Experimental and DFT generated Raman study of two bent-core monomeric liquid crystalline compounds

The temperature dependence Raman spectra of two liquid crystalline compounds defined by the chemical formula of 3,5-difluoro-4?-(4-pentylcyclohexyl)-(1,1?-biphenyl)-4-carbonitrile and 3,4,5-trifluoro-4?-(4-pentylcyclohexyl)-1,1?-biphenyl is being first reported in this study. These compounds are bent-core monomers and their bent nature has been verified by the Density Functional Theory (DFT). The temperature-dependent Raman spectroscopy has been widely used in understanding the effects of temperature-based phase transitions on the molecular vibrations. The same spectroscopic technique; helps to understand various phase transitions temperature in the liquid crystalline compounds (LC) and also their molecular arrangements during the phase transitions. This study has successfully revealed the nature of intermolecular interactions between the investigated compounds during the phase transitions and the correlation between the observed Raman spectra and the measurement temperature. The contributions of different types of chemical bonds in the investigated LC compounds to their recorded Raman spectra have also been discussed in detail. In predicting the observed Raman spectra, the theoretical Raman spectra obtained from the DFT calculation was used as a reliable tool. In the light of the calculated data, the peak position, line width, and integral intensity data for each band in the observed Raman spectra were reported.     

Phase transition sequence in the chiral compound 10FHBBBM7* studied by Raman scattering

Integrated Raman intensity of the stretching mode of the phenyl rings as a function of temperature has been studied in order to determine the phase sequence in a new liquid crystalline compound. It is shown that the Raman technique provides the identification of the phase transitions, although some of them were not observed by macroscopic techniques. In particular, two ferrielectric phases were found from Raman measurements between the SmC* and the SmC_A* phases, while by DSC and by dielectric measurements only one phase could be detected. A detailed Raman study under d.c. electric field has shown that in this compound the Raman intensity is weakly influenced by the appearance of disclination lines.     

The phase diagram of ammonium nitrate

The pressure-temperature (P-T) phase diagram of ammonium nitrate (AN) [NH(4)NO(3)] has been determined using synchrotron x-ray diffraction (XRD) and Raman spectroscopy measurements. Phase boundaries were established by characterizing phase transitions to the high temperature polymorphs during multiple P-T measurements using both XRD and Raman spectroscopy measurements. At room temperature, the ambient pressure orthorhombic (Pmmn) AN-IV phase was stable up to 45 GPa and no phase transitions were observed. AN-IV phase was also observed to be stable in a large P-T phase space. The phase boundaries are steep with a small phase stability regime for high temperature phases. A P-V-T equation of state based on a high temperature Birch-Murnaghan formalism was obtained by simultaneously fitting the P-V isotherms at 298, 325, 446, and 467 K, thermal expansion data at 1 bar, and volumes from P-T ramping experiments. Anomalous thermal expansion behavior of AN was observed at high pressure with a modest negative thermal expansion in the 3-11 GPa range for temperatures up to 467 K. The role of vibrational anharmonicity in this anomalous thermal expansion behavior has been established using high P-T Raman spectroscopy.     

Conformational and phase transformations of chlorocyclohexane at high pressures by Raman spectroscopy

Pressure induced conformational and phase transformations of chlorocyclohexane (CCH) were investigated in a diamond anvil cell by Raman spectroscopy at room temperature. Pure CCH was compressed up to 20 GPa and then decompressed to ambient pressure. The conformational equilibrium was shifted by pressure from equatorial to axial conformers in the fluid phase below 0.7 GPa, consistent with previous observations. Upon further compression, several solid-to-solid phase transitions were identified by the observation of markedly different Raman patterns as well as different pressure dependences of characteristic Raman modes. The possible structures of these phases were analyzed in correlation with previously observed solid phases at low temperatures. Finally, CCH exhibits pressure hysteresis and partial reversibility upon decompression which result in the formation of the phases with different Raman patterns from those obtained upon compression. The difference can be interpreted as conformational contribution as well as the intrinsic plasticity of CCH crystals.     

