Stacking Axis Research Articles

CONDENSATION OF SELF-TRAPPED CHARGE-TRANSFER EXCITATIONS AND INTERPLAY BETWEEN QUANTUM AND THERMAL EFFECTS AT THE NEUTRAL-TO-IONIC TRANSITION

The cooperativity between self–trapped electronic excitations is carried to extreme in the case of the neutral–to–ionic phase transition which is observed in some organic mixed–stack charge–transfer crystals. This electronic–structural phase transition manifests itself by a change of the degree of charge–transfer and a dimerization distorsion along the stacking axis in the ionic phase. Thermal charge–transfer excitations associated with the formation of structurally relaxed ionic strings along neutral chains are at the heart of mechanism of this uncommon phase transition. Symmetry and thermodynamics analysis of the neutral–to–ionic transition in the prototype compound, tetrathiafulvalene–p–chloranil, in particular the recent determination of the pressure–temperature phase diagram, make possible to present a consistent picture of this phase transition. Supported by a phenomenological approach, taking into account the quasi–one–dimensional nature of the system and the interplay between quantum and thermal effects, the experimental results show that the neutral–to–ionic transition results from the condensation and the ordering (crystallization) of charge–transfer excitations, following a phase diagram analogous to the solid–liquid–gas one.

Physics of Neutral-to-Ionic Phase Transition in Organic Charge Transfer Semiconducting Compounds

An uncommon excitonic instability takes place in some exotic semiconducting compounds. Indeed, the equilibrium neutral-to-ionic (N-I) phase transition, as well as the non-equilibrium photo-induced phase transforma tion, observed in some organic charge-transfer complexes, originate from intraand inter-chain cooperative effects between structurally relaxed charge-transfer excitations. This electronic-structural phase transition manifests itself by a change of the degree of charge-transfer and a dimerization distortion with the formation of donor-acceptor pairs along the stacking axis in the I phase. Thermal charge-transfer excitations associated with the formation of I strings along N chains are at the heart of the mechanism of this phase transition. These relaxed electronic excitations, which are an intrinsic feature of low-dimensional systems with strong electron-phonon coupling, can be described in terms of self-trapping and self-multiplication of charge-transfer excitons. Precise structural studies on the prototype compound, tetrathiafulvalene-p-chloranil allow to highlight the respective pole taken by the ionicity and the dimerization. Symmetry and thermodynamics analysis of the N-I transition, based on recent determination of the pressure-temperature phase diagram, make possible to present a consistent picture of this phase transition. Supported by theoretical considerations taking into account the interplay between quantum and thermal effects, the experimental observations show that the N-I transition results from the condensation and the ordering (crystallization) of charge-transfer excitations, following a phase diagram analogous to the solid—liquid—gas one.

Compressibility of one dimensional phthalocyanine conductors, NiPc(AsF 6) 0.5 and CoPc(AsF 6) 0.5

Powder X-ray diffraction patterns of phthalocyanine conductors, NiPc(AsF 6) 0.5 and CoPc(AsF 6) 0.5, were measured under high pressure. The crystal symmetries of these compounds were maintained in the pressure range of 0 − 4.1 GPa. The contraction along the stacking axis is larger than that perpendicular to the axis. This difference becomes clear at the pressure where the metal-ligand charge transfer occurs. The relation between the lattice contraction and the pressure-induced metal-ligand charge transfer is discussed.

Temperature-dependent IR reflectance investigations of the organic superconductor (BEDO-TTF) 2 ReO 4(H 2O)

Polarized reflectivity spectra of single crystals of (BEDO-TTF) 2ReO 4(H 2O) (BEDO-TTF = bis(ethylenedioxy)tetrathiafulvalene) were measured in the spectral range from 500 to 8500 cm −1 at six temperatures between 300 and 20 K. The polarization of the incident radiation was arranged parallel and perpendicular to the stacking axis of the donor molecules. The temperaturedependent reflectivity and the optical conductivity are presented. Changes at the metal-metal phase transition at 213 K and the metalsemiconductor phase transition at 35 K are discussed. The Drude parameters of the conduction electrons were obtained by fitting the reflectance data with a DrudeLorentz model, indicating that both phase transitions are accompanied by an abrupt change of the relaxation constant of the free charge carriers. Both phase transitions have an effect on the vibrational spectra and possible changes of the crystal structure at the phase transitions are discussed.

