Theoretical investigation on the linear and nonlinear susceptibilities of urea crystal

The linear and second-order nonlinear susceptibilities of the urea crystal have been evaluated by applying the supermolecule approach. Calculations performed at the time-dependent Hartree-Fock (TDHF) level using the Austin model (AM1) semiempirical Hamiltonian have first demonstrated the almost additive character of the essential polarizability and first hyperpolarizability components. In fact, the only exception concerns the chi(cc) ((1)) component when stacking urea molecules along the c axis, i.e., the axis of the hydrogen bonds. This behavior has been confirmed by ab initio calculations on small clusters. The macroscopic quantities have then been determined by adopting the multiplicative scheme and by correcting the TDHF/AM1 values for missing electron correlation by means of density functional theory and coupled cluster method. The reliability of the multiplicative scheme was demonstrated for clusters as large as 3ax3bx3c. While the electron correlation correction factors are similar for a single molecule and different small clusters, the global performance of the scheme differs for the linear and nonlinear responses. For the second-order nonlinear susceptibility, our predictions are in good agreement with experiment, while for the linear susceptibility and the associated refractive index, our predictions underestimate the experimental values. The limitations of our approach may be attributed to its inability to account for more subtle cooperative effects, like those associated with a network of hydrogen bonds. Together with other works, the supermolecule calculations confirm that the sign of chi(abc) ((2)) is negative, contrary to an estimate from band structure calculation.     

Liquid crystal phases of achiral banana-shaped molecules: a computer simulation study

The phase behaviour of achiral banana-shaped molecules was studied by computer simulation. The banana-shaped molecules were described by model intermolecular interactions based on the Gay-Berne potential. The characteristic molecular structure was considered by joining two calamitic Gay-Berne particles through a bond to form a biaxial molecule of point symmetry group C 2v with a suitable bending angle. The dependence on temperature of systems of N =1024 rigid banana-shaped molecules with bending angle ϕ=140° has been studied by means of Monte Carlo simulations in the isobaric-isothermal ensemble ( NpT ). On cooling an isotropic system, two phase transitions characterized by phase transition enthalpy, entropy and relative volume change have been observed. For the first time by computer simulation of a many-particle system of banana-shaped molecules, at low temperature an untilted smectic phase showing a global phase biaxiality and a spontaneous local polarization in the layers, i.e. a local polar arrangement of the steric dipoles, with an antiferroelectric-like superstructure could be proven, a phase structure which recently has been discovered experimentally. Additionally, at intermediate temperature a nematic-like phase has been proved, whereas close to the transition to the smectic phase hints of a spontaneous achiral symmetry breaking have been determined. Here, in the absence of a layered structure a helical superstructure has been formed. All phases have been characterized by visual representations of selected configurations, scalar and pseudoscalar correlation functions, and order parameters.     

NMR determination of the ordering of the molecules in cholesteryl esters liquid crystal phases

A procedure based on the method of moments of NMR line is applied to determine the order parameter tensor for cholesteryl esters. The order parameter dependence on the length of alkyl chain is obtained. The proposed method can be used to estimate the concentration of two independent conformations.     

Mechanism of linear and nonlinear optical properties of the urea crystal family

First-principles calculations of the second-order optical response functions and the dielectric functions of urea [CO(NH(2))(2)] and some of its derivatives such as monomethylurea (H(2)NCONHCH(3), MMU), and N,N'-dimethylurea (H(3)CHNCONHCH(3), DMU) crystals are performed. On the basis of the density functional theory (DFT) in the local-density approximation (LDA), the highly accurate full-potential projected augmented wave (FP-PAW) method was used to obtain the electronic structure. Over a wide frequency range (0.0-10.0 eV), the dielectric constants and second-harmonic generation (SHG) susceptibilities of the urea crystal family have been obtained, and the results are in good agreement with the experimental values. The origin of the linear and nonlinear optical (NLO) properties of the urea crystal family has been analyzed by coupling the calculated electronic structure and optical spectrum. The prominent spectra of χ((2)) are successfully correlated with the dielectric function ε(ω) in terms of single-photon and double-photon resonances. The virtual electron (VE) and virtual hole (VH) processes have also been performed for the urea crystal family. From the research into the electron deformation density, crystal configuration, substitutional group, and so forth, it is found that the origin of the SHG of the urea crystal family is the charge transfer due to the strong "(?)push-pull" effect along the hydrogen bond, which favors a head-to-tail arrangement of the molecules and enhances the SHG response. The electron-donating substitutional group supplies more electrons to the electron-accepting group, and helps to form large dipoles in molecules. The influence on the NLO properties of the local symmetry of the substitutional group is also discussed in detail.     

