Intramolecular Spin Exchange Research Articles

Specific Features of the Intramolecular Spin Exchange in a Novel Nitroxide Triradical

An X-band EPR spectroscopic study was performed of the temperature dependence of the dynamics of the intramolecular electron spin exchange in a triradical where the radical rings are linked by acetylene groups –C≡C–. A thermodynamic characteristic was determined for the conformational transition in the triradical between conformers where the spin exchange is of the triradical or biradical type. The geometric structure of the novel stable nitroxide triradical was determined using density functional theory calculations.

Tuning the Magnetic Exchange Interactions in Organic Biradical Networks

An overview of organic biradicals directed towards triplon excitations in higher ordered crystalline states is provided. The topic started from the idea to control the intramolecular spin exchange interaction in a sizable but not too large extent, in order to provide also decent intermolecular spin interactions in a crystalline lattice. The predictability of exchange couplings through DFT calculations of optimized geometries turns out to be limited, such that the molecules need to be synthesized and crystallized for further in‐depth characterizations. For crystal structure geometries, most often then a good agreement of calculated intra‐ and intermolecular exchange interactions with those later determined experimentally is found. Finally, it is demonstrated that by choosing well balanced size of both interactions a field‐induced higher ordered magnetic state becomes accessible.

Dynamics of intramolecular spin exchange interaction of a nitronyl nitroxide diradical in solution and on surfaces

In this paper we report the study of the dynamics of a thermally modulated intramolecular spin exchange interaction of a novel diradical nitronyl nitroxide-substituted disulfide in solution and when it is grafted on a gold surface. The structure of this diradical was designed to have flexible chains leading to intramolecular collisions and hence spin exchange interaction, and with an appropriate binding group to be grafted on the gold surface to study its behavior on the surface. In solution, this diradical shows a strong spin exchange interaction between both radicals which is modulated by temperature, but also gold nanoparticles (AuNPs) functionalized with this diradical permit investigation of such a phenomenon in surface-grafted radicals. The spin-labelled AuNP synthesis was optimized to obtain high coverage of spin labels to lead to high spin exchange interaction. The obtained AuNPs were studied by Electron Paramagnetic Resonance (EPR), UV-Vis, and IR spectroscopies, HR-TEM microscopy, Cyclic Voltammetry (CV), Energy Dispersive X-ray analysis (EDX) and Thermogravimetric Analysis (TGA). This inorganic-organic hybrid material also showed dipolar interactions between its radicals which were confirmed by the appearance in the EPR spectra of an |Δms| = 2 transition at half-field. This signal gives direct evidence of the presence of a high-spin state and permitted us to study the nature of the magnetic coupling between the spins which was found to be antiferromagnetic. Self-Assembled Monolayers (SAMs) of these radicals on the Au (111) substrate were also prepared and studied by contact angle, X-Ray Photoelectron Spectroscopy (XPS), Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS), Cyclic Voltammetry and EPR. The magnetic as well as the electrochemical properties of the hybrid surfaces were studied and compared with the properties of this diradical in solution. Analogies between the properties of AuNPs with high coverage of radicals and those of SAM were observed.

Intramolecular Spin Exchange in Flexible PEG-based Nitroxide Biradicals in Aqueous Solutions

Intramolecular spin exchange in four flexible PEG-based bis-nitroxides was studied by X-band electron paramagnetic resonance (EPR) spectroscopy as a function of temperature and solvent viscosity. A series of biradicals with different lengths of the ethylene glycol bridge connecting the two nitroxide groups was investigated in aqueous and mixed 1:1 i-propanol:water solutions. Conformational transitions in liquid solutions of the biradicals were analyzed, and thermodynamic parameters of the conformational reorganization were calculated from the EPR spectra. Intramolecular distances r between paramagnetic >N–O• fragments in biradicals in frozen solutions were calculated and compared with dynamic properties of the biradicals.

