One-dimensional Stacks Research Articles

Nature of the Magnetic Interaction in Organic Radical Crystals. 4. Magnetic Interaction in Mixed Molecular Radical Crystals

The nature of the spin exchange interaction within the half-filled band of one-dimensional stacks of 1-D mixed molecular radical crystals (MMRC) with the general formula: ······PAH····R•·····PAH····R•·····PAH····R•·····PAH····R•·····(R•, radical; PAH, diamagnetic polycyclic aromatic hydrocarbon) was investigated theoretically. The various contributions to the Heisenberg effective exchange integral, the direct (Coulomb), kinetic, and indirect spin exchange for the MMRCs, are evaluated quantitatively, using band theory, and are compared with those of the corresponding parent molecular radical crystals (MRC, without hydrocarbons). The nature and the magnitude of the exchange interaction in MMRCs are significantly different in comparison with the parent MRCs: the ferromagnetic interaction in MRC changes into antiferromagnetic in MMRCs and vice versa.

Anomalous Valence Fluctuation Near a Ferroelectric Transition in an Organic Charge-Transfer Complex

Dielectric susceptibility, X-ray diffraction and infrared molecular vibrational spectra have been investigated for a charge-transfer complex, tetrathiafulvalene (TTF)-2-bromo-3,5,6-trichloro- p -benzoquinone (QBrCl 3 ) to probe a ferroelectric valence transition at T c =66 K in comparison with the typical first-order neutral-ionic transition in the isostructural analog, TTF- p -chloranil (QCl 4 ). Around T c , the anomalously rapid fluctuation of molecular valence between the quasi-neutral and quasi-ionic state is evidenced by the temperature variation of infrared spectra, signaling the quantum motion of the neutral-ionic domain walls on the one-dimensional molecular stacks.

Control of Hierarchical Order in Crystalline Composites of Diblock Copolymers and a Molecular Chromophore

Addition of 2-chloro-4-nitroaniline (CNA) to diblock copolymers of poly(ethylene oxide) (PEO) and polystyrene (PS), poly(ethylethylene) (PEE), or poly(ethylenepropylene) (PEP) results in selective partitioning of CNA into the polar PEO domains. Calorimetry, infrared spectrosocpy, density measurements, and wide-angle X-ray diffraction support the formation of a crystalline molecular complex, comprising two ethylene oxide repeat units per one CNA. The structure of the complex is the same for PEO homopolymer and PEO-based diblocks. Wide-angle X-ray diffraction from uniaxially aligned samples of the PEO:CNA suggests a triclinic unit cell for the complex with a = 9.08 Å, b = 10.48 Å, c = 7.01 Å; and α = 90.98°, β = 88.38°, and γ = 116.72°. The data are consistent with a structural model in which the PEO chains adopt a nominally all-trans zigzag configuration, the chains organized as (100) layers separated by layers of one-dimensional stacks of CNA molecules. Polarized infrared measurements indicate that the molecular planes of the CNA molecules are nominally perpendicular to the PEO chains. The metrics associated with the zigzag PEO configuration appear to allow for optimal hydrogen bonding between the PEO oxygen atoms and the amine protons of the CNA chromophores as well as hydrogen bonding between CNA molecules in adjacent stacks. The arrangement of the CNA molecules in the crystalline PEO:CNA complex differs from the structure of bulk CNA, clearly indicating that host−guest interactions play a major role in chromophore alignment. Small-angle X-ray scattering from a series of samples reveal changes in the block copolymer microstructure as the effective volume of the PEO block is altered by the inclusion of CNA. Whereas the SAXS data for PS-PEO and PEP−PEO copolymers used here reveal hexagonally packed cylinder microstructures in which cylinders of the minority PEO block are surrounded by PS or PEP, the lamellar microstructure is observed for all three block copolymers at the composition of the 2:1 complex. Consequently, these materials can be described as rigid crystalline molecular complexes embedded in robust, ordered polymer microstructures. This control of hierarchical order over length scales, spanning several orders of magnitude, suggests a route to permanent macroscopic ordering of functional molecules, a desirable feature for applications such as optoelectronics. The conformational rigidity associated with these systems offers considerable advantages for the design of SHG materials as entropically driven disordering is inhibited compared to noncrystalline polymer−chromophore materials.

