Leokadiya V. Zorina

Ultra-fast Rotors for Molecular Machines and Functional Materials via Halogen Bonding: Crystals of 1,4-Bis(iodoethynyl)bicyclo 2.2.2 octane with Distinct Gigahertz Rotation at Two Sites

As a point of entry to investigate the potential of halogen-bonding interactions in the construction of functional materials and crystalline molecular machines, samples of 1,4-bis(iodoethynyl)bicyclo[2.2.2]octane (BIBCO) were synthesized and crystallized. Knowing that halogen-bonding interactions are common between electron-rich acetylenic carbons and electron-deficient iodines, it was expected that the BIBCO rotors would be an ideal platform to investigate the formation of a crystalline array of molecular rotors. Variable temperature single crystal X-ray crystallography established the presence of a halogen-bonded network, characterized by lamellarly ordered layers of crystallographically unique BIBCO rotors, which undergo a reversible monoclinic-to-triclinic phase transition at 110 K. In order to elucidate the rotational frequencies and the activation parameters of the BIBCO molecular rotors, variable-temperature (1)H wide-line and (13)C cross-polarization/magic-angle spinning solid-state NMR experiments were performed at temperatures between 27 and 290 K. Analysis of the (1)H spin-lattice relaxation and second moment as a function of temperature revealed two dynamic processes simultaneously present over the entire temperature range studied, with temperature-dependent rotational rates of k(rot) = 5.21 × 10(10) s(-1)·exp(-1.48 kcal·mol(-1)/RT) and k(rot) = 8.00 × 10(10) s(-1)·exp(-2.75 kcal·mol(-1)/RT). Impressively, these correspond to room temperature rotational rates of 4.3 and 0.8 GHz, respectively. Notably, the high-temperature plastic crystalline phase I of bicyclo[2.2.2]octane has a reported activation energy of 1.84 kcal·mol(-1) for rotation about the 1,4 axis, which is 24% larger than E(a) = 1.48 kcal·mol(-1) for the same rotational motion of the fastest BIBCO rotor; yet, the BIBCO rotor has three fewer degrees of translational freedom and two fewer degrees of rotational freedom! Even more so, these rates represent some of the fastest engineered molecular machines, to date. The results of this study highlight the potential of halogen bonding as a valuable construction tool for the design and the synthesis of amphidynamic artificial molecular machines and suggest the potential of modulating properties that depend on the dielectric behavior of crystalline media.

Large Spontaneous Polarization and Clear Hysteresis Loop of a Room-Temperature Hybrid Ferroelectric Based on Mixed-Halide [BiI3Cl2] Polar Chains and Methylviologen Dication

The search for hybrid organic-inorganic materials, which have the great advantage that they can be synthesized at moderate temperature (T < 200 °C), remains a great challenge in the field of ferroelectrics. Here, a room-temperature ferroelectric material with interesting characteristics, (MV)[BiI(3)Cl(2)] (MV(2+) = methylviologen), is reported. Its structure is based on polar inorganic chains resulting from a remarkable Cl/I segregation induced by methylviologen entities, which coincide with the fourfold polar axis of the tetragonal structure. Of great importance is that this room-temperature hybrid ferroelectric displays a clear electrical hysteresis loop with a large spontaneous polarization (>15 μC·cm(-2)).

Crystalline Arrays of Pairs of Molecular Rotors: Correlated Motion, Rotational Barriers, and Space-Inversion Symmetry Breaking Due to Conformational Mutations

The rod-like molecule bis((4-(4-pyridyl)ethynyl)bicyclo[2.2.2]oct-1-yl)buta-1,3-diyne, 1, contains two 1,4-bis(ethynyl)bicyclo[2.2.2]octane (BCO) chiral rotators linked by a diyne fragment and self-assembles in a one-dimensional, monoclinic C2/c centrosymmetric structure where two equilibrium positions with large occupancy imbalance (88% versus 12%) are identified on a single rotor site. Combining variable-temperature (70-300 K) proton spin-lattice relaxation, (1)H T1(-1), at two different (1)H Larmor frequencies (55 and 210 MHz) and DFT calculations of rotational barriers, we were able to assign two types of Brownian rotators with different activation energies, 1.85 and 6.1 kcal mol(-1), to the two (1)H spin-lattice relaxation processes on the single rotor site. On the basis of DFT calculations, the low-energy process has been assigned to adjacent rotors in a well-correlated synchronous motion, whereas the high-energy process is the manifestation of an abrupt change in their kinematics once two blades of adjacent rotors are seen to rub together. Although crystals of 1 should be second harmonic inactive, a large second-order optical response is recorded when the electric field oscillates in a direction parallel to the unique rotor axle director. We conclude that conformational mutations by torsional interconversion of the three blades of the BCO units break space-inversion symmetry in sequences of mutamers in dynamic equilibrium in the crystal in domains at a mesoscopic scale comparable with the wavelength of light used. A control experiment was performed with a crystalline film of a similar tetrayne molecule, 1,4-bis(3-((trimethylsilyl)ethynyl)bicyclo[1.1.1]pent-1-yl)buta-1,3-diyne, whose bicyclopentane units can rotate but are achiral and produce no second-order optical response.

