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The crystal structure of the radical ferromagnet 1a at 2 K reveals a contraction in the unit cell c constant which, at the molecular level, translates into a decrease in slippage of the radical π-stacks and an increase in ferromagnetic exchange interactions along the stacking axis. The results of BS-DFT calculations using long-range corrected functionals are consistent with an overall ferromagnetic topology.

The nitroxyl radical 1-methyl-2-azaadamantane N-oxyl (Me-AZADO) exhibits magnetic bistability arising from a radical/dimer interconversion. The transition from the rotationally disordered paramagnetic plastic crystal, Me-AZADO, to the ordered diamagnetic crystalline phase, (Me-AZADO)2, has been conclusively demonstrated by crystal structure determination from high-resolution powder diffraction data and by solid-state NMR spectroscopy. The phase change is characterized by a wide thermal hysteresis with high sensitivity to even small applied pressures. The molecular dynamics of the phase transition from the plastic crystal to the conventional crystalline phase has been tracked by solid-state (1H and 13C) NMR and EPR spectroscopies.

The isostructural dimers of the 1,4-phenylene-bridged bis-1,2,3,5-dithia- and bis-1,2,3,5-diselenadiazolyl diradicals 1,4-S/Se are small band gap semiconductors. The response of their molecular and solid state electronic structures to pressure has been explored over the range 0-10 GPa. The crystal structures, which consist of cofacially aligned (pancake) π-dimers packed into herringbone arrays, experience a continuous, near-isotropic compression. While the intramolecular covalent E-E (E = S/Se) bonds remain relatively unchanged with pressurization, the intradimer E···E separations are significantly shortened. Molecular and band electronic structure calculations using density functional theory methods indicate that compression of the π-dimers leads to a widening of the gap Δ E between the highest occupied and lowest unoccupied molecular orbitals of the dimer, an effect that offsets the expected decrease in the valence-to-conduction band gap E

Materials with magnetic properties that can be modulated via optical excitation offer enticing opportunities for the development of next generation technologies. An exciting new class of photomagnetic materials based on bisdithiazolyl radicals has recently been developed. The dimorphic system in this study crystallizes in two phases, one composed of diamagnetic dimers and the other of paramagnetic radicals. Here we report on the use of high-field electron paramagnetic resonance spectroscopy to characterize both the thermally- and light-induced transitions in the dimer phase. During the course of this study we show that signals originating from residual radical defects in the dimer phase can be differentiated from those arising from the radical phase.

Crystals of the heterocyclic radical naphtho-1,3,2-dithiazolyl NDTA display magnetic bistability with a well-defined hysteretic phase transition at Tc↓ = 128(2) K and Tc↑ = 188(2) K. The magnetic signature arises from a radical/dimer interconversion involving one of the two independent π-radicals in the P1 unit cell. Variable temperature X-ray crystallography has established that while all the radicals in HT-NDTA serve as paramagnetic (S = 1/2) centers, half of the radicals in LT-NDTA form closed-shell N–N σ-bonded dimers (S = 0) and half retain their S = 1/2 spin state. The wide window of bistability (60 K) may be attributed to the large structural changes that accompany the phase transition.

The exceptional performance of oxobenzene-bridged bis-1,2,3-dithiazolyls 6 as single-component neutral radical conductors arises from the presence of a low-lying π-lowest unoccupied molecular orbital, which reduces the potential barrier to charge transport and increases the kinetic stabilization energy of the metallic state. As part of ongoing efforts to modify the solid-state structures and transport properties of these so-called multiorbital materials, we report the preparation and characterization of the acetoxy, methoxy, and thiomethyl derivatives 6 (R = OAc, OMe, SMe). The crystal structures are based on ribbonlike arrays of radicals laced together by S···N' and S···O' secondary bonding interactions. The steric and electronic effects of the exocyclic ligands varies, affording one-dimensional (1D) π-stacked radicals for R = OAc, 1D cofacial dimer π-stacks for R = SMe, and a pseudo two-dimensional (2D) brick-wall arrangement for R = OMe. Variable-temperature magnetic and conductivity measurements reveal strong antiferromagnetic interactions and Mott insulating behavior for the two radical-based structures (R = OAc, OMe), with lower room-temperature conductivities (σRT ≈ 1 × 10-4 and ∼1 × 10-3 S cm-1, respectively) and higher thermal activation energies ( Eact = 0.24 and 0.21 eV, respectively) than found for the ideal 2D brick-wall structure of 6 (R = F), where σRT ≈ 1 × 10-2 S cm-1 and Eact = 0.10 eV. The performance of R = OMe, OAc relative to that of R = F, is consistent with the results of density functional theory band electronic structure calculations, which indicate a lower kinetic stabilization energy of the putative metallic state arising from their reduced electronic dimensionality.

The high-pressure magnetic properties of the iodo-substituted bisdiselenazolyl radical ferromagnet IBPSSEt have been studied by vibrating-coil SQUID magnetometry. The magnetic state at a pressure (P) of approximately 2 GPa has the highest Curie temperature (TC) of 27.5 K, and displays an ideal three-dimensional (3D) ferromagnetic interaction network. The value of TC observed by ac magnetic susceptibility measurements is consistent with that obtained from dc measurements below approximately 4 GPa. Field-cooled dc measurements at more elevated pressures reveal a slow evolution of magnetic ordering, so that atP >6 GPa the structure may be described in terms of a 1D ferromagnetic chain with predominantly antiferromagnetic lateral (interchain) interactions, in accord with the results of density functional theory calculations.

