Molecular rectifiers based on donor/acceptor assemblies: effect of orientation of the components' magnetic moments

In forming donor/acceptor assemblies that act as molecular rectifiers, we have introduced magnetic organic molecules as electron-donating and electron-accepting moieties. We have oriented the magnetic moment of the donor and acceptor components separately and immobilized them (and their moments) so that the molecular assemblies that act as rectifiers could be formed with moments mutually parallel or anti-parallel to each other. We have characterized the molecular assemblies formed on an electrode with a scanning tunneling microscope tip. Such donor/acceptor assemblies with a control over the orientation of moments of the components provided unique systems to study the effect of the nature of alignment on molecular rectifiers. We have observed that the rectification ratio increased in junctions with moments of the components being parallel to each other. The improvement in the rectification ratio has been explained in terms of an efficient electron-transfer process in a moment-aligned junction between the donor and acceptor moieties.

d-Electron-Induced Negative Magnetoresistance of a π−d Interaction System Based on a Brominated-TTF Donor

A new pi-d interaction system (EDT-TTFBr2)2FeBr4 (EDT-TTFBr2 = 4,5-dibromo-4',5'-ethylenedithiotetrathiafulvalene) and its nonmagnetic anion analogue (EDT-TTFBr2)2GaBr4 based on a brominated TTF-type organic donor are investigated. The salts featured by quasi-1D pi-electronic systems are metallic with metal-insulator transitions taking place at about 20 and 70 K for the FeBr4- and GaBr4- salts, respectively, where the low-temperature insulating state is associated with charge ordering or a Mott insulator followed by an antiferromagnetic transition at lower temperatures. The FeBr4- salt is featured with an antiferromagnetic transition of the anion d spins at a Neel temperature (TN) = 11 K, which is significantly high despite its long anion-anion Br-Br contact, suggesting the importance of the pi-d interaction in the magnetism. The surprisingly strong pi-d interaction, ca. -22.3 K estimated from the magnetization curve, evidences the usefulness of the chemical modification of the donor molecule with bromine substitution to achieve strong intermolecular interaction. The antiferromagnetic state of the anion d spins affects the transport of the conducting pi electrons through the strong pi-d interaction, as evidenced by the presence of a resistivity anomaly of the FeBr4- salt at TN. Below TN, the FeBr4- salt shows negative magnetoresistance that reaches -23% at the highest magnetic field investigated (B=15 T), whereas only a small positive magnetoresistance is observed in the pi-electron-only GaBr4- salt. The mechanism of the negative magnetoresistance is explained by the stabilization of the insulating state of the pi electrons by the periodic magnetic potential of the anion d spins in the FeBr4- salt, which is modified by applying the external magnetic field.

Bulk Spontaneous Magnetization in the New Radical Cation Salt TM-TTF[Cr(NCS)4(isoquinoline)2] (TM-TTF = Tetramethyltetrathiafulvalene)

A new organic-inorganic hybrid salt [TM-TTF][Cr(NCS)(4)(isoquinoline)(2)] (1) (TM-TTF = Tetramethyltetrathiafulvalene) has been synthesized. Compound 1 crystallizes in the triclinic P space group with a = 8.269(1), b = 10.211(2), and c = 11.176(2) A, alpha = 89.244(9), beta = 88.114(6), and gamma = 74.277(7) degrees, V = 907.6(3) A(3), and Z = 1. The crystal structure was resolved in the temperature range between 223 and 123 K, showing that changes in the crystal structure at low temperature result in stronger interactions between anions and cations. The packing of 1 consists of mixed anion-cation layers in the bc plane containing S.S and pi-pi anion-cation interactions, the layers being connected by very short S.S contacts between anions and cations. Magnetic measurements in a small external field show bulk spontaneous magnetization with a T(c) of 6.6 K consistent with the presence of weakly coupled ferrimagnetic order in compound 1. The EPR measurements also demonstrate the interaction between the d and pi electrons and the presence of an internal magnetic field brought about by the magnetic ordering.

Novel ET-Coordinated Copper(I) Complexes: Syntheses, Structures, and Physical Properties (ET = BEDT-TTF = Bis(ethylenedithio)tetrathiafulvalene)

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+.

Metal-dependent ferro- versus antiferromagnetic interactions in molecular crystals of square Planar (M(II) imino-nitroxide radical) complexes (M = Pt, Pd)

