Radical Crystals Research Articles

New Method for Preparation of Isolable N-tert-Alkoxyarylaminyl Radicals

Abstract Stable N-tert-alkoxy-2,4-diphenyl-6-tert-butylphenylaminyl radicals were prepared by heating 2,4-diphenyl-6-tert-butylphenylnitrosobenzenes and azo compounds in refluxing benzene and isolated as radical crystals. Their structures were confirmed by the elemental analyses, ESR, and X-ray crystallography.

Organic Radical Crystals Based on a TEMPO–substituted Pyridinium with Radical Anions

Abstract Organic crystals of the TCNQ or TCNQF4 radical anion salts derived from 2,4,6-triphenylpyridinium carrying TEMPO radical were obtained as a couple of different forms and their magnetic properties were found to vary markedly depending on the forms.

Photoinduced diamagnetic to paramagnetic phase transition in organic radical crystals studied by microscopic IR measurements

Photoinduced magnetic phase transition in an organic radical 1,3,5-trithia–2,4,6-triazapentalenyl (TTTA) crystal has been studied by microscopic IR measurements. Both photoexcitations of the charge-transfer and intramolecular π–π transition bands induce a low-temperature diamagnetic to high-temperature paramagnetic phase transition but with different threshold photon densities. The behavior of the photoinduced magnetic phase transition in TTTA is discussed in comparison with previous results measured by the photoinduced Raman and ESR measurements.

Physical properties of electrically conducting and stable molecular neutral radical solid [Co(2,3-Nc)(CN)2]CH3CN (2,3-Nc = 2,3-naphthalocyanine)

Solid state properties of dicyano(2,3-naphthalocyaninato)cobalt(III) neutral radical crystal, [ Co (2,3- Nc )( CN )2] CH 3 CN , were characterized by measurements of the resistivity under high pressure and under uniaxial strain, thermoelectric power, magnetic susceptibility, ESR and polarized reflectance spectra. The title compound exhibited thermally activated-type electrical conductivity along the c-axis. The room temperature (RT) resistivity ρRT along the c-axis and activation energy Ea rapidly decreased with increasing pressure. The temperature-dependent thermoelectric power S was that of a typical one-dimensional (1D) semiconductor. However the high absolute value of S suggested that this electronic system should be strongly correlated. Although the electrical resistivity exhibited monotonous temperature-dependence, the magnetic susceptibility clearly indicated a Peierls-type transition and marked fluctuation from RT. Both Peierls-type transitions and fluctuations are characteristic phenomena to 1D conductors. Furthermore ESR spectra manifested that the Peierls-type transition occurred at 100 K. The inconsistency between the electrical behavior (without a phase-transition) and magnetic behavior (with a phase-transition) indicates separation of the degrees of freedom in spin and charge (spin-charge separation) of this material. Spin-charge separation is a theoretically predicted phenomenon peculiar to the 1D conductors with strong correlation. The reflectance spectra were quantitatively explained by a 1D Hubbard model, and manifested the existence of a structural fluctuation of this material from RT. Based on these observed physical properties it is concluded that [ Co (2,3- Nc )( CN )2] CH 3 CN is a strongly correlated 1D semiconductor with a Mott(Hubbard) type energy gap and characterised with a fluctuation and spin-charge separation.

Magnetic interactions in nonalternant mixed molecular radical crystals and mixed ion radical crystals

AbstractThe character and the magnitude of magnetic interaction in 1‐D stacks of mixed molecular radical crystals (MMRCs) and mixed molecular ion radical crystals (MMIRCs) of alternant aromatic hydrocarbons have already been studied theoretically. In this work nonalternant 1‐D stacks built of π‐electronic subunits are considered. Some of the elementary units are singly or doubly charged (MMIRCs), others are neutral (MMRCs); alternant and nonalternant subunits are used as building blocks. Qualitative investigations based on the band theory of exchange effects in solid organic open‐shell systems are carried out. The contributions to the effective exchange integral Jeff: direct (Hund's), kinetic and indirect, are calculated. Structures of 1:1 MMRCs and MMIRCs with positive value of the integral Jeff, accounting for the spin exchange of the electrons in the half‐filled band, i.e., with ferromagnetic ground state, are suggested. The influence of different parameters (molecular size, presence of heteroatoms, localization versus delocalization, alternant versus nonalternant, charge, etc.) on the magnitude of the net exchange is discussed. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004

