Dipolar Units Research Articles

2D hexagonal assembly of a dipolar rotor with a close interval of 0.8 nm using a triptycene-based supramolecular scaffold.

Controlling the rotation of carbon-carbon bonds, which is ubiquitous in organic molecules, to create functionality has been a subject of interest for a long time. In this context, it would be interesting to explore whether cooperative and collective rotation could occur if dipolar molecular rotors were aligned close together while leaving adequate space for rotation. However, it is difficult to realize such structures as bulk molecular assemblies, since molecules generally tend to assemble into the closest packing structure to maximize intermolecular forces. To tackle this question, we examined an approach using a supramolecular scaffold composed of a tripodal triptycene, which has been demonstrated to strongly promote the assembly of various molecular and polymer units into regular "2D hexagonal packing + 1D layer" structures. We found that a molecule (1) consisting of a dipolar 1,2-difluorobenzene rotor sandwiched by two 10-ethynyl-1,8,13-tridodecyloxy triptycenes, successfully self-assembles into the desired structure, where the dipolar rotor units align two-dimensionally at a close interval of approximately 0.8 nm while having a degree of freedom for rotational motion. Here we describe the self-assembly behavior of 1 in comparison with the general trend in molecular self-assembly, as well as the motility of the two-dimensionally aligned molecular rotors investigated using solid-state 19F-MAS NMR spectroscopy.

Benzothiadiazole-based rotation and possible antipolar order in carboxylate-based metal-organic frameworks

By modifying organic ligands of metal-organic framework with dipolar units, they turn suitable for various applications, e.g., in the field of sensor systems or switching of gas permeation. Dipolar linkers in the organic ligand are capable to rotate in certain temperature and frequency ranges. The copper-bearing paddlewheel shaped metal-organic frameworks ZJNU-40 and JLU-Liu30 possess such a polarizable dipole moment due to their benzothiadiazole moiety in the organic ligands. Here, we investigate the molecular rotor behavior of benzothiadiazole units of the two carboxylate-based MOFs by dielectric spectroscopy and computational simulation. Our dielectric results provide clear evidence for significant reorientational relaxation dynamics of these rotors, revealing various characteristics of glasslike freezing upon cooling. The calculated rotational energy barriers are consistent with experimentally determined barriers for single-dipole dynamics. Moreover, for JLU-Liu30 we find hints at antipolar ordering below about 300 K.

Azimuthal Dipolar Rotor Arrays on Surfaces.

A set of dipolar molecular rotor compounds was designed, synthesized and adsorbed as self‐assembled 2D arrays on Ag(111) surfaces. The title molecules are constructed from three building blocks: (a) 4,8,12‐trioxatriangulene (TOTA) platforms that are known to physisorb on metal surfaces such as Au(111) and Ag(111), (b) phenyl groups attached to the central carbon atom that function as pivot joints to reduce the barrier to rotation, (c) pyridine and pyridazine units as small dipolar units on top. Theoretical calculations and scanning tunneling microscopy (STM) investigations hint at the fact that the dipoles of neighboring rotors interact through space through pairs of energetically favorable head‐to‐tail arrangements.

A single-molecule conductance study on the rotational isomers of a hexaarylbenzene derivative carrying dipolar rotating units

Using a scanning tunneling microscopy-break junction (STM-BJ) technique, we evaluated the single-molecule conductance of stable but interconvertible two rotational isomers of a hexaarylbenzene derivative featuring dipolar rotatable difluorophenyl units and tripodal thiol groups. The STM-BJ measurements of a configuration of single-molecule junction, formed between a Au-STM tip and a Au(111)/mica substrate covered with the hexaarylbenzene molecule, showed that the conductance behaviors of the rotational isomers are different from one another. This result takes a step forward in our goal of developing single-molecule switching/memory systems driven by electric fields.