The N-methyl-4-phenyl-pyridine-bridged bisdithiazolyl radical PhBPMe is polymorphic, crystallizing from cold acetonitrile in a trigonal α-phase, space group P3121, and from hot dichloroethane in an orthorhombic β-phase, space group Pca21. The crystal structures of both phases consist of slipped π-stacks of undimerized radicals aligned laterally into herringbone arrays. In the β-phase, there are two independent radicals in the asymmetric unit, and the resulting π-stacks form corrugated layers interspersed by methyl and phenyl groups which block the approach of neighboring radicals. In the α-phase, the methyl/phenyl groups and the radical π-stacks separately form spirals about 31 axes, the latter giving rise to a 3D network of close radical/radical contacts. Variable temperature magnetic susceptibility measurements on the β-phase indicate strong antiferromagnetic coupling. Weaker but predominantly antiferromagnetic interactions (θ = -20.7 K) are observed in the α-phase. A high temperature series expansion analysis of the magnetic data for the α-phase affords antiferromagnetic exchange energies for the one- and two-step radical/radical interactions about the 31 spirals ( J1 = -1.2 K, J2 = -10.9 K, respectively), with weak ferromagnetic interactions along the π-stacks ( Jπ = +1.8 K). Despite the presence of a 3D network based on the dominant J2 interactions, which affords two independent bipartite sublattices, no evidence of bulk antiferromagnetic order has been observed above T = 2 K. The magnetic results are discussed in light of exchange energies calculated using density functional theory broken symmetry methods.

In the solid state the iodo-substituted bisdiselenazolyl radical 1c orders as a ferromagnet with TC = 10.5 K. With the application of pressure TC rises rapidly, reaching a value of 27.5 K at 2.4 GPa. The accompanying structural and magnetic changes have been examined by high resolution powder X-ray diffraction and by DFT calculations of magnetic exchange interactions.

International audience; Condensation of persilylated nicotinimideamide and isonicotinimideamide with sulfur monochloride affords double salts of the 3-, 4-pyridyl-substituted 1,2,3,5-dithiadiazolylium DTDA cations of the general formula [3-, 4-pyDTDA][Cl][HCl] in which the pyridyl nitrogen serves as a noninnocent base. Reduction of these salts with triphenylantimony followed by deprotonation of the intermediate-protonated radical affords the free base radicals [3-, 4-pyDTDA], the crystal structures of which, along with those of their diselenadiazolyl analogues [3-, 4-pyDSDA], have been characterized by powder or single-crystal X-ray diffraction. The crystal structures consist of “pancake” π-dimers linked head-to-tail into ribbonlike arrays by η2-S2---N(py) intermolecular secondary bonding interactions. Methylation of the persilylated (iso)nicotinimide-amides prior to condensation with sulfur monochloride leads to N-methylated double chloride salts Me[3-, 4-pyDTDA][Cl]2, which can be converted by metathesis into the corresponding triflates Me[3-, 4-pyDTDA][OTf]2 and then reduced to the N-methylated radical triflates Me[3-, 4-pyDTDA][OTf]. The crystal structures of both the N-methylated double triflate and radical triflate salts have been determined by single-crystal X-ray diffraction. The latter consist of trans-cofacial π-dimers strongly ion-paired with triflate anions. Variable temperature magnetic susceptibility measurements on both the neutral and radical ion dimers indicate that they are diamagnetic over the temperature range 2–300 K.

Pressure-induced changes in the solid-state structures and transport properties of three oxobenzene-bridged bisdithiazolyl radicals 2 (R = H, F, Ph) over the range 0–15 GPa are described. All three materials experience compression of their π-stacked architecture, be it (i) 1D ABABAB π-stack (R = Ph), (ii) quasi-1D slipped π-stack (R = H), or (iii) 2D brick-wall π-stack (R = F). While R = H undergoes two structural phase transitions, neither of R = F, Ph display any phase change. All three radicals order as spin-canted antiferromagnets, but spin-canted ordering is lost at pressures

Benzoquino-bis-1,2,3-dithiazole 5 is a closed shell, antiaromatic 16π-electron zwitterion with a small HOMO-LUMO gap. Its crystal structure consists of planar ribbon-like molecular arrays packed into offset layers to generate a "brick-wall" motif with strong 2D interlayer electronic interactions. The spread of the valence and conduction bands, coupled with the narrow HOMO-LUMO gap, affords a small band gap semiconductor with σRT = 1 × 10-3 S cm-1 and Eact = 0.14 eV for transport within the brick-wall arrays. Closure of the band gap to form an all-organic molecular metal with σRT > 101 S cm-1 can be achieved by the application of pressure to 8 GPa.

A critical feature of the electronic structure of oxobenzene-bridged bisdithiazolyl radicals 2 is the presence of a low-lying LUMO which, in the solid state, improves charge transport by providing additional degrees of freedom for electron transfer. The magnitude of this multiorbital effect can be fine-tuned by variations in the π-electron releasing/accepting nature of the basal ligand. Here we demonstrate that incorporation of a nitro group significantly stabilizes the LUMO, and hence lowers Ueff, the effective Coulombic barrier to charge transfer. The effect is echoed, at the molecular level, in the observed trend in Ecell, the electrochemical cell potential for 2 with R = F, H and NO2. The crystal structures of the MeCN and EtCN solvates of 2 with R = NO2 have been determined. In the EtCN solvate the radicals are dimerized, but in the MeCN solvate the radicals form superimposed and evenly spaced π-stacked arrays. This highly 1D material displays Pauli-like temperature independent paramagnetic behavior,...

