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The electron spin resonance (ESR) spectroscopy technique was used to study various organic radicals, such as 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO), 4-hydroxy-TEMPO (TEMPOL), 2-X-nitronylnitroxide (2-X-NN, X = Ph, NO2Ph, or cyclohexyl), 4-Y-benzonitronylnitroxide (4-Y-PhBzNN, Y = Ph or NO2Ph), and 2-Z-iminonitroxide (2-Z-IN, Z = Ph or NO2Ph) dispersed in a polymethylmethacrylate (PMMA) matrix. The experiments were conducted at room temperature. The complex nature of the recorded ESR spectra could be attributed to the superposition of the rotational diffusion component of TEMPO (or TEMPOL) in the nanospace of the PMMA matrix with the rigid-limit component. A single component of the rigid-limit was observed for 2-X-NN and 4-Y-PhBzNN radicals dispersed in the PMMA matrix. The isotropic components of g and hyperfine ( A ) tensor, estimated by analyzing the solution spectra, were used to determine the g and A components of 4-Y-PhBzNN. Only the rotational diffusion component was observed for the 2-Z-IN radical. These results demonstrated that the PMMA matrix contains cylindrical nanospaces. Various radicals other than TEMPO derivatives could be used in the ESR spin probe technique as probe molecules for determining the structures, sizes, and shapes of the nanospaces.

Imino and nitronyl nitroxide (IN and NN, respectively) radicals such as phenyliminonitroxide (PhIN) and phenylnitronylnitroxide (PhNN), respectively, were dispersed in the organic 1D nanochannels of tris( o-phenylenedioxy)cyclotriphosphazene (TPP). Electron spin resonance (ESR) measurements were conducted on these inclusion compounds (ICs) in the temperature range 4.2-300 K. The modulated-septet ESR spectra of TPP ICs using PhIN observed in the range 165-258 K were reproduced with the EasySpin program package using a model in which some of the PhIN molecules underwent uniaxial rotational diffusion in the TPP nanochannels around the molecular long axis corresponding to the principal y-axis of the g tensor. However, for the TPP IC using PhNN, complicated ESR spectra were observed, which were not consistent with the modulated quintet observed in solution or in the fast-motion limit in solids. These spectra were reproduced by the superposition of a quintet originating from rotational diffusion of PhNN molecules and a septet based on rotational diffusion of PhIN molecules generated in the synthetic process. The rotational diffusion activation energies of PhIN and PhNN in the TPP nanochannels were estimated to be 19 and 45 kJ mol-1 using an Arrhenius plot, respectively. These were consistent with that for NN radicals in the 1D nanochannels of 2,4,6-tris(4-chlorophenoxy)-1,3,5-triazine (CLPOT) (37-54 kJ mol-1) with a larger pore diameter than TPP reported in our previous study, or with that for 4-substituted-2,2,6,6-tetramethyl-1-piperidinyloxyl (4-X-TEMPO) in TPP nanochannels (5-26 kJ mol-1) with regard to molecular size or host-guest or guest-guest interactions. These results indicate that not only the NN group but also IN may be used for the clarification of chemical or biological structures of nanomaterials such as nanosized cavities.

The fast reversible photochromic molecule was synthesized with the aid of triphenylimidazolyl radicals (TPI˙) and naphthalene linker. The crystal structure, photochemical properties, and kinetics of the target compound (1,8-TPID-naphthalene) were investigated. 1,8-TPID-naphthalene photochemically cleaved into 1,8-bisTPI˙- naphthalene and the color of the solution changed from colorless to green. ESR spectroscopy detected the light-induced triplet radical pair in frozen matrix. After the UV irradiation is ceased, the back-reaction of 1,8-bisTPI˙-naphthalene occurred by thermal conversion in the dark. The kinetic study on the back-reaction revealed that the reaction obeys the first-order kinetics with 2.04 s of half-life time at 295 K. The activation energy and frequency factor of the back-reaction were determined as 42.0 kJ/mol and 9.25 × 106 s−1, respectively. Copyright © 2007 John Wiley & Sons, Ltd.