Electropolymers based on diphenylamine II-Stacking in cationic benzidine units

We have prepared a series of monomers with the general formula diphenylamine-R-diphenylamine. The electro-oxidation of these monomers in a series of non-aqueous solvents leads to para C-C coupling of the terminal phenyl rings to produce (R-diphenyl benzidine) n , an alternating main chain unit material. Polymeric material is formed from high-polarity solvents while low-polarity solvents produce precipitated oligomeric materials. Low-energy (~0.8eV) charge-transfer bands can be observed in the 1 e − per repeat unit oxidized thin film precipitated oligomeric material. The thin film polymeric material does not show these charge-transfer bands. These bands show polarization parallel with respect to the surface. Since these materials lack conjugation along the chain, π-stacking between chains with a stacking axis parallel to the surface is most likely responsible for these electronic transitions. Thus this system self-organizes upon formation at the surface.

Analysis of the structural and electronic properties of (fluoranthene) 2PF 6 and characterization of its (011) surface by scanning tunneling microscopy

The phase transitions of the conducting salts (FA) 2X (FA = fluoranthene; X = PF 6, AsF 6) at 200 and 180 K were probed by analyzing the short intermolecular contacts of their crystal structures and calculating their electronic band structures. The scanning tunneling microscopy (STM) images of (FA) 2PF 6 were interpreted by calculating the partial electron density plots for its (011) surface. Our study indicates that the 200 K phase transition leads to a more compact crystal packing in which all F atoms of a PF 6 − anion make short F···H contacts with the surrounding FA molecules. The 180 K phase transition originates from the charge density wave (CDW) instability of each FA stack. It is most likely that the dimerization expected from the CDW instability involves rotations of FA dimers around the stacking axis, and that the occurrence of domain structures in each FA stack prevents the observation of a dimerized structure by single-crystal X-ray diffraction. The STM images of (FA) 2PF 6 indicate that a few FA molecules under the STM tip apex undergo a reversible, rotational rearrangement.

Optical and transport studies of Ni(dmit)2-based organic conductors.

Single-crystal Ni(dmit${)}_{2}$ salts, (${\mathrm{Ph}}_{4}$P)[Ni(dmit${)}_{2}$${]}_{3}$, (${\mathrm{Bu}}_{4}$N${)}_{2}$[Ni(dmit${)}_{2}$${]}_{7}$\ensuremath{\cdot}${2\mathrm{C}\mathrm{H}}_{3}$CN, and (${\mathrm{Me}}_{3}$S)[Ni(dmit${)}_{2}$${]}_{2}$, have been synthesized. All show semiconducting behavior in their temperature-dependent dc conductivities. Room-temperature polarized reflectance measurements have been made over the range between 100 and 32 000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ (12 meV--4 eV). For light polarized along the Ni(dmit${)}_{2}$ stacking axis, all spectra show an energy gap, with superimposed vibrational fine structure at low frequencies and charge-transfer bands at high frequencies. Band gaps determined from the optical conductivities are consistent with thermal activation energies from dc transport measurements. The stacking-axis conductivity shows the effect of electron-molecular vibration interaction; analysis for (${\mathrm{Me}}_{3}$S)[Ni(dmit${)}_{2}$${]}_{2}$ yields a dimensionless electron-phonon coupling constant \ensuremath{\lambda}\ensuremath{\sim}0.27. For light polarized perpendicular to the stacking axis, only weak vibrational features are observed. \textcopyright{} 1996 The American Physical Society.

Solid State Behaviour of Vinyl Quinones

Abstract 2,5-Bisvinyl-1,4-benzoquinones 1 and 2-vinyl-1,4-bsnzoquinones 2 substituted with aryl or ester end groups have been synthesized. The 2,5-bisstyryl-1,4-benzoquinones 1 (R=phenyl, p-tolyl, o-tolyl) crystallize with a 7 A-stacking axis. But only for the o-tolyl derivative the contacts between the vinyl groups are close enough to allow a four-center type photopolymerization. The ester derivative 1 (R=COOEt) has a layer structure and can be photooligomerized in the crystal. The generated cyclobutane subgroups have twofold symmetry. The vinylquinones 2 (R=aryl) may be dimerized photochemically in the crystal at the vinyl groups to centrosymmetric cyclobutanes. Crystals with 4 A stacking axis are also photoreactive.

Structural aspects of the neutral-to-ionic transition in mixed-stack charge-transfer complexes

Original and intriguing features associated with the so-called neutral-to-ionic (N-I) transition have been examined in some organic mixed-stack charge-transfer (CT) complexes, involving simultaneously electronic, structural and magnetic aspects. The origin of the N-I transition and the essential parameters governing it are still controversial. Structural aspects of the N-I transition in the prototype compound tetrathiafulvalene- p-chloranil (TTF-CA) are reviewed. In the ionic phase, a ferroelectric arrangement is apparent between the dimerized chains. However, these long-range dipolar interactions are too weak to drive the phase transition. At atmospheric pressure, a significant volume variation occurs at T N-I with an abrupt change in the cell parameters in directions perpendicular to the stacking axis. This clear three-dimensional aspect of the compound is also evident from several close interchain contacts. Intramolecular deformations observed at the transition have been directly related to the ionicity change and the symmetry breaking, in agreement with the evolution of the 35Cl NQR spectrum. The phase diagram has been determined and a clear change in the nature of the transition has been observed with a tendency towards second-order character at high pressures.