Anomalous transmission through near unit index contrast dielectric photonic crystals

We present calculations that reveal a limitation for devices that demand photonic crystals with very narrow band gaps. The opaque regions of the transmission spectra through finite length crystals may be substantially broader than the band gap of the corresponding infinite crystals. This is a consequence of the crystals’ near unit dielectric index contrast. The opaque spectral width decreases gradually with increasing thickness. Many hundreds of layers may be needed to obtain a transmission null which approaches the width of the band gap for the infinite crystal. The analysis is done for two configurations of interest for optical applications due to their relative ease of manufacturer. The first is a two-dimensional triangular array of dielectric rods for which finite difference time domain simulations are done to determine the transmission spectra. The second is a one-dimensional dielectric stack for which transmission is analytically calculated.

Room-Temperature Magnetic Bistability in Organic Radical Crystals.

A large first-order magnetic phase transition in an organic radical, 1,3,5-trithia-2,4,6-triazapentalenyl, is described. The transition occurs with a wide thermal hysteresis loop over the temperature range 230 to 305 kelvin. The high-temperature phase is paramagnetic, and its structure consists of a uniform one-dimensional stacking of the radical. The low-temperature phase is diamagnetic because of strong dimerization along the stacking direction. The results may have applications in thermal sensors, switching units, and information storage media based on organic radical crystals.

Photocontrolled nanophase segregation in a liquid-crystal solvent

Materials whose structure or electrical or optical properties can be controlled with light are said to be photoactive. Liquid crystals are of interest as photoactive media, because their fluidity maintains the possibility of molecular motion in response to photon absorption, while their orientational and/or positional ordering offers the possibility of cooperative behaviour that can amplify relatively weak photochemical effects. Moreover, liquid crystals impose their own ordering on solutes. For example, smectic A liquid crystals, comprised of one-dimensional stacks of fluid layers with the molecular axes aligned normal to the layers1, produce a modulation in solute concentration with a period equal to the layer spacing2,3. Here we present computer simulations which show that the positional ordering of a photoactive solute (an azobenzene derivative, denoted 7AB) in a smectic host (denoted 8CB) depends sensitively on its photochemical state. The photoactive molecules are driven from within the smectic layers to locations between the layers by trans-to-cis photoisomerization. This would explain the recent observation4,5,6,7 of a reversible increase in the smectic A layer spacing of a solution of 7AB in 8CB accompanying the photoisomerization process. The effect might be exploited for low-power, high-resolution optical data storage, and more generally for the manipulation of organic materials at the nanometre scale.

HREM study of 3C–SiC nanoparticles: influence of growth conditions on crystalline quality

In this contribution, we report on the influence of the growth conditions on the final crystalline quality of 3C–SiC nanoparticles with dimensions around 3–10 nm. The nanoparticles are grown by chemical vapour synthesis in a hot wall reactor using tetramethylsilane [(CH 3) 4Si] as precursor. By altering the decomposition temperature and reactor pressure, the clusters may be amorphous at low temperatures or crystalline at elevated temperatures. Crystalline nanoparticles show a one-dimensional stacking disorder with slip planes lying on different families of {1 1 1} planes when prepared at high reactor pressure. By lowering the pressure, only one-dimensional disordered particles along single (1 1 1) planes are observed.

Rapid Micro-Raman Imaging Using Fiber-Bundle Image Compression

A new technique for rapid Raman imaging and chemical analysis of micro-composites and biomaterials, with potential applications in real-time robotic vision, automated manufacturing, and medical imaging, is described and demonstrated. The key feature of this new instrument is a fiber-optic bundle used to compress two-dimensional images onto a one-dimensional fiber stack, which serves as the entrance slit of an imaging optical spectrograph. Thus a complete Raman spectrum is simultaneously collected from every point within a sample in a single scan of a charge-coupled-device (CCD) detector. The method is demonstrated by using Raman imaging of a microscopic mixed-salt sample. Its efficiency relative to alternative Raman imaging methods is quantitatively evaluated, and potential applications in other spectral imaging measurements are discussed. Index Headings: Raman spectroscopy; Spectral imaging; Chemical imaging; Fiber optics; Chemical sensor.

Nature of the magnetic interaction of Wurster's radicals in the solid state

The nature of the spin exchange interaction within the half-filled band in one-dimensional stacks of Wurster's radicals in Wurster's red bromide has been investigated theoretically. The various contributions to the Heisenberg effective exchange integral: direct, kinetic, and indirect spin exchange are evaluated quantitatively. It has been shown that the direct (Coulomb) exchange which causes the ferromagnetic coupling is small. The negative value of the effective exchange integral between Wannier functions, localized within the elementary units, has been determined by the kinetic and indirect (antiferromagnetic) exchange interaction.