Perovskite Crystals: Unique Pseudo-Jahn–Teller Origin of Ferroelectricity, Multiferroicity, Permittivity, Flexoelectricity, and Polar Nanoregions

In a semi-review paper, we show that the local pseudo-Jahn–Teller effect (PJTE) in transition metal B ion center of ABO3 perovskite crystals, notably BaTiO3, is the basis of all their main properties. The vibronic coupling between the ground and excited electronic states of the local BO6 center results in dipolar distortions, leading to an eight-well adiabatic potential energy surface with local tunneling or over-the-barrier transitions between them. The intercenter interaction between these dipolar dynamic units results in the formation of the temperature-dependent three ferroelectric and one paraelectric phases with order–disorder phase transitions. The local PJTE dipolar distortion is subject to the presence of sufficiently close in energy local electronic states with opposite parity but the same spin multiplicity, thus limiting the electronic structure and spin of the B(dn) ions that can trigger ferroelectricity. This allowed us to formulate the necessary conditions for the transition metal perovskites to possess both ferroelectric and magnetic (multiferroic) properties simultaneously. It clarifies the role of spin in the spontaneous polarization. We also show that the interaction between the independently rotating dipoles in the paraelectric phase may lead to a self-assembly process resulting in polar nanoregions and relaxor properties. Exploring interactions of PJTE ferroelectrics with external perturbations, we revealed a completely novel property—orientational polarization in solids—a phenomenon first noticed by P. Debye in 1912 as a possibility, which was never found till now. The hindered rotation of the local dipole moments and their ordering along an external field is qualitatively similar to the behavior of polar molecules in liquids, thus adding a new dimension to the properties of solids—notably, the perovskite ferroelectrics. We estimated the contribution of the orientational polarization to the permittivity and flexoelectricity of perovskite crystals in different limiting conditions.

Impact of Scaling on the Dipolar Coupling in Magnet–Insulator–Magnet Structures

The ability to control the magnetization of individual nanomagnets has made possible a broad class of devices, which store and manipulate information in the form of magnetization. For these devices to serve as building blocks for computation, they must be concatenable while allowing the information to flow in a directional manner. One scheme that has been proposed for this is the use of a dipolar coupling unit, a vertical stack of magnet–insulator–magnet, in which the information is transmitted from one magnet to the other through dipolar interaction. In this paper, we have used a novel magnetic force microscopy-based approach to demonstrate that a vertical dipolar coupling unit can be used to transfer information. We also investigated how the strength of the interaction can be maximized while preserving the directionality. The experimental observations were understood using micromagnetic simulations, and a scaling approach has been proposed for ultra-thin magnets.

Capacitive Dark Currents, Hysteresis, and Electrode Polarization in Lead Halide Perovskite Solar Cells.

Despite spectacular advances in conversion efficiency of perovskite solar cell many aspects of its operating modes are still poorly understood. Capacitance constitutes a key parameter to explore which mechanisms control particular functioning and undesired effects as current hysteresis. Analyzing capacitive responses allows addressing not only the nature of charge distribution in the device but also the kinetics of the charging processes and how they alter the solar cell current. Two main polarization processes are identified. Dielectric properties of the microscopic dipolar units through the orthorhombic-to-tetragonal phase transition account for the measured intermediate frequency capacitance. Electrode polarization caused by interfacial effects, presumably related to kinetically slow ions piled up in the vicinity of the outer interfaces, consistently explain the reported excess capacitance values at low frequencies. In addition, current-voltage curves and capacitive responses of perovskite-based solar cells are connected. The observed hysteretic effect in the dark current originates from the slow capacitive mechanisms.