Charge ordering, symmetry and electronic structure issues and Wigner crystal structure of the quarter-filled band Mott insulators and high pressure metals δ-(EDT-TTF-CONMe2)2X, X = Br and AsF6

We report on the synthesis and application of an internal chemical pressure to effectively control, and reduce, the Mott gap in the system δ-(EDT-TTF-CONMe2)2X, X = Br, AsF6; the detailed accounts of its Pmna, averaged room temperature structure and reversible phase transition at ca. 190 K towards a low temperature P21/a structure; the synthesis of (13C-EDT-TTF-CONMe2)2Br, where one carbon atom of the inner double bond is 100% 13C-enriched and single crystal 13C solid state NMR spectroscopy and relaxation revealing that charge ordering occurs at room temperature and ambient pressure; the discovery of weak superstructure Bragg reflections in δ-(EDT-TTF-CONMe2)2Br and subsequent analysis of the superstructure symmetry and refinement of an exhaustive synchrotron radiation data set; suggesting an alternation at room temperature of neutral and oxidized molecules along both the stacking a and transverse b directions in orthorhombic, non-centrosymmetric space groupP2nn, a CO pattern compatible with ferroelectricity. The charge disproportionation and long range order crystallization of the electron gas onto every other molecular site within a three-dimensional Wigner lattice is coupled to a concerted activation-deactivation of large collections of transverse Csp2–H⋯O hydrogen bonds and an anti-phase, static modulation of the bromide anions displacements along b. Despite the occurrence of charge ordering, the stacks remain essentially uniform, in agreement with the rich low temperature Mott physics of the system.

A Neutral Zwitterionic Molecular Solid

We report on the acid ethylenedithiotetrathiafulvaleneamidoglycine (EDT-TTF-CO-NH-CH(2)-CO(2)H; 1; EDT-TTF=ethylenedithiotetrathiafulvalene) and the 1:1 adduct [(EDT-TTF)(·+)-CO-NH-CH(2)-(CO(2))(-)][(EDT-TTF)-CO-NH-CH(2)-(CO(2)H)]·CH(3)OH (2), a new type of hydrogen-bonded, 1:1 acid/zwitterion hybrid embrace of redox peptidics into a two-dimensional architecture, an example of a system deliberately fashioned so that oxidation of π-conjugated cores toward the radical-cation form would interfere with the activity of the appended ionizable residues in the presence of a templating base during crystal growth. First-principles calculations demonstrate that, notwithstanding preconceived ideas, a metallic state is more stable than the hole-localized alternatives for a neat 1:1 neutral acid/zwitterion hybrid. The inhomogeneous Coulomb field associated with proton-shared, interstacks O-H···O hydrogen bonds between the ionizable residues distributed on both sides of the two-dimensional π-conjugated framework leads, however, to a weak hole localization responsible for the activated but high conductivity of 1 S cm(-1). This situation is reminiscent of the role of the environment on electron transfer in tetraheme cytochrome c, in which the protonation state of a heme propionate becomes paramount, or ion-gated transport phenomena in biology. These observations open rather intriguing opportunities for the construction of electronic systems at the interface of chemistry and biology.

Band structure and Fermi surface of the (BEDT-TTF)4M[Fe(CN)5NO]2 (M=Na, K, Rb,…) molecular metals containing the photochromic nitroprusside anion

Abstract Tight-binding band structure calculations for the room temperature crystal structure of the new family of (BEDT-TTF)4M[Fe(CN)5NO]2 (M=Na, K, Rb, NH4, Cs, Tl) salts are reported. It is shown that the Fermi surface of these salts contains two contributions: (a) a strongly warped non-nested open part and (b) a contribution which can be open or closed depending on minor changes in the donor–donor interactions which in either case exhibits well-nested portions. Because of these features, the pressure dependence of both the magnetoresistance measurements and the possible occurrence of density wave instabilities would be very interesting to study.

