Determining Molecular Orientations in Disordered Materials from X-ray Linear Dichroism at the Iodine L1-Edge.

To demonstrate that measurements of X-ray linear dichroism are effective for determining bond orientations in disordered materials, we report the first observation of X-ray linear dichroism at the iodine L1-edge. The iodine-containing molecular solid studied in this work was the inclusion compound containing 4,4'-diiodobiphenyl guest molecules in the perhydrotriphenylene host structure. In this material, the guest substructure does not exhibit three-dimensional ordering, and thus diffraction-based techniques do not provide insights on the orientational properties of the guest molecules. Iodine L1-edge X-ray absorption spectra, recorded as a function of orientation of a single crystal of the material, exhibit significant dichroism (whereas no dichroism is observed at the iodine L2- and L3-edges). From quantitative analysis of the X-ray dichroism, the orientational properties of the C-I bonds within this material are established. The results pave the way for applying X-ray dichroism to determine molecular orientational properties of other materials, especially for partially ordered materials such as liquid crystals, confined liquids, and disordered crystalline phases, for which diffraction techniques may not be applicable.

Polarity formation in crystals with long range molecular interactions: A Monte Carlo study.

Stochastic formation of a bi-polar state in three dimensional arrays of polar molecules with full Coulomb interactions is reproduced by Monte Carlo simulation. The size of the system is comparable to that of a real crystal seed. The spatial decay of the average order parameter is significantly slowed down by the long range interactions and the exact representation of correlation effects in terms of a single characteristic length becomes impossible. Finite size effects and possible scale invariance symmetry of the order parameter are extensively discussed.

Absolute polarity determination of teeth cementum by phase sensitive second harmonic generation microscopy.

The absolute sign of local polarity in relation to the biological growth direction has been investigated for teeth cementum using phase sensitive second harmonic generation microscopy (PS-SHGM) and a crystal of 2-cyclooctylamino-5-nitropyridine (COANP) as a nonlinear optic (NLO) reference material. A second harmonic generation (SHG) response was found in two directions of cementum: radial (acellular extrinsic fibers that are oriented more or less perpendicular to the root surface) and circumferential (cellular intrinsic fibers that are oriented more or less parallel to the surface). A mono-polar state was demonstrated for acellular extrinsic cementum. However, along the different parts of cementum in circumferential direction, two corresponding domains were observed featuring an opposite sign of polarity indicative for a bi-polar microscopic state of cellular intrinsic cementum. The phase information showed that the orientation of radial collagen fibrils of cementum is regularly organized with the donor (D) groups pointing to the surface. Circumferential collagen molecules feature orientational disorder and are oriented up and down in random manner showing acceptor or donor groups at the surface of cementum. Considering that the cementum continues to grow in thickness throughout life, we can conclude that the cementum is growing circumferentially in two opposite directions and radially in one direction. A Markov chain type model for polarity formation in the direction of growth predicts D-groups preferably appearing at the fiber front.

Polar Nature of Biomimetic Fluorapatite/Gelatin Composites: A Comparison of Bipolar Objects and the Polar State of Natural Tissue.

The correspondence of the state of alignment of macromolecules in biomimetic materials and natural tissues is demonstrated by investigating a mechanism of electrical polarity formation: An in vitro grown biomimetic FAp/gelatin composite is investigated for its polar properties by second harmonic (SHGM) and scanning pyroelectric microscopy (SPEM). Hexagonal prismatic seed crystals formed in gelatin gels represent a monodomain polar state, due to aligned mineralized gelatin molecules. Later growth stages, showing dumbbell morphologies, develop into a bipolar state because of surface recognition by gelatin functionality: A reversal of the polar alignment of macromolecules, thus, takes place close to that basal plane of the seed. In natural hard tissues (teeth and bone investigated by SPEM) and the biomimetic FAp/gelatin composite, we find a surprising analogy in view of growth-induced states of polarity: The development of polarity in vivo and in vitro can be explained by a Markov-type mechanism of molecular recognition during the attachment of macromolecules.