// Oihana Murillo-Sauca 1,5,6 , Man Ki Chung 1,5,6 , June Ho Shin 1,5,6 , Christina Karamboulas 7 , Shirley Kwok 4 , Young Ho Jung 1,5,6 , Richard Oakley 1,5 , James R. Tysome 1,5 , Lovisa O. Farnebo 1,5,6 , Michael J. Kaplan 1,5 , Davud Sirjani 1,5 , Vasu Divi 1,5 , F. Christopher Holsinger 1,5 , Chafeek Tomeh 1,5 , Anthony Nichols 8 , Quynh T. Le 2,5 , A. Dimitrios Colevas 3,5 , Christina S. Kong 4,5 , Ravindra Uppaluri 9 , James S. Lewis Jr. 9,10 , Laurie E. Ailles 7 and John B. Sunwoo 1,5,6 1 Division of Head and Neck Surgery, Department of Otolaryngology, Stanford University School of Medicine, Stanford, CA 2 Department of Radiation Oncology, Stanford University School of Medicine, Stanford, CA 3 Department of Medicine, Stanford University School of Medicine, Stanford, CA 4 Department of Pathology, Stanford University School of Medicine, Stanford, CA 5 Stanford Cancer Institute, Stanford University School of Medicine, Stanford, CA 6 Stanford Institute for Stem Cell Biology and Regenerative Medicine, Stanford University School of Medicine, Stanford, CA 7 Ontario Cancer Institute, University Health Network, Toronto, Canada 8 Department of Otolaryngology – Head and Neck Surgery, Victoria Hospital, Schulich School of Medicine and Dentistry, London, Ontario, Canada 9 Department of Otolaryngology – Head and Neck Surgery, Washington University School of Medicine, St. Louis, MO 10 Department of Pathology and Immunology, Washington University School of Medicine, St. Louis, MO Correspondence: John B. Sunwoo, email: // Keywords : HNSCC, cancer stem cells, NGFR, p75NTR, cancer-initiating cell, CD44 Received : May 22, 2014 Accepted : July 26, 2014 Published : July 26, 2014 Abstract Tumor-initiating cells (TICs) in squamous cell carcinoma of the head and neck (SCCHN) are best characterized by their surface expression of CD44. Although there is great interest in identifying strategies to target this population, no marker of these cells has been found to be functionally active. Here, we examined the expression of the purported marker of normal human oral epithelial stem cells, CD271. We show that CD271 expression is restricted to a subset of the CD44 + cells. Using xenograft assays, we show that the CD44 + CD271 + subpopulation contains the most tumorigenic cells. Loss of CD271 function results in a block in the G2-M phase of the cell cycle and a profound negative impact on the capacity of these cells to initiate tumor formation in vivo . Incubation with recombinant NGF results in enhanced phosphorylation of Erk, providing additional evidence that CD271 is functionally active. Finally, incubation of SCCHN cells with antibody to CD271 results in decreased Erk phosphorylation and decreased tumor formation in vivo . Thus, our data are the first to demonstrate that CD271 more specifically identifies the TIC subpopulation within the CD44 + compartment in SCCHN and that this receptor is a functionally active and targetable molecule.

The heterocyclic bisdithiazolyl radical 1b (R1 = Me, R2 = F) crystallizes in two phases. The α-phase, space group P2₁/n, contains two radicals in the asymmetric unit, both of which adopt slipped π-stack structures. The β-phase, space group P2₁/c, consists of cross-braced π-stacked arrays of dimers in which the radicals are linked laterally by hypervalent 4-center 6-electron S···S-S···S σ-bonds. Variable-temperature magnetic susceptibility measurements on α-1b indicate Curie-Weiss behavior (with Θ = -14.9 K), while the dimer phase β-1b is diamagnetic, showing no indication of thermal dissociation below 400 K. High-pressure crystallographic measurements indicate that the cross-braced π-stacked arrays of dimers undergo a wine-rack compression, but the dimer remains intact up to 8 GPa (at ambient temperature). The resistance of β-1b to dissociate under pressure, also observed in its conductivity versus pressure profile, is in marked contrast to the behavior of the related dimer β-1a (R1 = Et, R2 = F), which readily dissociates into a pair of radicals at 0.8 GPa. The different response of the two dimers to pressure has been rationalized in terms of differences in their linear compressibilities occasioned by changes in the degree of cross-bracing of the π-stacks. Dissociation of both dimers can be effected by irradiation with visible (λ = 650 nm) light; the transformation has been monitored by optical spectroscopy, magnetic susceptibility measurements, and single crystal X-ray diffraction. The photoinduced radical pairs persist up to temperatures of 150 K (β-1b) and 242 K (β-1a) before reverting to the dimer state. Variable-temperature optical measurements on β-1b and β-1a have afforded Arrhenius activation energies of 8.3 and 19.6 kcal mol(-1), respectively, for the radical-to-dimer reconversion. DFT and CAS-SCF calculations have been used to probe the ground and excited electronic state structures of the dimer and radical pair. The results support the interpretation that the ground-state interconversion of the dimer and radical forms of β-1a and β-1b is symmetry forbidden, while the photochemical transformation is symmetry allowed.

In the solid state the iodo-substituted bisdiselenazolyl radical 1c orders as a ferromagnet with T

Irradiation in the solid state of the hypervalent 4c-6e S···S-S···S bridged σ-dimer of a bisdithiazolyl radical leads to its photodissociation into a pair of π-radicals. The transformation has been monitored by optical spectroscopy, single crystal X-ray diffraction, and magnetic susceptibility measurements. As a result of the large electronic reorganization involved in the dimer-to-radical interconversion, the photogenerated S = 1/2 radical state is remarkably thermally stable, persisting to 242 K before reverting to the S = 0 dimer.

Electro-oxidation of the quinoidal bisdithiazole BT in dichloroethane in the presence of [Bu4N][GaBr4] affords the 1:1 radical ion salt [BT][GaBr4], crystals of which belong to the trigonal space group P3. The packing pattern of the radical cations provides a rare example of an organic kagome basket structure, with S = 1/2 radical ion chains located at the triangular corners of a trihexagonal lattice. Magnetic measurements over a wide temperature range from 30 mK to 300 K suggest strongly frustrated AFM interactions on the scale of J/kb ∼ 30 K, but reveal no anomalies that would be associated with magnetic order. These observations are discussed in terms of the symmetry allowed magnetic interactions within and between the frustrated layers.