Interpretation of tectonic setting in the Phetchabun Volcanic Terrane, Northern Thailand: Evidence from enhanced airborne geophysical data

Re-processed aeromagnetic data with enhancement approaches of reduction to the pole, high pass filtering and shaded relief have been used to interpret complex subsurface structures of the Carboniferous to Triassic Phetchabun Volcanic Terrane (PVT) which is largely covered by thick Cenozoic sediment deposits. Interpretation of the enhanced aeromagnetic data reveals four distinct structural domains in the PVT, viz. Northern, Eastern, Central, and Western domains. Within these domains, high magnetic units are recognized, namely elongate, ring, circular, and dipolar spot units. The elongate unit in the Central domain is characterized by a deformation zone with northwest–southeast trending, sinistral shearing. East–west trending and the northeast–southwest trending faults cross-cut several magnetic units in the Central domain, with sinistral and dextral movements, respectively. Three major fault directions have been identified, including the northeast–southwest trending sinistral faults, north–south trending dextral faults, and northwest–southeast trending dextral faults. The younger spot units are small intrusive bodies largely situated along these latest fault segments.The aeromagnetic interpretation results, together with relevant current field verification, as well as previous geochronological and petrochemical investigations, have lead to the clarification of structural development in the PVT. The elongate units are interpreted to represent Late Carboniferous intrusive bodies. They occurred as a result of an eastward subduction of the Nakhonthai oceanic plate beneath the Indochina continental plate, along the Loei suture. The elongate units are also reflected in a north–south trending deformation zone formed by the east–west compressional tectonics. The ring units are considered to have formed in a Permo-Triassic volcanic arc, whereas the circular units represent equigranular intrusive bodies which formed in a response to the second phase of eastward subduction. East–west extensional tectonics formed during the Late Triassic period is inferred to overlapp with subsidence along the Permo-Triassic volcanic arc. The magnetic spot units represent shallow, porphyritic intrusive bodies occurring along downthrown fault zones during this stage.

Light-induced mass transport in amorphous chalcogenides/gold nanoparticles composites

We have established that mass-transport processes in two types of amorphous materials, based on light-sensitive inorganic compounds like Se and As20Se80 chalcogenide glasses (ChG), can be enhanced at the nanoscale in the presence of localized plasmonic fields generated by visible light in gold nanoparticles (GNPs), if the condition of surface plasmon resonance (SPR) is fulfilled. It was found that irradiation by light in the presence of SPR produces profound surface nanostructurizations, and variation in topography follows closely and permanently the underlying near field intensity pattern. We have proposed a model of mass-transport in which the existence of moving anisotropic dipolar units and internal electric field in ChG as a main driving force of this movement is suggested.

Noncentrosymmetric Organic Solid and Its Zinc Coordination Polymer with Diamonded Network Prepared from an Ionothermal Reaction: Syntheses, Crystal Structures, and Second-Order Nonlinear Optics Properties

An organic ligand 4-(1,2,4-triazol-1-yl) benzoic acid (Htzbc) with a nonlinear optical (NLO) chromophore has been synthesized. Reaction of Zn(NO3)2 with Htzbc ligand in 1-n-butyl-3-methylimidazolium tetrafluoroborate ionic liquid afforded compound Zn(tzbc)2 (1). The Htzbc organic solid and 1 crystallize in the acentric Pn and Cc space groups, respectively. The tetrahedral Zn2+ center in 1 coordinates to two triazole nitrogen atoms and two carboxylate oxygen atoms from four tzbc– ligands, leading to a noncentrosymmetric 5-fold interpenetrating diamondoid network. Second-harmonic generation (SHG) measurements revealed the Htzbc solid and 1 have SHG activities. The Htzbc compound has a large SHG effect, which is approximately 3 times that of KDP and is type-I phase matchable, while 1 also belongs to phase-matchable class with a SHG response of about 1.2 times that of KDP. The SHG active of the organic solid can be derived from the inherently dipolar Htzbc molecule as well as the optimal relative orientation of the molecular NLO-phores in the crystal packing. The odd number-fold interpenetrated diamondoid network with dipolar NLO chromophoric units based on asymmetrical bridging ligands and metal centers results in the SHG activity of 1. In addition, elemental analysis, IR spectra, powder X-ray diffraction patterns (PXRD), and thermogravimetric analysis of both compounds are described.