Formation of polypseudorotaxane networks by cross-linking the quadruple hydrogen bonded linear supramolecular polymers via bisparaquat molecules

The creation of novel crown ether-paraquat polypseudorotaxane networks, constructed by bisparaquat monomers threading into the cavity of the crown ether units of linear supramolecular polymers that are formed based on the quadruple hydrogen bonded unit ureidopyrimidinone (Upy) in the concentrated solution, is described.     

Nanochannels: hosts for the supramolecular organization of molecules and complexes

Nanochannels have been used as hosts for supramolecular organization for a large variety of guests. The possibilities for building complex structures based on 2D and especially 3D nanochannel hosts are larger than those based on 1D nanochannel hosts. The latter are, however, easier to understand and to control. They still give rise to a rich world of fascinating objects with very distinguished properties. Important changes are observed if the channel diameter becomes smaller than 10 nm. The most advanced guest-nanochannel composites have been synthesized with nanochannels bearing a diameter of about 1 nm. Impressive complexity has been achieved by interfacing these composites with other objects and by assembling them into specific structures. This is explained in detail. Guest-nanochannel composites that absorb all light in the right wavelength range and transfer the electronic excitation energy via FRET to well-positioned acceptors offer a unique potential for developing FRET-sensitized solar cells, luminescent solar concentrators, color-changing media, and devices for sensing in analytical chemistry, biology, and diagnostics. Successful 1D nanochannel hosts for synthesizing guest-host composites have been zeolite-based. Among them the largest variety of guest-zeolite composites with appealing photochemical, photophysical, and optical properties has been prepared by using zeolite L (ZL) as a host. The reasons are the various possibilities for fine tuning the size and morphology of the particles, for inserting neutral molecules and cations, and for preparing rare earth complexes inside by means of the ship-in-a-bottle procedure. An important fact is that the channel entrances of ZL-based composites can be functonalized and completely blocked, if desired, and furthermore that targeted functionalization of the coat is possible. Different degrees of organizational levels and prospects for applications are discussed, with special emphasis on solar energy conversion devices.     

Single dibenzoterrylene molecules in an anthracene crystal: spectroscopy and photophysics.

We study single dibenzoterrylene molecules in an anthracene single crystal at 1.4 K in two insertion sites at 785.1 and 794.3 nm. The single-molecule zero-phonon lines are narrow (about 30 MHz), intense (the detected fluorescence rates at saturation reach 100,000 counts s(-1)), and very photostable. The intersystem-crossing yield is extremely low (10(-7) or lower). All of these features are hallmarks of an excellent system for high-resolution spectroscopy and nanoscale probing at cryogenic temperatures.     

Single dibenzoterrylene molecules in an anthracene crystal: main insertion sites.

We present a spectroscopic study of the properties of the two principal insertion sites (at 785.1 and 794.3 nm) of single dibenzoterrylene molecules in anthracene single crystals at cryogenic temperatures. We measured the temperature dependence of the line width, the orientation of the transition dipole moments, and the Stark effect. We performed molecular dynamics simulations, which show that one dibenzoterrylene molecule preferably replaces three anthracene molecules. From simulated annealing, we derive the molecular conformations in the most stable insertion sites and the orientations of the transition dipole moments. The good agreement between the spectroscopic results and the simulations allows us to propose unambiguous structures for the two principal spectroscopic sites.     

Nature of the liquid-crystalline phase in supramolecular linear polymers

The development of the liquid crystal (LC) state in the melt of a supramolecular linear LC polymer based on a diacid and bipyridine base has been studied by IR spectroscopy and polarizing optical microscopy. Relying on experiments on the hydrogen-bonded complex formation and statistical analysis of the formation kinetics of the nematic phase, it is inferred that the system under study behaves like a binary mixture of weakly interacting components rather than a polydisperse polymer.     