The pressure dependence of the crystal and molecular structure of the bis-1,2,3-thiaselenazolyl radical dimer [1b]2 has been investigated over the range 0–11 GPa by powder diffraction methods using synchrotron radiation and diamond anvil cell techniques. At ambient pressure, the dimer consists of a pair of radicals linked by a hypervalent 4-center 6-electron S---Se–Se---S σ-bond in an essentially coplanar arrangement. The dimers are packed in cross-braced slipped π-stack arrays running along the x-direction of the monoclinic (space group P21/c) unit cell. Pressurization to 11 GPa causes the unit cell dimensions a and c to undergo a slow but uniform compression, while the b-axis is slightly elongated. There is virtually no change in the molecular structure or in the slipped π-stack crystal architecture. This behavior is in marked contrast to that of the isostructural radical dimer [1a]2, where the basal fluorine is replaced by hydrogen. Pressurization of this latter material induces a phase change near 4–5...

The preparation and structural characterization of the methyl-substituted oxobenzene-bridged bisdithiazolyl radical 3b is described. Crystals of 3b belong to the monoclinic space group C2/c and contain two distinct radical environments, A and B. There are eight A radicals in the unit cell, which occupy general positions and form alternating twisted π-stacks running parallel to the c-axis. The four B radicals also adopt an alternating π-stack pattern, but each molecule lies on a crystallographic 2-fold rotation axis, and the overlay of neighboring radicals is centrosymmetric. Stacks of A radicals are linked by close intermolecular S···O′ and S···N′ contacts into ribbon-like arrays that weave along the y-direction, and the B radical stacks are located in columnar cavities generated by the out-of-register alignment of the ribbons of A radicals. Variable temperature magnetic susceptibility measurements indicate a strongly antiferromagnetically coupled system, a result in accord with DFT estimated exchange ene...

Semiquinone-bridged bisdithiazolyls 3 represent a new class of resonance-stabilized neutral radical for use in the design of single-component conductive materials. As such, they display electrochemical cell potentials lower than those of related pyridine-bridged bisdithiazolyls, a finding which heralds a reduced on-site Coulomb repulsion U. Crystallographic characterization of the chloro-substituted derivative 3a and its acetonitrile solvate 3a·MeCN, both of which crystallize in the polar orthorhombic space group Pna2(1), revealed the importance of intermolecular oxygen-to-sulfur (CO···SN) interactions in generating rigid, tightly packed radical π-stacks, including the structural motif found for 3a·MeCN in which radicals in neighboring π-stacks are locked into slipped-ribbon-like arrays. This architecture gives rise to strong intra- and interstack overlap and hence a large electronic bandwidth W. Variable-temperature conductivity measurements on 3a and 3a·MeCN indicated high values of σ(300 K) (10(-3) S cm(-1)) with correspondingly low thermal activation energies E(act), reaching 0.11 eV in the case of 3a·MeCN. Overall, the strong performance of these materials as f = ½ conductors is attributed to a combination of low U and large W. Variable-temperature magnetic susceptibility measurements were performed on both 3a and 3a·MeCN. The unsolvated material 3a orders as a spin-canted antiferromagnet at 8 K, with a canting angle φ = 0.14° and a coercive field H(c) = 80 Oe at 2 K.

The hydrostatic pressure response of TC of the bisdiselenazolyl radical ferromagnet 1 up to 5 GPa was investigated by AC magnetic susceptibility measurements using a SQUID magnetometer and a miniature diamond anvil cell. It was found that the ambient pressure value of TC = 17 K could be raised to 21 K at a pressure of 0.9 GPa. The experimental technique is described in detail and the pressure response is compared to that observed in related systems.

High-field, single-crystal EPR spectroscopy on a tetragonal bisdiselenazolyl ferromagnet has provided evidence for the presence of easy-axis magnetic anisotropy, with the crystallographic c axis as the easy axis and the ab plane as the hard plane. The observation of a zero-field gap in the resonance frequency is interpreted in terms of an anisotropy field several orders of magnitude larger than that observed in light-heteroatom, nonmetallic ferromagnets and comparable (on a per-site basis) to that observed in hexagonal close packed cobalt. The results indicate that large spin-orbit-induced magnetic anisotropies, typically associated with 3d-orbital-based ferromagnets, can also be found in heavy p-block radicals, suggesting that there may be major opportunities for the development of heavy p-block organic magnetic materials.

Supramolecular CHπ interactions cause a ruffling of the otherwise coplanar ribbon-like arrays of radicals in the structure of the oxobenzene-bridged bisdithiazolyl . The material displays a conductivity σ(300 K) = 6 × 10(-3) S cm(-1) (Eact = 0.16 eV) and orders antiferromagnetically below 4 K. At applied fields above 1 kOe the material displays metamagnetic behavior.

Variable pressure and temperature conductivity measurements on the bisthiaselenazolyl radical dimer [1a](2) have established the presence of a weakly metallic state over the pressure range 5-9 GPa. To explore the origin of this metallization we have examined the crystal and molecular structure of [1a](2) as a function of pressure. At ambient pressure the dimer consists of two radicals linked by a hypervalent 4-center 6-electron S...Se-Se...S sigma-bond into an essentially coplanar arrangement. The dimers are packed in cross-braced slipped pi-stack arrays running along the x-direction of the monoclinic (space group P2(1)/c) unit cell. Pressurization to 4 GPa induces little change in the molecular structure of [1a](2) or in the slipped pi-stack crystal architecture. Near 5 GPa, however, stress on the dimer leads to buckling of the two halves of the molecule and a contraction in the metrics of the S...Se-Se...S unit. These structural changes can be understood in terms of an electronic configurational switch from a 4-center 6-electron sigma-bonded dimer to a more conventional pi-bonded arrangement. At the same time the slipped pi-stack arrays undergo a concertina-like compression, and the crystal structure experiences highly anisotropic changes in cell dimensions. DFT calculations on the molecular electronic structure of the dimer indicate a marked decrease in the HOMO-LUMO gap as the dimer buckles. Related solid-state calculations indicate a rapid closure of the valence/conduction band gap in the same pressure region and the formation of a quasi-metallic state. Metallization of [1a](2) thus arises as much from intramolecular changes, which give rise to a collapse of the HOMO-LUMO gap and near coalescence of the valence and conduction bands, as from increased intermolecular interactions, which cause widening and overlap of the band edges.