Scanning SQUID microscopy of local superconductivity in inhomogeneous combinatorial ceramics.

Although combinatorial solid-state chemistry promises to be an efficient way to search for new superconducting compounds, the problem of determining which compositions are strongly diamagnetic in a mixed-phase sample is challenging. By means of reactions in a system of randomly mixed starting components (Ca, Sr, Ba, La, Y, Pb, Bi, Tl, and Cu oxides), samples were produced that showed an onset of diamagnetic response above 115 K in bulk measurements. Imaging of this diamagnetic response in ceramic samples by scanning SQUID microscopy (SSM) revealed local superconducting areas with sizes down to as small as the spatial resolution of a few micrometers. In addition, locally formed superconducting matter was extracted from mixed-phase samples by magnetic separation. The analysis of single grains (d<80 µm) by X-ray diffraction, elemental analysis, and bulk SQUID measurements allowed Tl2Ca3Ba2Cu4O12, TlCaBaSrCu2O(7-δ), BaPb(0.5)Bi(0.25)Tl(0.25)O(3-δ), TlBa2Ca2Cu3O9, Tl2Ba2CaCu2O8, and YBa2Cu3O7 phases to be identified. SSM, in combination with other diagnostic techniques, is therefore shown to be a useful instrument to analyze inhomogeneous reaction products in the solid-state chemistry of materials showing magnetic properties.

All organic host-guest crystals based on a dumb-bell-shaped conjugated host for light harvesting through resonant energy transfer.

Together we glow: Fully organic host-guest crystals with two dyes inserted in their parallel nanochannels display broad emission in the visible range thanks to resonant energy transfer. The conjugated host crystal provides light harvesting in the UV region.

The new insight into dynamic crossover in glass forming liquids from the apparent enthalpy analysis.

One of the most intriguing phenomena in glass forming systems is the dynamic crossover (T(B)), occurring well above the glass temperature (T(g)). So far, it was estimated mainly from the linearized derivative analysis of the primary relaxation time τ(T) or viscosity η(T) experimental data, originally proposed by Stickel et al. [J. Chem. Phys. 104, 2043 (1996); J. Chem. Phys. 107, 1086 (1997)]. However, this formal procedure is based on the general validity of the Vogel-Fulcher-Tammann equation, which has been strongly questioned recently [T. Hecksher et al. Nature Phys. 4, 737 (2008); P. Lunkenheimer et al. Phys. Rev. E 81, 051504 (2010); J. C. Martinez-Garcia et al. J. Chem. Phys. 134, 024512 (2011)]. We present a qualitatively new way to identify the dynamic crossover based on the apparent enthalpy space (H(a)(') = dlnτ/d(1/T)) analysis via a new plot lnH(a)(') vs. 1∕T supported by the Savitzky-Golay filtering procedure for getting an insight into the noise-distorted high order derivatives. It is shown that depending on the ratio between the "virtual" fragility in the high temperature dynamic domain (m(high)) and the "real" fragility at T(g) (the low temperature dynamic domain, m = m(low)) glass formers can be splitted into two groups related to f < 1 and f > 1, (f = m(high)∕m(low)). The link of this phenomenon to the ratio between the apparent enthalpy and activation energy as well as the behavior of the configurational entropy is indicated.

Organic fluorine compounds: a great opportunity for enhanced materials properties.

Interactions of "organic fluorine" have gained great interest not only in the context of crystal engineering, but also in the systematic design of functional materials. The first part of this tutorial review presents an overview on interactions known by organic fluorine. This involves π-π(F), C-F···H, F···F, C-F···π(F), C-F···π, C-F···M(+), C-F···C=O and anion-π(F) interactions, as well as other halogen bonds. The effect of the exchange of H vs. F is discussed by means of several examples and a short introduction to the young field of "fluorous" chemistry is given. The second part is dedicated to numerous applications of fluorine and fluorous interactions. It is shown how application of fluorination is used to enable a number of reactions, to improve materials properties and even open up new fields of research.