ESR study of stereochemistry in chiral nitroxide radical crystals

The distinct differences in ESR spectra were revealed for the polycrystalline samples of the enantiomer and racemate of nitroxide 1, as well as of their diastereomeric salts with optically active amine 2.

Isolation and magnetic properties of heterocycle-carrying N-alkoxyarylaminyl radicals.

Two N-tert-butoxy-2,6-diaryl-4-(4-pyridyl)phenylaminyls (1) and three N-tert-butoxy-2,6-diaryl-4-(1H-imidazol-1-yl)phenylaminyls (2) were prepared by the reaction of the lithium salts of the corresponding anilines with tert-butyl peroxybenzoate. Although 1 could not be isolated as radical crystals, 2 was successfully obtained as red crystals. The X-ray crystallographic analysis and magnetic susceptibility measurements were performed for one isolated radical.

Ferromagnetic behavior of formyl-group-carrying stable thioaminyl radicals.

Four formyl-group-carrying thioaminyl radicals were generated, and one radical could be isolated as radical crystals. Magnetic susceptibility measurements of the isolated radical showed a ferromagnetic regular linear-chain interaction of 2J/k(B) = 3.2 K, which was explained in terms of the X-ray crystallographic results.

Photoinduced phase transition in an organic radical crystal with room-temperature optical and magnetic bistability.

A phase control by photoirradiation is successfully achieved in a spin-Peierls system of the organic radical crystal, 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA), which exhibits optical and magnetic bistability around room temperature with a large hysteresis loop. A nanosecond laser pulse is found to induce a transition from a diamagnetic low-temperature phase to a paramagnetic high-temperature phase both inside (296 K) and outside (11 K) the hysteresis loop. Comparison of the excitation energy dependence between transition efficiency and photoconductivity suggests that the photoinduced transition is driven by suppression of the spin-Peierls instability by the accumulation of photocarriers.

Heat capacity study of the doping effect of paramagnetic impurity in organic spin-Peierls system: p-CyDOV radical crystal

Heat capacity studies were performed for the doped verdazyl radical crystals, (p-CyDOV)1−x(p-CyDTV)x (x=0, 0.01, and 0.07), to clarify the effect of magnetic impurity (p-CyDTV) on the spin-Peierls (SP) transition (TSP=15.0 K) of 3-(4-cyanophenyl)-1,5-dimethyl-6-oxoverdazyl (p-CyDOV) radical crystal, where the >CO group in p-CyDOV is replaced by the >CS group in p-CyDTV. The antiferromagnetic (AFM) transitions were observed at TN=0.135, 0.290, and 0.164 K for the crystals with x=0, 0.01, and 0.07, respectively, indicating the coexistence of AFM order and SP state in the systems with x=0 and 0.01, and the appearance of the single phase of AFM order in the system with x=0.07 below TN=0.164 K. However, the heat capacity anomaly associated with the SP transition was not found at TSP=15.0 K for the pure p-CyDOV crystal (x=0), whereas the anomalous broad peak in the heat capacity was observed at T′SP=5.6 K. The entropy consumption in the above broad peak was about 3% of the total spin entropy Rln2, suggesting that this transition may relate to SP transition. The pressure induced by the solidification of the Apiezon-N grease, which was used as the thermal binder for the heat capacity measurement, at low temperature may affect the shift of the SP transition temperature. The doping effects for the organic SP system (p-CyDOV), which include the temperature (T: TSP and TN)—impurity (x) phase diagram, are similar to those for the inorganic SP system (CuGeO3), although the one-dimensionality, i.e., ∣zJ′/J∣≅6×10−4, in the organic p-CyDOV system is much higher than that (∼10−1) in the inorganic CuGeO3 system, where zJ′ and J are inter- and intra-chain exchange interactions. This work is the first report of the heat capacity study of the doping effect on the organic SP transition system.