Structural aspects of the neutral-to-ionic transition in mixed-stack charge-transfer complexes

Original and intriguing features associated with the so-called neutral-to-ionic (N-I) transition have been examined in some organic mixed-stack charge-transfer (CT) complexes, involving simultaneously electronic, structural and magnetic aspects. The origin of the N-I transition and the essential parameters governing it are still controversial. Structural aspects of the N-I transition in the prototype compound tetrathiafulvalene— p-chloranil (TTF—CA) are reviewed. In the ionic phase, a ferroelectric arrangement is apparent between the dimerized chains. However, these long-range dipolar interactions are too weak to drive the phase transition. At atmospheric pressure, a significant volume variation occurs at T N-I with an abrupt change in the cell parameters in directions perpendicular to the stacking axis. This clear three-dimensional aspect of the compound is also evident from several close interchain contacts. Intramolecular deformations observed at the transition have been directly related to the ionicity change and the symmetry breaking, in agreement with the evolution of the 35Cl NQR spectrum. The phase diagram has been determined and a clear change in the nature of the transition has been observed with a tendency towards second-order character at high pressures.

Single Crystal Magnetic Measurements of the Molecular Magnet NH4Ni(mnt)2.H2O

Abstract We report measurements of the magnetic properties of the molecular magnet NH4Ni(mnt)2.H2O. The magnetic exchange in this salt is ferromagnetic below 100K, with a Weiss constant of 7 K. Long range magnetic ordering is observed below 5 K. Single crystals of this salt have been studied down to 2 K in a SQUID magnetometer. The salt is found to have hard and easy magnetisation directions. Magnetisation of 1800 emuG/mol at 2 kOe was measured at 2 K for magnetic fields applied perpendicular to the molecular stacking axis.

Crystallographic data and magnetic properties of new CeFeSi-type RMnGe compounds (R = FaSm) studied by magnetization and neutron diffraction measurements

Investigations made by powder X-ray diffraction, susceptibility measurements and neutron diffraction experiments on new ternary RMnGe (R = LaNd) are reported. They crystallize in the well-known tetragonal CeFeSi-type structure (space group P4/nmm). This structure, which is closely related to the ThCr 2Si 2-type structure, can be described as isolated “ThCr 2Si 2 blocks” connected via R-R contacts. All these compounds are antiferromagnetic below T N ∼ 410 K (not detected in the bulk susceptibility measurements). At room temperature, the magnetic structures are characterized by a stacking of antiferromagnetic (001) Mn layers with Mn magnetic moments (about 3.6-3.2 μ B at 2 K) at 40° (CeMnGe) or along the c axis. In all cases, this antiferromagnetic arrangement of the Mn sublattice persists down to 2 K. At lower temperature, the R sublattice orders in ferromagnetic (001) layers, with various ordering schemes along the stacking axis. At 150 K, PrMnGe exhibits ferromagnetic behaviour followed by a ferro-antiferromagnetic transition below 80 K simultaneously with a crystallographic phase transition (tetragonal to orthorhombic symmetry). Below 200 K, NdMnGe exhibits a ferromagnetic ordering of the Nd sublattice which persists until 2 K. Within the complete ordering temperature range, the Pr and Nd magnetic moments are in the basal plane with a magnitude of 2.45(3) and 3(1) μ B for Pr and Nd, respectively, at 2K. The magnetic structures correspond to the sequence Mn(AF)R(+)R(+)Mn(AF)R(−or+). The Mn moments rotate at 50° from the c axis in PrMnGe, while they are in quadrature in NdMnGe. In CeMnGe, the Ce-sublattice orders antiferromagnetically below about 130 K (not detected in the magnetometric measurements), yielding the following ordering scheme: Mn(AF)Ce(+)Ce(+)Mn(AF)Ce(−)Ce(−); at 2 K the Ce moment value is 2.2(1) μ B. The Mn and Ce moment directions are almost parallel, and make an angle of about 35° with the c axis. The results are discussed and compared to those obtained previously on the corresponding ternary silicides and germanides of the CeFeSi- and ThCr 2Si 2-type structures.