We report the preparation, crystallization, and solid-state characterization of the first member of a new family of tris(1,9-disubstituted phenalenyl)silicon neutral radicals. In the solid state, the radical packs as weak partial pi-dimers with intermolecular carbon...carbon contacts that fall at the van der Waals atomic separation. Magnetic susceptibility measurements indicate approximately 0.7 Curie spins per molecule from room temperature down to 50 K, below which antiferromagnetic coupling becomes apparent; the compound has a room-temperature single-crystal conductivity of sigmaRT = 2.4 x 10(-6) S cm(-1).

Pyridyl-1,3,2-dithiazolyl PyDTA (1) has been synthesized and its electronic structure has been probed by cyclic voltammetry, electron paramagnetic resonance spectroscopy, and density functional theory calculations. The results demonstrate that the spin distribution and redox properties of the radical are intermediate between those observed for the isoelectronic benzoand pyrazino-1,3,2-dithiazolyl radicals. Crystals of 1 can be grown by vacuum sublimation as two polymorphs. The kinetically favored form 1r grows as purple/black blocks or rods and crystallizes in the noncentric space group P21, while the thermodynamically favored 1 phase forms green/black shards and crystallizes in the centric space group P21/n. The structures of both 1r and 1 comprise layers of head-to-head π*-π* dimers in the ab plane. The two structures differ in the orientation of the π-dimers along the stacks. In 1r dimers are aligned head-to-head, while in 1 the orientation of the dimers alternates in a head-over-tail fashion, so as to produce an approximate doubling of the crystallographic c-axis in relation to 1r. Variable temperature magnetic susceptibility measurements reveal that both phases are essentially diamagnetic at low temperatures. Above 180 K (for 1 ) and above 250 K (for 1r) the free Curie spin count for both polymorphs begins to rise, indicating a weakening of the π*-π* dimers in the solid state.

We report an investigation into the effect of oxygen and sulfur substitution on the dimerization of phenalenyl-based neutral radicals; these substituents are expected to electronically stabilize the free radical while sterically interfering with dimerization. The 1,3,4,9-tetramethoxyphenalenyl (7a) and 1-ethoxy-3,4,9-trimethoxyphenalenyl (7b) radicals are prepared by chemical reduction of the cation salts, but the crystals that are isolated from these solutions are shown to be composed of alkoxy-substituted peropyrenes by X-ray crystallography. Crystallization of 1,9-dithiophenalenyl (8) gives rise to a π-dimer, and this is the first example of a radical based on a single phenalenyl unit that has been stabilized against σ-dimerization in the solid state by electronic effects rather than by the presence of sterically bulky substituents.

Recent developments in stable radical chemistry have afforded "heavy atom" radicals, neutral open-shell (S = 1/2) molecular species containing heavy p-block elements (S, Se), which display solid-state magnetic properties once considered exclusive to conventional metal-based magnets. These highly spin-delocalized radicals do not associate in the solid state and yet display extensive networks of close intermolecular interactions. Spin density on the heavy atoms allows for increased isotropic and spin-orbit mediated anisotropic exchange effects. Structural variations induced by chemical modification and physical pressure, coupled with ab-initio methods to estimate exchange energies, have facilitated the development of predictive structure/property relationships. These results, coupled with detailed theoretical analyses and magnetic resonance spectroscopic measurements, have provided insight into the magnetic structure of ferromagnetic and spin-canted antiferromagnetic ordered materials as well as an understanding of the importance of spin-orbit coupling contributions to magnetic hysteresis and anisotropy. Isotropic and anisotropic ferromagnetic exchange can also be enhanced indirectly by the incorporation of heavy atoms into nonspin-bearing sites, where they can contribute to multi-orbital spin-orbit coupling.

The alternating ABABAB π-stacked architecture of the EtCN solvate of the iodo-substituted, oxobenzene-bridged bisdithiazolyl radical IBBO (space group Pnma) gives rise to strong ferromagnetic exchange along the π-stacks, and the material orders as a spin-canted antiferromagnet with T(N) = 35 K, with a spontaneous (canted) moment M(spont) = 1.4 × 10(-3) μB and a coercive field H(c) = 1060 Oe (at 2 K). The observation of spin-canting can only be understood in terms of multiorbital contributions to both isotropic and anisotropic exchange interactions, the magnitude of which are enhanced by spin-orbit effects arising from the heavy-atom iodine substituent. Pseudodipolar interactions lead to a net canted moment along the c-axis, while the sublattice magnetization is predicted to possess an easy a-axis.

The combination of high-pressure ferromagnetic resonance (FMR) and an ab-initio scheme suitable for calculation of spin-orbit mediated anisotropic exchange interactions in molecular materials provides insights into the role of spin-orbit coupling (SOC) in a Se-based organic ferromagnet. FMR measurements reveal a continuous increase in the magnetic anisotropy with increasing pressure (up to 2.2 GPa), in excellent agreement with calculations based on the known pressure dependence of the structure. The large value of anisotropic exchange terms in this heavy atom organic ferromagnet emphasizes the important role of SOC in a wide range of organics where this effect is usually considered to be small.