Raman and X-ray investigations of ferroelectric phase transition in NH4HSO4

Temperature-dependent Raman spectroscopy and X-ray diffraction studies have been carried out on NH(4)HSO(4) single crystals in the temperature range 77-298 K. Two structural transitions driven by the molecular ordering and change in crystal symmetries are observed below 263 and 143 K. These phase transitions are marked by the anomalies in the temperature dependence of wavenumber and fwhm of several internal vibrational modes. The Raman spectra and X-ray data enable us to understand the nature of the molecular ordering resulting in the ferroelectric phase below 263 K, sandwiched between two nonferroelectric phases. The crystal structure of the ferroelectric phase is determined correctly as Pc, which has been earlier solved in Ba symmetry. The temperature dependent Raman and X-ray results suggest that the disorder to order transition leading to lower symmetry below 263 K is driven by the change in HSO(4)(-) ions and that below 143 K is driven by the change in both HSO(4)(-) and NH(4)(+) ions.     

Structural and thermal characterization of the γ″ polymorph of Bi2MoO6

Among the different polymorphic phases of the Bi₂MoO₆ system, the intermediate γ″ compound has been characterized within a metastable state by high temperature X-ray diffraction and Raman spectroscopy. The transition γ ? γ″ is found to be partially displacive and partially reconstructive. In the context of the ferroelectricity of γ, the nature of the paraelectric phase is discussed with the help of space group considerations and Buerger's theory of phase transitions.     

LCST and UCST behavior of poly(N-isopropylacrylamide) in DMSO/water mixed solvents studied by IR and micro-Raman spectroscopy

Temperature-responsive phase separations of poly(N-isopropylacrylamide) (PNiPAm)/dimethylsulfoxide (DMSO)/water mixtures have been investigated by infrared and confocal micro-Raman spectroscopy. The ternary mixtures exhibited lower critical solution temperature (LCST) and upper critical solution temperature (UCST) phenomena at low and high DMSO concentrations, respectively. The amide I band of PNiPAm consists of two components; the intensity of the 1650 cm-1 component increased, and that of the 1625 cm-1 component decreased with increasing temperature during both LCST and UCST phase transitions. Gradual red shifts of the C-H stretching and the amide II bands with increasing temperature or increasing DMSO concentration indicate a removal of water molecules from the alkyl and N-H groups. Raman microscopic measurements showed that DMSO is excluded from the polymer-rich phases upon both LCST and UCST phase separation. On the basis of the experimental results and the quantum chemical calculations, a model that explains the solvation change of the polymer during phase transitions was proposed.     

Raman scattering study of high-pressure phase transition in thiourea

A high-pressure Raman spectroscopic study of phase transitions in thiourea is reported. The changes in the Raman spectra with increasing and decreasing pressure have been followed to a maximum pressure of approximately 11 GPa. We observe several changes in the spectra including splitting of modes, appearance of new modes, and sudden change in the slope of the frequency-pressure curve at several pressures. On the basis of this study, we propose the existence of three more transitions in this system to phases VII, VIII, and IX at approximately 1, 3, and 6.1 GPa, respectively, in addition to the V-VI phase transition at 0.35 GPa reported earlier. All the transitions have been found to be completely reversible. We interpret these changes in terms of symmetry-lowering phase transitions.     

Sn-containing liquid crystalline side group homopolymers with two glass transitions

Three homologous tin-containing homopolymers with a terminal CN-dipole in the side group have been synthesized and characterized by dynamical calorimetry, polarization microscopy, X-ray and dielectric methods. AFM was used to evaluate the texture at room temperature. Four different phase transitions were detected by DSC. The high temperature phases were identified by polarization microscopy as SmA and SmC. AFM-measurements show focal-conic domains at room temperature and confirm so the smectic nature of all phases. X-ray measurements on nonoriented samples give hints to a phase segregation on nanometer scale. Dielectric investigation and temperature-modulated DSC (TMDSC) confirm clearly phase separation by appearance of two glass transitions related to the liquid order of the main chains and the liquid crystalline of the side groups.     