1-Bromo-2,3,5,6-tetra-fluoro-4-nitro-benzene.

In the title compound, C(6)BrF(4)NO(2), the nitro group is twisted by 41.7 (3)° with reference to the arene ring mean plane. The main inter-actions stabilizing the crystal structure include O⋯Br contacts [3.150 (2) and 3.201 (2) Å], while F⋯F inter-actions are minor [2.863 (3)-2.908 (3) Å].

Theoretical characterization of charge transport in one-dimensional collinear arrays of organic conjugated molecules.

A great deal of interest has recently focused on host-guest systems consisting of one-dimensional collinear arrays of conjugated molecules encapsulated in the channels of organic or inorganic matrices. Such architectures allow for controlled charge and energy migration processes between the interacting guest molecules and are thus attractive in the field of organic electronics. In this context, we characterize here at a quantum-chemical level the molecular parameters governing charge transport in the hopping regime in 1D arrays built with different types of molecules. We investigate the influence of several parameters (such as the symmetry of the molecule, the presence of terminal substituents, and the molecular size) and define on that basis the molecular features required to maximize the charge carrier mobility within the channels. In particular, we demonstrate that a strong localization of the molecular orbitals in push-pull compounds is generally detrimental to the charge transport properties.

Alignment and relaxation dynamics of dye molecules in host-guest inclusion compounds as probed by dielectric spectroscopy.

The alignment and relaxation dynamics of a polar dye molecule, N,N-dimethyl-4(4-nitrophenylazo)aniline (DNAA), in zeolite L and perhydrotriphenylene (PHTP) channels were investigated by means of a combination of optical, dielectric, and quantum-chemical methods. Both the zeolite L and PHTP channels enable the dye molecules to align along the channel axis. An amplified net dipole moment of DNAA in PHTP is observed and attributed to enhanced 1D close alignment of dye molecules. In zeolite L channels, a concentration gradient is found with aggregation at the channel entrances. The dynamics of the dye in zeolite L channels reveals localized conical rotational fluctuation modes following Arrhenius-type activation with energy of 0.31 eV, which we assign to small noninteracting fluctuating polar units of the dyes being loosely aligned or isolated. Unlike zeolite L, relaxations in PHTP are characterized by cooperative wobbling motions interpreted as increased intermolecular dipole interaction due to a closely packed one-dimensional array. Temperature-dependent activation energies of 0.25 eV below 0 degrees C and 0.37 eV at ambient temperature reflect the role of the soft channel walls in the activation process.

Surface Ordering in Molecular Clusters by Interactions of Well-Defined Parity Multipoles.

The spontaneous alignment of dipoles in patterns tangential or transversal to the interface of molecular clusters is studied by molecular dynamics simulations throughout the entire aggregation process. Tangential ordering (TANGO) is found to rely on dispersion forces driven by dipolar fluctuations. Transversal ordering (TRANSO) results from the interplay of two conditions: the broken translational invariance at the cluster interface and the interactions of multipoles of opposite parity. In this case, the orientational order parameter at the interface follows a universal scaling law behavior. In the limiting case of strongly interacting particles, a sharp structural transition from a disordered into an ordered state is observed at a critical distance inside the cluster and at a critical magnitude of the dipole moment.

High vacuum equipment for the measurement of the macroscopic dipole moment of polar molecular crystals.

Polar crystals placed into a charged plate capacitor may experience a mechanical torque. Mounting crystals on a spiral spring allows us to measure the momentum and thus the macroscopic dipole moment of a crystal object. To exclude the influence of ambient conditions (air, ions, electrons), experiments are performed in a high vacuum. Following text book knowledge, polar crystals show a surface charge density especially for faces involving the polar crystal axis. For ambient conditions, it is assumed that external charge carriers compensate the surface charge. Here, we present unique equipment by which the dipole moment of crystals can be measured also when the charge carrying parts of a crystal were cut off by two metallic blades. Present measurements for α-resorcinol (mm2) allow us to conclude that (a) as grown polar crystals measured under the high vacuum condition show a macroscopic dipole moment, (b) the weighted dipole moment increases after cutting the main polar faces, and (c) the weighted dipole moment decreased after exposure to ambient air.