Preparation and Properties of Gadolinium Homoleptic Double-and Triple-Decker Complexes Containing Tetrabenzoporphyrin Ligand

Two novel gadolinium sandwich-type complexes containing tetrabenzoporphyrin (TBP) ligands-Gd (TBP)2 and Gd (TBP)3 were prepared from porphyrin 1 and Gd (acac)3. nH2O under Ar by boiling 1, 2, 4-Tcb for 15~17h and 45~48 h respectively. Their structures are characterized by Uv-Vis, HR-MS and IR. The spectroscopic studies show that their longest-wavelength visible and near-IR absorption bands are obviously red-shifted due to the extension of the π conjugated systems in the TBP ring. The magnetic studies indicate that the observed value of χMT for Gd (TBP)2 is close to the combined value of Gd (III) and porphyrin radical anion at room temperature and that antiferromagnetic interaction possibly results from the intramolecular spin exchange between the porphyrin π-radical electron and that the gadolinium f unpaired electrons dominates its magnetic properties over the whole temperature range while there is no magnetic interactions between two Gd (III) ions in Gd (TBP)3 in the range from room temperature down to 20K and very weak antiferromagnetic coupling exsits between two Gd (III) ions below 20K.

Synthesis, Properties of Gadolinium Double-Decker Sandwich-Type Complex Containing Tetrabenzoporphyrin Ligand

A novel gadolinium double-decker sandwich-type complex with tetrabenzoporphyrin (TBP) ligands-Gd(TBP)2 was synthesized by the reaction of porphyrin 1 with Gd(acac)3. nH2O in boiling 1, 2, 4-Tcb for 15~17 h under Ar. Its structure is characterized byUv-Vis-NIR and HR-MS. The spectroscopic properties show that the longest-wavelength visible band (600-800 nm) and the strong near-IR absorption band centered near 1340 nm of Gd (TBP)2 complex are obviously red-shifted due to the extension of the π conjugated systems derived from the linear benzoannulation in the TBP ring. The magnetic studies also indicate that the observed value of χMT of Gd(TBP)2 complex is close to the combined value of Gd(III) and porphyrin radical anion at room temperature, and antiferromagnetic interaction that possibly results from the intramolecular spin exchange between the porphyrin π-radical electron, and the gadolinium f unpaired electrons dominates its magnetic properties over the whole temperature range. Since the first report of bis(porphyrinato) metal complexes M(Por)2 such as a cerium(IV) bisporphyrinate, Ce(TPP)2 and hydrogen praseodymium(III), PrH(TPP)2 in 1983[1], sandwich-type rare earth complexes with porphyrin ligands have received increasing attentions due to the unusual physical, spectroscopic, redox, electron transfer, and/or conductivity properties resulting from their strong π-π interactions[2-4]. Whereas the chemistry and properties of homoleptic sandwich complexes containing the same pophyrinato ligands of various lanthanide, actinides and group 4 transition metals have been extensively studied[5-6], actually all of them are confined to the sandwich-type complexes containing meso-substituted porphyrin ligands except for octaethylporphyrin (OEP) or octamethylporphyrin (OMP). Little is known about sandwich-like complexes with aromatic ring-fused porphyrin ligands that contain a more delocalized π system. In this paper, we will describe the preparation of a novel gadolinium homoleptic double-decker sandwich-type complex with tetrabenzoporphyrin (TBP) ligand (see Fig.1) as well as their spectroscopic characterization and magnetic studies.

Effect of Ligand Substitution on the Exchange Interactions in {Mn12}-Type Single-Molecule Magnets

We investigate how ligand substitution affects the intramolecular spin exchange interactions, studying a prototypical family of single-molecule magnets comprising dodecanuclear cluster molecules [Mn(III)(8)Mn(IV)(4)O(12)(COOR)(16)]. We identify a simple scheme based on accumulated Pauling electronegativity numbers (AEN) of the carboxylate ligand groups (R). The redistribution of the electron density, controlled by the AEN of a ligand, changes the degree of hybridization between 3d electrons of manganese and 2p electrons of oxygen atoms, thus changing the exchange interactions. This scheme, despite its conceptual simplicity, provides a strong correlation with the exchange energies associated with carboxylate bridges and is confirmed by the electronic structure calculations taking into account the Coulomb correlations in magnetic molecules.