Mesomorphic Organization and Thermochromic Luminescence of Dicyanodistyrylbenzene‐Based Phasmidic Molecular Disks: Uniaxially Aligned Hexagonal Columnar Liquid Crystals at Room Temperature with Enhanced Fluorescence Emission and Semiconductivity

AbstractA new dicyanodistyrylbenzene‐based phasmidic molecule, (2Z,2′Z)‐2,2′‐(1,4‐phenylene)bis(3‐(3,4,5‐tris(dodecyloxy)phenyl)acrylonitrile), GDCS, is reported, which forms a hexagonal columnar liquid crystal (LC) phase at room temperature (RT). GDCS molecules self‐assemble into supramolecular disks consisting of a pair of molecules in a side‐by‐side disposition assisted by secondary bonding interactions of the lateral polar cyano group, which, in turn, constitute the hexagonal columnar LC structure. GDCS shows very intense green/yellow fluorescence in liquid/solid crystalline states, respectively, in contrast to the total absence of fluorescence emission in the isotropic melt state according to the characteristic aggregation‐induced enhanced emission (AIEE) behavior. The AIEE and two‐color luminescence thermochromism of GDCS are attributed to the peculiar intra‐ and intermolecular interactions of dipolar cyanostilbene units. It was found that the intramolecular planarization and restricted molecular motion associated with a specific stacking situation in the liquid/solid crystalline phases are responsible for the AIEE phenomenon. The origin of the two‐color luminescence was elucidated to be due to the interdisk stacking alteration in a given column driven by the specific local dipole coupling between molecular disks. These stacking changes, in turn, resulted in the different degree of excited‐state dimeric coupling to give different emission colors. To understand the complicated photophysical properties of GDCS, temperature‐dependent steady‐state and time‐resolved PL measurements have been comprehensively carried out. Uniaxially aligned and highly fluorescent LC and crystalline microwires of GDCS are fabricated by using the micromolding in capillaries (MIMIC) method. Significantly enhanced electrical conductivity (0.8 × 10−5 S•cm−1/3.9 × 10−5 S•cm−1) of the aligned LC/crystal microwires were obtained over that of multi‐domain LC sample, because of the almost perfect shear alignment of the LC material achieved in the MIMIC mold.

Optical Properties of Poly(p-phylene ethylene)s with Laterally Attached Dipolar Unit

Poly ( p-phenylene ethylene) s containing laterally attached push-pull unit were synthesized by Sonogashira coupling reaction in the presence of different Pd catalysts( Pd( PPh3 )4 and PdCl2 ( PPh3 )2,PPh: P(C6H5)3). The structures and properties of the intermediate were identified by elemental analysis,IR, NMR,and so on. The polymers are soluble in common organic solvents and exhibits blue fluorescence. It has been found that the polymers prepared in the presence of different Pd catalysts have similar molecular mass and optical properties. The fact that the absorption maximum wavelength and emission maximum wavelength do not chang with the increase of the concentration of the polymers implies the electrostatic repulsion among the side chains can restrain the formation of aggregates. The optical study of the solid film of the polymers has supported the result.

Self-assembled monolayers of polar molecules on Au(111) surfaces: distributing the dipoles

Quantum-mechanical calculations are performed to investigate the interface between Au(111) surfaces and self-assembled monolayers (SAMs) of organic thiols. Dipolar pyrimidine units act as building blocks to systematically tune the molecular dipole moments via the number of repeat units. The resulting work-function modifications and the energetic alignment of the frontier electronic states in the SAM with the Fermi level are analyzed. Compared to SAMs where strong dipole moments are realized only by end-group substitutions on otherwise non-polar molecules, an entirely different evolution with backbone length is found for the present systems, where dipoles are built directly into the backbone. In particular, the achievable work-function modifications depend on peculiarities in the relative alignment of the energy levels in the SAM and in the metal. We thus introduce an additional degree of freedom for tuning surface and interface electronic properties with functional self-assembled monolayers.