Stochastic polarity formation in molecular crystals, composite materials and natural tissues.

This topical review summarizes the theoretical and experimental findings obtained over the last 20 years on the subject of growth-induced polarity formation driven by a Markov chain process. When entering the growing surface of a molecular crystal, an inorganic-organic composite or a natural tissue, the building blocks may undergo 180° orientational disorder. Driven by configurational entropy, faulted orientations can promote the conversion of a growing non-polar seed into an object showing polar domains. Similarly, orientational disorder at the interface may change a polar seed into a two-domain state. Analytical theory and Monte Carlo simulations were used to model polarity formation. Scanning pyroelectric, piezoresponse force and phase-sensitive second-harmonic microscopies are methods for investigating the spatial distribution of polarity. Summarizing results from different types of materials, a general principle is provided for obtaining growth-induced polar domains: a non-zero difference in the probabilities for 180° orientational misalignments of building blocks, together with uni-directional growth, along with Markov chain theory, can produce objects showing polar domains.

Peculiar orientational disorder in 4-bromo-4'-nitrobiphenyl (BNBP) and 4-bromo-4'-cyanobiphenyl (BCNBP) leading to bipolar crystals.

180° orientational disorder of molecular building blocks can lead to a peculiar spatial distribution of polar properties in molecular crystals. Here we present two examples [4-bromo-4'-nitrobiphenyl (BNBP) and 4-bromo-4'-cyanobiphenyl (BCNBP)] which develop into a bipolar final growth state. This means orientational disorder taking place at the crystal/nutrient interface produces domains of opposite average polarity for as-grown crystals. The spatial inhomogeneous distribution of polarity was investigated by scanning pyroelectric microscopy (SPEM), phase-sensitive second harmonic microscopy (PS-SHM) and selected volume X-ray diffraction (SVXD). As a result, the acceptor groups (NO2 or CN) are predominantly present at crystal surfaces. However, the stochastic process of polarity formation can be influenced by adding a symmetrical biphenyl to a growing system. For this case, Monte Carlo simulations predict an inverted net polarity compared with the growth of pure BNBP and BCNBP. SPEM results clearly demonstrate that 4,4'-dibromobiphenyl (DBBP) can invert the polarity for both crystals. Phenomena reported in this paper belong to the most striking processes seen for molecular crystals, demonstrated by a stochastic process giving rise to symmetry breaking. We encounter here further examples supporting the general thesis that monodomain polar molecular crystals for fundamental reasons cannot exist.

Insertion of dipolar molecules in channels of a centrosymmetric organic zeolite: molecular modeling and experimental investigations on diffusion and polarity formation.

The mechanism of insertion of p-nitroaniline (PNA) and its diffusion behavior in channels of the hexagonal host structure of tris(o-phenylenedioxy)cyclotriphosphazene (TPP) was investigated by means of molecular modeling tools. Strong preferential sites in the bulk were found to be due to pi-pi and NH-pi interactions between PNA and channel walls of TPP. MD simulations showed that diffusion can be characterized by jumps from one site to the next, occurring mainly because of the dynamic flexibility of the host structure. Calculations of host-guest interactions between the TPP surface and PNA approaching the entrance of channels with its terminal H2N-first or O2N-first revealed that the H2N-first insertion is clearly preferred. Preferential insertion of PNA is found to be the reason for polar effects, observed experimentally. Because of a distinct guest-host recognition at the surface, guest-guest interactions were found to have a minor influence on polarity.

Crystal structure of 4'-bromo-2,3,5,6-tetra-fluoro-biphenyl-4-carbo-nitrile.