New Transparent Metal‐like Bilayer Composite Films with Highly Conducting Layers of θ‐(BET‐TTF)2Br·3H2O Nanocrystals

A flexible low-density metallic material, which is extremely transparent, was obtained using as active component the highly conducting molecular metal θ-(BET-TTF)2Br·3H2O, BET-TTF = bis(ethylenethio)tetrathiafulvalene. This material is a bilayer (BL) film that was prepared by treating a polycarbonate film containing 2 wt.-% of molecularly dispersed BET-TTF with vapor of a Br2/CH2Cl2 solution. Optimum conditions for the preparation of very transparent metallic materials were established. The X-ray diffraction patterns indicate that the conducting layer of the BL films is formed by well a* oriented θ-(BET-TTF)2Br·3H2O nanocrystals, which are clearly observed in the SEM images. Conductivity measurements confirm that the nanocrystalline layers have the same transport properties as those of the single crystals, displaying metal-like behavior down to He temperature and the highest room temperature conductivity (120 Ω–1 cm–1) reported so far for this kind of film.

Coexistence of two donor packing motifs in the stable molecular metal α-‘pseudo-κ’-(BEDT-TTF)4(H3O)[Fe(C2O4)3]·C6H4Br2

The crystal and electronic structure of a new radical cation salt α-‘pseudo-κ’-(BEDT-TTF)4H3O[Fe(C2O4)3]·C6H4Br2 have been studied. The new triclinic crystals contain two conducting organic layers which are characterized by different BEDT-TTF packing motifs: a ‘pseudo-κ’-layer which is composed of charged dimers and neutral monomers of BEDT-TTF orthogonal to each other and an α-layer which consists of inclined, uniformly charged BEDT-TTF stacks. According to electronic band structure calculations, the ‘pseudo-κ’ layer has a large gap between the HOMO bands at the Fermi level and should be associated with an activated conductivity. In contrast, the α-layer is a strongly two-dimensional electronic system with uniform intermolecular interactions. The absence of any nesting in the Fermi surface of the α-layer suggests that this salt should be a stable metal down to low temperatures. Metallic properties have been observed in the crystals in the 300–0.4 K temperature range. Besides, well pronounced Shubnikov–de Haas oscillations of the magnetoresistance have been revealed at B > 8 T. The salt investigated is a new phase in the (BEDT-TTF)4AI[MIII(C2O4)3]G family of organic molecular conductors with paramagnetic anions and different guest solvent molecules G in the anion layer. Structural features of the new α-‘pseudo-κ’-crystals and other known phases of the family (β″, ‘pseudo-κ’ and α-β″) have been compared.

Structural phase transition in the β′′-(BEDT-TTF)4H3O[Fe(C2O4)3]·G crystals (where G is a guest solvent molecule)

A structural phase transition from monoclinic C2/c to triclinic P symmetry has been found by X-ray diffraction in a number of single crystals of the known family of organic metals and superconductors β′′-(BEDT-TTF)4H3O[Fe(C2O4)3]·G where G stands for halogenated benzene derivatives and their mixtures with benzonitrile. The transition occurs upon lowering the temperature at 180–230 K. Comparison of the crystal and electronic structure of the monoclinic and triclinic phases reveals details of the structural transformations in the (PhCl + PhCN)-containing superconducting β′′-crystal, as an example. It is shown that the transition concerns mainly the anion layer and has a weak influence on the structure of the BEDT-TTF layer and, consequently, on the conducting properties of the single crystals.

Terpyridine–tetrathiafulvalene hybrid ligands and their electroactive metal complexes

The synthesis and full characterization (including X-ray structures) of two redox-active terpyridine–tetrathiafulvalene ligands namely (4′-amido-2,2′:6′,2′′-terpyridyl)-6,7-ethylenedithiotetrathiafulvalene (1) and 2-(4′-thioacetamide-2,2′:6′,2′′-terpyridyl)-3,6,7-tris(methylsulfanyl)-6,tetrathiafulvalene (2) are described. The binding properties of these multifunctional systems for various transition metal cations (Ni2+, Zn2+, Cd2+ and Fe2+) are analyzed in solution by cyclic voltammetry and UV-visible spectroscopy. In addition, a tetrahedral neutral zinc metal complex of ligand (2) formulated as (MeS)3-TTF-SCH2CONH-Tpy-ZnCl2·MeOH [complex (3)] and an octahedral nickel complex formulated as [{(MeS)3-TTF-SCH2CONH-Tpy}2Ni]·(ClO4)2·0.5(H2O) [complex (4)] are characterized in the solid state by X-ray diffraction.