Reaction of N-alkylated pyridine-bridged bisdithiazolylium cations [1]+ (R1 =Me, Et; R2 =Ph) with selenium dioxide in acetic acid provides a one-step high-yield synthetic route to bisthiaselenazolylium cations [2]+ (R1 = Me, Et; R2 = Ph). The corresponding radicals 1 and 2 can be prepared by chemical or electrochemical reduction of the cations. Structural analysis of the radicals has been achieved by a combination of single-crystal and powder X-ray diffraction methods. While the two sulfur radicals 1 adopt different space groups (P3(1)21 for R1 = Me and P(-)1 for R1 = Et), the two selenium radicals 2 (space groups P3(1)21 for R1 = Me and P3(2)21 for R1 =Et) are isostructural with each other and also with 1 (R1 = Me, R2 = Ph). Variable-temperature magnetic measurements on all four compounds confirm that they are undimerized S = 1/2 systems, with varying degrees of weak intermolecular antiferromagnetic coupling. Variable-temperature electrical conductivity measurements on the two selenium radicals provide conductivities sigma(300 K) = 7.4 x 10-6 (R1 = Et) and 3.3 x 10-5 S cm-1 (R1 = Me), with activation energies, E(act), of 0.32 (R1 = Et) and 0.29 eV (R1 = Me). The differences in conductivity within the isostructural series is interpreted in terms of their relative solid-state bandwidths, as estimated from Extended Hückel band-structure calculations.

The germanium (IV) and silicon (IV) complexes, tris-(9-oxophenalenone)-germanium (IV) tetrakis[3,5-bis(trifluoromethyl) phenyl] borate (7+TFPB−) and tris-(9-oxophenalenone)-silicon (IV) tetrakis[3,5-bis(trifluoromethyl) phenyl] borate (8+TFPB−), derived from 9-hydroxyphenalenone have been synthesized and characterized by elemental analyses, infrared spectra and as well as by single crystal X-ray study. The crystal structure of 7+TFPB− and 8+TFPB− shows a one-dimensional chain like structure and π-overlap between the neighbouring phenalenyl planes. The cyclic voltammograms of 7+TFPB− and 8+TFPB− show three reversible one-electron reduction steps due to the formation of the radical, anion and dianion as each phenalenyl moiety accepts an electron.

We report the preparation and solid state characterization of a silicon (IV) salt based on a new redox active tripodal hexadentate ligand [H3L = l,l,l-Tris(l′-oxophenalenyl-9′-N-methyl) ethane]. X-ray analysis of the complex reveals a short Si-N bond distance (av 1.878(16)). The compound is shown to undergo three reversible one-electron reduction steps (without decomposition) due to the formation of radical, anion and dianion of the phenalenyl moiety.

An organic material composed of neutral free radicals based on the spirobiphenalenyl system exhibits a room temperature conductivity of 0.3 siemens per centimeter and a high-symmetry crystal structure. It displays the temperature-independent Pauli paramagnetism characteristic of a metal with a magnetic susceptibility that implies a density of states at the Fermi level of 15.5 states per electron volt per mole. Extended Hückel calculations indicate that the solid is a three-dimensional organic metal with a band width of ∼0.5 electron volts. However, the compound shows activated conductivity (activation energy, 0.054 electron volts) and an optical energy gap of 0.34 electron volts. We argue that these apparently contradictory properties are best resolved in terms of the resonating valence-bond ground state originally suggested by Pauling, but with the modifications introduced by Anderson.

We report on the FT-Raman spectroscopic characterization, aided by DFT model chemistry calculations, of a bunch of a,u-dicyano endcapped oligothiophenes, ranging in length from the dimer to the hexamer, which have been previously shown to display quite promissing amphoteric redox properties for their potential use in new technological applications. The attention is mainly focused to the role played by the CN/H hydrogen bonding interactions between adjacent molecules on various molecular properties, such as the minimum-energy geometry, atomic charges equilibrium distribution,topologies/energies of the frontier molecular orbitals and the Raman scattering spectral profile. To this end we have performed to two sets of quantum chemical calculations, at the DFT//B3LYP/3-21G* level, on: (a) a single terthiophene in the vacuum and (b) a suited dimeric model with a fully coplanar array of the two terthienyl segments interacting through the cyano end-caps. q 2004 Elsevier B.V. All rights reserved.

We report the preparation, crystallization, and solid-state characterization of the first members of a new family of spiro-bis-(1,9-diamino-substituted-phenalenyl)boron neutral radicals. The crystal structures show that the three radicals are monomeric and without close contacts in the crystal lattice. In all cases magnetic susceptibility measurements confirm the presence of free radicals with one unpaired spin per molecule. Two of the new radical compounds are among the most highly conducting neutral organic solids, with room-temperature conductivities reaching sigma(RT) = 4 x 10(-2) S/cm. The measured conductivities correlate with the closest intermolecular contacts in the solid state and with the calculated band dispersions, even though the bandwidths are much smaller than those found in other organic conductors.

The use of neutral π-radicals as building blocks for molecular conductors holds both appeal and challenge. Such systems obviate the need for counterions, as charge transfer is not required to generate charge carriers. Essentially an array of π-radicals should function like atoms in an elemental metal, e.g., sodium, affording a half-filled energy band. Most radicals, however, tend to dimerize, and even when association can be suppressed the resulting low bandwidth W, coupled with a high on-site Coulomb repulsion U, leads to a Mott insulating state. We are pursuing the design and synthesis of stable heterocyclic thiazyl radicals, with a view to generating stable, crystalline materials with a high W/U ratio. The search for these new radicals, the molecular analogues of sodium, is the subject of this presentation.