The title compound, C13H4BrF4N, synthesized from 1,4'-bromo-iodo-benzene and 4-bromo-2,3,5,6-tetra-fluoro-benzo-nitrile in a coupling reaction was found to crystallize in the ortho-rhom-bic space group P212121. The two phenyl rings are rotated with respect to each other by 40.6 (6)°. The mol-ecules inter-act via aryl-perfluoroaryl stacking [3.796 (2) and 3.773 (2) Å], resulting in inter-molecular chains along the a-axis direction. C-H⋯F contacts of about 2.45 Šconnect these chains. In contrast to the structure of the parent compound 4'-bromo-biphenyl-4-carbo-nitrile, CN⋯Br contacts that could have given rise to a linear arrangement of the biphenyl mol-ecules desirable for non-linear optical (NLO) materials are not observed in the packing. Instead, several Br⋯F [3.2405 (17) and 3.2777 (18) Å] and F⋯F [2.894 (2) Å] contacts of side-on type II form an inter-molecular network of zigzag chains. The crystal studied was refined as an inversion twin.

Crystal structure of 2,4,6-tris-(cyclo-hex-yloxy)-1,3,5-triazine.

The title compound, C21H33N3O3, is a tri-substituted cyclo-hex-yloxy triazine. In the crystal, the triazine rings form (C3i-PU) Piedfort units. The inter-centroid distance of the π-π inter-action involving the triazine rings is 3.3914 (10) Å. In the crystal, mol-ecules are linked by C-H⋯O hydrogen bonds, forming ribbons propagating along [1-10]. There are also weak C-H⋯N and C-H⋯O contacts present, linking inversion-related ribbons, forming a three-dimensional structure.

Markov-type evolution of materials into a polar state.

Assembling polar building blocks into a solid material by a Markov-chain process of unidirectional growth principally results in a metastable state that shows effects of macroscopic polarity. Stochastic polarity formation can be described by probabilities for the attachment of building blocks to a surface. Because of the polar symmetry of the building blocks, there is a fundamental difference in the probabilities for attaching them "tip-first" or "back-first" to growth sites at a surface. A difference in the corresponding probabilities drives the evolution of a vectorial property through a gain in configurational entropy. Examples from the mechanical, the crystalline and the biological world demonstrate growth-induced macroscopic polarity. In crystals, growth upon centrosymmetric seeds can produce twinned crystals with a "sectorwise" pyroelectric effect. Polarity formation in connective tissues is explained by a Markov-chain mechanism, which drives the self-assembly of collagen fibril segments. An unified stochastic growth model brings up a general concept for the formation of materials with polar properties.

Barium hydrogen phosphate/gelatin composites versus gelatin-free barium hydrogen phosphate: synthesis and characterization of properties.

Recently, attention has been spent on crystal growth of phosphate compounds in gels for studying the mechanism of in vitro crystallization processes. Here, we present a gel-based approach for the synthesis of barium hydrogen phosphate (BHP) crystals using single and double diffusion techniques in gelatin. The composite crystals were compared with analytical grade BHP powder, single and polycrystalline BHP materials using Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), scanning pyroelectric microscopy (SPEM), optical microscopy (OM), thermal gravimetric analysis (TGA) and X-ray diffraction (XRD). FTIR spectra showed surface adsorption of gelatin molecules by using BHP stacked sheets due to CH2 stretching, CH2 bending and amide I vibrations are found in a gelatin content of about 2% determined by dissolution. SEM shows various crystal morphologies of the BHP/gelatin composites forming bundled micro-flakes to irregular bundled needles and spheres different from gel-free crystals. The variety in morphology depends on the ion concentration, pH of gel as well as the method of crystal growth. SPEM investigation of BHP/gelatin aggregates revealed polar domains showing alteration of the polarization. Moreover, BHP/gelatin composite crystals showed a higher thermal stability in comparison with analytical grade BHP or/and BHP single crystals due to strong interactions between gelatin and BHP. The XRD diffraction analysis demonstrated that the single and double diffusion techniques in gelatin led to the formation of orthorhombic BHP. This study demonstrates that gelatin is a useful high molecular weight biomacromolecule for controlling the crystallization of a composite material by producing a variety of morphological forms.