Polarized IR reflectance studies of the organic superconductor (BEDO-TTF) 2ReO 4(H 2O)

Polarized reflectance spectra of single crystals of (BEDO-TTF) 2ReO 4(H 2O) (BEDO-TTF, bis(ethylenedioxy)tetrathiafulvalene were measured at room temperature over the spectral range 500–8300 cm −1. The incident radiation was polarized parallel and perpendicular to the stacking axis of the donor molecules. By fitting a Drude model to the dispersion of reflectance the Drude parameters were obtained and, starting from the plasma frequency, the bandwidth 4 t of the electronic system was derived. The real part of the dielectric function and the optical conductivity were determined by a Kramers-Kronig transformation. Both the optical conductivity and the bandwidth 4 t are compared with values obtained by a band-structure calculation. In the frequency region below 2000 cm −1 the origin of the vibrational transitions is discussed and compared to transmission measurements of optical density.

Self-Assembly of Low-Dimensional Molecular Nanoclusters on Au(111) Surfaces

Two-dimensional nanoclusters of (TTF)(TCNQ) (TTF=tetrathiafulvalene, TCNQ=tetracyanoquinodimethane) and Li + TCNQ - , formed on Au(111) surfaces by vapor phase sublimation under ambient conditions prior to growth of bulk crystals of these low-dimensional organic conductors, have been observed with scanning tunneling microscopy (STM) and scanning electron microscopy (SEM). The molecular planes of the constituents in individual nanoclusters are oriented perpendicular to the Au(111) substrate, while the clusters exhibit azimuthal orientations conforming to the 3-fold Au directions. The nanocluster morphology and structure suggest that self-assembly of the nanoclusters is governed by specific interactions between the molecular species and the substrate and molecular diffusion along troughs on the Au(111) substrate surface. In the case of the (TTF)(TCNQ) nanoclusters, TTF and TCNQ molecules assemble into molecular rows normal to the stacking direction, with intermolecular distances along the stacking direction which are nearly identical to those observed in bulk (TTF)(TCNQ). In contrast, the intermolecular spacings between TCNQ molecules along the molecular stacking axis in Li + TCNQ - nanoclusters are substantially larger than that observed in bulk M + TCNQ - salts. The large intermolecular spacing in Li + TCNQ - nanoclusters is consistent with Coulomb repulsion between fully reduced p=1 TCNQ - anion sites (p=formal charge). The smaller spacings observed for (TTF)(TCNQ) nanoclusters are consistent with reduced Coulomb repulsion, in agreement with the fractional charge known to exist in the bulk material (p=0.59 + and 0.59 - for TTF and TCNQ sites, respectively). The preferred direction of growth of the nanoclusters is transverse to the molecular stacking axes in both compounds, whereas the macroscopic morphologies reflect preferred growth parallel to the stacking direction. These observations indicate that morphology and molecular packing of crystal nuclei at the nanoscale are not necessarily identical to the corresponding characteristics observed at the macroscopic scale

Phasenumwandlung von 3,6-Diphenyl pyrrolo[3,4-c]pyrrole-1,4-dithione für optische Speicherungsanwendungen

'Diketopyrrolopyrroles' (DPP) are industrially important red pigments which resist bleaching. Thionation (replacement of the O- by the S-atom; the title compound (DTPP)) brings about an intense near-IR absorption in the solid state. Because of this, DTPP has recently attracted attention as a material for laser printers and optical information-storage systems based on GaAsAl laser diodes. Three crystal modifications I, II, and III are known to exist for DTPP. Among these, only modification III exhibits an intense near-IR absorption (860 nm), whereas the absorption band appears only at 690–710 nm in modifications I and II. The near-IR absorption is due to interplanar π-π interactions of the alternating N-atom of one DTPP molecule and thiocarbonyl C-atom of the neighboring molecule along the stacking axis. The formation or blocking of the π-π interaction ('phase change') by means of optical energy can be applied to an optical information storage system. The optical disk (structure: substrate/hydrazone/DTPP/Al) exhibits a reflectivity change from ca. 30 to 45% on writing with a power of ca. 9 mW at 780 nm.

Molecular structures and packing arrangements of five 2,5-bis(aryl-2-vinyl)-1,4-dimethoxybenzene derivatives

The crystal and molecular structures of five 2,5-bis(aryl-2-vinyl)-1,4-dimethoxybenzene derivatives1a-1e were determined by X-ray diffraction with respect to topochemical aspects. We found three types of packing arrangements: (1)α-type packing with 7 A stacking axes, (2)β-type packing with 4 A stacking axes and (3) a third intermediate packing type. The intermolecular distances between the vinylic double bonds of all derivatives exceed the limits of 4 A in the crystals. Therefore photochemical [2+2]cycloadditions were not observed in the crystals of these compounds. A correlation between the inclination angle of the molecular plane to the stacking axis and the separation between potentially reactive double bonds was detected.