A general synthetic route to the pyridine-bridged dithiazolodithiazolyl framework R2BPR1, involving N-alkylation of a 4-substituted 2,6-dichloropyridine, followed by amination and double Herz cyclization with S2Cl2, has been developed. The radicals R2BPMe (R2 = Me, Ph) have been prepared and characterized by EPR spectroscopy and cyclic voltammetry. Their crystal structures have been determined by X-ray crystallography. Both structures consist of undimerized slipped radical π-stacks. Lateral interactions in MeBPMe (space group P21/c) generate chainlike arrays with radicals linked by inversion centers; there are no close interchain S−S contacts. By contrast, in PhBPMe (space group P3121) the radical π-stacks are nested about 31 axes, so as to produce an extensive 2-dimensional network of intermolecular S−S interactions. Variable-temperature magnetic susceptibility measurements reveal that MeBPMe is essentially paramagnetic, whereas PhBPMe displays strong antiferromagnetic coupling. A value of J = −149 cm-1 ...

We report the preparation, crystallization, and solid-state characterization of a benzyl-substituted spirobiphenalenyl radical. The crystal structure shows that the radical is monomeric in the solid state, with the molecules packed in an unusual one-dimensional (1-D) fashion that we refer to as a pi-step stack. This particular mode of 1-D stacking is forced on the lattice arrangement by the presence of the orthogonal phenalenyl units that were specifically incorporated to prevent the crystallization of low-dimensional structures. The structure shows that this strategy is effective, and neighboring molecules in the stack can only interact via the overlap of one pair of active (spin-bearing) carbon atoms per phenalenyl unit, leading to the pi-step structure in which the remaining four active carbon atoms per phenalenyl unit do not interact with nearest neighbor molecules. The magnetic susceptibility data in the temperature range 4-360 K may be fit to an antiferromagnetic Heisenberg S = 1/2 linear chain model with intrachain spin coupling J = -52.3 cm(-1). Despite the uniform stacking, the material has a room temperature conductivity of 1.4 x 10(-3) S/cm and is best described as a Mott insulator.

New synthetic routes to 1,2,3-dithiazolo-1,2,3-dithiazolylium salts, based on double Herz condensations of N-alkylated 2,6-diaminopyridinium salts with sulfur monochloride, have been developed. The two prototypal 1,2,3-dithiazolo-1,2,3-dithiazolyl radicals HBPMe and HBPEt have been prepared and characterized in solution by cyclic voltammetry and EPR spectroscopy. Measured electrochemical cell potentials and computed (B3LYP/6-31G) gas-phase disproportionation enthalpies favor a low on-site Coulombic repulsion energy U in the solid state. The crystal structures of HBPR (R = Me, Et) have been determined by X-ray crystallography (at 293 K). Both consist of slipped pi-stacks of undimerized radicals, with many close intermolecular S- - -S contacts. Magnetic, conductivity, and optical measurements have been performed and the results interpreted in light of extended Hückel band calculations. The crystalline materials are paramagnetic above 100 K, with room-temperature conductivities sigma(RT) of 10(-5)-10(-6) S cm(-1); the slightly greater conductivity of the R = Et compound can be associated with a more well developed band structure. We suggest a Mott-Hubbard insulator ground state for these materials, with an on-site Coulomb repulsion energy U of about 1.0 eV.

A 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 (RT 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 95: 853- 865, 2003

The passage of a current through a solid requires a pathway for the movement of electrons or holes. A new class of phenalenyl-based neutral radical molecular conductors has recently been reported; a distinctive feature of these molecular solids is the absence of any obvious conducting pathway(s). 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 the first compound to be isolated behaves as a free radical with one spin per molecule, apparently supporting the idea that there is little interaction between the molecules in the solid state. Nevertheless, this radical shows the highest conductivity (σRT = 0.05 S/cm), of any neutral organic solid, and the conduction mechanism is presently unresolved. In the present study, we report a structurally related radical with a resistivity 4 orders of magnitude higher than that of the previously reported radicals. We analyze th...

Alkylation of the zwitterionic heterocycle 8-chloro-bis[1,2,3]dithiazolo[4,5-b:5',4'-e]pyridine (ClBP) with alkyl triflates affords 8-chloro-4-alkyl-4H-bis[1,2,3]dithiazolo[4,5-b:5',4'-e]pyridin-2-ium triflates [ClBPR][OTf] (R = Me, Et, Pr). Reduction of these salts with decamethylferrocene affords the corresponding ClBPR radicals as thermally stable crystalline solids. The radicals have been characterized in solution by cyclic voltammetry and EPR spectroscopy. Measured electrochemical cell potentials and computed (B3LYP/6-31G) gas-phase disproportionation enthalpies are consistent with a low on-site Coulombic barrier U to charge transfer in the solid state. The crystal structures of ClBPR (R = Me, Et, Pr) have been determined by X-ray crystallography (at 293 K). All three structures consist of slipped pi-stacks of undimerized radicals, with many close intermolecular S.S contacts. ClBPMe undergoes a phase transition at 93 K to a slightly modified slipped pi-stack arrangement, the structure of which has also been established crystallographically (at 25 K). Variable-temperature magnetic and conductivity measurements have been performed, and the results interpreted in light of extended Hückel band calculations. The room-temperature conductivities of ClBPR systems (sigma(RT) approximately 10(-)(5) to 10(-)(6) S cm(-)(1)), as well as the weak 1D ferromagnetism exhibited by ClBPMe, are interpreted in terms of weak intermolecular overlap along the pi-stacks. The latter is caused by slippage of the molecular plates, a feature necessitated by the steric size of the R and Cl groups on the pyridine ring.

