Hanhua Zhao

New Insight into the Nature of Cu(TCNQ): Solution Routes to Two Distinct Polymorphs and Their Relationship to Crystalline Films That Display Bistable Switching Behavior

Syntheses and characterization of two polymorphs of Cu(TCNQ) have been carried out and the results correlated to films of the materials. Reactions of CuI with TCNQ or [Cu(CH3CN)4][BF4] with TCNQ- lead to blue-purple needles of Cu(TCNQ) phase I (1). A slurry of this kinetic product in CH3CN yields a second crystalline phase of Cu(TCNQ), phase II (2), which exhibits a platelet morphology. Powder X-ray diffraction and scanning electron microscopy data revealed that both phases are present in films of Cu(TCNQ) formed by oxidation of Cu foil by TCNQ in CH3CN. X-ray photoelectron spectra of the two phases are indistinguishable from each other and are indicative of the presence of Cu(I). Single-crystal X-ray studies were undertaken on very small crystals of the two samples, the results of which reveal that subtle geometrical changes for the nitrile arrangements around the four-coordinate Cu(I) centers lead to major changes in the architectural framework of the polymers. Phase I was indexed in the tetragonal crys...

A Highly Anisotropic Cobalt(II)-Based Single-Chain Magnet: Exploration of Spin Canting in an Antiferromagnetic Array

In this article we report for the first time experimental details concerning the synthesis and full characterization (including the single-crystal X-ray structure) of the spin-canted zigzag-chain compound [Co(H2L)(H2O)]infinity [L = 4-Me-C6H4-CH2N(CPO3H2)2], which contains antiferromagnetically coupled, highly magnetically anisotropic Co(II) ions with unquenched orbital angular momenta, and we also propose a new model to explain the single-chain magnet behavior of this compound. The model takes into account (1) the tetragonal crystal field and the spin-orbit interaction acting on each Co(II) ion, (2) the antiferromagnetic Heisenberg exchange between neighboring Co(II) ions, and (3) the tilting of the tetragonal axes of the neighboring Co units in the zigzag structure. We show that the tilting of the anisotropy axes gives rise to spin canting and consequently to a nonvanishing magnetization for the compound. In the case of a strong tetragonal field that stabilizes the orbital doublet of Co(II), the effective pseudo-spin-1/2 Hamiltonian describing the interaction between the Co ions in their ground Kramers doublet states is shown to be of the Ising type. An analytical expression for the static magnetic susceptibility of the infinite spin-canted chain is obtained. The model provides an excellent fit to the experimental data on both the static and dynamic magnetic properties of the chain.

Dramatically Different Conductivity Properties of Metal–Organic Framework Polymorphs of Tl(TCNQ): An Unexpected Room-Temperature Crystal-to-Crystal Phase Transition†