A sum-over-state scheme of analysis of hyperpolarizabilities and its application to spiroconjugated molecular system

The static first and second hyperpolarizabilities of a number of spiromolecules with varying degree of polarity have been calculated at the HF and MP2 level using the 6-31+G* basis set and the B3LYP/6-31+G* optimized geometry. The variation of mean second hyperpolarizability in these molecular systems has been explained in terms of the ground state dipole moment, mean linear polarizability and second-order polarizability. A number of relationships among these quantities have been derived in the framework of the sum-over-state scheme and the generalized Thomas–Kuhn sum rule. The spiroconjugation results in the significant increase of the mean polarizability. The appreciable enhancement of first hyperpolarizability due to the spiroconjugation between two dipolar monomer units has been accounted for the rather significant increase of the mean polarizability tensor and the ground state dipole moment. The relatively larger value of the average second hyperpolarizability of the spiroconjugated molecules compared to that of the corresponding monomers arises from the rather significant increase of the nonaxial component γ xxyy . The replacement of spirocarbon by spirosilicon results in the enhancement of the cubic polarizability manifold. The donor–acceptor substituted spirocompounds are predicted to be the superior third-order nonlinear optical (NLO) phores. The nature of π-conjugation in the monomer units around the spirocenter shows a strong modulation of the NLO properties of spirocompounds. The influence of electron correlation on the NLO properties at the MP2 level has been found to be rather significant.

Temperature-Dependent Tunability in the Paraelectric State of BaTiO3-Based Solid Solutions

The electric field dependence of the dielectric constant (dc-tunability) as a function of composition at a few temperatures was investigated for BaTiO 3 -based solid solutions. Dense and homogeneous BaZr x Ti 1 − x O 3 ceramics (ferroelectric: x = 0.10 and x = 0.15 and relaxor: x = 0.18) with high dielectric constant and low losses were prepared by solid state reaction and their dc-tunability characteristics were investigate at a few temperatures from 20 to 70°C, including the Curie range. The nonlinear ϵ (E) dependences were described using the Johnson's approach plus an additional term. A model of random uncorrelated non-interacting dipolar units in a double well potential was employed for the ϵ (E) in the paraelectric state of the composition x = 0.18. For this composition, the average dipole moment of the nanopolar regions as a function of temperature was determined.

Molecular Dynamics and Quantum Chemistry Study on Conformations and Optical Properties of Hydrogen Bonded Dipolar Merocyanine Dyes

An earlier proposed strategy to orient two merocyanine dyes in the desirable head-to-tail parallel fashion through multiple hydrogen bonds has been examined by molecular dynamics simulations and quantum chemical calculations. Two different merocyanine dyes dissolved in chloroform solution under various conditions are simulated by molecular dynamics. It is found that two dipolar units can be well connected through various numbers of hydrogen bonds. Although the probability to form the desirable head-to-tail dimer is unfortunately small even under strong poling electric field and low temperature, the formation of unwanted antiparallel structure has been effectively reduced. Typical hydrogen bonded dimers obtained from molecular dynamics simulations have been studied by hybrid density functional calculations. It is found that only the most probable complex can lead to the optical absorption spectrum that is in close agreement with the corresponding experiments. Calculated results for dipole moments of ground and charge transfer states, as well as first hyperpolarizabilities, of three typical complexes have also been provided.