Influence of the liquid crystal behaviour on the Langmuir and Langmuir-Blodgett film supramolecular architecture of an ionic liquid crystal

Novel fluorescent poly(2-(acetoacetoxy)ethyl methacrylate)(PAAEMA) latexes have been synthesized by miniemulsion polymerization employing a polymeric costabilizer. Nanoscale aggregates of macromolecules bearing β-dicarbonyl are formed in the prepared latex particles. Ammonia and the β-dicarbonyl aggregates assemble a supramolecular complex, which exhibits strong visible fluorescence under UV light. The formation of the complex is confirmed by the characteristic absorption peak located at about 275 nm in UV-Vis spectra. The absorption spectrum has been found to be applicable for ammonia detection. Atomic Force Microscopy (AFM) studies of surface morphology reveal that gas-sensing properties of the PAAEMA thin films involve the reversible absorption and desorption of ammonia. PAAEMA thin films are sensitive to ammonia gas and have a short response time of 80s when exposed to 54 ppm of ammonia gas concentration.     

A new series of pyrimidine-containing linear molecules: their elegant crystal structures and intriguing photophysical properties

A new series of aza-substituted analogues 3-5 based on the 1,4-bis(phenylethynyl)benzene moiety have been synthesized by the selective Pd-catalyzed Sonogashira coupling reaction from 5-bromo-2-iodopyrimidine (1). In these linear molecules, the dipolar pyrimidine moiety is introduced as a probe to investigate factors that control the intermolecular interactions over the crystal engineering. The results reveal that the manner of packing changes both dipolar interactions between linear pyrimidine-containing molecules and transition moments simultaneously, resulting in remarkably different photophysical properties. Due to their versatile dipole-dipole and face-to-face pi-piinteractions in a crystal motif, further applications on the design of ordered crystalline materials for the field effect transistors are promising.     

Crystal hydrates: an investigation of the use of crystal hydrates as stationary phases in gas chromatography

Summary The use of crystallohydrates, crystallohydrate solutions and melts as stationary phases in gas chromatography has been proposed. Crystallohydrates have been shown to display high selectivity in the separation of polar organic compounds when use is made of water vapours as the mobile phase. Some aspects of gas-liquid chromatography in water vapours and with the stationary phase comprising crystallohydrates have been studied, and the preliminary results obtained point to the desirability of further progress in this field.Presented at the 14th International Symposium on Chromatography London, September, 1982     

Fluorescence behaviour of cyanobiphenyl liquid crystal molecules in liquid crystal/polymer composite films

The fluorescence behaviour of the liquid crystal, 4-cyano-4-pentylbiphenyl (5CB), in composite thin films prepared by the photopolymerization of 5CB/diacrylate mixtures, was investigated by means of three different excitation methods, in which the total-internal-reflection or surface-limited excitation method was used for analysis of the fluorescence from an ultra-thin interface layer ( 100nm) in contact with the substrate surface, whereas the fluorescence from the interior bulk was analysed by the through-film excitation method. It was found that intensity ratios of the monomer and excimer emissions of 5CB are significantly lower in the interface layer than in the interior bulk, depending upon photopolymerization conditions as well as upon the structures of the diacrylates used. Scanning electron microscopic observations and light-scattering measurements of some typical composite films showed possible relationships between morphological features and fluorescence characteristics depending upon the diacrylate structures and polymerization conditions. The different fluorescence behaviour has been discussed in terms of differences in mobility and/or aggregation degrees of 5CB molecules arising from dominant molecular interactions with the substrate and polymer surfaces.     

Self-organization of polybases neutralized with mesogenic wedge-shaped sulfonic acid molecules: an approach toward supramolecular cylinders

Complexes consisting of poly(4-vinylpyridine) and mesogenic wedge-shaped ligands 4'-[3",4",5"-tris(dodecyloxy)benzoyloxy]azobenzene-4-sulfonic acid and 4'-[3",4",5"-tris(octyloxy)benzoyloxy]azobenzene-4-sulfonic acid have been prepared with different monomer/ligand ratios. Upon protonation of the poly(4-vinylpyridine) chains by the wedge-shaped sulfonic acid molecules a hypsochromic and hyperchromic effect was observed with the pi-pi* transition of the azo-chromophor, allowing us to monitor the neutralization process by means of UV-vis spectroscopy in solution. The changes of the absorption characteristics implied a conformational change of the polymer backbone. In the bulk the interaction between pyridine and sulfonic acid moieties was proved by FT-IR spectroscopy. Polarizing optical microscopy, differential scanning calorimetry, and X-ray diffraction measurements were used to study the bulk structure of the complexes. The complexes formed a liquid crystalline lamellar phase at low degrees of substitution, while a hexagonal columnar mesophase was observed at degrees of neutralization of 80% and higher.     