The synthesis and fabrication of nanoscale materials for new types of electronic and magnetic devices is a central theme in materials science research in this second decade of the 21st century. Given that conventional storage materials are estimated to approach their miniaturization limit by 2016, heightened efforts are being directed at the design and synthesis of new types of bistable nanoscale materials, including those capable of undergoing a change from low to high resistance under the application of an electric field. Such nonvolatile memory devices are capable of operating at increased speeds and require less energy than conventional memory devices. Among the materials being investigated for resistance-based memory are materials that contain organic components and whose properties are influenced by magnetic or electric fields. Materials that respond to the application of an electric field or changes in light, pressure, or temperature are being sought for incorporation into electronic devices with ultrafast operating speeds. Examples of molecule-based materials that exhibit fascinating properties are the spin-crossover complex [Fe(picolylamine)3Cl2(C2H5OH)], [5, 6] the neutral– ionic transition system TTF–chloranil (TTF = tetrathiafulvalene), the metallo-organic conductor Cu(DMDCNQI)2, [10–16] (DM-DCNQI = dimethyl-N,N’-dicyanoquinonediimine) and the salt (EDO-TTF)2PF6, [17, 18] (EDO-TTF = ethylenedioxytetrathiafulvalene). These materials provide compelling evidence for the contention that molecular solids may eventually be useful in device applications. In terms of electric-field-induced behavior, the most extensively studied examples are the organocyanide-based materials Cu(TCNQ) (TCNQ = 7,7,8,8-tetracyanoquinodimethane), which exhibits reversible switching from a highresistance state to a conducting state promoted by the application of an electric field or upon irradiation, and the current-driven conductor K(TCNQ) salt. The latter material is a key member of the binary series of alkali-metal salts of TCNQ that behave as so-called “Mott insulators” at high temperatures, in which the fully reduced radical anions are arranged in columns with evenly spaced TCNQ units. At lower temperatures, these “soft” materials undergo a phase transition in which the TCNQ units are brought into close proximity as a result of p dimerization. The electrons are then trapped in the dimers, the conductivity drops, and the materials pass into the spin-Peierls insulating state. An approach that we have adopted for discovering conducting TCNQ phases is to capitalize on the rich chemistry of alkali metals while circumventing some issues that hinder their conductivity. In this vein, thallium is an interesting element, since it can behave as a pseudo-alkali metal. In contrast to other Group 13 elements, Tl prefers the 1 + oxidation state (although Tl is known), and many similarities between the chemistry of alkali-metal ions and Tl have been noted. The electronegativity of Tl (2.04) is much higher than that of any alkali metal, which should lead to less ionic compounds with smaller band gaps and thus higher carrier mobility. Moreover, unlike alkali metals, Tl possesses a stereoactive lone pair, which is expected to lead to a greater diversity of structures. Indeed, the viability of this idea was demonstrated by H nig et al., who reported Tl(DMDCNQI)2, which adopts a 3D metal–organic framework structure and behaves as a one-dimensional metal-like semiconductor (s300K = 50 Scm ). With the exception of the aforementioned material, there are no other reported main-group binary phases based on weak interactions with organocyanide molecules. In fact, main-group supramolecular chemistry is largely underdeveloped as compared to that of transition-metal ions. Herein we describe the first chemistry of the Tl cation with TCNQ radical anions, the result of which is the discovery of two polymorphs with very different conducting properties. Slow diffusion of a methanol solution of Li(TCNQ) and an aqueous solution of TlPF6 leads to the isolation of single crystals of the product Tl(TCNQ), phase I (1). A typical bulk stoichiometric reaction leads to crystals of a second product Tl(TCNQ), phase II (2). An X-ray structural determination revealed that 1 crystallizes in the P21/c space group as a 3D network structure consisting of metal ions arranged in linear strings, each surrounded by four stacks of TCNQ acceptor molecules (Figure 1a) and with adjacent TCNQ stacks [*] Dr. C. Avendano, Z. Zhang, Dr. A. Ota, Dr. H. Zhao, Prof. K. R. Dunbar Department of Chemistry, Texas A&M University P.O. Box 30012, College Station, TX 77842 (USA) Fax: (+ 1)979-845-7177 E-mail: dunbar@mail.chem.tamu.edu

Lanthanide–3d cyanometalate chains Ln(III)–M(III) (Ln = Pr, Nd, Sm, Eu, Gd, Tb; M = Fe) with the tridentate ligand 2,4,6-tri(2-pyridyl)-1,3,5-triazine (tptz): evidence of ferromagnetic interactions for the Sm(III)–M(III) compounds (M = Fe, Cr)

A series of cyanide-bridged chain mixed Fe(III)/Ln(III) (Ln=Pr, Nd, Sm, Eu, Gd, Tb) complexes with the tridentate ligand 2,4,6-tri(2-pyridyl)-1,3,5-triazine (tptz) used as a capping group has been prepared. Reactions of tptz and LnCl3 with K3Fe(CN)6 yield a family of air-stable 1-D compounds {[Pr(tptz)(H2O)4Fe(CN)6].8H2O}infinity, {[Nd(tptz)(H2O)4Fe(CN)6].8H2O}infinity, {[Sm(tptz)(H2O)4Fe(CN)6].8H2O}, {[Eu(tptz)(H2O)4Fe(CN)6].6H2O}infinity, {[Gd(tptz)(H2O)4Fe(CN)6].6H2O}infinity, and {[Tb(tptz)(H2O)4Fe(CN)6].8H2O}infinity. Temperature dependent magnetic susceptibility studies of reveal that in , the Sm(III) and Fe(III) ions are ferromagnetically coupled with 3-D ordering occurring below 3.5 K. The appearance of the frequency dependent out-of-phase signal is explained in terms of an ordering with a spin glass-like behavior. To compare the magnetic behavior of with related compounds, {[Sm(tptz)(H2O)4Co(CN)6].8H2O}infinity and {[La(tptz)(DMF)(H2O)3Fe(CN)6].5H2O}infinity, {[Sm(tmphen)(DMF)3(H2O)Fe(CN)6].2H2O}infinity, {[Sm(tmphen)2(H2O)2Fe(CN)6].MeOH.13H2O}infinity and {[Sm(tmphen)2(H2O)2Cr(CN)6].MeOH.9H2O}infinity with 3,4,7,8-tetramethyl-1,10-phenanthroline (tmphen) were also prepared.