A mixed-valence Mn6 cluster, [Mn6O2(t-BuCO2)10(H2O)(NNPy)2] (2), has been synthesized by the assembly of [Mn6O2(t-BuCO2)10(THF)4] · THF (1) with a nitronyl nitroxide radical molecule, 2-(p-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (NNPy). Complex 2 consists of [Mn6O2]10+ core and two NNPy radical attachments, where the [Mn6O2]10+ core is composed of two MnIII ions (i.e., the inner Mn ions) bridged by two μ4-O2− and four MnII ions (i.e., the outer Mn ions) surrounding the inner MnIII ions via the two μ4-O2−, forming an edge-shared double-tetrahedral polygon (vertices: Mn ions, centers: μ4-O2−). Bridges by pivalate groups in μ2- and μ3-modes assist the coordination spheres around these Mn ions. Two of the outer MnII ions in trans position are, to the sixth coordination site, occupied by NNPy molecule (pyridine-N coordination). One of the outer MnII ions is occupied by a water molecule to complete octahedral geometry, but the rest of them is vacancy taking a square-pyramidal geometry. The variable-temperature direct current (DC) magnetic susceptibility was collected in the temperature range of 1.81 to 300 K at a 1 kOe appled field. The molar magnetic susceptibility (χM) steadily increases from 0.053 emu mol−1 at 300 K to 0.29 emu mol−1 at 20 K, showing a kink (peak), and then increases rapidly to 0.70 emu mol−1 at 1.81 K. The profile of this magnetic behavior indicates that the intracluster spin cancellation takes place bellow 20 K to derive an SMn6 = 0 by strong antiferromagnetic interaction between Mn ions. This is consequently leading to paramagnetic isolation of only the two nitronyl nitroxide units as if remaining as exchange-uncorrelated spin carriers. The variable-temperature ESR spectra show a sharp signal below 30 K with g = 2.00, proving the presence of free nitronyl nitroxide radical, in which double integral of such signal obeys the Curie–Weiss law that follows the susceptibility result in the low-temperature region.

A novel photochromic hexaarylbiimidazole derivative with azobenzene moieties was synthesised and the molecular structure was determined by X-ray diffraction. The spectroscopic investigations for the photochromic behaviours revealed the presence of the independent four photochromic states, viz, triphenylimidazole dimers with trans- and cis-azobenzenes, and triphenylimidazolyl radicals with trans- and cis-azobenzenes.

The trinuclear complexes are synthesized by Cu(hfac) 2 and 5-substituted-1,3-bis(N-tert-butylaminoxyl)benzene (1 x ; X= H, F and Cl), π-conjugated bis-aminoxyl radical with a triplet ground state (1) . It is found that the [Cu(hfac) 2 ] 3 (1 x ) 2 complexes are paramagnetic and have a trinuclear cluster crystal structure. They have symmetric structure whereas the complex with X = Br is asymmetric. (2) While the complex with X = Br shows a ferromagnetic interaction within central three spins (J/k B = 33.9 K) and exhibits a first-order type structural transition around 48 K and below this temperature it has an S = 1/2 ground state, the complexes with X = F and Cl do not show any transformation. The intracluster exchanges for X = F and Cl are characterized by larger exchange parameters J/k B = 40.5 K and 41.6 K, respectively. They keep ferromagnetic interaction within the trimers down to lowest temperatures and their ground states have S = 3/2.

Bis(hexafluoroacetylacetonato) manganese(II), Mn(hfac) 2 , reacts with the bisnitroxide radicals, 5- R -1,3-bis( N - tert -butyl- N -oxyamino)benzene ( 1 R ) (R=H, Cl and Br), yielding one-dimensional (1-D) polymeric complexes of formula [Mn(hfac) 2 1 R ] n . X-ray analysis of the complexes has shown that they crystallize in the monoclinic space group P 2 1 / n . In this structure, the manganese(II) ions and 1 R molecules make up one-dimensional chains with the bisnitroxide radical serving as a bidentate ligand to Mn(II)(hfac) 2 . The 1 H complex orders antiferromagnetically at 5.5 K, while the 1 Cl and 1 Br complexes show ferrimagnetic order at 4.8 and 5.3 K, respectively. The intrachain exchange interaction parameters for a model of S =3/2 ferromagnetic chains were found to be, 2 J eff / k =23±2 K in all the compounds. J eff is the effective magnetic exchange interaction between units of NOMnNO. A change in the sign of the interchain exchange interaction is referred to as the change of the shortest exchange path, from the MnFN( tert -Bu)O ( 1 Cl and 1 Br ) to N( tert -Bu)O FN( tert -Bu)O ( 1 H )

We have designed and synthesised a new carborane derivative containing two nitronyl nitroxides. This molecule can be considered as a prototype of a three-dimensional analogue of o-benzoquinodimethane. The magnetic susceptibility of a crystalline sample revealed that this biradical possesses a weak antiferromagnetic interaction (theta = -1.05(4) K) and an ESR study in frozen matrix in the 5.4-104 K range gave an intramolecular antiferromagnetic interaction of -27(2) K. The synthesis, physical properties and DFT calculation result are also reported.

