nb(HL)4] moieties for compounds 1, 2, and 4, or the anionic [Nb(qui)(Hqui)3]− moiety for compound 3. The complexes are observed in various molecular arrangements, involving intercalated solvent molecules such as acetonitrile in compound 1 ([Nb(Hqui)4·0.8(CH3CN)], obtained at room temperature), a mixture of acetonitrile and pyridine in compound 2 ([Nb(Hqui)4·0.7CH3CN·2PYR], obtained via the solvothermal reaction at 80 °C), a mixture of pyridine and triethylamine, in addition with water and chloride species, in compound 3 ([Nb(qui)(Hqui)3·Cl·HPYR·HTEA·1.5H2O], obtained via solvothermal reaction at 80 °C), and N,N-dimethylformamide in compound 4 ([Nb(Hicc)4·6DMF], obtained at room temperature). The d1 configuration expected for the niobium(IV) centers has been analyzed by magnetic measurements, as well as by EPR and XPS. An anti-ferromagnetism transition has been observed at very low temperatures for complexes 1 (3.6 K) and 4 (3.3 K), for which the shortest Nb···Nb interatomic lengths occur

Low-Potential Sodium Insertion in a NASICON-Type Structure through the Ti(III)/Ti(II) Redox Couple

We report the direct synthesis of powder Na₃Ti₂(PO₄)₃ together with its low-potential electrochemical performance and crystal structure elucidation for the reduced and oxidized phases. First-principles calculations at the density functional theory level have been performed to gain further insight into the electrochemistry of Ti­(IV)/Ti­(III) and Ti­(III)/Ti­(II) redox couples in these sodium superionic conductor (NASICON) compounds. Finally, we have validated the concept of full-titanium-based sodium ion cells through the assembly of symmetric cells involving Na₃Ti₂(PO₄)₃ as both positive and negative electrode materials operating at an average potential of 1.7 V

Carl von Linné fil. to Peter Simon Pallas

Square planar cobalt­(II) complexes of salen ligands <i>N</i>,<i>N</i>′-bis­(3-<i>tert</i>-butyl-5<i>R</i>-salicylidene)-1,2-cyclohexanediamine), where R = OMe (<b>1</b>) and <i>tert</i>-butyl (<b>2</b>), were prepared. <b>1</b> and <b>2</b> were electrochemically reversibly oxidized into cations <b>[1-H</b>_<b>2</b><b>O]</b>^<b>+</b> and <b>[2-H</b>_<b>2</b><b>O]</b>^<b>+</b> in CH₂Cl₂. The chemically generated <b>[1-H</b>_<b>2</b><b>O]­(SbF</b>_<b>6</b><b>)·0.68 H</b>_<b>2</b><b>O·0.82CH</b>_<b>2</b><b>Cl</b>_<b>2</b> and <b>[2-H</b>_<b>2</b><b>O]­(SbF</b>_<b>6</b><b>)·0.3H</b>_<b>2</b><b>O·0.85CH</b>_<b>2</b><b>Cl</b>_<b>2</b> were characterized by X-ray diffraction and NIR spectroscopy. Both complexes are paramagnetic species containing a square pyramidal cobalt ion coordinated at the apical position by an exogenous water molecule. They exhibit remarkable NIR bands at 1220 (7370 M^–1 cm^–1) and 1060 nm (5560 M^–1 cm^–1), respectively, assigned to a CT transition. DFT calculations and magnetic measurements confirm the paramagnetic (<i>S</i> = 1) ground spin state of the cations. They show that more than 70% of the total spin density in <b>[1-H</b>_<b>2</b><b>O]</b>^<b>+</b> and <b>[2-H</b>_<b>2</b><b>O]</b>^<b>+</b> is localized on the metal, the remaining spin density being distributed over the aromatic rings (30% phenoxyl character). In the presence of <i>N</i>-methylimidazole <b>1</b> and <b>2</b> are irreversibly oxidized by air into the genuine octahedral cobalt­(III) bis­(phenolate) complexes <b>[1-im</b>_<b>2</b><b>]</b>^<b>+</b> and <b>[2-im</b>_<b>2</b><b>]</b>^<b>+</b>, the former being structurally characterized. Neither <b>[1-im</b>_<b>2</b><b>]</b>^<b>+</b> nor <b>[2-im</b>_<b>2</b><b>]</b>^<b>+</b> exhibits a NIR feature in its electronic spectrum. <b>1</b> and <b>2</b> were electrochemically two-electron oxidized into <b>[1]</b>^<b>2+</b> and <b>[2]</b>^<b>2+</b>. The cations were identified as Co­(III)–phenoxyl species by their characteristic absorption band at ca. 400 nm in the UV–vis spectrum. Coordination of the phenoxyl radical to the cobalt­(III) metal ion is evidenced by the EPR signal centered at <i>g</i> = 2.00