Single‐Chain Magnetic Behavior in a Hetero‐Tri‐Spin Complex Mediated by Supramolecular Interactions with TCNQF.− Radicals

The self-assembly of organic TCNQF˙⁻ radicals (2-fluoro-7,7,8,8-tetracyano-p-quinodimethane) and the anisotropic [Tb(valpn)Cu](3+) dinuclear cations produced a single-chain magnet (SCM) involving stacking interactions of TCNQF˙⁻ radicals (H2valpn is the Schiff base from the condensation of o-vanillin with 1,3-diaminopropane). Static and dynamic magnetic characterizations reveal that the effective energy barrier for the reversal of the magnetization in this hetero-tri-spin SCM is significantly larger than the barrier of the isolated single-molecule magnet based on the {TbCu} dinuclear core.

New type of single chain magnet based on spin canting in an antiferromagnetically coupled Co(II) chain

The new cobalt diphosphonate compound with a 1D zig–zag chain structure is the first example of its kind that has been recognized to exhibit single-molecule magnets behavior. The slow paramagnetic relaxation of the magnetization is explained on the basis of Ising anisotropy resulting from spin canting of antiferromagnetically coupled Co ions. The energy gap is in accord with the predictions of Glauber’s theory for a one-dimensional Ising system.

Squaring the cube: a family of octametallic lanthanide complexes including a Dy8 single-molecule magnet.

A series of isostructural octanuclear lanthanide complexes of general formula [Ln8(sao)4(μ3-OH)4(NO3)12(DMF)12] (Ln = Nd (), Sm (), Eu (), Gd (), Tb (), Dy (), Ho (), Er (); DMF = dimethylformamide) have been prepared via reactions of salicylaldoxime (saoH2), tetramethylammonium hydroxide (Me4NOH) with the appropriate lanthanide nitrate salt (Ln(NO3)3·6H2O). The metallic skeletons of the complexes describe [Ln4] tetrahedra encapsulated inside a [Ln4] square with the inner core stabilised through μ3-OH(-) ions and the periphery by μ4-sao(2-) ligands. The magnetic properties of compounds were investigated by dc and ac magnetometry. Temperature dependent ac magnetic susceptibility data reveal that the dysprosium analogue () displays an out-of-phase signal in the absence of an applied magnetic field indicative of slow relaxation of the magnetization typical of a Single-Molecule Magnet (SMM). Micro-SQUID measurements reveal temperature and sweep rate dependent hysteresis below 1.0 K.

Conducting Organic Frameworks Based on a Main-Group Metal and Organocyanide Radicals

Reactions of the main-group cation Tl(I) with anions of 2,5-derivatives of TCNQ (TCNQ = 7,7,8,8-tetracyanoquinodimethane) have led to the isolation of a family of unprecedented semiconducting main-group-metal-organic frameworks, namely, [Tl(TCNQX(2))], (X = H, Cl, Br, I). A comparison of single-crystal and powder X-ray diffraction data revealed the existence of a third polymorph of the previously reported material Tl(TCNQ)] and two distinct polymorphs of [Tl(TCNQCl(2))], whereas only one phase was identified for [Tl(TCNQBr(2))] and [Tl(TCNQI(2))]. These new results are described in the context of the structures of other known binary metal-TCNQ frameworks that display a variety of coordination environments for the central cation, namely, four-, six-, and eight-coordinate, and different arrangements of the adjacent TCNQ radicals-parallel versus perpendicular-in the stacked columns. The halogen substituents affect the structures and the properties of these compounds, owing to both steric and electronic effects as evidenced by the semiconducting properties of crystals of [Tl(TCNQCl(2))] phase I, [Tl(TCNQBr(2))], and [Tl(TCNQI(2))], which correlate well with the distances of adjacent TCNQ radicals in the columns. 1D infinite Hückel model simulations of the band structures of [Tl(TCNQCl(2))] phase I, [Tl(TCNQBr(2))], and [Tl(TCNQI(2))] were conducted with and without consideration of the Tl(I) cations, the results of which indicate that the charge mobility does not strictly occur in one dimension. The modulations of the band structures with various assumptions of the energy difference (Δ) between the Tl(I) 6s orbital and the TCNQ LUMO orbital were calculated and are discussed in light of the observed properties.

Unprecedented Binary Semiconductors Based on TCNQ: Single‐Crystal X‐ray Studies and Physical Properties of Cu(TCNQX2) X=Cl, Br