Supramolecular Self-Assembled Polynuclear Complexes from Tritopic, Tetratopic, and Pentatopic Ligands: Structural, Magnetic and Surface Studies

Polymetallic, highly organized molecular architectures can be created by "bottom-up" self-assembly methods using ligands with appropriately programmed coordination information. Ligands based on 2,6-picolyldihydrazone (tritopic and pentatopic) and 3,6-pyridazinedihydrazone (tetratopic) cores, with tridentate coordination pockets, are highly specific and lead to the efficient self-assembly of square [3 x 3] Mn9, [4 x 4] Mn16, and [5 x 5] Mn25 nanoscale grids. Subtle changes in the tritopic ligand composition to include bulky end groups can lead to a rectangular 3 x [1 x 3] Mn9 grid, while changing the central pyridazine to a more sterically demanding pyrazole leads to simple dinuclear copper complexes, despite the potential for binding four metal ions. The creation of all bidentate sites in a tetratopic pyridazine ligand leads to a dramatically different spiral Mn4 strand. Single-crystal X-ray structural data show metallic connectivity through both mu-O and mu-NN bridges, which leads to dominant intramolecular antiferromagnetic spin exchange in all cases. Surface depositions of the Mn9, Mn16, and Mn25 square grid molecules on graphite (HOPG) have been examined using STM/CITS imagery (scanning tunneling microscopy/current imaging tunneling spectroscopy), where tunneling through the metal d-orbital-based HOMO levels reveals the metal ion positions. CITS imagery of the grids clearly shows the presence of 9, 16, and 25 manganese ions in the expected square grid arrangements, highlighting the importance and power of this technique in establishing the molecular nature of the surface adsorbed species. Nanoscale, electronically functional, polymetallic assemblies of this sort, created by such a bottom-up synthetic approach, constitute important components for advanced molecule-based materials.

Solvent effects on the intramolecular spin exchange in biradicals at room temperature

Solvent effects on intramolecular electron spin exchange in biradicals have been investigated in various solvents by electron spin resonance (ESR) spectroscopy at room temperature. Biradicals containing different radical fragments and bridges of different length and composition were used. Interactions between solvent molecules and biradicals were found to take place at the radical fragment as well as at the functional group in the connecting bridge and have been proved to have an influence on the intramolecular spin exchange. The experimentally measured parameters, the exchange integral (J) and the characteristic time of an intramolecular movement (τ eff), have been compared with solvent viscosity and polarity parameters (macroscopic level) and the longitudinal solvent relaxation time (microscopic level). The dependencies on viscosity and polarity were in good agreement with reported results. In addition, strong evidence of hydrogen bonding between the solvent and different sites of the biradicals was found, in some cases even surpassing the other effects.

Intramolecular spin exchange in flexible short-chain biradicals in solutions with various viscosity

Intramolecular electron spin exchange as a function of temperature and solvent viscosity has been studied by X-band electron paramagnetic resonance (EPR) spectroscopy in two short-chain flexible nitroxide biradicals. Certain thermodynamic parameters of the conformational rearrangements were calculated from the EPR spectra. The process of spin exchange in these biradicals dissolved in six different alcohols is compared with that in a nonpolar solvent (toluene) and with the thermodynamic characteristics of the solvents. No correlation is found on a macroscopic level (solvent viscosity) but on a microscopic level (solvent longitudinal relaxation times) a distinct correlation is seen. The original results are compared with literature data.

Photoswitching of the magnetic interaction between a copper(II) ion and a nitroxide radical by using a photochromic spin coupler.

Photoswitching of an intramolecular spin exchange interaction between a copper(II) ion and a nitroxyl radical by using a metal complex of diarylethene has been studied by means of ESR spectroscopy. As a coordination ligand, a diarylethene with a 1,10-phenanthroline ring and nitronyl nitroxide radical was synthesized. Mixing the diarylethene ligand with [Cu(hfac)(2)] (hfac=hexafluoroacetylacetone) in toluene led to a hypsochromic shift of the absorption maxima of the closed-ring isomer due to complexation. ESR measurement in toluene at room temperature of the open-ring isomer of the Cu(II) complex gave a spectrum that is a superposition of the spectra from the nitroxide radical and Cu(II). When the sample was irradiated with 366 nm light, a new peak due to large exchange interaction appeared between those of the nitroxyl radical and Cu(II). This ESR measurement revealed that the magnitude of the spin exchange interaction was changed by more than 160-fold by photoirradiation. This is the largest magnetic photoswitching phenomenon recorded in diarylethene systems.