Importance of Short-Range Order in Governing Thin Film Morphology and Electronic Properties of Polymeric Organic Semiconductors

Semiconducting polymers provide a ubiquitous platform for a range of applications in molecular electronics and photovoltaics, but the ordered and disordered regions of these materials impart different optoelectronic properties. By resolving local morphology using solid-state magnetic resonance spectroscopy and modeling techniques, here, we demonstrate that the PTB7-Th donor–acceptor (D–A) copolymer and P3HT and MEH-PPV homopolymers exhibit different degrees of the short-range order, which can be associated with the large differences in their charge carrier mobilities. The high degree of local order in PTB7-Th (84–99%) is facilitated by noncovalent interactions between D and A moieties. In contrast to this, the reduced local order in P3HT (30–44%) and MEH-PPV (39–43%) homopolymers is due to the distortions in the vicinities of backbone and side chain moieties that lead to conformationally tilted polymer chains. Combined solid-state NMR and density functional theory (DFT) modeling allows the degree of backbone torsion in these materials to be determined, and insights into packing interactions are obtained by two-dimensional (2D) 1H–1H, 1H–13C, and 1H–19F correlation NMR spectroscopy. In addition, the different paramagnetic species and hyperfine interactions are analyzed by EPR spectroscopy and are expected to influence the charge carrier mobilities. A detailed analysis of the local structures presented in this study helps explain the morphological anomalies and their impact on bulk charge carrier mobilities and electronic density of states, thus providing essential insights into the morphology–property relationships in polymeric organic semiconductors

ZnO Oxygen Vacancies Formation and Filling Followed by in Situ Photoluminescence and in Situ EPR

Oxygen vacancies of zinc oxide were followed by photoluminescence (PL) and electron paramagnetic resonance (EPR) spectroscopies. The green PL emission was associated with oxygen vacancies: its intensity is enhanced upon static thermal treatment under inert or under vacuum, whereas it decreases upon oxygen treatment. A unique EPR signal at <i>g</i> = 1.96 was measured at room temperature after thermal in situ treatment under flow of inert or oxygenated atmospheres, its double integration follows the same trends than the green PL emission and its evolution was shown to probe the oxygen vacancy concentrations. The relative concentration of the related paramagnetic species would be increased/decreased upon trapping/release of the electron associated to the formation/filling of oxygen vacancy. The influence of Ti impurities on the PL and RPE signals was investigated. Finally, it is concluded that the EPR signal is related to oxygen vacancies and its position shift could be explained by the involvement of some mixing orbitals. Thanks to static (PL and EPR) and dynamic (EPR) in situ characterizations, the conditions of formation or filling of oxygen vacancies are discussed depending of the atmosphere and temperature of the pretreatment of kadox and ex-carbonate zinc oxide. High temperature treatments, inert atmospheres, and vacuum lead to the formation of new oxygen vacancies. This process is reversible upon oxygenated atmospheres. The efficiency of such filling up depends on the temperature and starts to prevail on the oxygen vacancy formation below 500 K. It is also shown that few native oxygen vacancies can also be filled up