2010 WILEY-VCH Verlag Gm Much current research in science is being directed at the synthesis and fabrication of nanoscale materials for new types of electronic and magnetic devices. Pressure to reduce the size and improve the response times of electronic components has always existed in information technology, but as we approach the miniaturization limits of traditional charge storage estimated to occur by 2016, the global quest for faster andmore efficient data storage and processing is heightening. One strategy that is being explored for the development of new device components is the pursuit of materials whose bistability is based on a resistance change rather than current flow. Such ‘‘nonvolatile’’ memory devices are capable of operating at increased speeds with reduced energy expenditure. Gigantic nonlinear responses (or switching phenomena) of materials have been observed in molecule-based organiccontaining materials in response to short pulses of low-power external stimuli. In this vein, materials are being vigorously pursued that respond to the application of an electric field, light, pressure, or temperature as the basis for electronic devices with ultrafast operating speeds. In terms of electric-field-induced behavior, one of the most extensively studied examples is Cu(TCNQ) where TCNQ is 7,7,8,8-tetracyanoquinodimethane. One phase of this material exhibits reversible switching from a high resistive state to a conducting state promoted by the application of an electric field or upon irradiation, which makes it an excellent candidate for nonvolatile memory. The promise for commercial applications is sufficiently high such that researchers have fabricated devices with nanowires, nanorods, and nanoribbons of Cu(TCNQ) as well as Ag(TCNQ). Although a vast amount of research has been directed at understanding the Cu(TCNQ) system, analogous materials based on TCNQ derivatives are surprisingly scarce. Given this situation, we recently initiated a broad survey of binary metal-containing TCNQ derivatives in order to probe the steric and electronic influences of the substituent on the structure and properties of these materials. Herein we report large high-quality crystals of two new isostructural semiconductors based on Cu ions. The materials are Cu(TCNQCl2) (1) and Cu(TCNQBr2) (2), where TCNQCl2 is 2,5-dichloro-7,7,8,8-tetracyanoquinodimethane and TCNQBr2 is 2,5-dibromo-7,7,8,8-tetracyanoquinodimethane. The conductivity of compound 1 is the highest in the family of 1:1 Mþ:(TCNQ) salts whereas the conductivity of 2 is comparable to that of Cu(TCNQ) phase I (3). The 3D architecture of the Cu ions coordinated to the m4-TCNQX2 (X1⁄4Cl, Br) ligands is unprecedented among the widely studied Cu(TCNQ) and Ag(TCNQ) compounds and derivatives. The extraordinary properties observed for Cu(TCNQ) have spurred the exploration of numerous strategies to obtain crystalline phases of Cu(TCNQ); these efforts include spontaneous electrolysis, reduction of TCNQ with CuI, vapor deposition of TCNQ on Cu, photocrystallization, electrocrystallization, physical chemical vapor combined deposition and vacuum co-deposition. The only known instance wherein crystals sufficiently large for single-crystal data collection were obtained is the work from our laboratories a number of years ago in which we reported marginal structures obtained from very tiny crystals of Cu(TCNQ). In this study, we discovered the previously unrecognized existence of polymorphism in Cu(TCNQ). The materials, which we dubbed phase I and phase II, exhibit marked differences involving not only the arrangements of TCNQ ligands, but also the infrared spectral, conducting, andmagnetic properties. It was noted in this study that Cu(TCNQ) phase I and the only other previously analogue that had been structurally characterized, namely Ag(TCNQ), adopt a common structure, referred to hereafter as type A. The structure involves metal ions in a highly distorted tetrahedral environment with m4-TCNQ ligands arranged in segregated stacks of TCNQ along the short axis; the adjacent stacks of TCNQ are rotated by 908 with respect to each other (Fig. 1c). Cu(TCNQ) phase II, defined as structure type B, differs from phase I in that the TCNQ ligands are parallel to each other throughout the extended framework and do not form close contacts that signify p-stacking of the TCNQ ligands. Crystals of 1 and 2 were obtained by slow diffusion of acetonitrile solutions of CuI and the respective TCNQX2 derivative (X1⁄4Cl (1), Br (2)), in a manner akin to the method used to prepare Cu(TCNQ) phase I. The X-ray crystal structures of 1 and 2 at 110K revealed that they also crystallize as 3D frameworks with Cu ions coordinated to four different [TCNQX2] anions (Fig. 1a). The Cu ions are in a highly distorted tetrahedral environment as evidenced by the N Cu N angles of 1: 94.738, 101.778, 130.898, and 139.688; and of 2: 93.218,

Magnetic ordering in TCNQ-based metal–organic frameworks with host–guest interactions

Host–guest interactions between the aromatic molecules benzene, toluene, aniline and nitrobenzene and the redox-active TCNQ-based metal–organic framework (MOF), Fe(TCNQ)(4,4′-bpy) (1) (TCNQ = 7,7,8,8-tetracyanoquinodimethane), have been found to modulate spontaneous magnetization behaviours at low temperatures. An analogous MOF, Mn(TCNQ)(4,4′-bpy) (2) with isotropic Mn(II) ions as well as the two-dimensional compound Fe(TCNQ)(DMF)2·2DMF (3·2DMF), were also prepared as models for studying the effects of single-ion magnetic anisotropy and structural distortion on spin canting. The results indicate guest-dependent long range magnetic ordering occurs at low temperatures, which correlates with the electrostatic and steric effects of the incorporated aromatic guests.