Structure, Magnetic Properties and Catalase-like Activity of Asymmetric Dimanganese(III, III) Carboxylate Complexes with Salicylaldimine Ligands

The crystal structures, magnetic properties, and catalase-like activities of assymmetric dinuclear manganese(III, III) complexes, [Mn2III, III(spa)2(μ-Me3CCO2)(Me3CCO2)(CH3OH)] (1) and [Mn2III, III(vpa)2(μ-Me3CCO2)(Me3CCO2)(CH3OH)] (2), (H2spa = 3-salicyclideneamino-1-propanol, H2vpa = O-vanillin), were reported. The crystal structures of complexes 1 and 2 consist of the same discrete asymmetric coordination environment of dinuclear clusters, where the two manganese atoms are bridged by two alkoxo oxygens of the spa or vpa ligands and one bidentate carboxylate ion, whereas an additional oxygen atom of monodentate carboxylate coordinated to the first metal ion, and the second metal ion was coordinated by one oxygen atom of the solvent CH3OH. Magnetic investigations (2–300 K) reveal an intramolecular antiferromagnetic spin exchange interaction with axial-field splittings: J = −12.3 cm−1 (D = −0.10 cm−1) and J = −13.3 cm−1 (D = −0.15 cm−1) for complexes 1 and 2, respectively. The complexes should show catalase-like activity for H2O2 disproportionation in CH3OH solvent at 25° with rate constants of k = 6.35 dm3moI−1s−1 and 6.20 dm3mol−1s−1 for complexes 1 and 2, respectively.

Effect of a metal ion on intramolecular spin exchange in spin-labeled complexes

The characteristic features of intramolecular spin exchange in 14 complexes of AgI, HgII, NiII, PdII, PtII, AuIII, and PtIV with spin-labeled ligands were studied by ESR spectroscopy. The measured values of the exchange integral ‖J‖ and the differences between the enthalpies of the efficient conformations (ΔH) were compared with the electronic polarization (refraction)R f of the NiII, PdII, and PtII ions and Klopman's rigidity parameters σK, which characterize the total polarazibility of the ions and the degree of covalence of the bond between the metal atom and the donor atom of the ligand, respectively. Delocalization of the electron spin density and the efficiency of spin exchange are determined by the relative contributions of the s, p, and d orbitals, which produce the overlap integral of wave functions, ‖J‖, and by the geometric features of the coordination polyhedron, which affect the mutual orientation of the N−O fragments.

Synthesis, Crystal Structure, Cryomagnetic Properties and Catalytic Activity for H2O2 Disproportionation of New Dinuclear and Polynuclear Bis(2,4‐pentanedionate)manganese(II) Complexes with Diazine Ligands

AbstractThe structures, magnetic properties, and catalytic activity for H2O2 disproportionation are reported for three new complexes [Mn2(acac),(pydz)] (1), [Mn(acac)2(pym)] (2), and (Mn(acac)2(pyz)] (3) (acac = 2,4‐pentanedionate, pydz = pyridazine, pym = pyrimidine, pyz = pyrazine). The X‐ray crystal structures of 1 and 3 have been determined. Complex 1 crystallizes as a binuclear complex with η2‐pyridazine and acac bridging ligands 3 crystallizes as a linear polymeric chain with bridging pyrazine molecule. Cryomagnetic investigations (4‐300 K) reveal a weak intramolecular antiferromagnetic spin exchange with J = −2.05, −0.04, and −0.05 cm−1 for the complexes of 1, 2, and 3, respectively. The complexes 1–3 showed two‐step catalytic activity for H2O2 disproportionation in pyridine solution at 0 °C.