The crystal structure and charge transport properties of the prototypal oxobenzene-bridged 1,2,3-bisdithiazolyl radical conductor 3a are strongly dependent on pressure. Compression of the as-crystallized α-phase, space group Fdd2, to 3-4 GPa leads to its conversion into a second or β-phase, in which F-centering is lost. The space group symmetry is lowered to Pbn2₁, and there is concomitant halving of the a and b axes. A third or γ-phase, also space group Pbn2₁, is generated by further compression to 8 GPa. The changes in packing that accompany both phase transitions are associated with an "ironing out" of the ruffled ribbon-like architecture of the α-phase, so that consecutive radicals along the ribbons are rendered more nearly coplanar. In the β-phase the planar ribbons are propagated along the b-glides, while in the γ-phase they follow the n-glides. At ambient pressure 3a is a Mott insulator, displaying high but activated conductivity, with σ(300 K) = 6 × 10(-3) S cm(-1) and E(act) = 0.16 eV. With compression beyond 4 GPa, its conductivity is increased by 3 orders of magnitude, and the thermal activation energy is reduced to zero, heralding the formation of a metallic state. High pressure infrared absorption and reflectivity measurements are consistent with closure of the Mott-Hubbard gap near 4-5 GPa. The results are discussed in the light of DFT calculations on the molecular and band electronic structure of 3a. The presence of a low-lying LUMO in 3a gives rise to high electron affinity which, in turn, creates an electronically much softer radical with a low onsite Coulomb potential U. In addition, considerable crystal orbital (SOMO/LUMO) mixing occurs upon pressurization, so that a metallic state is readily achieved at relatively low applied pressure.

The preparation and crystal structure of 5-cyanofuran-2-[1,2,3,5-diselenadiazolyl], [RCN(2)Se(2)] (R = 5-cyanofuran), is reported. Crystal data for C(6)H(2)ON(3)Se(2): monoclinic, space group P2(1), a = 7.1121(7), b= 20.541(2), c =20.923(2) A, beta = 99.785(1) degrees, Z = 16. The crystal structure consists of diselenadiazolyl pi-dimer stacks running parallel to the x direction; the asymmetric unit consists of four pi-dimer units. The dimers are aligned into snakelike ribbons along the y direction, with consecutive dimers linked by head-to-tail CN-Se contacts. Each pi-dimer stack is bordered by two out-of-register stacks, but most interstack Se-Se contacts lie outside the van der Waals separation. Along the pi-dimer stacks, the intradimer Se-Se distances range from 3.183(10) to 3.294(1) A, and the interdimer Se-Se distances range from 3.826(1) to 3.945(1) A. Like other pi-dimer stacked diselenadiazolyls, [C(6)H(2)ON(3)Se(2)](2) is diamagnetic over the temperature range 4-380K. Variable temperature single-crystal conductivity measurements reveal a room-temperature conductivity near 10(-5) S cm(-1) and provide a calculated band gap of 0.72 eV. The structural results and transport properties are interpreted in the light of Extended Hückel band structure calculations.

The S-Se-N-based heterocycle benzo[2,1-c:3,4-c']bis(1,2,3-thiaselenazole) (BSe) can be prepared by the condensation of 1,4-diaminobenzene-2,3-dithiol with selenium tetrachloride. Crystals of this compound are not isomorphous with the related benzo[2,1-c:3,4-c']bis(1,2,3-dithiazole) (BT); a structure is adopted that allows for more extensive intermolecular Se- - -Se contacts. Electro-oxidation of BSe in the presence of [n-Bu4N][ClO4] affords metallic green needles of the charge transfer salt [BSe]3[ClO4]2, which exhibit a pressed pellet conductivity sigma(RT) = 10(-1) S cm(-1). The crystal structure of [BSe]3[ClO4]2 consists of slipped pi-stacks based on the triple-decker closed shell [BSe]3(2+) building block. The packing is analogous to that found for the charge transfer salt [BT]3[FSO3]2, for which sigma(RT) = 10(-2) S cm(-1). Extended Hückel band structure calculations on these two (sulfur- and selenium-based) 3:2 salts reveal more extensive intermolecular interactions in the selenium compound. As a result, the latter has a more two-dimensional electronic structure. Crystal data for Se2S2N2C6H2, a = 4.103(2) A, b = 12.159(2) A, c = 16.171(2) A, orthorhombic, Pbnm, Z = 4. Crystal data for Se6S6N6C18H6Cl2O4, a =17.00(1) A, b = 18.36(1) A, c = 10.679(4) A, 110.27(3), monoclinic, C2/c, Z = 4.

Bifunctional 1,2,3-dithiazoles bridged with azine and phenylenediamine spacers have been prepared, with a view to determining the extent of communication between the two dithiazole rings as a function of the electronic and steric demands of the bridge. The crystal structure of the closed-shell diazine derivative [S(2)NClC(2)=NN=C(2)ClNS(2)] is rigorously planar. Cyclic voltammetry on this compound indicates two reversible one-electron oxidations. The radical cation state has been characterized by EPR spectroscopy and by crystal structure determination of its 1:1 PF(6)(-) salt. The latter reveals little interaction between neighboring radical cations; consistently, the material exhibits a conductivity of sigma10(-5) S cm(-1). Insertion of a phenylene group into the diazine bridge to afford [S(2)NClC(2)=NC(6)H(4)N=C(2)ClNS(2)] leads to significant torsional motion between the phenylene ring and the two end groups, as a result of which the two DTA rings are electronically independent; no radical cation state has been observed for this species. Crystal data for Cl(2)S(4)N(4)C(4): a = 5.1469(15), b = 13.343(2), c = 14.2031(17), orthorhombic, Pbca, Z = 4. Crystal data for Cl(2)S(4)N(4)C(4)PF(6): a = 11.699(4), b = 12.753(5), c = 10.461(4), beta = 112.17(1) degrees, monoclinic, C2/c, Z = 4. Crystal data for C(l2)S(4)N(4)C(10)H(4): a = 3.9477(6), b = 23.790(3), c = 7.3769(9), beta =90.793(12) degrees, monoclinic, P2(1)/c, Z = 2.