Control of the mesoscopic organization of conjugated thiophene oligomers, induced by self-assembly properties

In order to analyse the dependence of charge transport in conjugated materials on their structural organization, three sexithiophene derivatives were studied, unsubstituted 6T and its di-hexyl substituted compounds, both on the terminal α positions (αωDH6T) and as pendant groups in β position (β, β′DH6T). X-ray diffraction characterization of vacuum evaporated thin films of these oligomers shows that 6T and α,ωDH6T consist of layered structures in a monoclinic arrangement, with all-trans planar molecules standing on the substrate. A large increase of molecular organization at the mesoscopic level is observed when passing from 6T to α,ωDH6T, as shown by a much longer range ordering. Electrical characterizations show that the conductivity of α,ωDH6T is anisotropic, with a ratio of 120 in favor of the conductivity parallel to the substrate plane, ie along the stacking axis. The charge carrier mobility, determined on field-effect transistors realized from these conjugated oligomers, also shows a large increase when passing from 6T to α,ωDH6T, reaching a value of 5 × 10 −2 cm 2 V −1 s −1, in contrast with β,β′DH6T which presents very low conductivity and mobility. These results are attributed to the self-assembly properties brought by alkyl groups in the α,ω position.

Propagation d'ondes acoustiques dans des empilements faiblement désordonnés

This work is devoted to a numerical study of transverse waves localization propagating along the stacking axis of random alternating Fe-Cu layers. The randomness is introduced through fluctuations of the layer thickness. For a weak disorder, the calculated localization length is in good agreement with the theoretical (asymptotic) predictions as far as a large number of plies (5000) is considered. For a more realistic number of plies, from an experimental point of view (200<N<500), large discrepancies appear due to the weak acoustic contrast at the interface Cu-Fe

Electronic spectra of 7,14‐dithioketo‐5,7,12,14‐tetrahydroquinolino‐[2,3‐b]‐acridine in solution and in the solid state

Abstract7,14‐Dithioketo‐5,7,12,14‐tetrahydroquinolino‐[2,3‐b]‐acridine (DTQ) is a thionated derivative of quinacridone (QA) that is known as a red pigment. Thionation of QA brings about an intense near‐IR absorption in the solid state which is of practical importance for GaAsAl‐laser printers and optical information storage systems. The electronic properties of DTQ have been investigated in solution and in the solid state in order to characterize the near‐IR absorption from the standpoint of intermolecular hydrogen bonding. The electronic state of the DTQ‐chromophore is found to be most sensitively affected by an environment of proton acceptors which interact with the NH group of the chromophore. In solution, the hydrogen bonds between the NH group and the O atom of a solvent cause a bathochromic shift by 5 – 15 nm relative to the absorption spectrum in dioxane. This is interpreted as being due to electron transfer from the proton to the nitrogen atom, thereby increasing the electron density in the chromophore. A similar increase in electron density is also operative in the solid state through the intermolecular hydrogen bonds between the NH group of one DTQ and the S atom of the neighboring molecule. This is mainly responsible for the bathochromic displacement of DTQ in going from solution to the solid state. The near‐IR absorption is, however, not explicable in terms of the hydrogen bonding interaction alone. Another intermolecular interaction, such as π‐π interactions along the stacking axis, appears to be operative that might lead to a near‐IR absorption.

Phase change of 1,4-dithioketo-3,6-diphenyl-pyrrolo-[3,4-c]-pyrrole for information storage applications

Three crystal modifications I, II, and III are known to exist for 1,4-dithioketo-3,6-diphenyl-pyrrolo-[3,4-c]-pyrrole (DTPP; blue pigment). Among these, only modification III exhibits an intense near-IR absorption due to interplanar π-π interactions of the alternating nitrogen and thiocarbonyl carbon atoms along the stacking axis. An optical information storage system of the ‘‘write once’’ type has been developed in which the near-IR absorption (‘‘on’’ state) is switched-off (‘‘off’’ state) on irradiation with GaAs laser diodes in the presence of a hydrazone compound. The near-IR absorption ceases to exist due to a molecular rearrangement caused by laser-generated vapors of hydrazone molecules. X-ray-diffraction analysis revealed that the present phenomenon is due to the phase change from modification quasi-III to modification II. The optical disk (structure: substrate/hydrazone/DTPP/Al) exhibits a reflectivity change from about 30% to 45% on writing with a power of about 9 mW at 780 nm.