Structure and reactivity of pyridine crystal under pressure

In this work we have performed an extensive high pressure study of the condensed phases of pyridine by Raman and IR spectroscopy. We have evidenced three different polymorphs, two crystalline, and one glassy and established the pressure conditions in which they exist as stable or metastable phases by several compression/decompression experiments both on annealed and not annealed samples. Crystallization and phase transitions are found to be kinetically driven. The vibrational spectra are extremely complex due to the low symmetry of the crystals, which implies a large number of crystal components. This complexity required a careful analysis of both IR and Raman data that led to the identification of 20 out of 21 external modes expected for phase II. We did not find any conclusive indication of phase transitions on compressing phase II thus indicating that phase II is likely the stable phase at the onset pressure of the chemical transformation of pyridine. The latter starts at 18 GPa and relevant differences from the well characterized benzene reaction suggest that it is likely driven by crystal defects.     

Solid-state transitions in crystalline tetracene

Solid-state transitions in crystalline tetracene have been investigated by Raman spectroscopy and by anisotropy of the effect of a magnetic field on its prompt fluorescence. Discontinuous changes in the temperature dependence of Raman lattice modes and new phonon lines occur near 182 and 144 K, signifying the occurrence of two phase transitions. The magnetic anisotropy effect shows a gradual change with temperature, suggesting the transition at 182 K is preceded by a continuous rotation of the intermolecular plane.     

Nanoscale architectures for molecular electronics: vibrational spectroscopy and structure of solid hexa-n-dodecyl-hexa-peri-hexabenzocoronene

The solid columnar discotic and liquid-crystalline phases formed by hexa-n-dodecyl-hexa-peri-hexabenzocoronene (HBC-C12) have been investigated by IR and Raman vibrational spectroscopies. IR spectra clearly show the two phase transitions at 42 and at 107 degrees C already reported in literature and allow us to understand the conformational modifications of the n-alkyl chains that take place through the transitions. Thanks to the collected data, we propose a model of the structure of HBC-C12 in the solid-crystalline phase below 42 degrees C which includes the structure of the alkyl chains. This model is also confirmed by dichroic infrared microscopy measurements on highly oriented samples.     

Raman and X-ray scattering studies of high-pressure phases of urea

Single-crystal and polycrystalline urea samples were compressed to 12 GPa in a diamond-anvil cell. Raman-scattering measurements indicate a sequence of four structural phases occurring over this pressure range at room temperature. The transitions to the high-pressure phases take place at pressures near 0.5 GPa (phase I --> II), 5.0 GPa (II --> III), and 8.0 GPa (III --> IV). Lattice parameters in phase I (tetragonal, with 2 molecules per unit cell, space group P42(1)m (D3(2d))) and phase II (orthorhombic, 4 molecules per unit cell, space group P2(1)2(1)2(1) (D2(4))) were determined using angle-dispersive X-ray diffraction experiments. For phases III and IV, the combined Raman and diffraction data indicate that the unit cells are likely orthorhombic with four molecules per unit cell. Spatially resolved Raman measurements on single-crystal samples in phases III and IV reveal the coexistence of two domains with distinct spectral features. Physical origins of the spatial domains in phases III and IV are examined and discussed.     

Phase behaviour of poly-(λ-benzyl-L-glutamate) in benzyl alcohol

Polarized optical microscopy has been used to investigate phase transitions in the poly-(λ-benzyl-L-glutamate) benzyl alcohol system and these have been compared with the predictions of Flory. All of the samples studied form gels at room temperature. The behaviour of the lowest concentration studied, 5 per cent by volume, shows transitions in the optical microscope compatible with the Flory phase diagram, becoming isotropic at elevated temperatures. Gels of higher concentrations exhibit bulk phase separation into an isotropic liquid phase and an anisotropic phase at room temperature, also in accord with the Flory predictions; the texture of the anisotropic phase varies with concentration. At higher temperatures these concentrations exhibit two coexistent anisotropic phases.     

Adiabatic calorimetry of the metallomesogen purple cobalt stearate Co(O2CC17H35)2

The heat capacity of the metallomesogen purple cobalt stearate Co(O₂CC₁₇H₃₅)₂ has been measured by adiabatic calorimetry at temperatures between 16 and 420 K. This compound exhibits two crystalline phases (low temperature Cr2 and high temperature Cr1 phases), mesophase (M phase), and isotropic liquid (I phase). A third crystalline phase Cr3, which is entirely metastable with respect to all the others, is suggested by DSC studies. The Cr2-to-Cr1, Cr1-to-M, and M-to-I phase transitions occurred at 362.1, 380.9, and 400.4 K, respectively. The enthalpy and entropy gains at these phase transitions were determined. The mesophase is either smectic A or nematic.     