Micro-Raman spectroscopic identification of natural mineral phases and their weathering products inside an abandoned zinc/lead mine

Mining activities provide a good source of minerals of different nature. On the one hand, the primary minerals for whose formation a geological time-scale is required. On the other hand, secondary minerals, formed from removed products after the earlier weathering and alteration states. These are characteristic of the local geology and the environment context that commonly appears due to the low chemical stability of their original primary minerals. This work shows how quickly the reactions promoting secondary minerals may have taken place, due to the fact that these were found in newly formed solid materials called efflorescences. To achieve this purpose, the sampling is crucial. It was carried out in such a way that tried to guarantee that the samples collected consisted in the very top soil matter (first 2 cm depth). Thus, unlike the deeper soil, the material analysed may have been newly formed due to the interactions that they had with the place weathering agents (i.e. air oxygen, humidity, and microbial activities). Raman spectroscopy has emerged as a good and fast non-destructive technique that provides molecular information of the local mineralogy without the need of any pre-treatment of the samples. At the same time, the work looked for information on the variety of non-stable lead and-or zinc containing minerals due to the possible health and environmental risks they convey. Among the different minerals identified, 16 were of primary nature while 23 may be classified as secondary minerals, probably formed in the last decades as the result of the extractive activities.     

Conformation of BEDT-TTF molecules in the crystal

Based on comparison of low-temperature x-ray diffraction data for BEDT-TTF salts and quantum chemical MNDO calculations, we have shown that the most energetically favorable conformation of the BEDT-TTF molecule is not realized in the crystal. A local energy minimum 0.035 eV above the ground state corresponds to the conformation detected in the crystal. The x-ray diffraction data and the MNDO results suggest weak participation of the heteroatoms of the six-membered rings in the conjugation with the -system of the TTF-fragment in the BEDT-TTF and BEDO-TTF molecules.Institute of Chemical Physics at Chernogolovka, Russian Academy of Sciences, Chernogolovka 142432. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 4, pp. 890–894, April, 1992.     

Dynamics in ferro- and antiferroelectric phases of a liquid crystal with fluorinated molecules as studied by dielectric spectroscopy

For 1-[3-fluoro-4-(1-methylheptyloxycarbonyl)phenyl]-2-[4-2,2,3,3,4,4,4-heptafluorobutoxybutoxy)biphenyl-4-yl]ethane (1F7), built of chiral molecules, results of dielectric measurements of liquid-crystalline and solid phases are presented. Rich polymorphism of liquid-crystalline (SmC*, SmC*_A and SmI*_A) phases as well as of solid (Cr1 and Cr2) phases were observed down to –130°C. At a frequency range from 0.1 Hz to 3 MHz, the relaxation processes were detected in ferroelectric SmC*, antiferroelectric SmC*_A and highly ordered SmI*_A smectic phases. The mechanism of complex dynamics (moleculear and collective) was identified with the help of the bias field. Vitrification of conformationally disordered crystal phase Cr2 was found in accordance with calorimetric observations.     

Concerning the existence of linear carbon molecules in soot obtained in an electric arc

The presence of soluble linear carbon molecules in fullerene-containing soot obtained by arc spraying of graphite was confirmed by Fourier IR spectroscopy, electron diffraction, mass spectrometry, and NMR spectrometry data. Carbon chains are oriented in the layer perpendicular to the support surface and are packed in a hexagonal crystal lattice with d₁₁₀ = 4.35 Å.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1387–1390, July, 1995.The present work was supported by the Russian Foundation for Basic Research (Project Nos. 93-03-4492 and 93-03-18705).     

Rotational viscosity of fluids composed of linear molecules: an equilibrium molecular dynamics study

In this paper, we investigate the rotational viscosity for a chlorine fluid and for a fluid composed of small linear molecules by using equilibrium molecular dynamics simulations. The rotational viscosity is calculated over a large range of state points. It is found that the rotational viscosity is almost independent of temperature in the range studied here but exhibits a power-law dependency on density. The rotational viscosity also shows a power-law relationship with the molecular length, and the ratio between the shear and rotational viscosities approaches 0.5 for the longest molecule studied here. By changing the number of atoms or united atomic units per molecule and by keeping the molecule length fixed, we show that fluids composed of molecules which have a rodlike shape have a lower rotational viscosity. We argue that this phenomenon is due to the reduction in intermolecular connectivity, which leads to larger fluctuations around the values possessed by the fluid on average. The conclusions here can be extended to fluids composed of uniaxial molecules of arbitrary length.