Magnetic phase transitions in organic radical crystals studied by electron spin resonance

Electron spin resonance spectra of a single crystal of organic radical, 5-carboxy-2-thienyl nitronyl nitroxide, exhibiting a magnetic phase transition were measured over the temperature range, which includes the transition temperature of the radical. The radical shows discontinuous changes in the g factor and the line width at the transition temperature accompanied with considerable thermal hysteresis.

Magnetic interaction of pyridyl-substituted thioaminyl stable free radicals.

N-(2-Pyridylthio)-2,6-diaryl-4-R-phenylaminyls (R = Ph, 4-ClC(6)H(4), MeCO, CN, EtOCO) and N-(4-pyridylthio)-2,6-diaryl-4-R-phenylaminyls (R = Ph, 4-ClC(6)H(4), EtOCO) were prepared and isolated as radical crystals. Their ESR spectra were measured, and the NS and pyridyl nitrogen and anilino meta and pyridyl ortho and para proton hyperfine coupling constants were determined. The spin-density calculations based on the density functional theory were performed by the UBecke 3LYP hybrid method using the STO 6-31G basis set. X-ray crystallographic analyses were performed for three radicals, and their structures were discussed in detail. The magnetic susceptibility measurements were carried out for the nine kinds of isolated radicals with a SQUID magnetometer. One radical showed ferromagnetic coupling (2J/k(B) = 44 K), and the others showed antiferromagnetic behavior. The magnetic interactions observed are explained on the basis of the crystal structures.

Synthesis, structure, and physical properties of a new phenalenyl‐based neutral radical crystal: Correlation between structure and transport properties in carbon‐based molecular conductors

AbstractA new class of phenalenyl‐based neutral radical molecular conductors has recently been reported; a distinctive feature of these molecular solids is the absence of the well‐defined conducting pathway(s) that are characteristic of the organic charge‐transfer salts. These radicals do not stack in the solid state, and the requisite carbon–carbon contacts are all larger than the sum of the van der Waals distances. Magnetic susceptibility measurements show that these compounds usually exist as isolated free radicals with one spin per molecule (at least over some temperature range), apparently supporting the idea that there is little interaction between the molecules in the solid state. Nevertheless, these radicals show the highest conductivity (σRT = 0.05 S/cm) of any neutral organic solid and the conduction mechanism is at present unresolved. In this article we report a structurally related radical with a resistivity five orders of magnitude higher than that of the previously reported radicals. We analyze the crystallographic and electronic structure of this solid in detail and point out some of the challenges that remain in relating structure, magnetism, and conductivity in this new class of organic solids. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2003

N-tert-Butoxy-1-aminopyrenyl Radicals. Isolation, Electronic Structure, and Magnetic Characterization.