The synthesis and structural, spectral, and magnetic characterizations of two new complexes of formula [Pt(IM(2)Py)Cl(2)] (A) and [Pd(IM(2)Py)Cl(2)] (B) are reported. IM(2)Py stands for the imino-nitroxide radical ligand 2-(ortho-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl. Their crystal structures were solved at room temperature and at 120 K revealing structural phase transitions from pseudo-orthorhombic to monoclinic systems for the two compounds which remain isostructural in the whole temperature range explored. Structural parameters for A: T = 293 K [120 K], monoclinic (P2(1)/n) [P2(1)/c], a = 7.906(2) [7.989(3)] A, b = 17.872(9) [10.168(4)] A, c = 10.357(3) [17.623(6)] A, beta = 90.732(13) degrees [95.940(2)] degrees, Z = 4 [4]. Structural parameters for B: T = 293 K [120 K], monoclinic (P2(1)/n) [P2(1)/c], a = 7.900(3) [7.9730(2)] A, b = 17.907(9) [10.1806(3)] A, c = 10.299(3) [17.7171(4)] A, beta = 90.524(14) degrees [95.747(2)] degrees, Z = 4 [4]. In both complexes, the metal coordination is essentially planar. The average Pt-N, Pt-Cl and Pd-N, Pd-Cl bond lengths are 1.996(6) [1.88], 2.295(2) [2.248(8)] A and 2.015(7) [2.029(8)], 2.287(3) [2.294(3)] A, respectively. The solid state structure is characterized by a pairlike molecular packing stacked in columns parallel to the a axis; this dimer character is reinforced at low temperature. Despite their structural similarity, the investigation of the magnetic properties revealed that dominant ferromagnetic interactions govern the behavior of the Pt derivative A, whereas antiferromagnetic interactions take place for the Pd compound B. A rationalization for this rather intriguing difference is proposed in light of the spin population deduced from density functional theory calculations. The electronic absorption spectra of A and B present structured absorption bands in the visible which are attributed to MLCT transitions. Both compounds are nonluminescent at room temperature. However, a weak emission is detected for A in butyronitrile glasses at 77 K, indicating that the MLCT excited state is strongly quenched at low temperature.

New tetrathiapentalene-derived charge transfer salts with paramagnetic transition metal complex anion: kappa-(EDDH-TTP)(3)[Cr(phen)(NCS)(4)] x 2CH(2)Cl(2) and kappa(21)-(BDH-TTP)(5)[Cr(phen)(NCS)(4)](2) x 2CH(2)Cl(2)

The preparation, crystal structures, and optical and magnetic properties of two new charge-transfer salts kappa-(EDDH-TTP)(3)[Cr(phen)(NCS)(4)] x 2CH(2)Cl(2) (1) and kappa(21)-(BDH-TTP)(5)[Cr(phen)(NCS)(4)](2) x 2CH(2)Cl(2) (2), where phen = 1,10- phenanthroline, EDDH-TTP = 2-(4,5-ethylenedithio-1,3-dithiol-2-ylidene)-5-(1,3-dithiolan-2-ylidene)-1,3,4,6-tetrathiapentalene, and BDH-TTP = 2,5-bis(1,3-dithiolan-2-ylidene)-1,3,4,6-tetrathiapentalene, are reported. Crystal data: (1) monoclinic P2(1)/a, a = 25.0752(5) A, b = 10.6732(3) A, c = 28.1601(6) A, beta = 95.195(2) degrees, Z = 4, R = 0.0585 for 6741 independent reflections with I > 3 sigma(I); (2) monoclinic P2(1)/a, a = 23.8275(4) A, b = 9.1015 (2) A, c = 27.0420(1) A, beta = 99.9297(8) degrees, Z = 4, R = 0.0530 for 4565 independent reflections with I > 2 sigma(I). The crystal structures for both compounds consist of alternating organic and inorganic layers. The organic layer in compound 1 is characterized as kappa-type, while the organic layer in 2 resembles the kappa-type but it contains orthogonal dimers and monomers, and it is therefore called kappa(21). Compound 1 shows metallic behavior down to low temperature. Salt 2 shows semiconductive behavior, which is explained as the result of either charge ordering owing to the kappa(21)-type structure or Peierls distortion due to the one-dimensional electronic nature. However, weak metallic behavior could be observed at 10 kbar above ca. 150 K and at 15 kbar above 170 K. The magnetic susceptibilities for both compounds show Curie-Weiss behavior, showing that the exchange interactions between the magnetic anions are weak. Polarized reflectance spectra of single crystals were measured over the spectral range from 650 to 7000 cm(-1). Moreover, absorption and diffusion reflectance spectra of powdered crystals dispersed in KBr (from 400 to 7000 cm(-1)) were recorded. Vibrational and electronic features are discussed.

Spin distributions, ring conformations, and spiroconjugation in "phosphaverdazyl" radicals

Reaction of P-dimethylaminophosphonic acid bis(1-methylhydrazide) (6) with trimethyl orthobenzoate gave 1,2,5,6-tetrahydro-1,5-dimethyl-6-(N,N-dimethylamino)-6-phenyl-1,2,4,5,6-tetrazaphosphorine-6-oxide (7), which was subsequently oxidized to the corresponding P-diemthylamino-6-phosphaverdazyl (5) as a persistent radical. Analysis of the electron paramagnetic resonance spectrum of 5 revealed significant spin density on the exocyclic nitrogen but very little spin density on the phosphorus, in contrast to the previously reported P-phenyl-6-phosphaverdazyl (4). Density functional theory calculations on simplified models of 4, 5, and related radicals were performed and revealed that spin polarization effects and the nature of the substituents on phosphorus have significant effects on the structures and spin distributions of these radicals. The spin transfer to the dimethylamino group in 5 was revealed to arise from spiroconjugation-type overlap between the nitrogen 2p orbital with the verdazyl radical singly occupied molecular orbital.