New molecular charge-transfer complexes of bis(ethylenedithio)tetrathiafulvalene (ET), (ET)Cu(2)Br(4) (1), (ET)(2)Cu(6)Br(10) (2), (ET)(2)[Cu(4)Br(6)ET] (3), (ET)(2)Cu(2)Br(4) (4), (ET)(2)Cu(3)Br(7)(H(2)O) (5), and (ET)(2)Cu(6)Br(10)(H(2)O)(2) (6), have been synthesized by diffusing reaction of ET and Cu(II)Br(2). Their crystal structures and physical properties have been investigated. X-ray analyses revealed that ET molecules coordinated to the copper ions with the sulfur atoms of the ethylenedithio groups in all compounds. The Cu-S distances are found in the range 2.268(5)-2.417(8) A, being close to the typical Cu(I)-S coordination bond distances. Strong d-pi interactions between d-electrons of the copper ions and pi-electrons of the ET molecules can be expected through the Cu-S coordination bonds. ET molecules behave as trans-bidentate ligands bonding to two different copper ions in 1 and 3, as cis-bidentate ligands in 2, 5, and 6, and as monodentate ligands in 4. In the crystal structure of 3, there are two types of ET molecules in the crystal structure, where the first type is a trans-bidentate ligand and the second forms a stacking structure by itself. Compounds 1, 2, 4, and 6 show semiconducting behavior down to low temperature (sigma(RT) = 1.6 x 10(-2) S cm(-1) and E(a) = 122 meV for 1, sigma(RT) = 2.1 S cm(-1) and E(a) = 21 meV for 2, sigma(RT) = 5.4 x 10(-4) S cm(-1) and E(a) = 239 meV for 4, and sigma(RT) = 5.1 x 10(-2) S cm(-1) and E(a) = 207 meV for 6). On the other hand, in 3, a metal-like region is observed along the b-axis and c-axis, due to the contribution of stacked ET molecules, and a metal-semiconductor transition occurs at 280 and 270 K, respectively. Also, 5 exhibits metallic conductivity in the temperature ranges 240-300 and 200-300 K along the b-axis and c-axis, respectively, despite the oxidation state of ET with 2+.

International audience; The new coordination complexes M(hfac)2(TTF–py)2 (M = CuII, MnII) in which the paramagnetic transition metal and the organic donor are covalently linked through delocalized π system are prepared aiming with a new approach for the occurrence of π–d interactions in conducting and magnetic molecule based materials. The structures, magnetic properties and electrochemical properties were investigated. X-ray structure analysis revealed its structure is favorable for electrical conductivity.

The novel complex trans-Cu(hfac)2(TTF-py)2 (hfac = hexafluoroacetylacetonate; TTF-py = 4-(2-tetrathiafulvalenyl-ethenyl)pyridine) in which the paramagnetic transition metal and the organic donor are covalently linked through a conjugated bridge is synthesized and characterized aiming at a new approach for the occurrence of π−d interactions in conducting and magnetic molecule-based materials.

A carborane derivative containing nitronyl nitroxide group was synthesized and was crystallographically and magnetically characterized. The X-ray structure analysis revealed that the molecules were...

This study provides conclusive proof that the thermally excited open-shell state with biradical character is contributing to the ground state of a closed-shell molecule, tF-BDPI-2Y, where four hydrogen atoms at the central phenylene ring are substituted with four fluorine atoms of 1,4-bis-(4,5-diphenylimidazol-2-ylidene)cyclohexa-2,5-diene (BDPI-2Y). A small increase in the population of biradical species of tF-BDPI-2Y results in the formation of the dimer form by the radical recombination reaction. Controlling the equilibrium between a closed-shell diamagnetic-quinoid state and an open-shell paramagnetic-biradical state will provide significant progress in the field of pi-conjugated delocalized biradical chemistry.

Bis(hexafluoroacetylacetonato)copper(II) Cu(hfac)2(=M) reacts with 5-bromo-1,3-phenylenebis(N-tert-butylaminoxyl) 1 to yield a complex formula [M3·12]. The X-ray crystal structure shows that it crystallizes in the space group P1, with a = 12.469(2) A, b = 15.278(2) A, c = 11.602(2) A, α = 104.59(1)°, β = 111.86(1)°, γ = 88.32(1)°, and Z = 1, and contains molecules -M-1-M-1-M-aligned in a polymeric chain (Scheme 1). The magnetic susceptibility measurements revealed a structural transition at ca. 48 K accompanied by a temperature hysteresis.