Intramolecular Long Range Electronic Spin Exchange Coupling in Diradicals: A New Magnetic Coupling Unit

Abstract A phenyl diethynyl bridge is used as a magnetic coupling unit (MCU) between two iminonitroxide π-radicals. The ethynyl moiety is attached to the central phenyl ring either in the para or in the meta position. The magnetic properties of the resulting bis-imino nitroxide diradicals (p-BIN and m-BIN) are investigated both at the molecular scale and in the condensed phases. An intramolecular through bond spin coupling is observed at an unusual long distance (about 20 ÅS) in both derivatives. However, the ground state is found to be a singlet spin state in both compounds. The results suggest an interplay between the spin polarization and the spin delocalization (π-conjugation). An intermolecular weak antiferromagnetic spin coupling is occuring in both compounds.

An electron spin polarization study of the interaction of photoexcited triplet molecules with mono- and polynitroxyl stable free radicals

Time-resolved electron spin resonance (TR ESR) has been used to investigate the chemically induced dynamic electron polarization (CIDEP) generated by the interaction of stable free radicals with the triplet states of benzophenone, benzil, and 2-acetylnaphthalene. The stable radicals were mono-, di-, tri-, and tetranitroxyl free radicals possessing the 2,2,6,6-tetramethylpiperidine-N-oxyl moiety. All of the stable radical systems investigated were found to be emissively polarized by interaction with the triplet states, and the phase of polarization was independent of the sign of zero-field splitting (D) of the interacting triple molecule. Possible and likely mechanisms of polarization transfer (creation) resulting from the interaction of photoexcited triplet molecules with nitroxyls in the strong electron exchange are discussed. The emissive CIDEP of nitroxyls observed in the interactions with triplet benzil, which has D > 0, provides strong support for the operation of the radical-triplet pair mechanism. Within the time scale of TR ESR experiments ([approximately]10[sup [minus]7]--10[sup [minus]6] s) no significant variation in the shape of the CIDEP spectra of the nitroxyls was observed, either in viscous media or in micelles. It is concluded that intramolecular spin exchange (or conformational change) of polynitroyls occurs much faster than the time resolution of the experiment. 24 refs., 6more » figs., 1 tab.« less

Optical And Magnetic Properties Of Rigid Rod Conjugated Molecules

ABSTRACTA series of new stable fiinctionalised aryl ethynyl compounds bearing nitronyl-nitroxy and galvinoxyl radicals were synthesised as potential organic magnets with optical properties. Our goal is to construct biradicals in linear or broken rod molecules with the aim of probing i) the efficiency of the conjugation for the intramolecular spin exchange, and / or ii) the inter-Molecular Magnetic interaction between the neighboring radicals in the solid state Materials.We focused our attention on the preparation of phenoxy, and galvinoxyl biradicals of the conjugated phenyl and thienyl diethynyl Moieties. The synthesis of these new components will be presented. Their optical and magnetic properties will be discussed both in solution and in the solid state.

Unexpected Influence of Viscosity on Intramolecular Spin Exchange in Nitroxide Biradicals

The spin exchange dynamics and the rotational mobility of rigid-frame nitroxide biradicals have been studied by EPR; despite the intramolecular character of the exchange, its rate correlates with molecular mobility, being damped in viscous media, thus proving that even the simplest intramolecular reactions are controlled by molecular dynamics.

Effect of electron-withdrawing substituents in the pyrrolidine ring of the nitroxyl radical on intramolecular spin exchange in spin-labelled nickel (II) ?-dioximates

The value of the ¦J¦ integral in biradical spin-labelled metal complexes increases upon the introduction of electron-withdrawing substituents in the immediate vicinity of the fragment of the ligand.

Effect of adduct formation and solvation on intramolecular spin exchange in a spin-labeled nickel(II) ?-dioximate

1. The low-spin Ni(II) ion in the square planar spin-labeled α-dioximate NiL2 effectively exchanges spin density between the groups of the ligands. 2. Stepwise adduct formation NiL2 → NiL2B → NiL2B2 in the Ni-HL-toluene-neutral electron-donor compound system is observed and studied. Adduct formation is found to lead to a decrease of the value of the spin exchange integral ¦j¦ as a result of a decrease of the degree of covalency of the Ni bond with the ligand donor atoms in the equatorial plane.