Tunability properties of the Pb(Mg 1/3Nb 2/3O 3) relaxors and theoretical description

In the present paper, the dc-tunability properties of two types of Pb(Mg 1/3Nb 2/3O 3) relaxor ceramics in their paraelectric state are investigated. By using two different precursors: MgO and (MgCO 3) 4·Mg(OH) 2·4H 2O in preparing the ceramics, a difference of the temperature corresponding to the relaxor-paraelectric transition of ∼35 K was obtained. Their permittivity-field dependences at a few temperatures in their paraelectric state were accurately determined. The tunability data were discussed in terms of the Johnson model completed with a Langevin term to describe the reorientation of the polar nanoregions existing in relaxors above their transition temperature. The experimental and computed ɛ( E) dependences at a few temperatures above T m were compared. Two types of temperature dependences of ɛ( E) were found, as predicted also by the theoretical approach: a faster ɛ( E) decrease closer to the transition temperature, where the mobility of the nanodomains is higher and a non-intersecting small ɛ( E) variation far away from the transition temperature. Another approach for experimental data is a simple model considering random non-interacting dipolar units in a double-well potential was considered. This model allowed to determine the average size of the nanopolar domains around 7.6–8.2 nm, which are values in agreement with experimental values reported for PMN in the literature.

Tunability properties in the paraelectric state of the Pb(Mg 1/3Nb 2/3)O 3 ceramics

Pb(Mg 1/3Nb 2/3)O 3 ceramics prepared via columbite method exhibit high crystallinity and high density after sintering at 1200 °C. Typical relaxor behavior are demonstrated by the dielectric data. Although showing a diffuse phase transition, at room temperature the system is in its paraelectric state, i.e. the PMN structure is fully cubic. The dc-tunability was investigated above the room temperature, when other field-induced contributions than ferroelectric polarization might cause non-linearity. A random non-interacting dipolar unit in a double well potential was employed to describe the ɛ( E) non-linearity. The temperature-dependence of the average polarization corresponding to the polar nanoregions in the paraelectric state of the PMN relaxor was calculated from the ɛ( E) data at various temperatures above T m. A similar trend of decreasing as increasing temperature shows the spin-glass local order parameter determined from the dielectric constant data in the paraelectric state. The local order parameter in the paraelectric state is determined by the nanopolar domains size and correlations.

Temperature dependent tunability data and modeling in the paraelectric Ba0.7Sr0.3TiO3 solid solutions

The field dependence of the dielectric constant (dc-tunability) as a function of temperature in (Ba,Sr)TiO3 ceramics was investigated. Dense and homogeneous BaxSr1-xTiO3 ceramics prepared by Pechini method show low losses, high dielectric constant of ~4000 and typical ferroelectric properties at room temperature. The dc-tunability at various temperatures, including the Curie range, was obtained. A single polarization mechanism was used in the polar state of the ceramics to describe the tunability data. A good correlation between theory and experiment was found. In the paraelectric phase, a model of random uncorrelated non-interacting dipolar units in a double well potential was considered.

Structured diffuse scattering and the fundamental 1-d dipolar unit in PLZT (Pb 1−yLa y) 1−α(Zr 1−xTi x) 1−βO 3 (7.5/65/35 and 7.0/60/40) transparent ferroelectric ceramics

The observation via electron diffraction of relatively sharp, G±{111}* sheets of diffuse intensity arising from the large amplitude excitation of inherently polar, transverse optical modes of distortion in [(Pb 1− y La y ) 1− α □ α ][(Zr 1− x Ti x ) 1− β □ β ]O 3 (PLZT), 7.5/65/35 and 7.0/60/40, samples close to the morphotropic phase boundary in this system shows that the fundamental dipolar units in these materials correspond to highly anisotropic 〈111〉 chain dipoles formed from off-centre Pb/La and coupled Ti/Zr displacements. The correlation length along the chain of these 1-d dipoles can, in principle, be determined from the width of the observed {111}* diffuse sheets in reciprocal space and is estimated to be at least 2–3 nm. The primary role of the dopant La ions appears to be to set up random local strain fields preventing the condensation of long wavelength homogeneous strain distortions of the unit cell thereby suppressing transverse correlations of the fundamental 〈111〉 chain dipoles and the development of macro-, or even nano-scale, ordered ferroelectric domain state/s in the absence of an applied external electric field.