Quinaldine: Accessing two crystalline polymorphs via the supercooled liquid

Quinaldine (2-methyl quinoline) is a liquid at room temperature, which can be supercooled to reach finally the glassy state. By heating the glass above the glass transition temperature T(g) = 180 K the sample performs two subsequent transitions into, likewise, dielectrically active phases. Thus, the reorientational relaxations of these phases as well as the kinetics of the phase transitions can be tracked in a highly resolved way by dielectric spectroscopy. X-ray diffraction analysis clearly shows two structurally different crystalline phases in addition to the supercooled liquid. Calorimetric measurements support the notion of first order phase transitions, occurring irreversibly in the supercooled regime, and suggest that the intermediate crystalline phase is metastable, too. Analyzing the quite distinct dielectric relaxation strengths, we discuss the possible nature of the two crystalline phases. Additionally, a very similar behavior to quinaldine is observed for 3-methyl quinoline, indicating a broad field of polymorphism among the quinoline derivatives.     

Spinodal and ideal glass limits for metastable liquids

A spinodal line provides a high-temperature limit to the stability of a superheated liquid and a line of ideal glass transitions may provide a low-temperature limit for a supercooled liquid. For models in which the thermodynamic properties depend on only one independent external variable the line of ideal glass transition can be viewed as a second-order transition between two equilibrated phases and it meets the spinodal with the same slope at the maximum tension a stretched liquid can sustain. For real materials there are two independent external variables, temperature and pressure; the line of ideal glass transitions cannot be viewed as a second-order phase transition and the two lines need not have the same slope where they meet.     

High‐ and low‐temperature phases in isostructural 4‐chloro‐3‐nitroaniline and 4‐iodo‐3‐nitroaniline

The structures of 4‐chloro‐3‐nitroaniline, C₆H₅ClN₂O₂, (I), and 4‐iodo‐3‐nitroaniline, C₆H₅IN₂O₂, (II), are isomorphs and both undergo continuous (second order) phase transitions at 237 and 200 K, respectively. The structures, as well as their phase transitions, have been studied by single‐crystal X‐ray diffraction, Raman spectroscopy and difference scanning calorimetry experiments. Both high‐temperature phases (293 K) show disorder of the nitro substituents, which are inclined towards the benzene‐ring planes at two different orientations. In the low‐temperature phases (120 K), both inclination angles are well maintained, while the disorder is removed. Concomitantly, the b axis doubles with respect to the room‐temperature cell. Each of the low‐temperature phases of (I) and (II) contains two pairs of independent molecules, where the molecules in each pair are related by noncrystallographic inversion centres. The molecules within each pair have the same absolute value of the inclination angle. The Flack parameter of the low‐temperature phases is very close to 0.5, indicating inversion twinning. This can be envisaged as stacking faults in the low‐temperature phases. It seems that competition between the primary amine–nitro N—H...O hydrogen bonds which form three‐centred hydrogen bonds is the reason for the disorder of the nitro groups, as well as for the phase transition in both (I) and (II). The backbones of the structures are formed by N—H...N hydrogen bonding of moderate strength which results in the graph‐set motif C(3). This graph‐set motif forms a zigzag chain parallel to the monoclinic b axis and is maintained in both the high‐ and the low‐temperature structures. The primary amine groups are pyramidal, with similar geometric values in all four determinations. The high‐temperature phase of (II) has been described previously [Garden et al. (2004). Acta Cryst. C 60 , o328–o330].     

Temperature behavior of the AlH3 polymorph by in situ investigation using high resolution Raman scattering

A Raman investigation of the AlH(3) polymorph has been carried out at a low temperature (20 K) under helium atmosphere (2 bar). The pristine material was composed of three polymorphs, namely, the α, β, and γ phases. The β phase has been removed by warming the sample to 70 °C, while further heating at 100 °C was used to remove the γ phase. This allowed us to evidence, on a purely experimental basis, the characteristic Raman spectrum for each phase. Raman spectra, for the three phases, have been also calculated using density functional theory, and the results have been compared to the present experimental data, allowing for a univocal assignment, to each phase, of its characteristic spectral features.