Defect Studies in a One-Dimensional Photonic Band Gap Structure

We calculate defect modes in a finite one-dimensional quarter-wavelength stack by the transfer matrix technique, and find that the defect mode frequency depends on the refractive index as well as on the thickness of the defect layer and the transmission coefficient depends on its position. Then, by using the extended-beam-propagation method, we investigate the short pulse transmission. It is found that the amplitude of the electromagnetic field in the defect layer is much larger than that of the incident light, which can be used to enhance nonlinear effects if the defect layer is replaced by a nonlinear material.

Electrooptic lens stacks on LiTaO/sub 3/ by domain inversion

The design and analysis of one-dimensional (1-D) electrooptic lens stacks is presented. Experimental results of a seven-element stack fabricated on Z-cut LiTaO/sub 3/ substrates using domain inversion support the analysis. The optical focusing power of these bulk stacks is shown to be voltage-controlled.

Electromagnetic localization in one-dimensional stacks with random loss and gain.

We study both analytically and numerically the properties of a one-dimensional stack consisting of layers with the same thickness, but random complex refractive indices. Thus the stack as a whole may exhibit loss or gain. We present a simple expression for the localization length in the domain of intermediate and long wavelengths. For short wavelengths we show that, in contrast to the case of a stack with layers having real refractive indices, the localization length tends to zero as a linear function of the wavelength, rather than approaching a nonzero constant. Numerical calculations confirm the analysis. \textcopyright{} 1996 The American Physical Society.

Molecular Conductors Comprised of Organic Cations and Phthalocyanines

Abstract The axially substituted dicyanophthalocyaninatocobalt(III) anion, [Co(Pc)(CN)2]−, has been utilized for constructing molecular conductors. Highly conducting partially oxidized salt can be obtained from the combination with a closed-shell cation of [PPh4]+. The crystal is composed of one-dimensional stacks of partially oxidized phthalocyanine. On the other hand, phthalocyanine is found to be not oxidized in the salts with open-shell cations such as TTF or TMTTF.

Irregular mixed-layer structure of Ba-behenate multibilayer film

The one-dimensional stacking disorder in the structure of a Ba-behenate multibilayer film prepared by the Langmuir-Blodgett (LB) technique was studied by X-ray diffraction pattern simulation. The theory of X-ray diffraction by disordered lamellar structures was used for intensity calculations. The formation of an irregular mixed-layer structure, consisting of layers with different heights and different Ba contents, was demonstrated.

Stratigraphlc Hierarchy and Cycle Stacking, Facies Distribution, and Interwell-Scale Heterogenelty: Grayburg Formation, New Mexico: ABSTRACT

The Grayburg Formation (middle Guadaluplan) of southeastern New Mexico is a mixed carbonate-siliciclastic succession that represents predominantly shallow water sedimentation on the Northwest Shelf of the Delaware Basin. Exceptional Grayburg outcrops in the Brokeoff Mountains permit high-resolution correlation and mapping of interwell-scale facies complexity both along depositional dip and across strike. Three orders of stratigraphic hierarchy are recognized using vertical facies successions and transgressive-regressive relationships expressed along a dip cross section. This stratigraphic organization cannot be fully defined from a vertical section. Cycles (3-30 ft thick) are the smallest-scale, upward-shoaling facies successions that can be correlated across facies tracts and constitute the basic chronostratigraphic unit. Cycles are organized into transgressive-regressive cycle sets (15-40 ft thick). High-frequency sequences (100-160 ft thick) are unconformity-bound successions composed of 10 to 20 cycles arranged into transgressive and highstand systems tracts. One-dimensional cycle thickness stacking patterns do not reflect the long-term accommodation trends imposed by high-frequency sequences because most cycles did not fully aggrade to sea level. Interwell-scale heterogeneity (hundreds of ft) is high in transgressive ooid grainstones, which formed dip-elongate tidal channels and shoals. Highstand ooid grainstones formed strike-elongate shoals that are laterally continuous along dip and across strike (thousands of feet) due tomore » stacking and amalgamation of grainstone bodies in an accommodation-limited highstand setting. The well-defined relationship between stratigraphic hierarchy and facies composition and heterogeneity can be used to improve correlations and better predict lateral dimensions of reservoir facies.« less

Crystal structures of nickel, copper and zinc naphthalocyaninates

The crystal and molecular structures of [M(nc)](M = Ni, Cu or Zn; H2nc = naphthalocyanine) have been determined. The bond lengths Ni–N 1.929(7), Cu–N 1.951(4) and Zn–N 1.983(4)Å are nearly consistent with those in the phthalocyaninates of the same metals. The patterns of the perpendicular displacements of atoms from the mean plane for the three molecules, although small, are identical with each other and also quite similar to those commonly observed in the β polymorphs of phthalocyanines. The three molecules pack in similar fashion. The molecules stack within the ab plane, forming a two-dimensional sheet, contrary to the one-dimensional stacking of the phthalocyaninates of the same metals. The azamethine N atoms are not in close contact with the central metal atoms of adjacent molecules, although their deviations from the mean plane are appreciable.