N-tert-Butoxy-2,7-di-tert-butyl-1-pyrenylaminyl (4), N-tert-butoxy-2-tert-butyl-1-pyrenylaminyl (5), and N-tert-butoxy-7-tert-butyl-1-pyrenylaminyl (6) free radicals were generated by the reaction of the lithium amides of the corresponding 1-aminopyrenes with tert-butyl peroxybenzoate in THF at -78 degrees C. Although 6 could not be isolated due to the gradual decomposition in solution, 4 and 5 were quite persistent and could be isolated as monomeric radical crystals. The X-ray crystallographic analyses for the isolated free radicals were successfully carried out, indicating that the N and O atoms are almost coplanar with the pyrene ring. The ESR spectra of 4 and 5 were very complex due to the presence of many magnetically unequivalent protons. Therefore, the proton hyperfine coupling (hfc) constants were determined by (1)H ENDOR/TRIPLE resonance spectroscopy. To assign the hfc constants for the pyrene ring protons, a partially deuterated radical, 4-d(4), was prepared and the ENDOR and ESR spectra were measured. To discuss the spin density distribution for 4 and 5 ab initio molecular orbital calculations were performed by the DFT UBecke 3LYP method, using the STO 6-31G basis set. Magnetic susceptibility measurements were carried out for 4 and 5 with a SQUID magnetometer. For 4 a weak antiferromagnetic interaction was observed, and for 5 a very strong antiferromagnetic interaction was observed. The antiferromagnetic interactions were explained by their crystal structures.

High-Field Magnetization Study of the Doping Effect of Magnetic Impurity on the Organic Spin-Peierls System:p-CyDOV Radical Crystal

The field dependence of the magnetization ( H =0- 55 T) of the mixed verdazyl radical crystals, ( p -CyDOV) 1- x ( p -CyDTV) x ( x =0, 0.01, 0.02, 0.03, 0.05, and 0.07) was measured to study the doping effect of magnetic impurity ( p -CyDTV) on the organic spin-Peierls (SP) system, 3-(4-cyanophenyl)-1,5-dimethyl-6-oxoverdazyl ( p -CyDOV) radical crystal, where the >C=O group in p -CyDOV is replaced by the >C=S group in p -CyDTV. The effect of doping on SP system is very remarkable. We have found that the high-field magnetization M ( x , H ) curves of the doped organic SP system below T SP =15.0 K can be well explained by the sum of the magnetization ( M ) of the spin-Peierls state ( C SP ( x ) M SP ( H )), one dimensional (1D) Heisenberg antiferromagnet ( C 1D-HAF ( x ) M 1D-HAF ( H )), and Curie impurity ( C Curie ( x ) M Curie ( H )), where C i ( x ) is the fraction of the i th contribution. The result suggests the coexistence of SP state and antiferromagnetic long-range order in the doped p -CyDOV radi...

Anomalous Magnetic Behavior in Three Kinds of 3-(Aryl-substituted)-1,5-diphenylverdazyl Radical Crystals (p-FPDV, p-PyDV and m-PyDV) Induced by Frustrated Spin Interaction

Magnetic susceptibilities (χM) of three kinds of 3-(aryl-substituted)-1,5-diphenyl-verdazyl radical crystals (aryl: 4-fluorophenyl (p-FPDV), 4-pyridyl (p-PyDV), and 3-pyridyl (m-PyDV)) have been measured between 1.8 and 300 K, showing a broad maximum Tmax at 200, 272, and 157 K, respectively. The values of Tmax observed are 1 or 2 orders of magnitude larger than those for usual organic neutral radical crystals, suggesting strong intermolecular antiferromagnetic (AFM) exchange interactions (J) in the crystals. The temperature dependence of the susceptibilities cannot be explained by a simple model, such as the one-dimensional (1D) Heisenberg AFM linear- and alternating-chain model. The values of χM observed for the p-FPDV, p-PyDV, and m-PyDV radicals are about 42, 76, and 35% larger than the corresponding values calculated by a 1D Heisenberg AFM linear-chain model at each temperature. The result of the crystal structure analysis of p-FPDV indicates the existence of butterfly-type spin frustration in the s...