Theoretical determination of the molecular and solid-state electronic structures of phthalocyanine and largely extended phthalocyanine macrocycles

The molecular and solid-state electronic structures of metal-free phthalocyanine and a series of linearly benzoannulated phthalocyanines have been investigated using the valence effective Hamiltonian (VEH) quantum-chemical method. Geometry optimizations show that, from the molecular structure standpoint, phthalocyanine-based macrocycles are the result of joining four polyacenic units to the C8N8 central ring. The electronic structure calculated for the parent phthalocyanine is compared with that of porphyrin, and the consequences of benzoannulation and meso-tetraaza substitution on the optical properties of phthalocyanines are discussed. The VEH results obtained for extended phthalocyanines are in agreement with available photoemission, cyclic voltammetry and optical absorption data and help to rationalize the evolution of the electronic properties. The first ionization energy is predicted to decrease with linear benzoannulation and asymptotically converges to an extrapolated value of –5.7 eV. Strikingly, a non-convergent behaviour is obtained for the HOMO–LUMO energy gap and very low excitation energies are predicted for extended phthalocyanines. Band-structure calculations have been performed for one-dimensional stacks of the molecules investigated. The variation of the bandwidth with the staggering angle and the intermolecular separation provides a coherent picture of the electrical conductivities observed experimentally in crystals and polymers. Very small bandgaps lower than 0.5 eV are predicted for extended phthalocyanines.

Stacking sequence changes in topologically close-packed phases

We demonstrate that the seemingly complex stacking sequence changes observed in topologically close-packed phases can be interpreted in terms of the ground state phases derived from a simple one-dimensional stacking model involving only a few interplanar interactions. We also show that the energetics of creating various possible stacking defects in such phases can be assessed from this model.

A Theoretical Study of Crystallochromy. Quantum Interference Effects in the Spectra of Perylene Pigments

The crystallochromy of perylene pigments has been explored using tight-binding extended Hiickel calculations on a one-dimensional infinite stack of perylenes as a function of transverse and longitudinal offset. Although the smallest band gap occurs for perylenes when the molecules are completely eclipsed in the solid state, the general perception in the literature that the degree of of successive perylenes in the solid state determines simply the shift of the absorption maximum is not supported by the calculations. The nodal character of the HOMO and LUMO wave functions may be such that the area overlap is large, but the band gap and band broadening is small. This is a quantum interference effect. Calculations on one-dimensional stacks of perylenes of known structure can reproduce the trend both in the wavelength of the absorption maximum and the change in width of the absorption. An analysis of the relationship of band broadening to the photogeneration efficiencies of pigments in photoconductors shows that pigments with substantial band broadening by aggregation are precisely those which benefit most significantly by separating the hole and electron on excitation. Examples of band-broadened pigments with high photogenerating efficiencies are taken from perylenes, squaraines, thiopyrylium, and diketopyrrole photogenerators.

Highly Conductive 1 : 1 Radical Cation Salts of Anthra[1,9-cd : 4,10-c′d′]bis[1,2]dichalcogenoles

Abstract Various radical cation salts of 2,3-dimethyl-tetrathioanthracene (DMTTA) and -tetraselenoanthracene (DMTSA) with NO3−, BF4−, ClO4−, PF6−, AsF6−, and Br− counter anions were prepared by an electrocrystallization technique. It has been found that these salts have either 1 : 1 or 2 : 1 stoichiometry depending on the volumes of the anions, but all are electrically conductive regardless of the stoichiometries. In particular, the NO3 and BF4 salts of DMTSA, in spite of 1 : 1 stoichiometry, recorded unusually high room temperature conductivities of 440 and 450 S cm−1, respectively, and had metallic behaviors down to around 200 K. In addition, the other salts were semiconductors with very low activation energies. X-Ray crystallographic analyses showed that the crystal structures of all the salts consist of one-dimensional stacking columns of the donor molecules, but the two metallic salts have more favorable uniform stacking columns in which there are not only effective π electronic interactions but also strong heteroatomic interactions. Furthermore, solid electronic spectra indicated that these 1 : 1 radical cation salts have the advantage of marked reduction of on-site Coulombic repulsion in the donor molecules, which serves to suppress a Mott transition.