High-Field Magnetization and Magnetic Susceptibility Studies of the Doping Effect of Nonmagnetic Impurities on the Organic Spin-Peierls System: p-CyDOV Radical Crystal

Doping effect of nonmagnetic impurities, p-CyDOV-H (reduced form of p-CyDOV), on the organic spin-Peierls (SP) system, 3-(4-cyanophenyl)-1,5-dimethyl-6-oxoverdazyl (p-CyDOV) radical crystal, has been studied by measuring the temperature dependence of the magnetic susceptibility (T = 2.0−300 K) and the field dependence of the magnetization (H = 0−55 T) of the diluted p-CyDOV radical crystals, (p-CyDOV)1-x(p-CyDOV-H)x (x = 0, 0.01, 0.03, 0.05, 0.07, and 0.10). The effect of doping on SP system is very remarkable. The SP transition temperature (TSP = 15.0 K) decreases rapidly upon p-CyDOV-H doping and the SP state almost disappears in the lightly doped sample with x = 0.05. The susceptibility and magnetization of diluted radical crystal with x = 0.01 and 0.03 below TSP can be explained by the sum of the susceptibilities and magnetizations of the spin-Peierls state, antiferromagnet, and Curie impurity, respectively, indicating the coexistence of SP and antiferromagnetic phases in the doped SP system at low temperature. This work is the first report of the high-field magnetization study of the doping effect of nonmagnetic impurities on the organic SP transition system.

Structure and Magnetic Interaction in Organic Radical Crystals. 6. Spin-Transfer Crystals: A Theoretical Study

The nature of the spin exchange interaction in half-filled bands of one-dimensional (1-D) stacks of mixed molecular radical crystals (MMRC) with the general formula ...R * ....H....R * ....H.... (R * is a π radical, and H is a diamagnetic polycyclic molecule) has been investigated theoretically. The intermolecular interaction is accompanied by spin transfer from R * to H and determines the spin-spin interaction within the 1-D stacks of the MMRCs. The nature and the magnitude of the magnetic interaction in these structures are investigated using the band theory. At a given topology and geometry of the MMRC stacks the antiferromagnetic interaction in the parent molecular radical crystals...R * ....R * .... becomes ferromagnetic in the MMRCs and vice versa. It was found that the effective exchange integral J e f f of MMRCs, belonging to a given group with one and the same radical but different H, is proportional to the diamagnetic susceptibility Χ(H). A new topological effect of the exchange interaction in MMRCs is formulated.

Controlled π-π Stacking Structure in Phthalocyanine-based Conductors

Axially substituted metal-phthalocyanines are utilized for constructing molecular conductors. The starting component, [M(Pc)(CN) 2 ] − (Pc=phthalocyaninato), has been designed to achieve the multi-dimensional π-π interaction, and the conducting crystals have been obtained by the electrochemical oxidation. A variety of the π-π stacking structures, ranging from one- to three-dimensions, have been obtained depending on the crystal solvent for the neutral radical crystals of Co(Pc)(CN) 2 . They are semiconducting with relatively high conductivity (10 −2 -10 0 S cm −1 ) at room temperature. The partially oxidized salt crystals obtained have a narrow one-dimensional metallic band. The metallic nature has been confirmed by the thermoelectric power measurements, though the conductivity is not always clearly metallic. A dramatic change in the conductivity and magnetic properties has been observed by the replacement of the central metal ion from non-magnetic Co III to magnetic Fe III .

Nature of the Magnetic Interaction in Organic Radical Crystals. 5. Magnetic Interaction in Mixed Radical Ion Crystals

The nature and magnitude of the spin exchange interaction within the half-filled band of 1-D stacks of mixed radical ion crystals (MRIC) consisting of two hydrocarbons H1 and H2 with different ionization potentials and electron affinities have been investigated theoretically. In a fully reduced or oxidized 1-D crystal, each elementary unit is an anion radical or a cation radical: (H1 ...H2)•- - - or - - (H1 ...H2)•+ - -. In contrast to organic radical cation (anion) crystals which consist of identical polycyclic hydrocarbons −(Hδ···Hδ)•+− or −(Hδ···Hδ)•-− with a metalic or semiconducting ground state, the ground state of some classes of MRICs is a magnetic one. The band theory of magnetic interaction in many electron π-systems is applied to calculate the different contributions of the effective Heisenberg exchange integral.